Nat Hazards Earth Syst Sci 11 241ndash248 2011wwwnat-hazards-earth-syst-scinet112412011doi105194nhess-11-241-2011copy Author(s) 2011 CC Attribution 30 License

Natural Hazardsand Earth

System Sciences

Seismo-electromagnetic phenomena in the western part of theEurasia-Nubia plate boundary

H G Silva1 M Bezzeghoud1 J P Rocha1 P F Biagi2 M Tlemcani1 R N Rosa1 M A Salgueiro da Silva3J F Borges1 B Caldeira1 A H Reis1 and M Manso4

1Geophysical Centre ofEvora and Physics Department ECT University ofEvora Portugal2University of Bari and Inter-Department Centre for the Evaluation and Mitigation of the Volcanic and Seismic Risk Italy3Department of Physics and Astronomy FCUP University of Porto Portugal4EDISOFT Lazarim Portugal

Received 10 October 2010 ndash Accepted 10 December 2010 ndash Published 28 January 2011

Abstract This paper presents a work that aims to moni-tor seismo-electromagnetic phenomena in the Western Partof the Eurasia-Nubia Plate Boundary This region has asignificant tectonic activity combined with relatively lowelectromagnetic noise levels rendering high quality seismo-electromagnetic measurements possible An overview of theseismicity of this region is presented and the research plan isdiscussed accordingly

1 Introduction

It is known that low-frequency (ultra (ULF) very (VLF) andlow-frequency (LF)) electromagnetic waves produce moreconvincing earthquake precursors (compared to higher fre-quencies) because of less contamination large skin depthand low attenuation Chauhan et al (2009) Thus twoseismo-electromagnetic phenomena (SEM) will be con-sidered ULF electromagnetic field emissions Telescaet al (2008) and VLFLF radio broadcastings Biagi etal (2009) With respect to the ULF measurements magne-tometers are planned to be installed in the South of IberianPeninsula supported by the existing networks of seismic re-search and a portable ULF station is also under considera-tion Regarding the VLFLF radio broadcastings a receiveris presently under installation at the University ofEvora tomonitor radio signals from up to 10 VLFLF transmitters tostudy the seismic activity in the Western Part of the Eurasia-Nubia Plate Boundary (WENP) region The system inte-

Correspondence toH G Silva(hgsilvauevorapt)

grates the International Network for Frontier Research onEarthquake Precursors (INFREP) Details about this networkare presented in other paper submitted to the same issueand in this context it will not be restricted to WENP regionFurthermore these measurements are expected to be com-plemented with the monitoring of the atmospheric electricalfield and the radon levels since these parameters can pro-vide crucial insights into the physics of SEM phenomenol-ogy Smirnov (2008) and Harrison et al (2010)

With the development of this research plan it is aimed thecollection of novel SEM data emerging from the seismic ac-tivity in the WENP region We expect to address the timevariations of the electromagnetic properties of the crustplatewith the strain field and its relation with composition tem-perature and stress fields Moreover the interplay betweenatmospheric (and solar) perturbations with crust perturba-tions will be monitored to observe geomagnetic perturba-tions at different locations

In this paper we describe the seismicity of the WENP re-gion the micro-seismicity of the South of Portugal and theSEM monitoring future research plan together with a sum-mary of the expected results

2 Seismicity of WENP region

The western part of the Eurasia-Nubia plate boundary(WENP) starts in the Azores archipelago and goes to theStrait of Gibraltar This region encloses the transition froman oceanic boundary (between the Azores and the Gor-ringe Bank) to a continental boundary where Iberia andAfrica meet Grandin et al (2007a) In the oceanic partfrom the Azores archipelago to the Azores-Gibraltar fault

Published by Copernicus Publications on behalf of the European Geosciences Union

242 H G Silva et al Seismo-electromagnetic phenomena

15

Figure 3 Photograph of the VLF and LF antennas and the box where receiver is kept (pointed 1

with the blue arrow) at the University of Eacutevora The inset shows the front panel of the 2

receiver 3

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Figure 4 Representative map of the INFREP showing the location (orange circles) of the five 5

VLFLF receivers presently in operation Some transmitters whose signals are sampled by the 6

different receivers are also indicated (light brown diamonds represent VLF transmitters and 7

yellow squares LF ones) The new receiver under installation in Mitra (Eacutevora) is pointed with 8

a red star 9

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Figure 5 Map of the planned VLFLF radio broadcast system three transmitters (represented 11

by yellow squares) located at Azores (Portugal) Madeira (Portugal) and Ceuta (Spain) and 12

the receiver (shown as a red star) installed at Eacutevora (Portugal) 13

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Figure 6 Photograph of the VAE sensor installed at the University of Eacutevora The inset is a 15

zoom into the head of the instrument 16

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Figure 1 31

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Fig 1 Seismicity of the western part of the Eurasia-Nubia plateboundary for the period 01011973ndash17062010 (M ge 30 USGSData File) Small yellow circles represent seismic events with3le M lt 4 small light brown ones correspond to 4le M lt 5 meanorange circles signify 5le M lt 6 and big red hollow circles standfor 6le M lt 9

approximately 12 W the plate boundary is very well delim-ited But from 12 W to 35 E the boundary is more dis-perse and embraces a larger area of deformation this regioncomprises the Iberia-African area and prolongs to the west-ern part of Algeria Buforn et al (2004) Borges et al (20072008) and Bezzeghoud et al (2008 2010) The WENP re-gion can be separated into six fundamental zones (from theMid-Atlantic Ridge in the west to Algeria in the east) withdifferent seismtectonic behaviors zone I ndash The Azores re-gion zone II ndash The central region zone III ndash The Ibero-Maghrebian region In Fig 1 a picture of this region is pre-sented Bezzeghoud et al (2008 2010)

21 The Azores region

The seismicity of the Azores is related with the triple pointEurasian African and American plate boundaries (Fig 1)Typically it presents a complex seismicity composed ofmoderate magnitudes and shallow depth (h lt40 km) eventsIn fact just two earthquakes withMs ge7 occurred the first ateast of Santa Marıa Island (8 May 1939) and the second be-tween Terceira and Graciosa Islands (1 January 1980) Ac-tually the major seismic activity is developed on the Mid-Atlantic Ridge and the North Azores fracture zone andgoes on into the Terceira Ridge Furthermore it is knownfrom historical seismicity that large earthquakes occurred inAzores (with maximum modified Mercalli intensities of X)Nunes and Ribeiro (2001)

In short the seismicity of this region trails much the trendof the islands roughly ENE from the Mid-Atlantic Ridge toTerceira Island the 1980 and 1998 earthquakes took place inthis zone and SE from Terceira Island to San Miguel Islandwhere the 1997 shock appended Seismic activity then van-ishes at 24 W where the Terceira Ridge joins with the Gloriafault that is believed to be seismically dormant as shown inFig 1

22 The central region

The central region also represented in Fig 1 is normally ac-cepted to be an extension of the East Azores Fracture Zonethat starts in the Gloria Fault between the longitudes 23 Wand 11 W and coincides with the previously defined zoneIII The East Azores rupture zone whose direction is aroundEW is evidently pertained by the ocean bottom topogra-phy until 18 W longitude From this longitude a complexbathymetry is observed where it is difficult to identify a com-prehensible alignment since it is fulfilled by large submarinemountains and abyssal plains Along this fracture variousNS similar fractions are recognized with profiles containingsignificant recent scarps Searle (1980) Besides Gloria Faultis a right lateral transform fault Buforn et al (1988a) thatseems to be presently inactive (Fig 1) This is probably dueto the high return period of earthquakes of high magnitudethat can be more than 80 years which is comparable with theexistence time of instrumental data from this region Arguset al (1989) Moreover numerous seismic events occurredin the east of the island Nunes and Ribeiro (2001) As amatter of fact the occurrence on 25 November 1941 of aMs = 84 earthquake with epicenter placed at the eastern endof the Gloria Fault claims in support of this interpretationThe strong magnitude event and the associated seismic de-formation could be a consequence of the relative and longstanding quietness of this fault

Summing up the Gloria Fault is characterized by a seis-mic gap that extends from the 23 W to 20 W From thislongitude to the 11 W it presents an EW orientation that en-gaged different earthquakes as the strong magnitude eventsoccurred on the 20 May 1931 (Ms = 71) 25 November 1941(Ms = 84) and 25 May 1975 (Ms = 79) Bezzeghoud etal (2008 2010)

23 The Ibero-Maghrebian region

This part of WENP can be subdivided into three parts zoneA ndash Gorringe Bank to Cadiz zone B ndash enclosing Betic RifCordilleras and Alboran Sea zone C ndash northwest Algeriaand Tell Mountains Similar division was made in Bufornet al (2004) Bezzeghoud et al (2008 2010) In effect in-termediate depth activity 40lt h lt 150 km is located in re-gions A and B and the deep earthquakesh sim 650 km arelocated to the south of Granada an example of such eventsis the recentMs = 63 earthquake on 11 May 2010 with623 km depth that as a result of being so deep are not of in-terest for SEM monitoring The zone intricacy reflects itsbathymetry seismicity stress regime and tectonics Indeedthe bathymetry evidences the Gorringe Bank the HorseshoeAbyssal Plain and the Guadalquivir bank (located west ofthe Strait of Gibraltar) with a number of seamounts banksand submarine ridges with important regional crustal thick-ness variations Torne et al (2000) as fundamental aspectsBesides in land the main geological features are the Bet-ics the Rif Cordilleras and the Tell Mountains that are a

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 243

consequence of the collision between Eurasia and Africaand form the Alpine domain In the Gibraltar western partfrom Gulf of Cadiz to Gorringe bank epicenters are scatteredin an EW direction across aboutsim100 km wide with fociat shallow and intermediate depth Four significant earth-quakes have occurred in this region in the last 50 years onein the Gulf of Cadiz (15 March 1964Ms = 64) and threeprobably along the Horseshoe fault at west of the San Vi-cente Cape (12 December 2009Mw = 56 2 February 2007Mw = 59 28 February 1969Ms = 80) Udıas et al (1976)Buforn et al (1988b) Bezzeghoud et al (2010) Since 2003an increase in the seismicity of the area enclosing the Gor-ringe Bank and the Horseshoe Fault (Zone A) has been ob-served In particular the East of 16 W region is controlledby a transpressive tectonic regime with a very low conver-gence rate of 55 mmyr trending NW to NNW Bezzeghoudet al (2008 2010) Such regime conforms the observed max-imum horizontal stress direction Borges et al (2001) andBezzeghoud et al (2008 2010) Moreover in this regionthe limit between plates occupies a large area with a dissem-inated deformation that even reaches a NS width of 300 kmclose to the Iberia continental margin Actually the seismic-ity is spread even though a vast number of events are con-centrated along asim100 km extensive band trending ESE-WNW from 16 W to 9 W In the area sequences of topo-graphic configurations tending WSW-ENE appear Borges etal (2007) Bezzeghoud et al (2008 2010) Parallel to theSan Vicente canyon the Horseshoe scarp and the Marquesde Pombal scarp since at least the Miocene have undergonedeformations This hypothesis is corroborated by the inci-dence of oceanic earthquakes with abnormally large magni-tude inside the area of dispersed seismic activity alike the1969 earthquake withMs = 80 and the historical 1755 Lis-bon earthquake Grandin et al (2007a 2007b) On the otherhand historical earthquakes with maximum intensity of IXor X occurred W of San Vicente Cape (Lisbon earthquakein 1755) southern Iberia (1829 and 1884) and in northernMorocco (1909) At least seismic activity with magnituderange from 35 to 5 at intermediate depth (40lt h lt 150 km)is further present in this region as extensively described inliterature Munuera (1963) Hatzfeld (1978) Grimison andCheng (1986) Buforn et al (1988b 1991a 1991b 19972004) Seber et al (1996) Serrano et al (1998) Bezzeghoudet al (2008 2010) Particularly in the area A intermediate-depth shocks are diffuse over asim100 km broad belt (between36 N and 37 N) that goes EW from 8 W to 11 W Nev-ertheless a large amount of foci (the majority in fact) atintermediate depth is found in area B at the eastern sideof the Strait of Gibraltar in a thin NS trending band lessthan 50 km wide concentrated at 45 W and expanding from35 N to 37 N Therefore the distribution of intermediate-depth earthquakes is different in area A (EW propensity)from that of area B (NS trend) No intermediate depth seis-micity is observed east of 3 W in Spain Morocco or Algeria

Summarizing the seismic activity of the Iberia-African re-

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Figure 2 17

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Figure 3 33

Fig 2 Yellow circles represent the micro-seismicity (1lt M lt 3) ofthe South of Portugal acquired with the network described in Rochaet al (2008) for nearly 4 years of observation Green circles standfor the seismic records from IM database from 1961ndash2009 The redstars show the planned locations to host the ULF three-componentmagnetometers

gion is dominated by the incidence of moderate magnitudeearthquakes the majority with shallow depth focus in therange 0lt h lt 40 km Nevertheless significant seismicity atintermediate depths 40lt hlt 150 km with an EW trend andconcentrated in Gulf of Cadiz is also present Yet another in-termediate depth seismicity spot between 36 N and 37 Nappears in a 100 km wide area and it also generates verydeep earthquakes (h sim650 km) Buforn et al (1991a 1997)

3 Micro-seismicity of the south of Portugal

The study of the micro-seismic activity of the south of Por-tugal is recent but is revealing rather interesting results Thedata here analysed were acquired using both the Portuguesenational network supported by Instituto de Meteorologia IPPortugal (IM) and a seismic monitoring network installedon purpose to this study in the mentioned region Rocha etal (2008) In Fig 2 the micro-seismicityM lt 3 of thisregion is presented It appears that the seismicity in both re-gions is characterized by low magnitude events Bezzeghoudet al (2008) It is clearly possible to define in the region ofMonchique epicenter alignments that suggest a correlationwith two possible faults newly discovered in this area Rochaet al (2008) In the NW region of Almodovar it is possibleto define a trend of the distribution of epicenters that mayindicate the occurrence of an uncharted fault in this zone

Interestingly the field compression is in agreement withthe regional stress field expected for this region and the re-sulting collision between the Eurasian plate and African

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

244 H G Silva et al Seismo-electromagnetic phenomena

31 Almodovar region

The obtained results show the occurrence of reduced mag-nitude events Using the proper programs of localizationKlein (1978) and relocation Waldhaus (2001) it was pos-sible to define a zone of seismic activity in Almodovar re-gion (see Fig 2) The seismicity detected in the region ofAlmodovar has a tendency according to NE-SW orientationwhich becomes more evident with the epicenter redistribu-tion obtained after the relocation

32 Monchique region

The studies undertaken allowed the identification of a rel-evant seismic activity with typical magnitudes 1lt M lt 3and constrained between 5 and 15 km depth around theMonchique region The seismicity map obtained after therelocation using the previous programs defines a clear align-ment of the seismic epicenters in this region which can berelated with two probable faults recently indentified in thisarea Rocha et al (2008) The faults have ENE-WSW andNNE-SSW directions

4 SEM monitoring study

As already pointed out in the introduction the present studyintegrates two main types of SEM phenomena one dedicatedto the recognition of ULF electromagnetic fields and theother to the detection of VLFLF radio broadcasting anoma-lies both related with seismic activities It has three struc-tural objectives (1) establish an Iberian monitoring networkof seismic related ULF electromagnetic emissions possi-bly integrated in the existing seismic networks Caldeira etal (2007) and the Western Mediterranean Broad-Band Seis-mological Network (WMBB) Martin et al (2007) (2) ex-tend the international INFREP network with a new receiverwhose installation is already in progress in the University ofEvora (3) contribute to the increase of both seismic relatedULF and VLFLF world database Moreover these measure-ments are expected to be complemented with the monitoringof the atmospheric electrical field and the radon levels sincethese parameters can give crucial insights into the physics ofSEM phenomenology In this way multi-parameter assess-ment of the preparatory stage of the earthquakes is envisaged

41 Observation of ULF magnetic field emissions

The detection of ULF electromagnetic fields aims to equipseismic stations in the south of Iberian Peninsula with three-component ULF magnetometers of type LEMI-30 (producedby the Lviv Centre of Space Research Ukraine) as com-monly used in ULF research Chauhan et al (2009) A pos-sible location for the initial two equipments is presented inFig 2 According to the micro-seismicity previously pre-sented the following potential regions have been selected

(1) offshore zone with intermediate and strong magnitude ndashbetween Gorringe Bank the Horseshoe Fault and the Gulf ofCadiz (2) onshore zone with low magnitude ndash Algarve andsouth Alentejo (between Monchique and Almodovar) Animportant point is that the sensors are installed in specificsites that exhibit significant seismic activity and simultane-ously low electromagnetic noise levels These conditionsfavour detection of SEM phenomena Moreover preferencewill be given to places that already host seismic stations Aportable three-component ULF magnetometer most proba-bly the LEMI-18 model will be used in the search for theproper location by monitoring the electromagnetic activityof the region of interest Once a specific site is chosen itwill be employed in calibration procedures Nonethelessthis portable magnetometer could also be used for in-situ af-tershock studies typicallyM sim 3minus 5 once the mainshockepicentre has been correctly identified Special attention willbe given to man-made noise sources such as electricity trans-mission grids factories roads etc that must be clearly iden-tified to ensure the reliability of the acquired data Prattes etal (2008) and Bleier et al (2009) Our ULF study will be fo-cused on the analysis of low magnitude earthquakes (LME)with M le 4 These events are frequent in the South of Por-tugal but have been almost completely disregarded in liter-ature as will be discussed in the next session Finally themagnetometers will integrate the South European GeoMag-netic Array (SEGMA) array

42 Observation of VLFLF radio signals

The recognition of VLFLF radio broadcasting anomaliesis already being performed through two antennas (one foreach band) connected to a receiver built by Elettronika(Italy) which is able to acquire up to 10 signals (dis-tributed in these bands) This equipment is frequently em-ployed in similar experiments Biagi et al (2009) In Fig 3a photograph of the apparatus installation in the Univer-sity of Evora (3834prime N 754prime W 300 m amsl) is pre-sented The system was recently installed in the contextof the present project and is in operation since 1 Septem-ber 2010 The receiver integrates the International Net-work for Frontier Research on Earthquake Precursors (IN-FREP) shown in Fig 4 and is expected to monitor signalsemitted from active transmitters to study the seismic activ-ity in the WENP region In fact the relevant issue whenchoosing a transmitter is that the radio path from the trans-mitter to the receiver crosses the epicentral area Biagi etal (2008 2009) so three transmitters out of the ten being ex-amined (already monitored by other INFREP receivers) arelocated in Sicily (Italy) 3707prime N126prime E Sardinia (Italy)4054prime N0942prime E Monaco (France) 4347prime N0609prime EOnce we assure a proper functioning of the system threefrequencies that are now being monitored will possiblybe substituted by three other emitters in the west ofthe Iberian Peninsula Ponta Delgada (Azores-Portugal)

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 245

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Fig 3 Photograph of the VLF and LF antennas and the box wherereceiver is kept (pointed with the blue arrow) at the University ofEvora The inset shows the front panel of the receiver

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Figure 4 17

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Figure 5 31

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Fig 4 Representative map of the INFREP showing the location(orange circles) of the five VLFLF receivers presently in opera-tion Some transmitters whose signals are sampled by the differentreceivers are also indicated (light brown diamonds represent VLFtransmitters and yellow squares LF ones) The new receiver underinstallation in Mitra (Evora) is pointed with a red star

Funchal (Madeira-Portugal) Ceuta (Spain) Figure 5 showsthe location of the mentioned emitters and the receiver It isexpected that the receiver is able to detect anomalies relatedwith high magnitude earthquakes (HME)M ge 5 in EuropeThe data acquired with this equipment will be available tothe INFREP community according to the regulation of thenetwork

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Figure 4 17

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Figure 5 31

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Fig 5 Map of the planned VLFLF radio broadcast system threetransmitters (represented by yellow squares) located at Azores (Por-tugal) Madeira (Portugal) and Ceuta (Spain) and the receiver(shown as a red star) installed atEvora (Portugal)

43 Observation of the Atmospheric Electrical field

Investigation of AE anomalies related with seismic activityis by now being preformed through a vertical component ofthe atmospheric electrical field (VAE) sensor Keithley Elec-trometer model JCI 131 installed at the University ofEvora(in the same coordinates as the VLFLF system) This equip-ment has been in operation during the period of December2003 to October 2004 and from February 2005 until now Itis prepared for continuous monitoring of the VAE and worksin four scales 2 20 200 and 2000 kVm with automaticcommutations respectively with the correspondent sensitiv-ity thresholds of 01 1 10 and 100 kVm Figure 6 presentsa picture of the apparatus Inspection of the data collecteduntil now is revealing interesting results presented in a pa-per submitted to this issue Silva et al (2010) In the futureit is planned that more VAE sensors could equip the seis-mic stations that will receive ULF magnetometers in orderto achieve multiple-parameter monitoring

44 Monitorization of the atmospheric Radon levels

The Radon levels are monitored using for the moment(the acquisition of new equipments is being considered) aldquoRadon Thoron Daughters Meter model 4Srdquo built by Silena(a former Italian company) that uses alpha spectrometryand has a sensitivity of 37 Bqm3 of equilibrium equiva-lent radon concentration (1 mWL) and an electronically-regulated flow-rate of 3 lmin with 5 precision The ap-paratus is now under installation at the University ofEvorain the same place of the VAE sensor and VLFLF system toallow multiple parameter assessment of the region Precur-sor anomalous Radon levels have been reported for variousseismic events Toutain et al (1999) Thus monitoring suchlevels deserves attention by itself Even so their direct cor-relation with SEM could open new insights into the physical

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

246 H G Silva et al Seismo-electromagnetic phenomena

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Figure 6 23 Fig 6 Photograph of the VAE sensor installed at the University ofEvora The inset is a zoom into the head of the instrument

mechanisms behind SEM that are the main purpose of thisproject Similarly to the VAE sensors new Radon detectorscould in the future equip the seismic stations that will hostULF magnetometers

5 Discussion

With this research effort it is likely that new SEM data linkedwith seismic activity in the WENP region could emerge andcontribute to a better understanding of SEM phenomenaFurthermore such data could be used in the following ob-jectives (1) perform the correlation between the ULF andVLFLF radio signals for different seismic events with theatmospheric electrical field and the radon levels (2) theoret-ical modelling of the observed phenomena (3) extend theempirical magnitude versus distance diagram for the detec-tion of abnormal ULF signals preceding HME presented byHattori et al (2004) to LME (working below the present de-tection threshold) In fact since LME were not considered inthe former diagram the extension of such diagram could pro-vide valuable information about the ULF precursory emis-sions Moreover the study of Gladychev et al (2010) re-veals that observation of LME is feasible if the seismic epi-centre is sufficiently close to the ULF station (4) ExploreVLFLF radio anomalies correlated with HME earthquakesand also LME already below the empirical threshold for itsdetection (similar to the ULF case) discussed by Rozhnoi etal (2004) (5) Search for a relation law between the strength

of SEM anomalies and the magnitude andor local (referredto the place where the sensors will be installed) ground mo-tion of the impending LME This objective could give an im-portant insight into the driving mechanisms of these phenom-ena and present a decisive contribution to the development ofthis field Obviously the SEM perturbations caused by LMEwill be in principle very faint and their detection and dis-crimination from other possible signal sources will representa real challenge

As a final remark it is important to mention that labo-ratorial experiments concerning the electrical properties ofthe rocks (for the moment the study is focusing on graniticrocks) which may contribute to a better understanding ofSEM phenomena are already being undertaken

AcknowledgementsThe Portuguese team acknowledges the sup-port of two Portuguese institutions QREN support (OperationalProgram ldquoThematic Factors of Competitivenessrdquo) co-fundedby the European Regional Development Fund (ERDF) for theresearch program SIRAS and the FCT (Science and TechnologyFoundation) for the grant SFRHBPD638802009 (HGS) Finallywe are grateful to the support from Samuel Barias e ClaudiaSerrano

Edited by K EftaxiasReviewed by C Nomicos and another anonymous referee

References

Argus D Gordon R DeMets C and Stein S Closureof the Africa-Eurasia-North America plate motion and tec-tonics of the Gloria fault J Geophys Res 94 5585ndash5602doi101029JB094iB05p05585 1989

Bezzeghoud M Borges J F Caldeira B Buforn E and UdıasA Seismic activity in the Azores Region in the context of thewestern part of the Eurasia-Nubia plate boundary InternationalSeminar on Seismic risk and rehabilitation on the 10th Anniver-sary of the July 9 1998 Azores Earthquake Horta-Azores 9ndash13July 27 2008

Bezzeghoud M Borges J F and Caldeira B Fontes sısmicasao longo da fronteira de placas tectonicas entre os Acores e aArgelia um modelo sismotectonico edited by Dias R AraujoA and Terrinha P (Escolar Editora Lisbon Portugal) in press2010

Biagi P F Castellana L Maggipinto T and Ermini A Anoverview on preseismic anomalies in LF radio signals revealed inItaly by wavelet analysis Ann Geophys Ann Geophys 51(1)237ndash246 2008

Biagi P F Castellana L Maggipinto T Loiacono D Schi-avulli L Ligonzo T Fiore M Suciu E and Ermini AA pre seismic radio anomaly revealed in the area where theAbruzzo earthquake (M = 63) occurred on 6 April 2009 NatHazards Earth Syst Sci 9 1551ndash1556 doi105194nhess-9-1551-2009 2009

Bleier T Dunson C Maniscalco M Bryant N Bambery Rand Freund F Investigation of ULF magnetic pulsations airconductivity changes and infra red signatures associated with

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 247

the 30 October Alum Rock M54 earthquake Nat Hazards EarthSyst Sci 9 585ndash603 doi105194nhess-9-585-2009 2009

Borges J Fitas A J S Bezzeghoud M and Teves-Costa PSeismotonics of Portugal and its adjacent Atlantic area Tectono-physics 331(4) 373ndash387 doi101016S0040-1951(00)00291-22001

Borges J F Bezzeghoud M Buforn E Pro C and FitasA The 1980 1997 and 1998 Azores earthquakes and itsseismotectonic implications Tectonophysics 435(1ndash4) 37ndash54doi101016jtecto200701008 2007

Borges J F Caldeira B Bezzeghoud M and Buforn E Seis-micity and Seismotectonics of Azores Geodynamic Implica-tions Ch 6 99-110 in A Book on the 1998 Azores earthquake -10 years after its occurrence edited by Oliveira C S Costa Aand Nunes J C ISBN 978989-20-1223-0 741 2008

Buforn E Udıas A and Colombas M A Seismic-ity source mechanisms and seismotectonics of the Azores-Gibraltar plate boundary Tectonophysics 152(1ndash2) 89ndash118doi1010160040-1951(88)90031-5 1988a

Buforn E Udıas A and Mezcua J Seismicity and focal mecha-nisms in south Spain Bull Seism Soc Am 78(6) 2008ndash2024(httpwwwbssaonlineorgcgicontentabstract7862008)1988b

Buforn E Udıas A and Madariaga R Intermediateand deep earthquakes in Spain Pageoph 136(4) 375-393doi101007BF00878576 1991a

Buforn E Udıas A Mezcua J and Madariaga R A deep earth-quake under south Spain 8 March 1990 Bull Seism Soc Am81(4) 1403ndash1407httpbssageoscienceworldorgcgicontentcitation8141403 1991b

Buforn E Coca P Udıas A and Lasa C Source mechanism ofintermediate and deep earthquakes in southern Spain J Seismol1(2) 113ndash130 doi101023A1009754219459 1997

Buforn E Bezzeghoud M Udıas A and Pro C Seis-mic Sources on the Iberia-African Plate Boundary and theirTectonic Implications Pure Appl Geophys 161(3) 623ndash646doi101007s00024-003-2466-1 2004

Caldeira B Carrilho F Miranda M Bezzeghoud M Alves PM Silveira G Villalonga F Pena J A Matias L BorgesJ F Vales D Corela C and Madureira G Recent improve-ments in the Broadband seismic networks in Portugal CSEM EMSC Newsletter 18-19 May 2007

Chauhan V Singh O P Kushwah V Singh V andSingh B Ultra-low-frequency (ULF) and total electron con-tent (TEC) anomalies observed at Agra and their associa-tion with regional earthquakes J Geodyn 48(2) 68ndash74doi101016jjog200906002 2009

Gladychev V Baransky L Schekotov A Fedorov EPokhotelov O Andreevsky S Rozhnoi A Khabazin YBelyaev G Gorbatikov A Gordeev E Chebrov V SinitsinV Lutikov A Yunga S Kosarev G Surkov V MolchanovO Hayakawa M Uyeda S Nagao T Hattori K and NodaY Study of electromagnetic emissions associated with seismicactivity in Kamchatka region Nat Hazards Earth Syst Sci 1127ndash136 doi105194nhess-1-127-2001 2001

Grandin R Borges J F Bezzeghoud M Caldeira B and Car-rilho F Simulations of strong ground motion in SW Iberia forthe 1969 February 28 (Ms = 80) and the 1755 November 1 (Msim 85) earthquakes ndash I Velocity model Geophys J Int 171(3)

1144-1161 doi101111j1365-246X200703570x 2007aGrandin R Borges J F Bezzeghoud M Caldeira B and Car-

rilho F Simulations of strong ground motion in SW Iberiafor the 1969 February 28 (Ms = 80) and the 1755 November1 (M sim 85) earthquakes ndash II Strong ground motion simula-tions Geophys J Int 171(2) 807-822 doi 101111j1365-246X200703571x 2007b

Grimison N and Cheng W The Azores-Gibraltar plate bound-ary focal mechanisms depths of earthquakes and their tec-tonic implications J Geophys Res 91(B2) 2029ndash2047doi101029JB091iB02p02029 1986

Harrison R G Aplin K L and Rycroft M J Atmo-spheric electricity coupling between earthquake regions andthe ionosphere J Atmos Sol-Terr Phys 72(5ndash6) 376ndash381doi101016jjastp200912004 2010

Hattori K Takahashi I Yoshino C C Isezaki N Iwasaki HHarada M Kawabata K Kopytenko E Kopytenko Y Malt-sev P Korepanov V Molchanov O Hayakawa M NodaY Nagao T and Uyeda S ULF geomagnetic field measure-ments in Japan and some recent results associated with IwatekenNairiku Hokubu earthquake in 1998 Phys Chem Earth 29(4ndash9) 481ndash494 doi101016jpce200309019 2004

Hatzfeld D Etude sismotectonique de la zone de collision Ibero-Maghrebine PhD Thesis Grenoble (France) 281 1978

Munuera J M Datos basicos para un estudio de sismicidad en laregion de la Penınsula Iberica Mem Inst Geog Cat Madrid32 93 1963

Nunes J C and Ribeiro E Caracterizacao da sismicidade instru-mental dos Acores no periodo 1950ndash1980 SISMICA EncontroNacional de Sismologıa e Enghenaria Sısmica Acores 2001

Prattes G Schwingenschuh K Eichelberger H U MagnesW Boudjada M Stachel M Vellante M WesztergomV and Nenovski P Multi-point ground-based ULF magneticfield observations in Europe during seismic active periods in2004 and 2005 Nat Hazards Earth Syst Sci 8 501ndash507doi105194nhess-8-501-2008 2008

Rocha J P Bezzeghoud M Caldeira B Araujo A Borges JF Vilallonga F and Dorbath C Microseismicity in the neigh-bourhood of ldquoAlmodovar faultrdquo 6a Assembleia Luso Espanholade Geodesia e Geofısica 2008 (in Portuguese)

Rozhnoi A Solovieva M S Molchanov O A andHayakawa M Middle latitude LF (40 kHz) phase varia-tions associated with earthquakes for quiet and disturbed ge-omagnetic conditions Phys Chem Earth 29(4ndash9) 589ndash598doi101016jpce200308061 2004

Searle R Tectonic pattern of the Azores spreading centreand triple junction Earth Plan Sci Lett 51(2) 415ndash434doi1010160012-821X(80)90221-6 1980

Seber D Barazangi M Ibenbrahim A and Demnati AGeophysical evidence for lithospheric delamination beneath theAlboran Sea and Rif-Betic mountains Nature 379 785ndash790doi101038379785a0 1996

Serrano I Morales J Zhao D Torcal F and Vidal F P-wave tomographic images in the Central Betics-Alboran Sea(south Spain) using local earthquakes contribution for a con-tinental collision Geophys Res Lett 25(21) 4031ndash4034doi1010291998GL900021 1998

Silva H G Bezzeghoud M Reis A H Rosa R N TlemcaniM Araujo A A Caldeira B Borges J F Serrano C and

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

H G Silva et al Seismo-electromagnetic phenomena 243

consequence of the collision between Eurasia and Africaand form the Alpine domain In the Gibraltar western partfrom Gulf of Cadiz to Gorringe bank epicenters are scatteredin an EW direction across aboutsim100 km wide with fociat shallow and intermediate depth Four significant earth-quakes have occurred in this region in the last 50 years onein the Gulf of Cadiz (15 March 1964Ms = 64) and threeprobably along the Horseshoe fault at west of the San Vi-cente Cape (12 December 2009Mw = 56 2 February 2007Mw = 59 28 February 1969Ms = 80) Udıas et al (1976)Buforn et al (1988b) Bezzeghoud et al (2010) Since 2003an increase in the seismicity of the area enclosing the Gor-ringe Bank and the Horseshoe Fault (Zone A) has been ob-served In particular the East of 16 W region is controlledby a transpressive tectonic regime with a very low conver-gence rate of 55 mmyr trending NW to NNW Bezzeghoudet al (2008 2010) Such regime conforms the observed max-imum horizontal stress direction Borges et al (2001) andBezzeghoud et al (2008 2010) Moreover in this regionthe limit between plates occupies a large area with a dissem-inated deformation that even reaches a NS width of 300 kmclose to the Iberia continental margin Actually the seismic-ity is spread even though a vast number of events are con-centrated along asim100 km extensive band trending ESE-WNW from 16 W to 9 W In the area sequences of topo-graphic configurations tending WSW-ENE appear Borges etal (2007) Bezzeghoud et al (2008 2010) Parallel to theSan Vicente canyon the Horseshoe scarp and the Marquesde Pombal scarp since at least the Miocene have undergonedeformations This hypothesis is corroborated by the inci-dence of oceanic earthquakes with abnormally large magni-tude inside the area of dispersed seismic activity alike the1969 earthquake withMs = 80 and the historical 1755 Lis-bon earthquake Grandin et al (2007a 2007b) On the otherhand historical earthquakes with maximum intensity of IXor X occurred W of San Vicente Cape (Lisbon earthquakein 1755) southern Iberia (1829 and 1884) and in northernMorocco (1909) At least seismic activity with magnituderange from 35 to 5 at intermediate depth (40lt h lt 150 km)is further present in this region as extensively described inliterature Munuera (1963) Hatzfeld (1978) Grimison andCheng (1986) Buforn et al (1988b 1991a 1991b 19972004) Seber et al (1996) Serrano et al (1998) Bezzeghoudet al (2008 2010) Particularly in the area A intermediate-depth shocks are diffuse over asim100 km broad belt (between36 N and 37 N) that goes EW from 8 W to 11 W Nev-ertheless a large amount of foci (the majority in fact) atintermediate depth is found in area B at the eastern sideof the Strait of Gibraltar in a thin NS trending band lessthan 50 km wide concentrated at 45 W and expanding from35 N to 37 N Therefore the distribution of intermediate-depth earthquakes is different in area A (EW propensity)from that of area B (NS trend) No intermediate depth seis-micity is observed east of 3 W in Spain Morocco or Algeria

Summarizing the seismic activity of the Iberia-African re-

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Figure 2 17

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23

24

25

26

27

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29

30

31

32

Figure 3 33

Fig 2 Yellow circles represent the micro-seismicity (1lt M lt 3) ofthe South of Portugal acquired with the network described in Rochaet al (2008) for nearly 4 years of observation Green circles standfor the seismic records from IM database from 1961ndash2009 The redstars show the planned locations to host the ULF three-componentmagnetometers

gion is dominated by the incidence of moderate magnitudeearthquakes the majority with shallow depth focus in therange 0lt h lt 40 km Nevertheless significant seismicity atintermediate depths 40lt hlt 150 km with an EW trend andconcentrated in Gulf of Cadiz is also present Yet another in-termediate depth seismicity spot between 36 N and 37 Nappears in a 100 km wide area and it also generates verydeep earthquakes (h sim650 km) Buforn et al (1991a 1997)

3 Micro-seismicity of the south of Portugal

The study of the micro-seismic activity of the south of Por-tugal is recent but is revealing rather interesting results Thedata here analysed were acquired using both the Portuguesenational network supported by Instituto de Meteorologia IPPortugal (IM) and a seismic monitoring network installedon purpose to this study in the mentioned region Rocha etal (2008) In Fig 2 the micro-seismicityM lt 3 of thisregion is presented It appears that the seismicity in both re-gions is characterized by low magnitude events Bezzeghoudet al (2008) It is clearly possible to define in the region ofMonchique epicenter alignments that suggest a correlationwith two possible faults newly discovered in this area Rochaet al (2008) In the NW region of Almodovar it is possibleto define a trend of the distribution of epicenters that mayindicate the occurrence of an uncharted fault in this zone

Interestingly the field compression is in agreement withthe regional stress field expected for this region and the re-sulting collision between the Eurasian plate and African

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

244 H G Silva et al Seismo-electromagnetic phenomena

31 Almodovar region

The obtained results show the occurrence of reduced mag-nitude events Using the proper programs of localizationKlein (1978) and relocation Waldhaus (2001) it was pos-sible to define a zone of seismic activity in Almodovar re-gion (see Fig 2) The seismicity detected in the region ofAlmodovar has a tendency according to NE-SW orientationwhich becomes more evident with the epicenter redistribu-tion obtained after the relocation

32 Monchique region

The studies undertaken allowed the identification of a rel-evant seismic activity with typical magnitudes 1lt M lt 3and constrained between 5 and 15 km depth around theMonchique region The seismicity map obtained after therelocation using the previous programs defines a clear align-ment of the seismic epicenters in this region which can berelated with two probable faults recently indentified in thisarea Rocha et al (2008) The faults have ENE-WSW andNNE-SSW directions

4 SEM monitoring study

As already pointed out in the introduction the present studyintegrates two main types of SEM phenomena one dedicatedto the recognition of ULF electromagnetic fields and theother to the detection of VLFLF radio broadcasting anoma-lies both related with seismic activities It has three struc-tural objectives (1) establish an Iberian monitoring networkof seismic related ULF electromagnetic emissions possi-bly integrated in the existing seismic networks Caldeira etal (2007) and the Western Mediterranean Broad-Band Seis-mological Network (WMBB) Martin et al (2007) (2) ex-tend the international INFREP network with a new receiverwhose installation is already in progress in the University ofEvora (3) contribute to the increase of both seismic relatedULF and VLFLF world database Moreover these measure-ments are expected to be complemented with the monitoringof the atmospheric electrical field and the radon levels sincethese parameters can give crucial insights into the physics ofSEM phenomenology In this way multi-parameter assess-ment of the preparatory stage of the earthquakes is envisaged

41 Observation of ULF magnetic field emissions

The detection of ULF electromagnetic fields aims to equipseismic stations in the south of Iberian Peninsula with three-component ULF magnetometers of type LEMI-30 (producedby the Lviv Centre of Space Research Ukraine) as com-monly used in ULF research Chauhan et al (2009) A pos-sible location for the initial two equipments is presented inFig 2 According to the micro-seismicity previously pre-sented the following potential regions have been selected

(1) offshore zone with intermediate and strong magnitude ndashbetween Gorringe Bank the Horseshoe Fault and the Gulf ofCadiz (2) onshore zone with low magnitude ndash Algarve andsouth Alentejo (between Monchique and Almodovar) Animportant point is that the sensors are installed in specificsites that exhibit significant seismic activity and simultane-ously low electromagnetic noise levels These conditionsfavour detection of SEM phenomena Moreover preferencewill be given to places that already host seismic stations Aportable three-component ULF magnetometer most proba-bly the LEMI-18 model will be used in the search for theproper location by monitoring the electromagnetic activityof the region of interest Once a specific site is chosen itwill be employed in calibration procedures Nonethelessthis portable magnetometer could also be used for in-situ af-tershock studies typicallyM sim 3minus 5 once the mainshockepicentre has been correctly identified Special attention willbe given to man-made noise sources such as electricity trans-mission grids factories roads etc that must be clearly iden-tified to ensure the reliability of the acquired data Prattes etal (2008) and Bleier et al (2009) Our ULF study will be fo-cused on the analysis of low magnitude earthquakes (LME)with M le 4 These events are frequent in the South of Por-tugal but have been almost completely disregarded in liter-ature as will be discussed in the next session Finally themagnetometers will integrate the South European GeoMag-netic Array (SEGMA) array

42 Observation of VLFLF radio signals

The recognition of VLFLF radio broadcasting anomaliesis already being performed through two antennas (one foreach band) connected to a receiver built by Elettronika(Italy) which is able to acquire up to 10 signals (dis-tributed in these bands) This equipment is frequently em-ployed in similar experiments Biagi et al (2009) In Fig 3a photograph of the apparatus installation in the Univer-sity of Evora (3834prime N 754prime W 300 m amsl) is pre-sented The system was recently installed in the contextof the present project and is in operation since 1 Septem-ber 2010 The receiver integrates the International Net-work for Frontier Research on Earthquake Precursors (IN-FREP) shown in Fig 4 and is expected to monitor signalsemitted from active transmitters to study the seismic activ-ity in the WENP region In fact the relevant issue whenchoosing a transmitter is that the radio path from the trans-mitter to the receiver crosses the epicentral area Biagi etal (2008 2009) so three transmitters out of the ten being ex-amined (already monitored by other INFREP receivers) arelocated in Sicily (Italy) 3707prime N126prime E Sardinia (Italy)4054prime N0942prime E Monaco (France) 4347prime N0609prime EOnce we assure a proper functioning of the system threefrequencies that are now being monitored will possiblybe substituted by three other emitters in the west ofthe Iberian Peninsula Ponta Delgada (Azores-Portugal)

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 245

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Figure 2 17

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33

Fig 3 Photograph of the VLF and LF antennas and the box wherereceiver is kept (pointed with the blue arrow) at the University ofEvora The inset shows the front panel of the receiver

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Figure 4 17

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30

Figure 5 31

32

33

Fig 4 Representative map of the INFREP showing the location(orange circles) of the five VLFLF receivers presently in opera-tion Some transmitters whose signals are sampled by the differentreceivers are also indicated (light brown diamonds represent VLFtransmitters and yellow squares LF ones) The new receiver underinstallation in Mitra (Evora) is pointed with a red star

Funchal (Madeira-Portugal) Ceuta (Spain) Figure 5 showsthe location of the mentioned emitters and the receiver It isexpected that the receiver is able to detect anomalies relatedwith high magnitude earthquakes (HME)M ge 5 in EuropeThe data acquired with this equipment will be available tothe INFREP community according to the regulation of thenetwork

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Figure 4 17

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Figure 5 31

32

33

Fig 5 Map of the planned VLFLF radio broadcast system threetransmitters (represented by yellow squares) located at Azores (Por-tugal) Madeira (Portugal) and Ceuta (Spain) and the receiver(shown as a red star) installed atEvora (Portugal)

43 Observation of the Atmospheric Electrical field

Investigation of AE anomalies related with seismic activityis by now being preformed through a vertical component ofthe atmospheric electrical field (VAE) sensor Keithley Elec-trometer model JCI 131 installed at the University ofEvora(in the same coordinates as the VLFLF system) This equip-ment has been in operation during the period of December2003 to October 2004 and from February 2005 until now Itis prepared for continuous monitoring of the VAE and worksin four scales 2 20 200 and 2000 kVm with automaticcommutations respectively with the correspondent sensitiv-ity thresholds of 01 1 10 and 100 kVm Figure 6 presentsa picture of the apparatus Inspection of the data collecteduntil now is revealing interesting results presented in a pa-per submitted to this issue Silva et al (2010) In the futureit is planned that more VAE sensors could equip the seis-mic stations that will receive ULF magnetometers in orderto achieve multiple-parameter monitoring

44 Monitorization of the atmospheric Radon levels

The Radon levels are monitored using for the moment(the acquisition of new equipments is being considered) aldquoRadon Thoron Daughters Meter model 4Srdquo built by Silena(a former Italian company) that uses alpha spectrometryand has a sensitivity of 37 Bqm3 of equilibrium equiva-lent radon concentration (1 mWL) and an electronically-regulated flow-rate of 3 lmin with 5 precision The ap-paratus is now under installation at the University ofEvorain the same place of the VAE sensor and VLFLF system toallow multiple parameter assessment of the region Precur-sor anomalous Radon levels have been reported for variousseismic events Toutain et al (1999) Thus monitoring suchlevels deserves attention by itself Even so their direct cor-relation with SEM could open new insights into the physical

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

246 H G Silva et al Seismo-electromagnetic phenomena

18

1

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Figure 6 23 Fig 6 Photograph of the VAE sensor installed at the University ofEvora The inset is a zoom into the head of the instrument

mechanisms behind SEM that are the main purpose of thisproject Similarly to the VAE sensors new Radon detectorscould in the future equip the seismic stations that will hostULF magnetometers

5 Discussion

With this research effort it is likely that new SEM data linkedwith seismic activity in the WENP region could emerge andcontribute to a better understanding of SEM phenomenaFurthermore such data could be used in the following ob-jectives (1) perform the correlation between the ULF andVLFLF radio signals for different seismic events with theatmospheric electrical field and the radon levels (2) theoret-ical modelling of the observed phenomena (3) extend theempirical magnitude versus distance diagram for the detec-tion of abnormal ULF signals preceding HME presented byHattori et al (2004) to LME (working below the present de-tection threshold) In fact since LME were not considered inthe former diagram the extension of such diagram could pro-vide valuable information about the ULF precursory emis-sions Moreover the study of Gladychev et al (2010) re-veals that observation of LME is feasible if the seismic epi-centre is sufficiently close to the ULF station (4) ExploreVLFLF radio anomalies correlated with HME earthquakesand also LME already below the empirical threshold for itsdetection (similar to the ULF case) discussed by Rozhnoi etal (2004) (5) Search for a relation law between the strength

of SEM anomalies and the magnitude andor local (referredto the place where the sensors will be installed) ground mo-tion of the impending LME This objective could give an im-portant insight into the driving mechanisms of these phenom-ena and present a decisive contribution to the development ofthis field Obviously the SEM perturbations caused by LMEwill be in principle very faint and their detection and dis-crimination from other possible signal sources will representa real challenge

As a final remark it is important to mention that labo-ratorial experiments concerning the electrical properties ofthe rocks (for the moment the study is focusing on graniticrocks) which may contribute to a better understanding ofSEM phenomena are already being undertaken

AcknowledgementsThe Portuguese team acknowledges the sup-port of two Portuguese institutions QREN support (OperationalProgram ldquoThematic Factors of Competitivenessrdquo) co-fundedby the European Regional Development Fund (ERDF) for theresearch program SIRAS and the FCT (Science and TechnologyFoundation) for the grant SFRHBPD638802009 (HGS) Finallywe are grateful to the support from Samuel Barias e ClaudiaSerrano

Edited by K EftaxiasReviewed by C Nomicos and another anonymous referee

References

Argus D Gordon R DeMets C and Stein S Closureof the Africa-Eurasia-North America plate motion and tec-tonics of the Gloria fault J Geophys Res 94 5585ndash5602doi101029JB094iB05p05585 1989

Bezzeghoud M Borges J F Caldeira B Buforn E and UdıasA Seismic activity in the Azores Region in the context of thewestern part of the Eurasia-Nubia plate boundary InternationalSeminar on Seismic risk and rehabilitation on the 10th Anniver-sary of the July 9 1998 Azores Earthquake Horta-Azores 9ndash13July 27 2008

Bezzeghoud M Borges J F and Caldeira B Fontes sısmicasao longo da fronteira de placas tectonicas entre os Acores e aArgelia um modelo sismotectonico edited by Dias R AraujoA and Terrinha P (Escolar Editora Lisbon Portugal) in press2010

Biagi P F Castellana L Maggipinto T and Ermini A Anoverview on preseismic anomalies in LF radio signals revealed inItaly by wavelet analysis Ann Geophys Ann Geophys 51(1)237ndash246 2008

Biagi P F Castellana L Maggipinto T Loiacono D Schi-avulli L Ligonzo T Fiore M Suciu E and Ermini AA pre seismic radio anomaly revealed in the area where theAbruzzo earthquake (M = 63) occurred on 6 April 2009 NatHazards Earth Syst Sci 9 1551ndash1556 doi105194nhess-9-1551-2009 2009

Bleier T Dunson C Maniscalco M Bryant N Bambery Rand Freund F Investigation of ULF magnetic pulsations airconductivity changes and infra red signatures associated with

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 247

the 30 October Alum Rock M54 earthquake Nat Hazards EarthSyst Sci 9 585ndash603 doi105194nhess-9-585-2009 2009

Borges J Fitas A J S Bezzeghoud M and Teves-Costa PSeismotonics of Portugal and its adjacent Atlantic area Tectono-physics 331(4) 373ndash387 doi101016S0040-1951(00)00291-22001

Borges J F Bezzeghoud M Buforn E Pro C and FitasA The 1980 1997 and 1998 Azores earthquakes and itsseismotectonic implications Tectonophysics 435(1ndash4) 37ndash54doi101016jtecto200701008 2007

Borges J F Caldeira B Bezzeghoud M and Buforn E Seis-micity and Seismotectonics of Azores Geodynamic Implica-tions Ch 6 99-110 in A Book on the 1998 Azores earthquake -10 years after its occurrence edited by Oliveira C S Costa Aand Nunes J C ISBN 978989-20-1223-0 741 2008

Buforn E Udıas A and Colombas M A Seismic-ity source mechanisms and seismotectonics of the Azores-Gibraltar plate boundary Tectonophysics 152(1ndash2) 89ndash118doi1010160040-1951(88)90031-5 1988a

Buforn E Udıas A and Mezcua J Seismicity and focal mecha-nisms in south Spain Bull Seism Soc Am 78(6) 2008ndash2024(httpwwwbssaonlineorgcgicontentabstract7862008)1988b

Buforn E Udıas A and Madariaga R Intermediateand deep earthquakes in Spain Pageoph 136(4) 375-393doi101007BF00878576 1991a

Buforn E Udıas A Mezcua J and Madariaga R A deep earth-quake under south Spain 8 March 1990 Bull Seism Soc Am81(4) 1403ndash1407httpbssageoscienceworldorgcgicontentcitation8141403 1991b

Buforn E Coca P Udıas A and Lasa C Source mechanism ofintermediate and deep earthquakes in southern Spain J Seismol1(2) 113ndash130 doi101023A1009754219459 1997

Buforn E Bezzeghoud M Udıas A and Pro C Seis-mic Sources on the Iberia-African Plate Boundary and theirTectonic Implications Pure Appl Geophys 161(3) 623ndash646doi101007s00024-003-2466-1 2004

Caldeira B Carrilho F Miranda M Bezzeghoud M Alves PM Silveira G Villalonga F Pena J A Matias L BorgesJ F Vales D Corela C and Madureira G Recent improve-ments in the Broadband seismic networks in Portugal CSEM EMSC Newsletter 18-19 May 2007

Chauhan V Singh O P Kushwah V Singh V andSingh B Ultra-low-frequency (ULF) and total electron con-tent (TEC) anomalies observed at Agra and their associa-tion with regional earthquakes J Geodyn 48(2) 68ndash74doi101016jjog200906002 2009

Gladychev V Baransky L Schekotov A Fedorov EPokhotelov O Andreevsky S Rozhnoi A Khabazin YBelyaev G Gorbatikov A Gordeev E Chebrov V SinitsinV Lutikov A Yunga S Kosarev G Surkov V MolchanovO Hayakawa M Uyeda S Nagao T Hattori K and NodaY Study of electromagnetic emissions associated with seismicactivity in Kamchatka region Nat Hazards Earth Syst Sci 1127ndash136 doi105194nhess-1-127-2001 2001

Grandin R Borges J F Bezzeghoud M Caldeira B and Car-rilho F Simulations of strong ground motion in SW Iberia forthe 1969 February 28 (Ms = 80) and the 1755 November 1 (Msim 85) earthquakes ndash I Velocity model Geophys J Int 171(3)

1144-1161 doi101111j1365-246X200703570x 2007aGrandin R Borges J F Bezzeghoud M Caldeira B and Car-

rilho F Simulations of strong ground motion in SW Iberiafor the 1969 February 28 (Ms = 80) and the 1755 November1 (M sim 85) earthquakes ndash II Strong ground motion simula-tions Geophys J Int 171(2) 807-822 doi 101111j1365-246X200703571x 2007b

Grimison N and Cheng W The Azores-Gibraltar plate bound-ary focal mechanisms depths of earthquakes and their tec-tonic implications J Geophys Res 91(B2) 2029ndash2047doi101029JB091iB02p02029 1986

Harrison R G Aplin K L and Rycroft M J Atmo-spheric electricity coupling between earthquake regions andthe ionosphere J Atmos Sol-Terr Phys 72(5ndash6) 376ndash381doi101016jjastp200912004 2010

Hattori K Takahashi I Yoshino C C Isezaki N Iwasaki HHarada M Kawabata K Kopytenko E Kopytenko Y Malt-sev P Korepanov V Molchanov O Hayakawa M NodaY Nagao T and Uyeda S ULF geomagnetic field measure-ments in Japan and some recent results associated with IwatekenNairiku Hokubu earthquake in 1998 Phys Chem Earth 29(4ndash9) 481ndash494 doi101016jpce200309019 2004

Hatzfeld D Etude sismotectonique de la zone de collision Ibero-Maghrebine PhD Thesis Grenoble (France) 281 1978

Munuera J M Datos basicos para un estudio de sismicidad en laregion de la Penınsula Iberica Mem Inst Geog Cat Madrid32 93 1963

Nunes J C and Ribeiro E Caracterizacao da sismicidade instru-mental dos Acores no periodo 1950ndash1980 SISMICA EncontroNacional de Sismologıa e Enghenaria Sısmica Acores 2001

Prattes G Schwingenschuh K Eichelberger H U MagnesW Boudjada M Stachel M Vellante M WesztergomV and Nenovski P Multi-point ground-based ULF magneticfield observations in Europe during seismic active periods in2004 and 2005 Nat Hazards Earth Syst Sci 8 501ndash507doi105194nhess-8-501-2008 2008

Rocha J P Bezzeghoud M Caldeira B Araujo A Borges JF Vilallonga F and Dorbath C Microseismicity in the neigh-bourhood of ldquoAlmodovar faultrdquo 6a Assembleia Luso Espanholade Geodesia e Geofısica 2008 (in Portuguese)

Rozhnoi A Solovieva M S Molchanov O A andHayakawa M Middle latitude LF (40 kHz) phase varia-tions associated with earthquakes for quiet and disturbed ge-omagnetic conditions Phys Chem Earth 29(4ndash9) 589ndash598doi101016jpce200308061 2004

Searle R Tectonic pattern of the Azores spreading centreand triple junction Earth Plan Sci Lett 51(2) 415ndash434doi1010160012-821X(80)90221-6 1980

Seber D Barazangi M Ibenbrahim A and Demnati AGeophysical evidence for lithospheric delamination beneath theAlboran Sea and Rif-Betic mountains Nature 379 785ndash790doi101038379785a0 1996

Serrano I Morales J Zhao D Torcal F and Vidal F P-wave tomographic images in the Central Betics-Alboran Sea(south Spain) using local earthquakes contribution for a con-tinental collision Geophys Res Lett 25(21) 4031ndash4034doi1010291998GL900021 1998

Silva H G Bezzeghoud M Reis A H Rosa R N TlemcaniM Araujo A A Caldeira B Borges J F Serrano C and

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

244 H G Silva et al Seismo-electromagnetic phenomena

31 Almodovar region

The obtained results show the occurrence of reduced mag-nitude events Using the proper programs of localizationKlein (1978) and relocation Waldhaus (2001) it was pos-sible to define a zone of seismic activity in Almodovar re-gion (see Fig 2) The seismicity detected in the region ofAlmodovar has a tendency according to NE-SW orientationwhich becomes more evident with the epicenter redistribu-tion obtained after the relocation

32 Monchique region

The studies undertaken allowed the identification of a rel-evant seismic activity with typical magnitudes 1lt M lt 3and constrained between 5 and 15 km depth around theMonchique region The seismicity map obtained after therelocation using the previous programs defines a clear align-ment of the seismic epicenters in this region which can berelated with two probable faults recently indentified in thisarea Rocha et al (2008) The faults have ENE-WSW andNNE-SSW directions

4 SEM monitoring study

As already pointed out in the introduction the present studyintegrates two main types of SEM phenomena one dedicatedto the recognition of ULF electromagnetic fields and theother to the detection of VLFLF radio broadcasting anoma-lies both related with seismic activities It has three struc-tural objectives (1) establish an Iberian monitoring networkof seismic related ULF electromagnetic emissions possi-bly integrated in the existing seismic networks Caldeira etal (2007) and the Western Mediterranean Broad-Band Seis-mological Network (WMBB) Martin et al (2007) (2) ex-tend the international INFREP network with a new receiverwhose installation is already in progress in the University ofEvora (3) contribute to the increase of both seismic relatedULF and VLFLF world database Moreover these measure-ments are expected to be complemented with the monitoringof the atmospheric electrical field and the radon levels sincethese parameters can give crucial insights into the physics ofSEM phenomenology In this way multi-parameter assess-ment of the preparatory stage of the earthquakes is envisaged

41 Observation of ULF magnetic field emissions

The detection of ULF electromagnetic fields aims to equipseismic stations in the south of Iberian Peninsula with three-component ULF magnetometers of type LEMI-30 (producedby the Lviv Centre of Space Research Ukraine) as com-monly used in ULF research Chauhan et al (2009) A pos-sible location for the initial two equipments is presented inFig 2 According to the micro-seismicity previously pre-sented the following potential regions have been selected

(1) offshore zone with intermediate and strong magnitude ndashbetween Gorringe Bank the Horseshoe Fault and the Gulf ofCadiz (2) onshore zone with low magnitude ndash Algarve andsouth Alentejo (between Monchique and Almodovar) Animportant point is that the sensors are installed in specificsites that exhibit significant seismic activity and simultane-ously low electromagnetic noise levels These conditionsfavour detection of SEM phenomena Moreover preferencewill be given to places that already host seismic stations Aportable three-component ULF magnetometer most proba-bly the LEMI-18 model will be used in the search for theproper location by monitoring the electromagnetic activityof the region of interest Once a specific site is chosen itwill be employed in calibration procedures Nonethelessthis portable magnetometer could also be used for in-situ af-tershock studies typicallyM sim 3minus 5 once the mainshockepicentre has been correctly identified Special attention willbe given to man-made noise sources such as electricity trans-mission grids factories roads etc that must be clearly iden-tified to ensure the reliability of the acquired data Prattes etal (2008) and Bleier et al (2009) Our ULF study will be fo-cused on the analysis of low magnitude earthquakes (LME)with M le 4 These events are frequent in the South of Por-tugal but have been almost completely disregarded in liter-ature as will be discussed in the next session Finally themagnetometers will integrate the South European GeoMag-netic Array (SEGMA) array

42 Observation of VLFLF radio signals

The recognition of VLFLF radio broadcasting anomaliesis already being performed through two antennas (one foreach band) connected to a receiver built by Elettronika(Italy) which is able to acquire up to 10 signals (dis-tributed in these bands) This equipment is frequently em-ployed in similar experiments Biagi et al (2009) In Fig 3a photograph of the apparatus installation in the Univer-sity of Evora (3834prime N 754prime W 300 m amsl) is pre-sented The system was recently installed in the contextof the present project and is in operation since 1 Septem-ber 2010 The receiver integrates the International Net-work for Frontier Research on Earthquake Precursors (IN-FREP) shown in Fig 4 and is expected to monitor signalsemitted from active transmitters to study the seismic activ-ity in the WENP region In fact the relevant issue whenchoosing a transmitter is that the radio path from the trans-mitter to the receiver crosses the epicentral area Biagi etal (2008 2009) so three transmitters out of the ten being ex-amined (already monitored by other INFREP receivers) arelocated in Sicily (Italy) 3707prime N126prime E Sardinia (Italy)4054prime N0942prime E Monaco (France) 4347prime N0609prime EOnce we assure a proper functioning of the system threefrequencies that are now being monitored will possiblybe substituted by three other emitters in the west ofthe Iberian Peninsula Ponta Delgada (Azores-Portugal)

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 245

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1

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Figure 2 17

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27

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32

33

Fig 3 Photograph of the VLF and LF antennas and the box wherereceiver is kept (pointed with the blue arrow) at the University ofEvora The inset shows the front panel of the receiver

17

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10

11

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Figure 4 17

18

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23

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25

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27

28

29

30

Figure 5 31

32

33

Fig 4 Representative map of the INFREP showing the location(orange circles) of the five VLFLF receivers presently in opera-tion Some transmitters whose signals are sampled by the differentreceivers are also indicated (light brown diamonds represent VLFtransmitters and yellow squares LF ones) The new receiver underinstallation in Mitra (Evora) is pointed with a red star

Funchal (Madeira-Portugal) Ceuta (Spain) Figure 5 showsthe location of the mentioned emitters and the receiver It isexpected that the receiver is able to detect anomalies relatedwith high magnitude earthquakes (HME)M ge 5 in EuropeThe data acquired with this equipment will be available tothe INFREP community according to the regulation of thenetwork

17

1

2

3

4

5

6

7

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11

12

13

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Figure 4 17

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24

25

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27

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29

30

Figure 5 31

32

33

Fig 5 Map of the planned VLFLF radio broadcast system threetransmitters (represented by yellow squares) located at Azores (Por-tugal) Madeira (Portugal) and Ceuta (Spain) and the receiver(shown as a red star) installed atEvora (Portugal)

43 Observation of the Atmospheric Electrical field

Investigation of AE anomalies related with seismic activityis by now being preformed through a vertical component ofthe atmospheric electrical field (VAE) sensor Keithley Elec-trometer model JCI 131 installed at the University ofEvora(in the same coordinates as the VLFLF system) This equip-ment has been in operation during the period of December2003 to October 2004 and from February 2005 until now Itis prepared for continuous monitoring of the VAE and worksin four scales 2 20 200 and 2000 kVm with automaticcommutations respectively with the correspondent sensitiv-ity thresholds of 01 1 10 and 100 kVm Figure 6 presentsa picture of the apparatus Inspection of the data collecteduntil now is revealing interesting results presented in a pa-per submitted to this issue Silva et al (2010) In the futureit is planned that more VAE sensors could equip the seis-mic stations that will receive ULF magnetometers in orderto achieve multiple-parameter monitoring

44 Monitorization of the atmospheric Radon levels

The Radon levels are monitored using for the moment(the acquisition of new equipments is being considered) aldquoRadon Thoron Daughters Meter model 4Srdquo built by Silena(a former Italian company) that uses alpha spectrometryand has a sensitivity of 37 Bqm3 of equilibrium equiva-lent radon concentration (1 mWL) and an electronically-regulated flow-rate of 3 lmin with 5 precision The ap-paratus is now under installation at the University ofEvorain the same place of the VAE sensor and VLFLF system toallow multiple parameter assessment of the region Precur-sor anomalous Radon levels have been reported for variousseismic events Toutain et al (1999) Thus monitoring suchlevels deserves attention by itself Even so their direct cor-relation with SEM could open new insights into the physical

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

246 H G Silva et al Seismo-electromagnetic phenomena

18

1

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Figure 6 23 Fig 6 Photograph of the VAE sensor installed at the University ofEvora The inset is a zoom into the head of the instrument

mechanisms behind SEM that are the main purpose of thisproject Similarly to the VAE sensors new Radon detectorscould in the future equip the seismic stations that will hostULF magnetometers

5 Discussion

With this research effort it is likely that new SEM data linkedwith seismic activity in the WENP region could emerge andcontribute to a better understanding of SEM phenomenaFurthermore such data could be used in the following ob-jectives (1) perform the correlation between the ULF andVLFLF radio signals for different seismic events with theatmospheric electrical field and the radon levels (2) theoret-ical modelling of the observed phenomena (3) extend theempirical magnitude versus distance diagram for the detec-tion of abnormal ULF signals preceding HME presented byHattori et al (2004) to LME (working below the present de-tection threshold) In fact since LME were not considered inthe former diagram the extension of such diagram could pro-vide valuable information about the ULF precursory emis-sions Moreover the study of Gladychev et al (2010) re-veals that observation of LME is feasible if the seismic epi-centre is sufficiently close to the ULF station (4) ExploreVLFLF radio anomalies correlated with HME earthquakesand also LME already below the empirical threshold for itsdetection (similar to the ULF case) discussed by Rozhnoi etal (2004) (5) Search for a relation law between the strength

of SEM anomalies and the magnitude andor local (referredto the place where the sensors will be installed) ground mo-tion of the impending LME This objective could give an im-portant insight into the driving mechanisms of these phenom-ena and present a decisive contribution to the development ofthis field Obviously the SEM perturbations caused by LMEwill be in principle very faint and their detection and dis-crimination from other possible signal sources will representa real challenge

As a final remark it is important to mention that labo-ratorial experiments concerning the electrical properties ofthe rocks (for the moment the study is focusing on graniticrocks) which may contribute to a better understanding ofSEM phenomena are already being undertaken

AcknowledgementsThe Portuguese team acknowledges the sup-port of two Portuguese institutions QREN support (OperationalProgram ldquoThematic Factors of Competitivenessrdquo) co-fundedby the European Regional Development Fund (ERDF) for theresearch program SIRAS and the FCT (Science and TechnologyFoundation) for the grant SFRHBPD638802009 (HGS) Finallywe are grateful to the support from Samuel Barias e ClaudiaSerrano

Edited by K EftaxiasReviewed by C Nomicos and another anonymous referee

References

Argus D Gordon R DeMets C and Stein S Closureof the Africa-Eurasia-North America plate motion and tec-tonics of the Gloria fault J Geophys Res 94 5585ndash5602doi101029JB094iB05p05585 1989

Bezzeghoud M Borges J F Caldeira B Buforn E and UdıasA Seismic activity in the Azores Region in the context of thewestern part of the Eurasia-Nubia plate boundary InternationalSeminar on Seismic risk and rehabilitation on the 10th Anniver-sary of the July 9 1998 Azores Earthquake Horta-Azores 9ndash13July 27 2008

Bezzeghoud M Borges J F and Caldeira B Fontes sısmicasao longo da fronteira de placas tectonicas entre os Acores e aArgelia um modelo sismotectonico edited by Dias R AraujoA and Terrinha P (Escolar Editora Lisbon Portugal) in press2010

Biagi P F Castellana L Maggipinto T and Ermini A Anoverview on preseismic anomalies in LF radio signals revealed inItaly by wavelet analysis Ann Geophys Ann Geophys 51(1)237ndash246 2008

Biagi P F Castellana L Maggipinto T Loiacono D Schi-avulli L Ligonzo T Fiore M Suciu E and Ermini AA pre seismic radio anomaly revealed in the area where theAbruzzo earthquake (M = 63) occurred on 6 April 2009 NatHazards Earth Syst Sci 9 1551ndash1556 doi105194nhess-9-1551-2009 2009

Bleier T Dunson C Maniscalco M Bryant N Bambery Rand Freund F Investigation of ULF magnetic pulsations airconductivity changes and infra red signatures associated with

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 247

the 30 October Alum Rock M54 earthquake Nat Hazards EarthSyst Sci 9 585ndash603 doi105194nhess-9-585-2009 2009

Borges J Fitas A J S Bezzeghoud M and Teves-Costa PSeismotonics of Portugal and its adjacent Atlantic area Tectono-physics 331(4) 373ndash387 doi101016S0040-1951(00)00291-22001

Borges J F Bezzeghoud M Buforn E Pro C and FitasA The 1980 1997 and 1998 Azores earthquakes and itsseismotectonic implications Tectonophysics 435(1ndash4) 37ndash54doi101016jtecto200701008 2007

Borges J F Caldeira B Bezzeghoud M and Buforn E Seis-micity and Seismotectonics of Azores Geodynamic Implica-tions Ch 6 99-110 in A Book on the 1998 Azores earthquake -10 years after its occurrence edited by Oliveira C S Costa Aand Nunes J C ISBN 978989-20-1223-0 741 2008

Buforn E Udıas A and Colombas M A Seismic-ity source mechanisms and seismotectonics of the Azores-Gibraltar plate boundary Tectonophysics 152(1ndash2) 89ndash118doi1010160040-1951(88)90031-5 1988a

Buforn E Udıas A and Mezcua J Seismicity and focal mecha-nisms in south Spain Bull Seism Soc Am 78(6) 2008ndash2024(httpwwwbssaonlineorgcgicontentabstract7862008)1988b

Buforn E Udıas A and Madariaga R Intermediateand deep earthquakes in Spain Pageoph 136(4) 375-393doi101007BF00878576 1991a

Buforn E Udıas A Mezcua J and Madariaga R A deep earth-quake under south Spain 8 March 1990 Bull Seism Soc Am81(4) 1403ndash1407httpbssageoscienceworldorgcgicontentcitation8141403 1991b

Buforn E Coca P Udıas A and Lasa C Source mechanism ofintermediate and deep earthquakes in southern Spain J Seismol1(2) 113ndash130 doi101023A1009754219459 1997

Buforn E Bezzeghoud M Udıas A and Pro C Seis-mic Sources on the Iberia-African Plate Boundary and theirTectonic Implications Pure Appl Geophys 161(3) 623ndash646doi101007s00024-003-2466-1 2004

Caldeira B Carrilho F Miranda M Bezzeghoud M Alves PM Silveira G Villalonga F Pena J A Matias L BorgesJ F Vales D Corela C and Madureira G Recent improve-ments in the Broadband seismic networks in Portugal CSEM EMSC Newsletter 18-19 May 2007

Chauhan V Singh O P Kushwah V Singh V andSingh B Ultra-low-frequency (ULF) and total electron con-tent (TEC) anomalies observed at Agra and their associa-tion with regional earthquakes J Geodyn 48(2) 68ndash74doi101016jjog200906002 2009

Gladychev V Baransky L Schekotov A Fedorov EPokhotelov O Andreevsky S Rozhnoi A Khabazin YBelyaev G Gorbatikov A Gordeev E Chebrov V SinitsinV Lutikov A Yunga S Kosarev G Surkov V MolchanovO Hayakawa M Uyeda S Nagao T Hattori K and NodaY Study of electromagnetic emissions associated with seismicactivity in Kamchatka region Nat Hazards Earth Syst Sci 1127ndash136 doi105194nhess-1-127-2001 2001

Grandin R Borges J F Bezzeghoud M Caldeira B and Car-rilho F Simulations of strong ground motion in SW Iberia forthe 1969 February 28 (Ms = 80) and the 1755 November 1 (Msim 85) earthquakes ndash I Velocity model Geophys J Int 171(3)

1144-1161 doi101111j1365-246X200703570x 2007aGrandin R Borges J F Bezzeghoud M Caldeira B and Car-

rilho F Simulations of strong ground motion in SW Iberiafor the 1969 February 28 (Ms = 80) and the 1755 November1 (M sim 85) earthquakes ndash II Strong ground motion simula-tions Geophys J Int 171(2) 807-822 doi 101111j1365-246X200703571x 2007b

Grimison N and Cheng W The Azores-Gibraltar plate bound-ary focal mechanisms depths of earthquakes and their tec-tonic implications J Geophys Res 91(B2) 2029ndash2047doi101029JB091iB02p02029 1986

Harrison R G Aplin K L and Rycroft M J Atmo-spheric electricity coupling between earthquake regions andthe ionosphere J Atmos Sol-Terr Phys 72(5ndash6) 376ndash381doi101016jjastp200912004 2010

Hattori K Takahashi I Yoshino C C Isezaki N Iwasaki HHarada M Kawabata K Kopytenko E Kopytenko Y Malt-sev P Korepanov V Molchanov O Hayakawa M NodaY Nagao T and Uyeda S ULF geomagnetic field measure-ments in Japan and some recent results associated with IwatekenNairiku Hokubu earthquake in 1998 Phys Chem Earth 29(4ndash9) 481ndash494 doi101016jpce200309019 2004

Hatzfeld D Etude sismotectonique de la zone de collision Ibero-Maghrebine PhD Thesis Grenoble (France) 281 1978

Munuera J M Datos basicos para un estudio de sismicidad en laregion de la Penınsula Iberica Mem Inst Geog Cat Madrid32 93 1963

Nunes J C and Ribeiro E Caracterizacao da sismicidade instru-mental dos Acores no periodo 1950ndash1980 SISMICA EncontroNacional de Sismologıa e Enghenaria Sısmica Acores 2001

Prattes G Schwingenschuh K Eichelberger H U MagnesW Boudjada M Stachel M Vellante M WesztergomV and Nenovski P Multi-point ground-based ULF magneticfield observations in Europe during seismic active periods in2004 and 2005 Nat Hazards Earth Syst Sci 8 501ndash507doi105194nhess-8-501-2008 2008

Rocha J P Bezzeghoud M Caldeira B Araujo A Borges JF Vilallonga F and Dorbath C Microseismicity in the neigh-bourhood of ldquoAlmodovar faultrdquo 6a Assembleia Luso Espanholade Geodesia e Geofısica 2008 (in Portuguese)

Rozhnoi A Solovieva M S Molchanov O A andHayakawa M Middle latitude LF (40 kHz) phase varia-tions associated with earthquakes for quiet and disturbed ge-omagnetic conditions Phys Chem Earth 29(4ndash9) 589ndash598doi101016jpce200308061 2004

Searle R Tectonic pattern of the Azores spreading centreand triple junction Earth Plan Sci Lett 51(2) 415ndash434doi1010160012-821X(80)90221-6 1980

Seber D Barazangi M Ibenbrahim A and Demnati AGeophysical evidence for lithospheric delamination beneath theAlboran Sea and Rif-Betic mountains Nature 379 785ndash790doi101038379785a0 1996

Serrano I Morales J Zhao D Torcal F and Vidal F P-wave tomographic images in the Central Betics-Alboran Sea(south Spain) using local earthquakes contribution for a con-tinental collision Geophys Res Lett 25(21) 4031ndash4034doi1010291998GL900021 1998

Silva H G Bezzeghoud M Reis A H Rosa R N TlemcaniM Araujo A A Caldeira B Borges J F Serrano C and

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

H G Silva et al Seismo-electromagnetic phenomena 245

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Figure 2 17

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27

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Fig 3 Photograph of the VLF and LF antennas and the box wherereceiver is kept (pointed with the blue arrow) at the University ofEvora The inset shows the front panel of the receiver

17

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Figure 4 17

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25

26

27

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29

30

Figure 5 31

32

33

Fig 4 Representative map of the INFREP showing the location(orange circles) of the five VLFLF receivers presently in opera-tion Some transmitters whose signals are sampled by the differentreceivers are also indicated (light brown diamonds represent VLFtransmitters and yellow squares LF ones) The new receiver underinstallation in Mitra (Evora) is pointed with a red star

Funchal (Madeira-Portugal) Ceuta (Spain) Figure 5 showsthe location of the mentioned emitters and the receiver It isexpected that the receiver is able to detect anomalies relatedwith high magnitude earthquakes (HME)M ge 5 in EuropeThe data acquired with this equipment will be available tothe INFREP community according to the regulation of thenetwork

17

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

Figure 4 17

18

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20

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22

23

24

25

26

27

28

29

30

Figure 5 31

32

33

Fig 5 Map of the planned VLFLF radio broadcast system threetransmitters (represented by yellow squares) located at Azores (Por-tugal) Madeira (Portugal) and Ceuta (Spain) and the receiver(shown as a red star) installed atEvora (Portugal)

43 Observation of the Atmospheric Electrical field

Investigation of AE anomalies related with seismic activityis by now being preformed through a vertical component ofthe atmospheric electrical field (VAE) sensor Keithley Elec-trometer model JCI 131 installed at the University ofEvora(in the same coordinates as the VLFLF system) This equip-ment has been in operation during the period of December2003 to October 2004 and from February 2005 until now Itis prepared for continuous monitoring of the VAE and worksin four scales 2 20 200 and 2000 kVm with automaticcommutations respectively with the correspondent sensitiv-ity thresholds of 01 1 10 and 100 kVm Figure 6 presentsa picture of the apparatus Inspection of the data collecteduntil now is revealing interesting results presented in a pa-per submitted to this issue Silva et al (2010) In the futureit is planned that more VAE sensors could equip the seis-mic stations that will receive ULF magnetometers in orderto achieve multiple-parameter monitoring

44 Monitorization of the atmospheric Radon levels

The Radon levels are monitored using for the moment(the acquisition of new equipments is being considered) aldquoRadon Thoron Daughters Meter model 4Srdquo built by Silena(a former Italian company) that uses alpha spectrometryand has a sensitivity of 37 Bqm3 of equilibrium equiva-lent radon concentration (1 mWL) and an electronically-regulated flow-rate of 3 lmin with 5 precision The ap-paratus is now under installation at the University ofEvorain the same place of the VAE sensor and VLFLF system toallow multiple parameter assessment of the region Precur-sor anomalous Radon levels have been reported for variousseismic events Toutain et al (1999) Thus monitoring suchlevels deserves attention by itself Even so their direct cor-relation with SEM could open new insights into the physical

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

246 H G Silva et al Seismo-electromagnetic phenomena

18

1

2

3

4

5

6

7

8

9

10

11

12

13

14

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17

18

19

20

21

22

Figure 6 23 Fig 6 Photograph of the VAE sensor installed at the University ofEvora The inset is a zoom into the head of the instrument

mechanisms behind SEM that are the main purpose of thisproject Similarly to the VAE sensors new Radon detectorscould in the future equip the seismic stations that will hostULF magnetometers

5 Discussion

With this research effort it is likely that new SEM data linkedwith seismic activity in the WENP region could emerge andcontribute to a better understanding of SEM phenomenaFurthermore such data could be used in the following ob-jectives (1) perform the correlation between the ULF andVLFLF radio signals for different seismic events with theatmospheric electrical field and the radon levels (2) theoret-ical modelling of the observed phenomena (3) extend theempirical magnitude versus distance diagram for the detec-tion of abnormal ULF signals preceding HME presented byHattori et al (2004) to LME (working below the present de-tection threshold) In fact since LME were not considered inthe former diagram the extension of such diagram could pro-vide valuable information about the ULF precursory emis-sions Moreover the study of Gladychev et al (2010) re-veals that observation of LME is feasible if the seismic epi-centre is sufficiently close to the ULF station (4) ExploreVLFLF radio anomalies correlated with HME earthquakesand also LME already below the empirical threshold for itsdetection (similar to the ULF case) discussed by Rozhnoi etal (2004) (5) Search for a relation law between the strength

of SEM anomalies and the magnitude andor local (referredto the place where the sensors will be installed) ground mo-tion of the impending LME This objective could give an im-portant insight into the driving mechanisms of these phenom-ena and present a decisive contribution to the development ofthis field Obviously the SEM perturbations caused by LMEwill be in principle very faint and their detection and dis-crimination from other possible signal sources will representa real challenge

As a final remark it is important to mention that labo-ratorial experiments concerning the electrical properties ofthe rocks (for the moment the study is focusing on graniticrocks) which may contribute to a better understanding ofSEM phenomena are already being undertaken

AcknowledgementsThe Portuguese team acknowledges the sup-port of two Portuguese institutions QREN support (OperationalProgram ldquoThematic Factors of Competitivenessrdquo) co-fundedby the European Regional Development Fund (ERDF) for theresearch program SIRAS and the FCT (Science and TechnologyFoundation) for the grant SFRHBPD638802009 (HGS) Finallywe are grateful to the support from Samuel Barias e ClaudiaSerrano

Edited by K EftaxiasReviewed by C Nomicos and another anonymous referee

References

Argus D Gordon R DeMets C and Stein S Closureof the Africa-Eurasia-North America plate motion and tec-tonics of the Gloria fault J Geophys Res 94 5585ndash5602doi101029JB094iB05p05585 1989

Bezzeghoud M Borges J F Caldeira B Buforn E and UdıasA Seismic activity in the Azores Region in the context of thewestern part of the Eurasia-Nubia plate boundary InternationalSeminar on Seismic risk and rehabilitation on the 10th Anniver-sary of the July 9 1998 Azores Earthquake Horta-Azores 9ndash13July 27 2008

Bezzeghoud M Borges J F and Caldeira B Fontes sısmicasao longo da fronteira de placas tectonicas entre os Acores e aArgelia um modelo sismotectonico edited by Dias R AraujoA and Terrinha P (Escolar Editora Lisbon Portugal) in press2010

Biagi P F Castellana L Maggipinto T and Ermini A Anoverview on preseismic anomalies in LF radio signals revealed inItaly by wavelet analysis Ann Geophys Ann Geophys 51(1)237ndash246 2008

Biagi P F Castellana L Maggipinto T Loiacono D Schi-avulli L Ligonzo T Fiore M Suciu E and Ermini AA pre seismic radio anomaly revealed in the area where theAbruzzo earthquake (M = 63) occurred on 6 April 2009 NatHazards Earth Syst Sci 9 1551ndash1556 doi105194nhess-9-1551-2009 2009

Bleier T Dunson C Maniscalco M Bryant N Bambery Rand Freund F Investigation of ULF magnetic pulsations airconductivity changes and infra red signatures associated with

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 247

the 30 October Alum Rock M54 earthquake Nat Hazards EarthSyst Sci 9 585ndash603 doi105194nhess-9-585-2009 2009

Borges J Fitas A J S Bezzeghoud M and Teves-Costa PSeismotonics of Portugal and its adjacent Atlantic area Tectono-physics 331(4) 373ndash387 doi101016S0040-1951(00)00291-22001

Borges J F Bezzeghoud M Buforn E Pro C and FitasA The 1980 1997 and 1998 Azores earthquakes and itsseismotectonic implications Tectonophysics 435(1ndash4) 37ndash54doi101016jtecto200701008 2007

Borges J F Caldeira B Bezzeghoud M and Buforn E Seis-micity and Seismotectonics of Azores Geodynamic Implica-tions Ch 6 99-110 in A Book on the 1998 Azores earthquake -10 years after its occurrence edited by Oliveira C S Costa Aand Nunes J C ISBN 978989-20-1223-0 741 2008

Buforn E Udıas A and Colombas M A Seismic-ity source mechanisms and seismotectonics of the Azores-Gibraltar plate boundary Tectonophysics 152(1ndash2) 89ndash118doi1010160040-1951(88)90031-5 1988a

Buforn E Udıas A and Mezcua J Seismicity and focal mecha-nisms in south Spain Bull Seism Soc Am 78(6) 2008ndash2024(httpwwwbssaonlineorgcgicontentabstract7862008)1988b

Buforn E Udıas A and Madariaga R Intermediateand deep earthquakes in Spain Pageoph 136(4) 375-393doi101007BF00878576 1991a

Buforn E Udıas A Mezcua J and Madariaga R A deep earth-quake under south Spain 8 March 1990 Bull Seism Soc Am81(4) 1403ndash1407httpbssageoscienceworldorgcgicontentcitation8141403 1991b

Buforn E Coca P Udıas A and Lasa C Source mechanism ofintermediate and deep earthquakes in southern Spain J Seismol1(2) 113ndash130 doi101023A1009754219459 1997

Buforn E Bezzeghoud M Udıas A and Pro C Seis-mic Sources on the Iberia-African Plate Boundary and theirTectonic Implications Pure Appl Geophys 161(3) 623ndash646doi101007s00024-003-2466-1 2004

Caldeira B Carrilho F Miranda M Bezzeghoud M Alves PM Silveira G Villalonga F Pena J A Matias L BorgesJ F Vales D Corela C and Madureira G Recent improve-ments in the Broadband seismic networks in Portugal CSEM EMSC Newsletter 18-19 May 2007

Chauhan V Singh O P Kushwah V Singh V andSingh B Ultra-low-frequency (ULF) and total electron con-tent (TEC) anomalies observed at Agra and their associa-tion with regional earthquakes J Geodyn 48(2) 68ndash74doi101016jjog200906002 2009

Gladychev V Baransky L Schekotov A Fedorov EPokhotelov O Andreevsky S Rozhnoi A Khabazin YBelyaev G Gorbatikov A Gordeev E Chebrov V SinitsinV Lutikov A Yunga S Kosarev G Surkov V MolchanovO Hayakawa M Uyeda S Nagao T Hattori K and NodaY Study of electromagnetic emissions associated with seismicactivity in Kamchatka region Nat Hazards Earth Syst Sci 1127ndash136 doi105194nhess-1-127-2001 2001

Grandin R Borges J F Bezzeghoud M Caldeira B and Car-rilho F Simulations of strong ground motion in SW Iberia forthe 1969 February 28 (Ms = 80) and the 1755 November 1 (Msim 85) earthquakes ndash I Velocity model Geophys J Int 171(3)

1144-1161 doi101111j1365-246X200703570x 2007aGrandin R Borges J F Bezzeghoud M Caldeira B and Car-

rilho F Simulations of strong ground motion in SW Iberiafor the 1969 February 28 (Ms = 80) and the 1755 November1 (M sim 85) earthquakes ndash II Strong ground motion simula-tions Geophys J Int 171(2) 807-822 doi 101111j1365-246X200703571x 2007b

Grimison N and Cheng W The Azores-Gibraltar plate bound-ary focal mechanisms depths of earthquakes and their tec-tonic implications J Geophys Res 91(B2) 2029ndash2047doi101029JB091iB02p02029 1986

Harrison R G Aplin K L and Rycroft M J Atmo-spheric electricity coupling between earthquake regions andthe ionosphere J Atmos Sol-Terr Phys 72(5ndash6) 376ndash381doi101016jjastp200912004 2010

Hattori K Takahashi I Yoshino C C Isezaki N Iwasaki HHarada M Kawabata K Kopytenko E Kopytenko Y Malt-sev P Korepanov V Molchanov O Hayakawa M NodaY Nagao T and Uyeda S ULF geomagnetic field measure-ments in Japan and some recent results associated with IwatekenNairiku Hokubu earthquake in 1998 Phys Chem Earth 29(4ndash9) 481ndash494 doi101016jpce200309019 2004

Hatzfeld D Etude sismotectonique de la zone de collision Ibero-Maghrebine PhD Thesis Grenoble (France) 281 1978

Munuera J M Datos basicos para un estudio de sismicidad en laregion de la Penınsula Iberica Mem Inst Geog Cat Madrid32 93 1963

Nunes J C and Ribeiro E Caracterizacao da sismicidade instru-mental dos Acores no periodo 1950ndash1980 SISMICA EncontroNacional de Sismologıa e Enghenaria Sısmica Acores 2001

Prattes G Schwingenschuh K Eichelberger H U MagnesW Boudjada M Stachel M Vellante M WesztergomV and Nenovski P Multi-point ground-based ULF magneticfield observations in Europe during seismic active periods in2004 and 2005 Nat Hazards Earth Syst Sci 8 501ndash507doi105194nhess-8-501-2008 2008

Rocha J P Bezzeghoud M Caldeira B Araujo A Borges JF Vilallonga F and Dorbath C Microseismicity in the neigh-bourhood of ldquoAlmodovar faultrdquo 6a Assembleia Luso Espanholade Geodesia e Geofısica 2008 (in Portuguese)

Rozhnoi A Solovieva M S Molchanov O A andHayakawa M Middle latitude LF (40 kHz) phase varia-tions associated with earthquakes for quiet and disturbed ge-omagnetic conditions Phys Chem Earth 29(4ndash9) 589ndash598doi101016jpce200308061 2004

Searle R Tectonic pattern of the Azores spreading centreand triple junction Earth Plan Sci Lett 51(2) 415ndash434doi1010160012-821X(80)90221-6 1980

Seber D Barazangi M Ibenbrahim A and Demnati AGeophysical evidence for lithospheric delamination beneath theAlboran Sea and Rif-Betic mountains Nature 379 785ndash790doi101038379785a0 1996

Serrano I Morales J Zhao D Torcal F and Vidal F P-wave tomographic images in the Central Betics-Alboran Sea(south Spain) using local earthquakes contribution for a con-tinental collision Geophys Res Lett 25(21) 4031ndash4034doi1010291998GL900021 1998

Silva H G Bezzeghoud M Reis A H Rosa R N TlemcaniM Araujo A A Caldeira B Borges J F Serrano C and

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

246 H G Silva et al Seismo-electromagnetic phenomena

18

1

2

3

4

5

6

7

8

9

10

11

12

13

14

15

16

17

18

19

20

21

22

Figure 6 23 Fig 6 Photograph of the VAE sensor installed at the University ofEvora The inset is a zoom into the head of the instrument

mechanisms behind SEM that are the main purpose of thisproject Similarly to the VAE sensors new Radon detectorscould in the future equip the seismic stations that will hostULF magnetometers

5 Discussion

With this research effort it is likely that new SEM data linkedwith seismic activity in the WENP region could emerge andcontribute to a better understanding of SEM phenomenaFurthermore such data could be used in the following ob-jectives (1) perform the correlation between the ULF andVLFLF radio signals for different seismic events with theatmospheric electrical field and the radon levels (2) theoret-ical modelling of the observed phenomena (3) extend theempirical magnitude versus distance diagram for the detec-tion of abnormal ULF signals preceding HME presented byHattori et al (2004) to LME (working below the present de-tection threshold) In fact since LME were not considered inthe former diagram the extension of such diagram could pro-vide valuable information about the ULF precursory emis-sions Moreover the study of Gladychev et al (2010) re-veals that observation of LME is feasible if the seismic epi-centre is sufficiently close to the ULF station (4) ExploreVLFLF radio anomalies correlated with HME earthquakesand also LME already below the empirical threshold for itsdetection (similar to the ULF case) discussed by Rozhnoi etal (2004) (5) Search for a relation law between the strength

of SEM anomalies and the magnitude andor local (referredto the place where the sensors will be installed) ground mo-tion of the impending LME This objective could give an im-portant insight into the driving mechanisms of these phenom-ena and present a decisive contribution to the development ofthis field Obviously the SEM perturbations caused by LMEwill be in principle very faint and their detection and dis-crimination from other possible signal sources will representa real challenge

As a final remark it is important to mention that labo-ratorial experiments concerning the electrical properties ofthe rocks (for the moment the study is focusing on graniticrocks) which may contribute to a better understanding ofSEM phenomena are already being undertaken

AcknowledgementsThe Portuguese team acknowledges the sup-port of two Portuguese institutions QREN support (OperationalProgram ldquoThematic Factors of Competitivenessrdquo) co-fundedby the European Regional Development Fund (ERDF) for theresearch program SIRAS and the FCT (Science and TechnologyFoundation) for the grant SFRHBPD638802009 (HGS) Finallywe are grateful to the support from Samuel Barias e ClaudiaSerrano

Edited by K EftaxiasReviewed by C Nomicos and another anonymous referee

References

Argus D Gordon R DeMets C and Stein S Closureof the Africa-Eurasia-North America plate motion and tec-tonics of the Gloria fault J Geophys Res 94 5585ndash5602doi101029JB094iB05p05585 1989

Bezzeghoud M Borges J F Caldeira B Buforn E and UdıasA Seismic activity in the Azores Region in the context of thewestern part of the Eurasia-Nubia plate boundary InternationalSeminar on Seismic risk and rehabilitation on the 10th Anniver-sary of the July 9 1998 Azores Earthquake Horta-Azores 9ndash13July 27 2008

Bezzeghoud M Borges J F and Caldeira B Fontes sısmicasao longo da fronteira de placas tectonicas entre os Acores e aArgelia um modelo sismotectonico edited by Dias R AraujoA and Terrinha P (Escolar Editora Lisbon Portugal) in press2010

Biagi P F Castellana L Maggipinto T and Ermini A Anoverview on preseismic anomalies in LF radio signals revealed inItaly by wavelet analysis Ann Geophys Ann Geophys 51(1)237ndash246 2008

Biagi P F Castellana L Maggipinto T Loiacono D Schi-avulli L Ligonzo T Fiore M Suciu E and Ermini AA pre seismic radio anomaly revealed in the area where theAbruzzo earthquake (M = 63) occurred on 6 April 2009 NatHazards Earth Syst Sci 9 1551ndash1556 doi105194nhess-9-1551-2009 2009

Bleier T Dunson C Maniscalco M Bryant N Bambery Rand Freund F Investigation of ULF magnetic pulsations airconductivity changes and infra red signatures associated with

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011

H G Silva et al Seismo-electromagnetic phenomena 247

the 30 October Alum Rock M54 earthquake Nat Hazards EarthSyst Sci 9 585ndash603 doi105194nhess-9-585-2009 2009

Borges J Fitas A J S Bezzeghoud M and Teves-Costa PSeismotonics of Portugal and its adjacent Atlantic area Tectono-physics 331(4) 373ndash387 doi101016S0040-1951(00)00291-22001

Borges J F Bezzeghoud M Buforn E Pro C and FitasA The 1980 1997 and 1998 Azores earthquakes and itsseismotectonic implications Tectonophysics 435(1ndash4) 37ndash54doi101016jtecto200701008 2007

Borges J F Caldeira B Bezzeghoud M and Buforn E Seis-micity and Seismotectonics of Azores Geodynamic Implica-tions Ch 6 99-110 in A Book on the 1998 Azores earthquake -10 years after its occurrence edited by Oliveira C S Costa Aand Nunes J C ISBN 978989-20-1223-0 741 2008

Buforn E Udıas A and Colombas M A Seismic-ity source mechanisms and seismotectonics of the Azores-Gibraltar plate boundary Tectonophysics 152(1ndash2) 89ndash118doi1010160040-1951(88)90031-5 1988a

Buforn E Udıas A and Mezcua J Seismicity and focal mecha-nisms in south Spain Bull Seism Soc Am 78(6) 2008ndash2024(httpwwwbssaonlineorgcgicontentabstract7862008)1988b

Buforn E Udıas A and Madariaga R Intermediateand deep earthquakes in Spain Pageoph 136(4) 375-393doi101007BF00878576 1991a

Buforn E Udıas A Mezcua J and Madariaga R A deep earth-quake under south Spain 8 March 1990 Bull Seism Soc Am81(4) 1403ndash1407httpbssageoscienceworldorgcgicontentcitation8141403 1991b

Buforn E Coca P Udıas A and Lasa C Source mechanism ofintermediate and deep earthquakes in southern Spain J Seismol1(2) 113ndash130 doi101023A1009754219459 1997

Buforn E Bezzeghoud M Udıas A and Pro C Seis-mic Sources on the Iberia-African Plate Boundary and theirTectonic Implications Pure Appl Geophys 161(3) 623ndash646doi101007s00024-003-2466-1 2004

Caldeira B Carrilho F Miranda M Bezzeghoud M Alves PM Silveira G Villalonga F Pena J A Matias L BorgesJ F Vales D Corela C and Madureira G Recent improve-ments in the Broadband seismic networks in Portugal CSEM EMSC Newsletter 18-19 May 2007

Chauhan V Singh O P Kushwah V Singh V andSingh B Ultra-low-frequency (ULF) and total electron con-tent (TEC) anomalies observed at Agra and their associa-tion with regional earthquakes J Geodyn 48(2) 68ndash74doi101016jjog200906002 2009

Gladychev V Baransky L Schekotov A Fedorov EPokhotelov O Andreevsky S Rozhnoi A Khabazin YBelyaev G Gorbatikov A Gordeev E Chebrov V SinitsinV Lutikov A Yunga S Kosarev G Surkov V MolchanovO Hayakawa M Uyeda S Nagao T Hattori K and NodaY Study of electromagnetic emissions associated with seismicactivity in Kamchatka region Nat Hazards Earth Syst Sci 1127ndash136 doi105194nhess-1-127-2001 2001

Grandin R Borges J F Bezzeghoud M Caldeira B and Car-rilho F Simulations of strong ground motion in SW Iberia forthe 1969 February 28 (Ms = 80) and the 1755 November 1 (Msim 85) earthquakes ndash I Velocity model Geophys J Int 171(3)

1144-1161 doi101111j1365-246X200703570x 2007aGrandin R Borges J F Bezzeghoud M Caldeira B and Car-

rilho F Simulations of strong ground motion in SW Iberiafor the 1969 February 28 (Ms = 80) and the 1755 November1 (M sim 85) earthquakes ndash II Strong ground motion simula-tions Geophys J Int 171(2) 807-822 doi 101111j1365-246X200703571x 2007b

Grimison N and Cheng W The Azores-Gibraltar plate bound-ary focal mechanisms depths of earthquakes and their tec-tonic implications J Geophys Res 91(B2) 2029ndash2047doi101029JB091iB02p02029 1986

Harrison R G Aplin K L and Rycroft M J Atmo-spheric electricity coupling between earthquake regions andthe ionosphere J Atmos Sol-Terr Phys 72(5ndash6) 376ndash381doi101016jjastp200912004 2010

Hattori K Takahashi I Yoshino C C Isezaki N Iwasaki HHarada M Kawabata K Kopytenko E Kopytenko Y Malt-sev P Korepanov V Molchanov O Hayakawa M NodaY Nagao T and Uyeda S ULF geomagnetic field measure-ments in Japan and some recent results associated with IwatekenNairiku Hokubu earthquake in 1998 Phys Chem Earth 29(4ndash9) 481ndash494 doi101016jpce200309019 2004

Hatzfeld D Etude sismotectonique de la zone de collision Ibero-Maghrebine PhD Thesis Grenoble (France) 281 1978

Munuera J M Datos basicos para un estudio de sismicidad en laregion de la Penınsula Iberica Mem Inst Geog Cat Madrid32 93 1963

Nunes J C and Ribeiro E Caracterizacao da sismicidade instru-mental dos Acores no periodo 1950ndash1980 SISMICA EncontroNacional de Sismologıa e Enghenaria Sısmica Acores 2001

Prattes G Schwingenschuh K Eichelberger H U MagnesW Boudjada M Stachel M Vellante M WesztergomV and Nenovski P Multi-point ground-based ULF magneticfield observations in Europe during seismic active periods in2004 and 2005 Nat Hazards Earth Syst Sci 8 501ndash507doi105194nhess-8-501-2008 2008

Rocha J P Bezzeghoud M Caldeira B Araujo A Borges JF Vilallonga F and Dorbath C Microseismicity in the neigh-bourhood of ldquoAlmodovar faultrdquo 6a Assembleia Luso Espanholade Geodesia e Geofısica 2008 (in Portuguese)

Rozhnoi A Solovieva M S Molchanov O A andHayakawa M Middle latitude LF (40 kHz) phase varia-tions associated with earthquakes for quiet and disturbed ge-omagnetic conditions Phys Chem Earth 29(4ndash9) 589ndash598doi101016jpce200308061 2004

Searle R Tectonic pattern of the Azores spreading centreand triple junction Earth Plan Sci Lett 51(2) 415ndash434doi1010160012-821X(80)90221-6 1980

Seber D Barazangi M Ibenbrahim A and Demnati AGeophysical evidence for lithospheric delamination beneath theAlboran Sea and Rif-Betic mountains Nature 379 785ndash790doi101038379785a0 1996

Serrano I Morales J Zhao D Torcal F and Vidal F P-wave tomographic images in the Central Betics-Alboran Sea(south Spain) using local earthquakes contribution for a con-tinental collision Geophys Res Lett 25(21) 4031ndash4034doi1010291998GL900021 1998

Silva H G Bezzeghoud M Reis A H Rosa R N TlemcaniM Araujo A A Caldeira B Borges J F Serrano C and

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

H G Silva et al Seismo-electromagnetic phenomena 247

the 30 October Alum Rock M54 earthquake Nat Hazards EarthSyst Sci 9 585ndash603 doi105194nhess-9-585-2009 2009

Borges J Fitas A J S Bezzeghoud M and Teves-Costa PSeismotonics of Portugal and its adjacent Atlantic area Tectono-physics 331(4) 373ndash387 doi101016S0040-1951(00)00291-22001

Borges J F Bezzeghoud M Buforn E Pro C and FitasA The 1980 1997 and 1998 Azores earthquakes and itsseismotectonic implications Tectonophysics 435(1ndash4) 37ndash54doi101016jtecto200701008 2007

Borges J F Caldeira B Bezzeghoud M and Buforn E Seis-micity and Seismotectonics of Azores Geodynamic Implica-tions Ch 6 99-110 in A Book on the 1998 Azores earthquake -10 years after its occurrence edited by Oliveira C S Costa Aand Nunes J C ISBN 978989-20-1223-0 741 2008

Buforn E Udıas A and Colombas M A Seismic-ity source mechanisms and seismotectonics of the Azores-Gibraltar plate boundary Tectonophysics 152(1ndash2) 89ndash118doi1010160040-1951(88)90031-5 1988a

Buforn E Udıas A and Mezcua J Seismicity and focal mecha-nisms in south Spain Bull Seism Soc Am 78(6) 2008ndash2024(httpwwwbssaonlineorgcgicontentabstract7862008)1988b

Buforn E Udıas A and Madariaga R Intermediateand deep earthquakes in Spain Pageoph 136(4) 375-393doi101007BF00878576 1991a

Buforn E Udıas A Mezcua J and Madariaga R A deep earth-quake under south Spain 8 March 1990 Bull Seism Soc Am81(4) 1403ndash1407httpbssageoscienceworldorgcgicontentcitation8141403 1991b

Buforn E Coca P Udıas A and Lasa C Source mechanism ofintermediate and deep earthquakes in southern Spain J Seismol1(2) 113ndash130 doi101023A1009754219459 1997

Buforn E Bezzeghoud M Udıas A and Pro C Seis-mic Sources on the Iberia-African Plate Boundary and theirTectonic Implications Pure Appl Geophys 161(3) 623ndash646doi101007s00024-003-2466-1 2004

Caldeira B Carrilho F Miranda M Bezzeghoud M Alves PM Silveira G Villalonga F Pena J A Matias L BorgesJ F Vales D Corela C and Madureira G Recent improve-ments in the Broadband seismic networks in Portugal CSEM EMSC Newsletter 18-19 May 2007

Chauhan V Singh O P Kushwah V Singh V andSingh B Ultra-low-frequency (ULF) and total electron con-tent (TEC) anomalies observed at Agra and their associa-tion with regional earthquakes J Geodyn 48(2) 68ndash74doi101016jjog200906002 2009

Gladychev V Baransky L Schekotov A Fedorov EPokhotelov O Andreevsky S Rozhnoi A Khabazin YBelyaev G Gorbatikov A Gordeev E Chebrov V SinitsinV Lutikov A Yunga S Kosarev G Surkov V MolchanovO Hayakawa M Uyeda S Nagao T Hattori K and NodaY Study of electromagnetic emissions associated with seismicactivity in Kamchatka region Nat Hazards Earth Syst Sci 1127ndash136 doi105194nhess-1-127-2001 2001

Grandin R Borges J F Bezzeghoud M Caldeira B and Car-rilho F Simulations of strong ground motion in SW Iberia forthe 1969 February 28 (Ms = 80) and the 1755 November 1 (Msim 85) earthquakes ndash I Velocity model Geophys J Int 171(3)

1144-1161 doi101111j1365-246X200703570x 2007aGrandin R Borges J F Bezzeghoud M Caldeira B and Car-

rilho F Simulations of strong ground motion in SW Iberiafor the 1969 February 28 (Ms = 80) and the 1755 November1 (M sim 85) earthquakes ndash II Strong ground motion simula-tions Geophys J Int 171(2) 807-822 doi 101111j1365-246X200703571x 2007b

Grimison N and Cheng W The Azores-Gibraltar plate bound-ary focal mechanisms depths of earthquakes and their tec-tonic implications J Geophys Res 91(B2) 2029ndash2047doi101029JB091iB02p02029 1986

Harrison R G Aplin K L and Rycroft M J Atmo-spheric electricity coupling between earthquake regions andthe ionosphere J Atmos Sol-Terr Phys 72(5ndash6) 376ndash381doi101016jjastp200912004 2010

Hattori K Takahashi I Yoshino C C Isezaki N Iwasaki HHarada M Kawabata K Kopytenko E Kopytenko Y Malt-sev P Korepanov V Molchanov O Hayakawa M NodaY Nagao T and Uyeda S ULF geomagnetic field measure-ments in Japan and some recent results associated with IwatekenNairiku Hokubu earthquake in 1998 Phys Chem Earth 29(4ndash9) 481ndash494 doi101016jpce200309019 2004

Hatzfeld D Etude sismotectonique de la zone de collision Ibero-Maghrebine PhD Thesis Grenoble (France) 281 1978

Munuera J M Datos basicos para un estudio de sismicidad en laregion de la Penınsula Iberica Mem Inst Geog Cat Madrid32 93 1963

Nunes J C and Ribeiro E Caracterizacao da sismicidade instru-mental dos Acores no periodo 1950ndash1980 SISMICA EncontroNacional de Sismologıa e Enghenaria Sısmica Acores 2001

Prattes G Schwingenschuh K Eichelberger H U MagnesW Boudjada M Stachel M Vellante M WesztergomV and Nenovski P Multi-point ground-based ULF magneticfield observations in Europe during seismic active periods in2004 and 2005 Nat Hazards Earth Syst Sci 8 501ndash507doi105194nhess-8-501-2008 2008

Rocha J P Bezzeghoud M Caldeira B Araujo A Borges JF Vilallonga F and Dorbath C Microseismicity in the neigh-bourhood of ldquoAlmodovar faultrdquo 6a Assembleia Luso Espanholade Geodesia e Geofısica 2008 (in Portuguese)

Rozhnoi A Solovieva M S Molchanov O A andHayakawa M Middle latitude LF (40 kHz) phase varia-tions associated with earthquakes for quiet and disturbed ge-omagnetic conditions Phys Chem Earth 29(4ndash9) 589ndash598doi101016jpce200308061 2004

Searle R Tectonic pattern of the Azores spreading centreand triple junction Earth Plan Sci Lett 51(2) 415ndash434doi1010160012-821X(80)90221-6 1980

Seber D Barazangi M Ibenbrahim A and Demnati AGeophysical evidence for lithospheric delamination beneath theAlboran Sea and Rif-Betic mountains Nature 379 785ndash790doi101038379785a0 1996

Serrano I Morales J Zhao D Torcal F and Vidal F P-wave tomographic images in the Central Betics-Alboran Sea(south Spain) using local earthquakes contribution for a con-tinental collision Geophys Res Lett 25(21) 4031ndash4034doi1010291998GL900021 1998

Silva H G Bezzeghoud M Reis A H Rosa R N TlemcaniM Araujo A A Caldeira B Borges J F Serrano C and

wwwnat-hazards-earth-syst-scinet112412011 Nat Hazards Earth Syst Sci 11 241ndash248 2011

248 H G Silva et al Seismo-electromagnetic phenomena

Biagi P F Atmospheric electrical field suppression in the timeof theM = 41 Sousel earthquake (Portugal) Nat Hazards EarthSyst Sci in review 2011

Smirnov S Association of the negative anomalies of the qua-sistatic electric field in atmosphere with Kamchatka seismicityNat Hazards Earth Syst Sci 8 745ndash749 doi105194nhess-8-745-2008 2008

Telesca L Lapenna V Macchiato M and Hattori K Investigat-ing non-uniform scaling behavior in Ultra Low Frequency (ULF)earthquake-related geomagnetic signals Earth Planet Sci Lett268(1ndash2) 219ndash224 doi101016jepsl200801033 2008

Torne M Fernandez M Comas M C and Soto J I Litho-spheric structure beneath the Alboran basin results from 3Dgravity modeling and tectonic revelance J Geophys Res105(B2) 3209ndash3228 doi1010291999JB900281 2000

Toutain J P and Baubron J C Gas geochemistry andseismotectonics a review Tectonophysics 304(1ndash2) 1ndash27doi101016S0040-1951(98)00295-9 1999

Udıas A Lopez Arroyo A and Mezcua J Seismotectonics ofthe Azores-Alboran region Tectonophysics 31(3ndash4) 259ndash289doi1010160040-1951(76)90121-9 1976

Nat Hazards Earth Syst Sci 11 241ndash248 2011 wwwnat-hazards-earth-syst-scinet112412011