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European Geosciences Union General Assembly 2012 Vienna | Austria | 22 – 27 April 2012

Seismotectonics and crustal deformation in Africa TS8.2/G3.8/NH4.4/SM2.11

Conveners: M. Meghraoui (IPG Strasbourg, France; [email protected]) R. Durrheim (Witwatersrand University, S. Africa), A. Ayele (Addis Abeba University,

Ethiopia), V. Midzi (Council for Geoscience, Pretoria, S. Africa)

VOLUME OF ABSTRACTS

http://meetingorganizer.copernicus.org/EGU2012/sessionprogramme/TS#TS8

European Geosciences Union General Assembly 2012 Vienna | Austria | 22 – 27 April 2012

Seismotectonics and crustal deformation in Africa TS8.2/G3.8/NH4.4/SM2.11 Conveners: M. Meghraoui (IPG Strasbourg, France; [email protected]) R. Durrheim (Witwatersrand University, S. Africa), A. Ayele (Addis Abeba University, Ethiopia), V. Midzi (Council for Geoscience, Pretoria, S. Africa) http://meetingorganizer.copernicus.org/EGU2012/sessionprogramme/TS#TS8

Session topic: The African continent is made of various geological structures that include zones

of active deformation. Seismically active regions are primarily located along rift zones, thrust and fold mountain belts, transform faults and volcanic fields. Several tectonic structures may generate large earthquakes with significant damage and economic losses in Africa. The development of thematic mapping with the identification and characterization of seismically active zones constitutes the framework for the seismic hazard assessment and mitigation of catastrophes.

The IGCP‐UNESCO programme recently launched the project‐601 (with the support of SIDA) titled "Seismotectonics and seismic hazards in Africa". The African plate was the site of numerous large earthquakes (Mw > 6.5) causing severe damage and tremendous economic losses. The most recent events being the 2006 Machaze earthquake (M 7.0) in Mozambique, the 2003 Zemmouri‐Boumerdes earthquake (M 6.8) in Algeria, and the 1990 Juba earthquake (M 7.1) in southern Soudan. The presence of major active faults that generate destructive earthquakes is among the most important geological and geophysical hazards for the continent.

In this session we seek contributions on the observation, analysis and modelling of the significant seismotectonic activity in the African continent and neighbouring islands. In particular, we encourage contributions on lithospheric and crustal deformation using interdisciplinary approach and field measurements such as GPS, InSAR and seismicity analysis (tomography) in contractional and extensional tectonic domains, and along main transform faults.

EGU General Assembly 2012

TS8.2/G3.8/NH4.4/SM2.11 Abstracts

05 Mar 2012 19:35:11

Geophysical Research AbstractsVol. 14, EGU2012-474, 2012EGU General Assembly 2012© Author(s) 2011

Rigidity of Nubia from combined GPS and DORIS solutions: Implicationto Africa Reference Frame (AFREF)E. Saria (1), E. Calais (1), Z. Altamimi (2), P. Willis (3), R. Fernandes (4), S. Stamps (1), and H. Farah (5)(1) Purdue University, Earth and Atmospheric Science, West Lafayette, IN 47907, United States ([email protected]), (2)Institut Geographique National (ENSG/LAREG ) Paris 77455, Champs-sur-Marne, FRANCE, (3) Institut de Physique duGlobe de Paris, Etudes Spatiales et Planetologie Paris 75013, France , (4) University of Beira Interior, Department ofComputer Sciences, Rua Marquês d’Ávila e Bolama, 6201-001 Covilhã, Portugal, (5) Regional Center for Mapping ofResources and Development, P. O. Box 632-00618, Kasarani, Nairobi, Kenya

As with many major tectonic plates, the interior of the Nubian plate is affected by (relatively rare) largeearthquakes and recent volcanism outside of its boundaries in northern or eastern Africa. This may indicate somelevel of internal deformation of the plate. Quantifying this internal deformation is important for studies relyingon stable Nubia as a reference frame, such as geophysical studies aimed at quantifying its relative motion tosurrounding plates or geodetic studies aimed at defining the upcoming Africa Reference Frame (AFREF). Mostother well-instrumented continental interiors seem to show very small – if any – internal deformation, in spite ofseismic activity. Thanks to a significant increase in space geodetic instrumentation in Africa and improved dataanalysis strategies, it is now possible to test this for Nubia and determine the amount of current deformation withinthe Nubian continental interior.We combine up to 16 years of GPS data and 17 years global DORIS solution to derive a new position/velocitysolution for Africa. The resulting velocity field describes horizontal plate motion at 120 GPS and 9 DORIS sitesoperating in Africa. We use our combined velocity field to estimate internal deformation within Nubia, bothhorizontal and vertical. As a by-product, we use the resulting velocity field to update the angular velocity estimatefor the Somalian plate and smaller sub-plates Victoria and Rovuma and compare the Somalia-Nubia relativevelocity with the current global velocity model GEODVEL.Our proposed model provides a robust bound on the rigidity of the Nubian plate and the kinematics of the EastAfrican Rift system, and is therefore an important contribution to AFREF.


African plate motions constraining and Euler pole determination usingpermanent GPS dataA. Deprez, F. Masson, C. Doubre, and P. UlrichFrance ([email protected])

IPGS-EOST, CNRS-Université de Strasbourg, 5 rue Descartes, 67084 Strasbourg Cedex France.

GPS data of permanent stations in Africa allowed us to calculate the position time series and the absolutevelocity of many points on this continent during a twelve-year period (1999-2011). The data processing was madethanks to the GAMIT/GLOBK software. The results lead us to select reliable sites presenting a quantity of datalarge enough to limit the uncertainty. We took care too that this sites did not undergo local deformation and, inparticular, that they were far from the plate boundaries. Then we adopted a technique based on William’s methodto remove the data jumps and the seasonal variations from the position time series. Station sets was broughttogether function of their position and of plate and micro-plate boundaries from precedents studies. The mainconclusions of this velocity field study were as follows: (1) We achieve, with coherent data samples, to find polecoordinates for plates and micro-plates defined by Stamps et al. (2007). These coordinates differ slightly fromvalues found in precedent studies (Altamimi et al., 2012 (in review); Stamps et al., 2007), which were taken as apriori values. (2) Many African stations were too recent to give reliable velocity and the pole coordinate accuracycould be improved in the next few years. (3) The presence of large zones of local deformation particularly on bothsides of the East African Rift prevents from the use of many data for the pole determination.

Geophysical Research AbstractsVol. 14, EGU2012-1474-1, 2012EGU General Assembly 2012© Author(s) 2012

Evidence of changes in the velocity field in the Asal-Ghoubbet rift usingGPS data: results of repeated GPS campaigns over a 13-year period.A. Deprez (1), C. Doubre (1), F. Masson (1), A. Socquet (2), P. Ulrich (1), C. Vigny (3), and J.C. Ruegg (4)(1) Strasbourg, EOST-IPGS, France ([email protected]), (2) ISTERRE, Grenoble, France, (3) ENS PARIS, (4) IPGP

The Asal-Ghoubbet rift is located at the western tip of the Aden Ridge, propagating into the eastern part of theAfar Depression (East Africa). The region has been intensevely studied to improve the understanding of the riftingprocess through a long series of geophysical and geodetic measurements. In particular, GPS campaigns have beenperformed since the early 90’s, with the repetition of measurements of a dense network of geodetic points in 1999,2001, 2003, 2010 and 2012 within the rift. We determine the horizontal velocity field associated with the openingrift over a total period of 13 years (1999-2012), and its evolution by the decomposition into two periods (1999-2003and 2003-2012). This data analysis leads us to list significant changes in the velocity field. First, the velocity fieldobtained during the second period appears more homogeneous with a regular gradient from the southern to northernmargins through the whole densely faulted rift. However, because of the important deformation in the southernmargin observed since 2003, the deformation associated with the divergent motion of the Arabia/Somalia platesappears more distributed after 2003. Indeed the sites located south of the Tadjoura Bay and the Asal Rift are affectedby slow (<3mm/yr) NE-trending movement during the first period and exhibit clear northwards displacementsduring the second period. The origin of such an important change is not clear: it could be related to the aseismicactivity of large normal faults forming the large scarps bounding the south of the Afar Depression and/or (very)far field effects of the Ethiopian Rifting of Dabbahu-Manda Harraro which started in 2005. These displacementsobserved in the southern part of the region under study reduces the opening velocity across the rift to ∼9 mm/yrinstead of the ∼15 mm/yr observed during the first period, or deduced from the long-term displacements. Thedynamics and the location of the opening of the western part of the Aden Ridge is clearly not steady-state, implyingthe important role of the volcanic systems on the localization/distribution of the extensive deformation related tothe plate boundary. At the regional level, this study suggests the contribution of the opening of the basins locatedwithin the transfer zone, between the Aden Ridge and the Red Sea branch into Afar, into the plate separation. Atthe rift scale, it suggests that such significant variations of velocity still bear witness to effects of the 1978 dikingevent.


Seismic constraints on a large dyking event in Western Gulf of AdenA. Ahmed (1,2), C. Doubre (3), S. Leroy (1), J. Perrot (4), L. Audin (5,6), F. Rolandone (1), D. Keir (7), I.Al-Ganad (8), K. Khanbari (9), K. Mohamed (10), J. Vergne (3), E. Jacques (11), and A. Nercessian (11)(1) ISTEP UPMC CNRS, Paris, France , (2) Seismological and Volcanological Observatory Center, Dhamar, Yemen , (3)EOST, IPGS, Université de Strasboug, France , (4) IUEM, Plouzane, France , (5) CNRS-LMTG, Universite PaulSabatier-Toulouse France , (6) IRD , (7) School of Earth and Environment, Leeds, United Kingdom , (8) Yemen GeologicalSurvey & Mineral Resources Board, Sana’a, Yemen , (9) Ministry of Telecomunication and Information Technology, Sana’a,Yemen, (10) Geophysical Observatory of Arta, CERD, Djibouti, (11) IPGP, France

In November 2010, a large number of events were recorded by the world seismic networks showing importantactivity occurring along the western part of the Aden Ridge. West of the Shulka El Sheik transform zone, eventsin this large seismic swarm (magnitudes above 5) occurred in a complex area, where the change of both the ridgedirection and the bathymetry suggest the propagation of the ridge into a continental lithosphere and the influenceof the thermal anomaly of the Afar Hot Spot. We combine several sets of data from permanent networks andtemporary 3C broad stations installed after the beginning of the event along the southern and eastern coasts ofYemen and Djibouti respectively, we located more than 600 earthquakes with magnitudes ranging from 2.5 to 5.6that occurred during the first months following the first event. The spatial distribution of the main seismicity revealsa very clear N115◦-trending alignment, parallel to the mean direction of the en-echelon spreading segments thatform the ridge at this longitude. Half of the events, which represent half of the total seismic energy released duringthe first months, are located in the central third section of the segment. Here several volcanic cones and recentlava flows observed from bathymetric and acoustic reflectivity data during the Tadjouraden cruise (Audin, 1999,Dauteuil et al., 2001) constitute the sea floor. In addition to this main activity, two small groups of events suggestthe activiation of landslides into a large fan and the activity in a volcanic area 50 km due east from the main activezone. The time evolution of the seismicity shows several bursts of activity. Some of them are clearly related tosudden activities within the volcanic areas, when others exhibit horizontal migration of the events, with velocityaround ∼ 1 km/h. The time-space evolution of the seismicity clearly reveals the intrusion of dykes associated withmagma propagation from the crustal magmatic centres into the rift zone.We use knowledge from spatial geodeticobservations of rifting episodes in the regions: the Harrat Al-Shaqah event in 2009 (Arabia Souadia), the Tanzanianone in 2007, and the major Dabahu Manda Harraro rifting episode in Afar. Taking into account that the geodeticmoment is one order of magnitude higher than the seismic moment during such events, the seismic activity of thisevent of the Aden ridge represents a major rifting episode certainly associated with the opening of the segment bydyking estimated to be higher than 10 m.


Complex seismicity patterns in the Rwenzori region: insights to riftingprocesses at the Albertine Rift.M. Lindenfeld (1), G. Rümpker (1), I. Wölbern (1), A.G. Batte (1,2), and A. Schumann (1)(1) Department of Geosciences, Goethe University, Frankfurt, Germany, (2) Department of Geology, Makerere University,Kampala, Uganda

Numerous seismological studies in East Africa have focused on the northern and eastern branches of the EastAfrican Rift System (EARS). However, the seismic activity along the western branch is much more pronounced.Here, the Rwenzori Mountains are located within the Albertine rift valley, at the border between Uganda and D.R.Congo.During a seismic monitoring campaign between February 2006 and September 2007 we have recorded morethan 800 earthquakes per month in the Rwenzori area. The earthquake distribution is highly heterogeneous.The majority of located events lie within faults zones to the East and West of the Rwenzoris with the highestseismic activity observed in the northeastern area, were the mountains are in contact with the rift shoulders. Thehypocentral depth distribution peaks at 16 km depth and extends down to the Moho which was found at 20 – 32km depths by teleseismic receiver functions. Local magnitudes range from -0.5 to 5.1 with a b-value of 1.1. Faultplane solutions of 304 events were derived from P-polarities and SV/P amplitude ratios. More than 70% of thesource mechanisms exhibit normal faulting. T-axis trends are highly uniform and oriented WNW-ESE, which isperpendicular to the rift axis and in good agreement with kinematic rift models.The area of highest seismic activity NE of the Rwenzoris is characterized by the occurrence of several earthquakeclusters in 5 -20 km depth. They have stable positions throughout time and form elongated pipes with 1-2 kmdiameter and vertical extensions of 3-5 km. From petrological considerations we presume that these earthquakeswarms are triggered by fluids and gasses which originate from a magmatic source below the crust. The existenceof a magmatic source within the lithosphere is supported by the detection of a shear-wave velocity reductionin 55-80 km depth from receiver-function analysis and the location of mantle earthquakes at about 60 km. Weinterpret these observations as indication for an initial rifting process that may eventually lead to the completedetachment of the Rwenzori block from the surrounding rift flanks.


Seismotectonics of North AfricaM. Meghraoui (1) and A. Ayadi (2)(1) EOST - Institut Physique du Globe Strasbourg (UMR 7516), Geodynamics and Active Deformation, Strasbourg, France([email protected]), (2) CRAAG, Bouzareah, Alger

In this work, the seismotectonic analysis is based on the compiled historical (from AD 42 to 1900) and instrumen-tal seismicity catalogue, a detailed identification of active faults and the study of individual large and moderateearthquakes. The active tectonics of the Tell and Sahara Atlas Mountains are directly related with convergentmovements along the Africa - Eurasia plate boundary. Neotectonic structures and significant seismicity (Ms>4.7)document the strain distribution obtained from fault and fold structures and P axes of focal mechanisms of largeearthquakes, illustrate the oblique convergence at the plate boundary. A catalogue of fault parameters has been im-plemented using mainshock and aftershock data, surface faulting and folding structures, InSAR and geophysicalresults associated with the significant earthquakes since mainly the El Asnam large seismic event (10/10/1980, Mw7.3). From the long-term deformation along the Africa – Eurasia plate boundary we observe that the shortening rateof the deforming zone that attains 2.2 mm/y is comparable to the compressional rate of 1.5 mm/yr obtained fromthe seismic moment tensor summation. The pattern of seismic deformation and faulting activity that constrainsthe seismic zoning in the Atlas Mountains (Sahara and Tell) is the basis for any seismic hazard assessment. Thisseismotectonic project is conducted with the support of the IGCP-UNESCO programme and Global EarthquakeModel for North-Africa.


Lateral extrusion of Tunisia : Contribution of Jeffara Fault (southernbranch) and Petroleum ImplicationsR. Ghedhoui (1), B. Deffontaines (1), and M.C Rabia (2)(1) (1) Laboratoire de Géomatique, Télédétection et Modélisation des Connaissances (GTMC) - Université Paris-Est MLV ; 5Bd Descartes 77454 Marne-la-Vallée, Marne-la-Vallée, 77454France ([email protected] & [email protected]),(2) Unité de Recherche « Géomatique et Géosystèmes » 02/UR/10-01, Université de la Mannouba,Tunisie([email protected])

Contrasting to the northward African plate motion toward Eurasia and due to its geographic position in the NorthAfrican margin, since early cretaceous, Tunisia seems to be submitted to an eastward migration. The aim ofthis work is to study the southern branch of this inferred tectonic splay that may guide the Tunisian extrusioncharacterised to the east by the Mediterranean sea as a free eastern boundary. The Jeffara Fault zone (southernTunisia), represent a case example of such deformation faced by Tunisia.

Helped by the results of previous researchers (Bouaziz, 1995 ; Rabiaa, 1998 ; Touati et Rodgers, 1998 ;Sokoutis D. et al., 2000 ; Bouaziz et al., 2002 ; Jallouli et al., 2005 ; Deffontaines et al., 2008. . . ), and newevidences developed in this study, we propose a geodynamic Tunisian east extrusion model, due to such thenorthern African plate migration to the Eurasian one.

In this subject, structural geomorphology is undertaken herein based on both geomorphometric drainagenetwork analysis (Deffontaines et al., 1990), the Digital Terrain Model photo-interpretation (SRTM) combinedwith photo-interpretation of detailed optical images (Landsat ETM+), and confirmed by field work and numerousseismic profiles at depth. All these informations were then integrated within a GIS (Geodatabase) (Deffontaines1990 ; Deffontaines et al. 1994 ; Deffontaines, 2000 ; Slama, 2008 ; Deffontaines, 2008) and are coherent with theeastern extrusion of the Sahel block. We infer that the NW-SE Gafsa-Tozeur, which continue to the Jeffara majorfault zone acting as a transtensive right lateral motion since early cretaceous is the southern branch of the Sahelblock extrusion. Our structural analyses prove the presence of NW-SE right lateral en-echelon tension gashes,NW-SE aligned salt diapirs, numerous folds offsets, en-echelon folds, and so on that parallel this major NW-SEtranstensive extrusion fault zone.These evidences confirm the fact that the NW-SE Jeffara faults correspond to thetectonic accident, located in the south of the Tunisian extrusion, in favour of the eastern migration of the Sahelblock toward the free Mediterranean sea boundary. Therefore this geodynamic movement explains the presence,in offshore area, of small elongated NW-SE, N-S &NE-SW petroleum transtensive basins and grabens.

To conclude, at the regional scale, the structural geomorphologic approach combined with both field workand reflexion seismic profile analyses appear to be an excellent tool to prove & confirm the east Sahel blockextrusion of the central Tunisian part caused by the northward migration of African plate.>>>>>>>>Keywords : Geodynamics, Neotectonics, right lateral transtensive fault, Extrusion, Petroleum exploration,Geomorphometry, Digital Elevation Model, Geographic Information System (GIS), Geodatabase, Jeffara, SouthTunisia.


Geodynamic significance of the TRM segment in the East African Rift:active tectonics and paleostress in western TanzaniaD. Delvaux (1), F. Kervyn (1), A.S. Macheyeki (2), and E.B. Temu (3)(1) Royal Museum for Central Africa, Geology - Mineralogy, Tervuren, Belgium ([email protected]), (2)Dept. of Geology, School of Mines and Petroleum Engineering, University of Dodoma, Tanzania ([email protected]),(3) Geological Survey of Tanzania, Dodoma, Tanzania

The Tanganyika-Rukwa-Malawi (TRM) rift segment in western Tanzania is a key sector for understanding theopening dynamics of the East African rift system (EARS). In an oblique opening model, it is considered as adextral transfer fault zone that accommodates the general opening of the EARS in a NW-SE direction. In anorthogonal opening model, it accommodates pure dip-slip normal faulting with extension orthogonal to the riftsegments and a general E-W extension for the entire EARS. We investigated the active tectonic architecture andpaleostress evolution of the Ufipa plateau and adjacent Rukwa basin and in order to define their geodynamic rolein the development of the EARS and highlight their pre-rift brittle tectonic history. The active fault architecture,fault-kinematic analysis and paleostress reconstruction show that the recent to active fault systems that control therift structure develop in a pure extensional setting with extension direction orthogonal to the trend of the TRMsegment. Two pre-rift brittle events are evidenced. An older brittle thrusting is related to the interaction betweenthe Bangweulu block and the Tanzanian craton during the late Pan-African (early Paleozoic). It was followed by atranspressional inversion during the early Mesozoic. This inversion stage caused dextral strike-slip faulting alongthe fault systems that now control the major rift structures. It has been erroneously interpreted as related to the lateCenozoic EARS which instead is characterized by pure normal faulting.


Pre-instrumental seismicity in Central Africa using felt seisms recordedmainly at the meteorological stations of DRC, Rwanda and Burundiduring the colonial periodJ.-L. Mulumba (1) and D. Delvaux (2)(1) Department of Geology, University of Lubumbashi, Katanga, DRC ([email protected]), (2) Royal Museum forCentral Africa, Geology - Mineralogy, Tervuren, Belgium ([email protected])

Seismic hazard assessment and mitigation of catastrophes are primarily based on the identification and charac-terization of seismically active zones. These tasks still rely heavily on the existing knowledge of the seismicactivity over the longest possible time period. The first seismic network in Equatorial Africa (IRSAC network)was operated from the Lwiro scientific base on the western shores of Lake Kivu between 1953 and 1963. Beforethis installation, the historical record of seismic activity in Central Africa is sparse. Even for the relatively shortperiod concerned, spanning only 50-60 years, the historical record is far from being complete. A first attempthas been made by Herrinckx (1959) who compiled a list 960 felt seisms recorded at the meteorological stationsbetween 1915 and 1954 in Congo, Rwanda and Burundi. They were used to draw a density map of felt seisms persquare degree. We completed this data base by exploiting the meteorological archives and any available historicalreport to enlarge the database which now reaches 1513 entries between 1900 and 1959.

These entries have been exanimate in order to identify possible historical seismic events. Those are definedby 3 or more quasi-simultaneous records observed over a relatively short distance (a few degrees of lati-tude/longitude) within a short time difference (few hours). A preliminary list of 115 possible historical seisms hasbeen obtained, identified by 3 to 15 different stations. The proposed location is taken as the average latitude andlongitude of the stations where the felt seisms were recorded. Some of the most important ones are associatedto aftershocks that have been felt at some stations after the main shocks. The most recent felt seisms havebeen also recorded instrumentally, which helps to validate the procedure followed. The main difficulties arethe magnitude estimation and the possible spatial incompleteness of the recording of felt seism evidence at themargin of the observation network. The distribution of these historical felt seisms mach the distribution of the in-strumental epicenters. The results obtained may be used to complete the existing catalogues of historical seismicity.

Herrinckx, P. (1959). Séismicité du Congo belge. Compilation des seismes observés aux stations clima-tologiques entre 1909 et 1954. Académie royale des Sciences coloniales. Classe des Sciences naturelles etmédicales. Mémoire in8◦. Nouvelle série, 11(5), 1-55


reevaluation of seismicity and seismotectonics of LibyaA. Ben-SulemanTripoli University, Geophysics, tripoli, ([email protected])

Libya is located at the northern margin of the African continent, which is bordered by the Alpine tectonic belt ofthe Atlas Mountain and by the active belt beneath the southern Mediterranean. Libya underwent many episodesof orogenic activity of the Caledonian and Hercynian in the Paleozoic during Cretaceous, Middle Tertiary, andHolocene time. These episodes of orogenic activity affected the region and shaped the geological setting of theCountry. As a result a number of sedimentary basins were formed separated by intervening arches.At the end of the year 2005 the Libyan national seismological network (LNSN) starts functioning with 15stations. In this study the seismic activity of Libya is reevaluated using the new data recorded by Libyan nationalseismological network. Fault plane solution was estimated for 17 earthquakes recorded by the Libyan NationalSeismograph Network in northwestern Libya.At first glance the seismic activity map shows dominant trends of seismicity with most seismic activity concen-trated along the northern coastal areas. Four major seismic trends were quite noticeable. A first trend is a NW-SEdirection coinciding with the eastern boarder of the Hun Graben. A second trend is also a NW-SE direction in thenortheastern offshore Tripoli area. The other two trends were located in the western Gulf of Sirt and Cyrenaicaplatform. The rest of seismicity is diffuse either offshore or in land, with no good correlation with well-mappedfaults.A result of fault plane solution suggests that normal faulting was dominant in the westernmost part of Libya; strikeslip faulting was dominant in northern-central part of Libya. The northern-eastern part of the country suggests thatdip-dip faulting were more prevalent.


Constraints to the strain field of Africa from geodetic solutions: acontribution for the Seismotectonic Map of AfricaR.M.S. Fernandes (1,2), M. Meghraoui (3), J.M. Miranda (4), M.S. Bos (5), A. Radwan (6), A. Tahayt (7), M.Muhammad (8,1)(1) UBI, IDL, Covilhã, Portugal ([email protected]), (2) TUDelft, Delft, The Netherlands, (3) IPG – UMR 7516, U.Strasbourg, France, (4) UL, IDL, Lisboa, Portugal, (5) CIIMAR, Porto, Portugal, (6) NRIAG, Helwan, Egypt, (7) CNRST,Rabat, Morocco, (8) CGG-NASDRA, Toro, Nigeria

This work presents the strain field of Africa derived from the current GNSS velocity field of Africa. It is beingcarried out in the framework of the IGCP Project 601 – “Seismotectonics and Seismic Hazards in Africa”, whichultimate goal is to create thematic maps of the earthquake hazards of Africa.

The geographical distribution of the existing network of permanent GNSS stations is still far from optimalwith significant gaps existing particularly on the Central and North (Sahara) Africa. This is even more evidentwhen we consider the stations with enough long data span and monument/equipment stability (no significantnumber of epochs with offsets) to produce reliable velocity solutions (we use here a threshold value of 2.5 yearsdata span to compute the station velocity). Nevertheless, the existing number of sites (∼100) already permits tocompute a velocity field that can be used to infer the current strain field for Africa. This provides us an indicativepicture of the main seismotectonic hazards areas of Africa.

We also compute reliable uncertainties associated with the strain velocity field in order to determine the re-gions where the estimated strain is significant. For this, our GNSS velocity solutions with respect to the latestglobal reference frame, ITRF2008, is computed taking into account the existing temporal correlations between thedaily solutions of the stations.

This strain field, derived solely from space-geodetic data is correlated with the seismicity map of Africa inorder to identify the regions prone to seismic hazard and risk and also to detect areas where aseismic processescan be the main cause to accommodate the observed deformation in Africa.


Seismotectonic Analysis for the KZN region of South AfricaM. SinghSchool of Engineering, University of KwaZulu Natal, Howard College Campus, Durban, 4041, South Africa

Recently, devastating earthquakes and tsunamis have shocked the modern world (Japan [April 7 2011, Mw 9.0,loss of life and destruction of infrastructure, 15,457 deaths 5,389 injured, US300$billion loss (Japanese NationalPolice Agency 2011)], New Zealand [21 February 2011, Mw 6.3, 148 killed], Haiti [12 January 2010, Mw7.0, estimated 316 000 killed and 300 000 injured]. These earthquakes have caused large scale damage to thebuilt environment not to mention the high number of fatalities. The KZN coastal region is also fast developingespecially towards the north of Durban CBD (Cornubia [New development near Umhlanga, 25 Billion Randsinvestment], Gateway/Umhlanga Business District, Moses Mabida Stadium (cost of R3.4 billion ), King ShakaInternational Airport at a cost of R6.8 billion, Dube Tradeport to be developed next to the airport at a cost ofR5 billion, as well as the development of the Richards Bay Industrial Development Zone . The KZN is home to10 million inhabitants with a relatively denser population distribution around the Durban and PietermaritzburgCBDs. With the increasing amount of investment towards the north coast of Durban, the population distributionwill migrate to these areas. These areas now become ‘vulnerable’ to rare, infrequent and potentially devastatingnatural disasters like earthquakes. One of the first steps to understand and plan for an earthquake occurrence isthrough a seismic hazard and risk assessment. The seismic hazard and risk method has well been establishedsince 1968 (see Cornell (1968); Veneziano et al., (1984); Bender and Perkins (1993); McGuire (1993); McGuireand Toro (2008); Kijko and Graham (1998); Kijko and Sellevoll, (1989, 1992)). The components of a seismicrisk assessment (SRA) include several building blocks namely: the development of the earthquake catalogue,seismotectonic model, attenuation models, seismic hazard assessment (SHA), vulnerability assessment andseismic risk computations. The seismotectonic model element will be explored in further detail for this research.Preliminary investigations into a seismotectonic investigation for the province have been undertaken by Singh etal. (2011). Under the framework of this research the following tasks are planned for the KZN coastal region: i)Development of a historical earthquake catalogue ii) Development of a GeoDatabase for Seismotectonic Zonationiii) Development of a Seismotectonic Model and iv) Development of an Earthquake Recurrence Model. Theauthor will present progress made to date towards this research.


Imaging the Lithosphere beneath Cameroon and implications to theorigin of the Cameroon Volcanic LineA. Tokam Kamga (1), C. T. Tabod (1), A. A. Nyblade (2), and S. S. Nguiya (3)(1) Dept. of Physics, University of Yaounde 1, Cameroon, (2) Dept of Geosciences, Penn State University, USA, (3) Faculty ofIndustrial Engineering, University of Douala, Cameroon

The origin of the Cameroon Volcanic Line (CVL), a N30E lineament of Cenozoic volcanism that extends fromthe Gulf of Guinea to Lake Chad (West Africa), is still an important science question. Among the models thathave been proposed to explain its origin the latest based on body wave tomography, suggested the edge-flowconvection as the viable model to explain its linearity. In this work, we explore the similarities and differencesbetween various existing geophysical and geodetic models computed at various regional scales and provide a newtopography of the Lithosphere, based on surface wave tomography and the joint inversion of surface wave groupvelocities and receiver functions. The data used in this study were obtained from a temporary deployment of anetwork of 32 temporary seismic stations deployed between January 2005 and January 2007.


Earthquake and Volcanic Hazard Mitigation and Capacity Building inSub-Saharan AfricaA. AyeleInstitute of Geophysics Space Science and Astronomy, Addis Ababa University, Addis Ababa

The East African Rift System (EARS) is a classic example of active continental rifting, and a natural laboratorysetting to study initiation and early stage evolution of continental rifts. The EARS is at different stages ofdevelopment that varies from relatively matured rift (16 mm/yr) in the Afar to a weakly extended OkavangoDelta in the south with predicted opening velocity < 3 mm/yr. Recent studies in the region helped researchersto highlight the length and timescales of magmatism and faulting, the partitioning of strain between faultingand magmatism, and their implications for the development of along-axis segmentation. Although the humanresource and instrument coverage is sparse in the continent, our understanding of rift processes and deep structurehas improved in the last decade after the advent of space geodesy and broadband seismology. The recent majorearthquakes, volcanic eruptions and mega dike intrusions that occurred along the EARS attracted several earthscientist teams across the globe.However, most African countries traversed by the rift do not have the full capacity to monitor and mitigateearthquake and volcanic hazards. Few monitoring facilities exist in some countries, and the data acquisition israrely available in real-time for mitigation purpose. Many sub-Saharan Africa governments are currently focusedon achieving the millennium development goals with massive infrastructure development scheme and urbanizationwhile impending natural hazards of such nature are severely overlooked. Collaborations with overseas researchersand other joint efforts by the international community are opportunities to be used by African institutions to bestutilize limited resources and to mitigate earthquake and volcano hazards.


Importance of macroseismic data from moderate local earthquakes forseismic microzoning effects distribution during the 2003 Bardo, Tunisia,earthquakeJ. Kacem (1) and M. Hfaiedh (2)(1) University of Sfax, Laboratory of water- energy- environmental, AD-10-02, BP. 1173 – 3038 Sfax, Tunisia, (2)Météorologie Nationale, now at STEG, Tunis

The area considered in this study is located in Northern Tunisia. Being part of the western Mediterranean region,the geodynamic evolution of Northern Tunisia is closely related to the convergence between the African andthe European tectonic plates. Numerous Quaternary fold, reverse and strike slip faults and historical earthquakeindicate that the seismic hazard of Tunisia is considerable and a better strategy for seismic risk evaluation needsto be developed. In fact, the recent Quaternary activity in Tunisia has been proved and described by numerousauthors. This activity sometimes affects Holocene to historic deposits. In particular, evidence of damage can beseen in several sites where constructions dating back to the Roman epoch have been affected. The large number ofsites showing Holocene to Historic tectonic deformations cannot be explained by the relatively weak magnitude(M< 5), which characterizes the seismicity of Tunisia. These results suggest that Tunisia is characterized either byrelatively important seismicity during the recent quaternary period or by a very shallow seismicity. The secondhypothesis is supported by the recent macroseismicity data where several surface effects are observed in manyexamples of moderate earthquakes.To verify the results of seismic microzoning and to improve techniques, the macroseismic data of past stronglyexpressed earthquakes is an important key reference. The macroseismic and accelerometric data of the 2003Bardo, Tunisia, earthquake in the epicentral region are collected and compiled to produce the most reliable anddetailed isoseismal map. The area enclosed in the isoseismal with IV EMS degree is not symmetric with respectto the isoseismal with higher degree (V EMS). From this point of view, we can affirm that the attenuation wasstronger on the western part than on the eastern one. Moreover, due to very local site effects, we found sporadicsmall areas with intensity up to IV EMS degree randomly distributed. Through the case study of the 2003Bardo earthquake, the results suggest that is important for more accurate microzoning to consider the regionalcharacteristics of soil conditions and that the damage data can be the basic input for verification and revision ofthe site amplification evaluation.


Witnessing the birth of a new ocean? The first 6 years of the Dabbahurifting episode, and other activity in AfarT. Wright (1), A. Ayele (2), T. Barnie (3), M. Belachew (4), E. Calais (5), L. Field (6), I. Hamling (7), J.Hammond (8), D. Keir (9), and the Afar Rift Consortium Team(1) School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK; Corresponding author: [email protected],(2) Institute of Geophysics, Space Science and Astronomy, Addis Ababa University, Ethiopia , (3) Department of Geography,Cambridge University, UK, (4) Department of Earth and Environmental Sciences, University of Rochester, Rochester, NewYork, USA, (5) Dept. of Earth & Atmospheric Sciences, Purdue University, West Lafayette, IN 47907-1397, USA, (6) Dept.of Earth Sciences, Univeristy of Bristol, UK , (7) ICTP, Trieste, Italy, (8) Dept. of Earth Science and Engineering, ImperialCollege, London, UK, (9) National Oceanography Centre Southampton, University of Southampton, UK

Intense earthquake activity and a small rhyolitic eruption in September 2005 heralded the onset of an unprece-dented period of geological activity in the Afar Depression. The seismic activity accompanied dyke intrusionin the upper 10 km of crust along 60 km of the Dabbahu (northern Manda-Hararo) Magmatic Segment (DMS)of the Nubia-Arabia plate boundary, a nascent seafloor spreading centre. InSAR observations of the resultingdeformation showed that the initial dyke was up to 8 m thick, with a total volume of 2-2.5 km3. Urgency fundingfrom the UK Natural Environmental Research Council (NERC) and US National Science Foundation (NSF)enabled us to deploy a local array of seismometers in October 2005, continuous GPS instruments in January 2006,and to acquire a dense time series of satellite radar images. The medium-term viability of these instruments wassecured with major follow-on funding from NSF and NERC; these projects supported the collection and analysis ofadditional unique data sets, including data from a broader array of seismic and GPS instruments, magneto-tellurictransects of the rift, airborne LiDAR, petrological sampling and micro-gravity work. The combination of thesedata has allowed us to quantify the processes associated with crustal growth at divergent plate boundaries for thefirst time.Here, we present a broad overview of geological activity in the Afar depression in the hyperactive 21st century.Activity in the DMS began after September 2000, when Gabho volcano at the north of the segment beganuplifting, as its magma chamber, ∼3 km below the surface, was replenished. It is likely that the inflation at Gabhoultimately triggered the onset of the Dabbahu rifting episode. The rifting episode began with intense seismicity atthe northern end of the DMS, before jumping to the Ado Ale Volcanic Complex at the segment centre. This initialdyking was fed from shallow (∼3 km) chambers at Gabho and Dabbahu as well as a deeper (∼10 km) sourceat Ado Ale. The initial dyke was followed by a sequence of smaller dyke intrusions, which began in June 2006.To date, there have been 14 dyke intrusions in total, with the most recent occurring in May 2010. These laterdykes were typically 2-3 m thick and 10-15 km long, and have a cumulative volume approaching 1 km3. Threedykes broke the surface to produce basaltic fissural eruptions. Seismicity data show that they were all fed from theAVC and propagated at rates of 0.2 – 0.5 m/s. Overall, the locations of the dyke intrusions appear to be guidedby tectonic driving stress, with the later dykes filling in areas that opened less in the initial dyke. However, thelocation of individual dyke intrusions is also influenced by their immediate predecessor.Activity in the 21st century in Afar has not been restricted to the Dabbahu Magmatic Segment. In the Erta AleMagmatic Segment (EMS), a large basaltic eruption occurred at the Alu-Dalafilla range in November 2008, andthe Erta Ale lava lake itself overflowed in November 2010. In addition, we have recently identified a shallow dykeintrusion at the north of the EMS, which propagated south from Dallol in 2004. Further afield, Nabro volcanoin Eritrea, near the border with Eritrea, began erupting explosively in June 2011. Many of the eruptions werefirst detected by satellite observations of SO2 emissions and thermal hotspots, which enabled reliable alerts to betransmitted to local and national authorities, sometimes before eye-witness reports were communicated. Nabrowas the 7th eruption in Afar in less than 6 years.An intense and immense collaborative effort involving numerous Ethiopian and international scientists, withcrucial support from local and national authorities, has enabled us to document and learn from this unique periodof activity in Afar.


Status of Seismotectonic and seismic hazard studies in South AfricaV MidziCouncil for Geoscience, Pretoria, South Africa

Though South Africa is considered to lie in a stable continental region, earthquakes are recorded and locateddaily. Large events have been recorded that resulted in severe damage to infrastructure in nearby towns, farms,underground mines and even death in some circumstances. Therefore, it is necessary that we consider the effectsof these events in the design of our infrastructure. This mitigation is done by carrying out reliable seismic hazardand risk studies of our regions using state of the art methodologies. In South Africa, several regional seismic hazardstudies have been carried out and published. Continental wide studies that include the South African region werealso published by various scientists from the continent (e.g. GSHAP). However, to ensure that we conform tointernational best practice in such studies, more studies need and are being done to improve data, knowledge andmethodologies used in the assessments. We continue to collect and improve collection methods of historical andinstrumental seismicity data. Available geological information is being used to identify and characterize active orcapable faults.

Geophysical Research AbstractsVol. 14, EGU2012-14174-1, 2012EGU General Assembly 2012© Author(s) 2012

Seismicity along the western part of the Eurasia-Nubian plate boundaryM. Bezzeghoud (1), C. Adam (1), E. Buforn (2), J.F. Borges (1), and B. Caldeira (1)(1) Centro de Geofísica de Évora e Depto. de Física, ECT, University of Évora, Portugal, (2) Dpt. de Geofísica yMeteorología, Fac. CC. Físicas, Universidad Complutense, 28040 Madrid, Spain

The seismicity along the western part of the Eurasia-Nubian plate boundary is characterized by a very complexpattern. In average, the motion is transtensional in the Azores, dextral along the Gloria transform zone and con-vergent between the SW Portuguese Atlantic margin and the Ibero-Maghrebian zone. To constraint the factorscontrolling the seismicity, we provide a new seismotectonic synthesis using several significant seismic events. Weshow that the studied area can be divided into six different regions, each one characterized by a coherent seismicitypattern. The total seismic moment tensor and the average slip velocities are provided for each one of them. Tounderstand the spatial distribution of the seismicity, we compute for each event from the focal mechanism the slipvector and compare it to the relative velocity between the Eurasia and Nubia plates, deduced from global kinemat-ics models. Despite local departures in the Alboran Sea and in the proximity of the Mid Atlantic Ridge, we finda good correlation between these two independent vectors sets. Quantitatively, the slip velocities display a linear,non-affine correlation with the norms of the relative kinematics velocities. The norm of the slip velocities seems toalso depend on the tectonic regime and on the morphology of the plates’ boundary.


Mapping the evolving strain field during continental breakup from crustalanisotropy in the Afar DepressionD. Keir (1), M. Belachew (2), C. Ebinger (2), M. Kendall (3), J. Hammond (4), G. Stuart (5), A. Ayele (6), and J.Rowland (7)(1) University of Southampton , (2) University of Rochester, (3) University of Bristol , (4) Imperial College London , (5)University of Leeds, (6) University of Addis Ababa , (7) University of Auckland

Rifting of the continents leading to plate rupture occurs by a combination of mechanical deformation and magmaintrusion, yet the spatial and temporal scales over which these alternate mechanisms localize extensional strainremains controversial. Here we quantify anisotropy of the upper crust across the volcanically active Afar TripleJunction using shear-wave splitting from local earthquakes to evaluate the distribution and orientation of strainin a region of continental breakup. The pattern of S-wave splitting in Afar is best explained by anisotropy fromdeformation-related structures, with the dramatic change in splitting parameters into the rift axis from the increaseddensity of dyke-induced faulting combined with a contribution from oriented melt pockets near volcanic centres.The lack of rift-perpendicular anisotropy in the lithosphere, and corroborating geoscientific evidence of extensiondominated by dyking, provide strong evidence that magma intrusion achieves the majority of plate opening in thiszone of incipient plate rupture.


Tectonic and hydrothermal activities in Debbagh, Guelma Basin, EasternAlgeriaS. Maouche (1), A. Abtout (1), N. Merabet (1), T. Aïfa (2), A. Lamali (1), B. Bouyahyaoui (1), S. Boughchiche(1), and M. Ayache (1)(1) CRAAG, Bouzareah BP 63, 16340 Alger, Algeria, (2) Geosciences-Rennes, CNRS UMR6118, Université de Rennes 1,France

Quaternary and Pliocene travertine, deposited from hot springs, can reveal much about tectonic and hydrothermalactivities. The aim of this work is to understand the actual tectonic activity in the Guelma Basin and in one of itsspas structure. Considering the fieldwork observations in the Hammam Debbagh area, gravity data were analyzedto better highlight the architecture of its subsurface underlying hydrothermal structures. Analysis of the gravitydata included the computation of a Bouguer anomaly, upward continuations, as well as residual and derivativemaps. Comparison of gravity maps, field geology, geomorphic observations and structural maps allowed us toidentify the major structural features. As a result we propose a model of three subsurface structure sources at 0.2, 1and 7 km depth from north to south, respectively. This confirms some structural elements collected from outcropsand defines subsurface structures, where the Hammam Debbagh active fault is superimposed to the hydrothermalactive source in the NW-SE direction characterized by a negative gravity anomaly.

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