induced seismicity around the tous new dam (spain)hera.ugr.es/doi/1577322x.pdf · 2005-12-19 ·...

Geophys. J. Int. (2005) 160, 144–160 doi: 10.1111/j.1365-246X.2005.02459.xG

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Induced seismicity around the Tous New Dam (Spain)

Federico Torcal,1,2 Inmaculada Serrano,1,∗ Jens Havskov,3 Jose Luis Utrillas4

and Jose Valero4

1Instituto Andaluz de Geofısica y Prevencion de Desastres Sısmicos, Observatorio de Cartuja, Cuesta del Observatorio no 1, Campus Universitario deCartuja, 18071 Granada, Spain. E-mails: [email protected] (FT); [email protected] (IS)2Departamento de Ciencias Ambientales, Universidad Pablo de Olavide, Sevilla, Carretera de Utrera km 1, 41013 Sevilla, Spain. E-mail: [email protected] of Solid Earth Physics, University of Bergen, Allegaten 41, 5007 Bergen, Norway. E-mail: [email protected] Hidrografica del Jucar, Avenida Blasco Ibanez 48, 46010 Valencia, Spain

Accepted 2004 August 13. Received 2004 August 4; in original form 2001 October 22

S U M M A R YInduced seismicity around the Tous New Dam, located 45 km southwest of Valencia, Spain,is studied from a detailed analysis of seismic data from a 16-month period, starting in 1999January, recorded by the Tous New Dam Seismic Network (TDSN), which comprises a three-component station located in the dam and six vertical-component outstations within 25 km ofthe dam. On the basis of the spatial 2D–3D and temporal distribution of this seismicity, wehave found 24 microearthquakes, of magnitudes 0.4–3.8, most of which appear correlated intime and space with changes in the water level in the Tous New Dam reservoir. An initial groupof earthquakes (all within 12 km or less of the reservoir) occurred over roughly 1 yr, starting inmid-1999 while the water level was at its maximum, continuing during draining in late 1999and subsequent refilling in early 2000. Thereafter, seismicity was more widespread, reachingup to 40 km from the dam and with the greatest frequency of occurrence coinciding with therapid draining of the reservoir in mid-2000. The few focal mechanisms able to be found arenormal or strike-slip. In general, it is clear that a relationship exists between reservoir waterlevel, which probably modifies the pre-existing tectonic stress field and pore fluid pressures,but no specific mechanism for the induced seismicity has yet been identified.

Key words: induced seismicity, microearthquakes, reservoirs, Tous (Valencia, Spain).

1 I N T RO D U C T I O N

Almost all studies of induced seismicity are consistent in their con-clusions about the existence of earthquakes or microearthquakeslocated beneath the reservoir and at shallow depth, after the initialimpoundment (Simpson et al. 1988). Other authors recognize thatas well as initial seismicity subsequent to impoundment, further in-duced earthquakes occur as a consequence of substantial water levelchanges or later filling above the previous high water level (Talwani1997). Reservoir filling is known to perturb the stress field and/orpore fluid pressure sufficiently to trigger earthquakes in the vicinityof reservoirs (Kafka & Driscoll 1999).

2 C H A R A C T E R I S T I C S O F T H E T O U SN E W DA M A N D T H E T O U S N E WDA M S E I S M I C N E T W O R K

Unlike many other dams, the Tous New Dam was built on the Jucar(or Xuquer) River, between 1990 and 1996, on a site occupied by an

∗Formerly at: Department of Earth Sciences, School of Sciences,Ehime University, Bunkyo-Cho 2-5, Matsuyama 790-8577, Japan. E-mail:[email protected]

earlier dam (early 1970s–1982). The original Tous Dam suffered se-vere failure in 1982 October, produced by a major flood of 10 000 m3

s−1 of water of the Jucar River. Serious, widespread damage to peo-ple and property was reported, and extensive flooding of agriculturalland took place.

The new dam was designed to avoid flooding, guarantee a fullwater supply to the metropolitan area of Valencia, provide water forthe Jucar–Turia River irrigation channel and irrigated lands in theLa Ribera region, and produce hydroelectric energy through twopower plants.

The Tous New Dam is located in the province of Valencia (easternSpain), in the southeast Iberian Cordillera, near to the northern limitof the Betic Cordilleras (Fig. 1). Neogene basins of unconsolidatedrocks lie to the east of the dam, along the Mediterranean coast inthe Valencia trough. The study area is in the eastern Iberian margin,which is characterized by the superposition of Cenozoic structuralunits developed on a Hercynian basement and a sedimentary covermade up of deposits ranging from Permian to Upper Oligocene.Details of geology can be found in Pomar et al. (1983), Guimera(1984), Moissenet (1985), Pierson d’Autrey (1987), Ott d’Estevouet al. (1988), Ramos-Guerrero et al. (1989), Fontbote et al. (1990),Guimera & Alvaro (1990) and Gelabert et al. (1992).

144 C© 2005 RAS

Induced seismicity around the Tous New Dam 145

Figure 1. Geological setting of eastern Iberian Peninsula, around the TousNew Dam showing the main geological units and features (modified fromJulivert et al. 1977).

The Tous New Dam is an embankment-type dam with a clay core.The maximum length of the crest is 1025 m and the height of thedam between the lowest point of foundation upriver and the crest is102.50 m. The reservoir of the Tous New Dam reaches a standard

Table 1. Some measurements characteristics of the Tous New Dam andreservoir, Valencia (Spain).

Tous New Dam

Volume of dam 7 730 000 m3

Length of dam 1025 mElevation of roadway 162.5 mElevation of lowest point of foundation upriver 60 mHeight of dam 102.5 mElevation of lowest point of foundation downriver 53 mHeight of the dam over groundworks 135.5 m

Tous New Dam reservoir

Standard maximum elevation 130 mMinimum height from the crest of the dam to water level 32.5 mVolume of the reservoir at the standard maximum elevation 340.4 Hm3

Reservoir extended area at the standard maximum elevation 980 HaMaximum volume of the reservoir 700 Hm3

Note: the elecations are referenced to mean Mediteranean sea level.

maximum water surface elevation of 32.50 m below the crest ofthe dam. At this level, the reservoir has a 3.4 · 108 m3 or 340 Hm3

total capacity of water, occupying an area of 9.8 · 108 m2 or 980 Ha(hectares). The maximum possible reservoir volume is 7.0 · 108 m3

or 700 Hm3. These and other characteristics are summarized inTable 1.

The Tous New Dam Seismic Network (TDSN) was set up aroundthe dam (Torcal & Serrano 1995), to provide accurate, reliabledata on earthquake parameters. One three-component digital station(VTOU), with continous analogue recording also, is located in thedam and the other six one-component digital stations are distributedaround the dam site at a maximum distance of 25 km (Fig. 2). Thishas enabled us to calculate epicentre—Tous New Dam distances forall microearthquakes recorded to date (Table 2). Consequently, weare confident that all microseismic activity around the Tous Dam andreservoir during the study period (1999 January 1–2000 May 14) hasbeen fully recorded. However, not all seismic events are recordedat every station. In many cases, fewer than three stations registeredactivity and the minimum number of records necessary to calcu-late an accurate location was set at three. Furthermore, during thestudy period, failures occurred in some stations, which preventedus from obtaining enough data to calculate an accurate location oraccurate focal mechanisms for some of the microearthquakes. Thisstudy also includes data from seismic stations operated by the Insti-tuto Geografico Nacional (IGN; the institute that controls the mainseismic network covering all Spain) nearest the Tous New Dam tocomplement TDSN data (Figs 3 and 4).

The maps in Figs 2, 3 and 4, show the location of known quar-ries, many of which were active during the present study. We haverecorded numerous blasts from these quarries, recognizable by spe-cific characteristics (Reamer et al. 1992; Kitov et al. 1997) that haveallowed us to distinguish them from natural microearthquakes. Typ-ical records of a quarry blast and a microearthquake are shown inFig. 5.

3 E V O L U T I O N O F S E I S M I C A C T I V I T YR E C O R D E D A RO U N D T H E T O U S N E WDA M A N D R E S E RV O I R B E T W E E N1 9 9 9 JA N UA RY A N D T H E F I R S TH A L F O F 2 0 0 0 M AY

A month-by-month summary of the number and type of seis-mic events recorded appears in Table 3. The majority of events

C© 2005 RAS, GJI, 160, 144–160

146 F. Torcal et al.

Figure 2. Map of the Tous Dam and reservoir location, through the Jucar River stream. In this figure, coordinates are UTM zone 30 eastings and northings,in metres. The map is centred on the Tous Dam, about 45 km to the SW of Valencia, the main city in the area on the east coast of Iberian Peninsula, betweenthe Iberian and Betic Cordilleras. Triangles show the location of the Tous New Dam Seismic Network stations. Station VTOU is located inside the Tous Dam.Squares stand for the nearest Instituto Geografico Nacional (IGN) seismic stations. The full circle limits the area of the present study. Crosses represent thelocations of the active quarries catalogued in this zone.

Table 2. Criteria used to classify seismic events according to their epicentral distance from VTOUseismic station, or the dam, their predominant frequency and the variability of the seismogram graph.

Criteria Type of seismic event

Local Near-regional Regional Teleseism

� (km) � < 50 50 ≤ � < 100 100 ≤ � < 1000 � > 1000T S−P (s) T S−P < 7 7 ≤ T S−P < 13 13 ≤ T S−P < 125 T S−P > 125F (Hz) F > 4 4 ≤ F < 3 3 ≤ F < 2 F < 2Variability Great Medium Slight None or minimal

�: epicentral distance (km).T S−P: P–S time separation (s).F: predominant frequency (Hz).Variability: variability of waveform between stations.

(49 per cent) are classified as noise. Explosions from nearby quar-ries account for 18 per cent of the total amount. Teleseisms rep-resent some 15 per cent, regional seismic events 9 per cent, andnear-regional seismic events 8 per cent. Local microearthquakescorrespond to 1 per cent of events recorded.

Teleseismic recordings have been used to test the polarity of theseismic sensors. Some regional and near-regional earthquakes andmicroearthquakes were used to calculate a more specific, accuratecrust model for the area (Torcal & Serrano 1999; Table 4). The lackof sufficient data to adjust a magnitude-scale law for this area obligedus to use duration magnitude M D scales from the two nearest seismicnetworks: (eq. 1) the Andalusian Seismic Network [Red Sısmica deAndalucıa (RSA)] (De Miguel et al. 1988) and (eq. 2) the El CabrilSeismic Network (Bernal et al. 1992a,b):

MD = (1.67 ± 0.11) log t − (0.43 ± 0.19), (1)

MD = 1.96 log t + 0.0029� − 1.79, (2)

where t is total signal duration in seconds, following the conventionalcriterion of Real & Teng (1973), and �, is epicentral distance inkilometres.

4 A N A LY S I S O F M I C RO S E I S M I CA C T I V I T Y R E C O R D E D A RO U N D T H ET O U S N E W DA M A N D R E S E RV O I R

For local earthquakes and microearthquakes, epicentral distance isthe only parameter we have been able to calculate directly and with

C© 2005 RAS, GJI, 160, 144–160


Figure 3. Previous seismicity registered around the Tous Dam and reservoir by the IGN, from 1993 to 1998. In this figure, coordinates are UTM zone 30eastings and northings, in metres. Triangles are the Tous New Dam Seismic Network stations. Squares are the nearest IGN seismic stations. Crosses are locationsof catalogued quarries in the area. Small full circles denote earthquake and microearthquake locations.

Figure 4. Previous seismicity registered around the Tous Dam and reservoir by IGN during 1999 and until 2000 May. Symbols as for Fig. 3.

C© 2005 RAS, GJI, 160, 144–160


-2.0E+03

-1.6E+03

-1.2E+03

-8.0E+02

-4.0E+02

0.0E+00

4.0E+02

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1.2E+03

1.6E+03

2.0E+03

Am

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coun

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0 5 10 15 20 25 30 35 40 45 50 55 60 65

Time (s)

P

S

-2.0E+03

-1.6E+03

-1.2E+03

-8.0E+02

-4.0E+02

0.0E+00

4.0E+02

8.0E+02

1.2E+03

1.6E+03

2.0E+03

Am

plitu

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coun

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0 5 10 15 20 25 30 35 40 45 50 55 60 65

Time (s)

PS

Figure 5. Comparison between two seismic events registered by the Tous New Dam Seismic Network, one for a quarry blast/explosion, at 10:20 h UTC on1999 June 22 (top) and another for a microearthquake, at 01:22 h UTC on 1999 June 23 (bottom). Both seismic events were recorded at the VPEM seismicstation.

some degree of confidence, from P–S wave arrival time differences(T S−P) measured from analogue records. For those earthquakeswhere available data were not enough to calculate this directly, focaldepth was inferred from that of the nearest, similar earthquakes.Thus, the 12.5 km depth assigned to some earthquakes is the meanvalue of a 10–15 km depth range but it could have been located atany depth within this range. The location parameters for earthquakenumber 6 were also allocated to earthquakes numbers 7, 8, 9 and10, which all have the same T S−P parameter and similar waveformcharacteristics at all stations that detected them. For earthquakes1 and 22 only analogue records from VTOU are available. Conse-quently, it is not possible to determine their precise location; onlyepicentral distance to the Tous New Dam is known. Similarly, we areunable to calculate depth for earthquake 5. Overall, between 1999January 1 and 2000 May 14, we obtained hypocentral coordinates

for 25 of the 27 local earthquakes/microearthquakes recorded by theTDSN (Table 5).

To complement TDSN data, we have compiled hypocentral datafor all earthquakes/quarry blasts located by the IGN near the TousDam from 1993 to date. Construction of the Tous New Dam wascompleted during the last months of 1998. Consequently, we havedata for 5 yr before first impoundment of the reservoir (Figs 3 and4). Fig. 3 contains all data from 1993 to 1998, prior to the firstimpoundment after the construction of the new dam. Fig. 4 con-tains data from 1999 to 2001 May, the period covered by TDSNrecords. We have identified (Torcal 1999, 2000a,b,c,d,e) a numberof quarry blasts/explosions that were considered microearthquakesby the IGN. Specifically, some of the microearthquakes locatedthroughout the northwestern zone (Figs 3 and 4). Therefore, somemicroearthquakes reported by the IGN during 1993–1998 might in

C© 2005 RAS, GJI, 160, 144–160


Table 3a. Basic statistics for the various types of seismic events registered by the Tous New Dam (Spain) Seismic Network, between 1999 January and 2000May.

Date Type of seismic events No. of events

Year & month Teleseisms Regionals Near-regionals Explosions Noises Microearthquakes No. Cumulative No.

1999 January 8 1 0 19 45 0 73 731999 February 4 11 0 17 38 0 70 1431999 March 97 22 15 27 48 1 210 1531999 April 44 9 4 38 74 0 169 5221999 May 13 1 3 32 97 0 146 6681999 June 8 2 9 39 50 1 109 7771999 July 8 3 5 52 47 0 103 8801999 August 16 11 6 15 53 2 103 9831999 September 16 13 8 26 88 8 159 11421999 October 24 16 15 34 132 0 221 13631999 November 42 58 35 34 168 0 337 17001999 December 31 34 17 29 133 0 244 19442000 January 21 21 35 25 52 0 154 20982000 February 20 23 36 44 106 3 232 23302000 March 27 15 17 44 71 4 178 25082000 April 7 10 2 12 91 2 124 26322000 May 9 2 3 9 22 4 49 2681

Total number 395 252 210 496 1315 25 2681

Total number 14.7 9.4 7.8 18.5 49.0 0.9 100.0(per cent) (per cent) (per cent) (per cent) (per cent) (per cent) (per cent) (per cent)

fact have been quarry blasts/explosions. However, we do not haveenough information and criteria to resolve this issue definitively.

Fig. 6 shows the geographic distribution of all earthquake epicen-tres recorded by the TDSN between 1999 January and 2000 May.The spatial distribution of seismicity enables us to distinguish sixseismogenic zones (A–F) by clustering the earthquakes (Table 6).

The 3-D hypocentral distribution of seismicity is shown in Figs 7,8, 9 and 10. A nominal depth of 10.0 km has been assigned to earth-quakes 13 and 14, whose location has been taken from IGN data. Thedepths of these earthquakes were not given nor could they be calcu-lated. Given that earthquake 21 is the nearest to these and its depthis 10.6 km, it is reasonable to assume that all three earthquakes hada similar depth. It was not possible to calculate the depth for earth-quake 5 and only its epicentral location is available. For the samereason, earthquakes 1 and 22 do not appear in the 3-D distributionmaps. Hypocentres of earthquakes located with accuracy and cer-tainty are displayed as filled circles (•). Hypocentres of earthquakeslocated with a lower degree of accuracy and certainty are displayedas open circles (◦). We have divided the earthquakes into shallow(0–10 km depth), intermediate (10–15 km depth) and deep (≥15 kmdepth), although the terms are simply relative in the context of thestudy.

5 S PAT I A L A N D T E M P O R A LE V O L U T I O N O F S E I S M I C I T Y A RO U N DT H E T O U S N E W DA M A N D R E S E RV O I RB E T W E E N 1 9 9 9 JA N UA RY 1 A N D 2 0 0 0M AY 1 4 . C O M PA R I S O N W I T H O T H E RDA M S A RO U N D T H E W O R L DA N D D I S C U S S I O N

First, earthquakes 13, 14 and 21 are located in the Gulf of Valencia,just outside the area studied and in zone A. We infer that theseearthquakes are not associated in any way with seismicity inducedby the Tous Dam and reservoir because their epicentres are located

some 50 km from the dam and some 5 km from the coastline beneaththe Mediterranean sea, while the seismic activity induced by theimpoundment of a reservoir is typically located within a 12–25 kmradius of the dam (Simpson 1976; Zhang et al. 1997). Furthermore,the IGN had previously located seismicity in this zone. All threeearthquakes took place in the middle and towards the end of theperiod studied. Thus, we believe that they are not causally relatedto the Tous New Dam and reservoir.

Excluding earthquake 4, located 38.7 km SSE of the Tous NewDam and probably a natural tectonic earthquake, all other earth-quakes analysed to date are located within a 25 km radius of thedam. This coincides with the majority of observations from otherdams worldwide, such as the Shenwo Dam (China), which has areservoir of maximum height 97.4 m where most induced earth-quake epicentres were concentrated in a small area within some12 km of the dam (Zhang et al. 1997). In the Maharashtra regionof India, most earthquakes associated with the Dhamni Dam werewithin some 10 km of the reservoir (Rastogi et al. 1997). Simpson(1976) reports that epicentres in most cases of reservoir-inducedseismicity have occurred within 25 km of the reservoir area. All ofthis strongly suggests that, in our case, seismic activity is closelyrelated to reservoir impoundment.

One of most commonly used parameters to characterize the in-duced seismicity of a given zone is the ratio between the largestaftershock magnitude and main shock magnitude. This ratio is 0.84both for the Shenwo Dam (Zhang et al. 1997) and the Koyna reser-voir (Gupta & Rastogi 1976). For the Tous New Dam and reser-voir, disregarding the earthquakes in the Gulf of Valencia, the ra-tio is 0.83–0.89, depending which magnitude definition is used(Bernal et al. 1992a,b, or De Miguel et al. 1988 or IGN magni-tude). These values are not significantly different from those of otherdams. For mainly tectonic seismicity, this parameter is clearly overunity. Therefore, this is further evidence that the seismicity analysedaround the Tous New Dam and reservoir is induced by reservoirimpoundment.

C© 2005 RAS, GJI, 160, 144–160


Table 3b. Basic statistics of various types of seismic events registered by the Tous New Dam (Spain) Seismic Network between 1999 January and 2002December.

Date Type of seismic events No. of events

Year & month Teleseisms Regionals Near-regionals Explosions Noises Microearthquakes No. Cumulative No.

1999 January 8 1 0 19 45 0 73 731999 February 4 11 0 17 38 0 70 1431999 March 97 22 15 27 48 1 210 3531999 April 44 9 4 38 74 0 169 5221999 May 13 1 3 32 97 0 146 6681999 June 8 2 9 39 50 1 109 7771999 July 8 3 5 52 47 0 103 8921999 August 16 11 6 15 53 2 103 9951999 September 16 13 8 26 88 8 159 11541999 October 24 16 15 34 132 0 221 13751999 November 42 58 35 34 168 0 337 17121999 December 31 34 17 29 133 0 244 19562000 January 21 21 35 25 52 0 154 21102000 February 20 23 36 44 106 3 232 23422000 March 27 15 17 44 71 4 178 25202000 April 7 10 2 12 91 2 124 26442000 May 9 2 3 9 22 4 49 27522000 Jun 22 14 7 24 33 3 103 28552000 Jul 13 10 12 13 12 3 63 29182000 Aug 14 15 19 22 39 0 109 30272000 Sep 13 21 14 25 27 0 100 31272000 Oct 9 12 19 18 63 9 130 32572000 Nov 20 26 17 53 82 1 199 34562000 Dec 12 8 3 32 16 1 72 35282001 Jan 16 13 3 43 140 0 215 37432001 Feb 13 11 12 53 138 2 229 39722001 Mar 11 14 28 37 61 7 158 41302001 Apr 13 15 29 43 29 2 131 42612001 May 12 13 7 58 68 0 158 44192001 Jun 30 8 9 22 33 0 102 45212001 Jul 21 7 21 28 4 2 83 46042001 Aug 5 5 14 7 0 0 31 46352001 Sep 9 9 28 15 0 0 61 46962001 Oct 6 3 13 14 3 1 40 47362001 Nov 6 4 5 17 2 1 35 47712001 Dec 12 3 6 16 2 2 41 48122002 Jan 13 12 38 22 16 6 107 49192002 Feb 13 21 19 45 10 0 108 50272002 Mar 18 32 13 42 2 2 109 51362002 Apr 22 23 31 40 4 5 125 52612002 May 13 29 17 48 4 0 111 53722002 Jun 20 11 21 49 6 1 108 54802002 Jul 14 27 29 32 30 3 135 56152002 Aug 15 63 31 21 33 2 165 57802002 Sep 19 28 32 25 10 3 117 58972002 Oct 18 38 25 36 0 3 120 60172002 Nov 24 15 35 35 0 3 112 61292002 Dec 14 36 31 30 0 0 111 6240

Total number 868 816 809 1473 2184 90 6240

Total number 13.9 13.1 13 23.6 35 1.4 100(per cent)

It is very important to remember that the period covered by thepresent study starts from the building of the Tous New Dam. Anotherdam stood on the same site between the 1970s and 1982, so thisis not an example of initial reservoir impoundment. Worldwide,many cases of induced seismicity starting after first impoundmentare known. Nevertheless there are other dams where a notable timehas elapsed between first impoundment and the start of recorded

seismicity: for example Oroville (USA), Aswan (Egypt) and Koyna(India; Simpson et al. 1988).

Considering both the evolution of the water level of the reser-voir and the occurrence of the seismicity, three periods (initialimpoundment-draining cycle, second impoundment and seconddraining, respectively) in the evolution of the induced seismicityof the Tous New Dam and reservoir can be distinguished (Fig. 11).

C© 2005 RAS, GJI, 160, 144–160


Table 4. Crustal structure under the vicinity of the Tous New Dam (Spain).

Layer Depth (km) Vp (km s−1) Vs (km s−1)

1 0–10 5.9 3.42 10–30 6.6 3.83 >30 8.1 —Gradient 0.003 km s−1 km−1

This shows Tous reservoir water level between 1998 September and2000 May, together with earthquakes recorded during the same pe-riod. The TDSN started recording regularly from 1998 December.Earthquakes are shown as bars, located on the days when they oc-curred. Each bar is labelled with the number of earthquakes occur-ring that day. In periods (1) and (2), the earthquakes were in zonesE and F as defined above. From 2000 March to 2000 May, period(3), seismicity was more widespread (between 9 and 23 km fromthe dam).

Only a few focal mechanism solutions are available and only ina few cases has it been possible to match computed mechanisms tothe predominant, known fault systems within the epicentral zone.Thus, some earthquakes could have originated in faults that were notconspicuous at the surface and whose characteristics are unknown.Of the focal mechanisms currently available (Table 7), most arenormal faulting events and some show a strike-slip characteristic.

Table 5a. Data of the microearthquakes registered around the Tous New Dam (Spain) between 1999 January and the first half of 2000 May.

MagnitudesOrigin time, Latitude Longitude Depth � to dam Focal SeismicNo. Day Month Year or first P time (◦N) (◦E) (km) (km) mb Md Md Md mechanism network

(IGN) (RSA) (IGN) (RSC) available

1 30 3 1999 03:44:32.9 ? ? ? 19.3 — 0.2 0.5 0.9 No TDSN2 23 6 1999 01:22:05.6 38.966 −0.622 5.7 18.8 — 2.3 2.9 2.2 No TDSN3 7 8 1999 21:09:14.7 39.113 −0.666 14.9 2.9 — 2.5 3 2.2 Yes TDSN4 12 8 1999 00:31:30.0 38.797 −0.534 6.9 38.7 2.6 2.6 3.1 2.3 Yes TDSN5 2 9 1999 13:10:00.3 39.102 −0.71 ? 6.5 — 1.2 1.8 1.4 No TDSN6 2 9 1999 13:10:13.60 39.17 −0.66 15.2 4.1 2.7 2.6 3.1 2.3 Yes IGN7 2 9 1999 13:11:56.0 39.17 −0.66 15.2 8.3 — 0.3 0.9 0.7 No TDSN8 2 9 1999 13:12:47.6 39.17 −0.66 15.2 8.3 — 0.5 1.1 0.9 No TDSN9 2 9 1999 21:58:27.3 39.17 −0.66 15.2 8.3 — 1.5 2.1 1.6 No TDSN10 5 9 1999 22:37:50.9 39.17 −0.66 15.2 8.9 — 1.5 2.1 1.6 No TDSN11 28 9 1999 00:52:35.17 39.026 −0.458 10 20.3 2.8 2.5 3.1 2.3 No IGN12 28 9 1999 05:28:39.35 39.041 −0.434 5.6 21.2 2.8 2.5 3.1 2.3 No IGN13 19 12 1999 11:20:34.81 39.496 −0.272 — 51.6 2.6 — — — No IGN14 19 12 1999 12:02:29.99 39.51 −0.317 — 50.5 2.4 — — — No IGN15 1 2 2000 05:17:57.6 39.233 −0.7 12.5 11.8 — 1.8 2.4 1.8 No TDSN16 13 2 2000 15:28:25.24 39.022 −0.698 15.1 13.2 2.4 2.9 3.5 2.6 No IGN17 14 2 2000 12:23:45.2 39.113 −0.666 14.9 2.4 — 2.4 3.1 2.3 No TDSN18 3 3 2000 13:38:04.8 39.203 −0.713 12.5 9.4 — 2.3 2.9 2.1 No TDSN19 18 3 2000 06:27:23.1 39.01 −0.568 14.2 15.4 — 2.2 2.8 2.1 No TDSN20 18 3 2000 11:57:19.8 39.01 −0.568 14.2 15.4 — 2 2.6 2 No TDSN21 31 3 2000 21:26:59.99 39.426 −0.268 10.7 46.1 2.1 — — — No IGN22 27 4 2000 19:30:05.5 ? ? ? 4.1 — 1.1 1.7 1.3 No TDSN23 30 4 2000 14:10:57.60 39.239 −0.418 11.9 23 3.5 2.6 3.2 2.3 Yes TDSN23∗ 30 4 2000 14:11:01.09 39.224 −0.575 19.8 11.8 3.5 2.6 3.2 2.3 No IGN24 1 5 2000 13:55:12.9 39.231 −0.439 11.9 21 — 2.5 3.1 2.3 Yes TDSN25 8 5 2000 12:04:08.1 39.134 −0.525 12.5 10.6 — 1.1 1.7 1.3 No TDSN26 8 5 2000 13:02:33.4 39.129 −0.441 16.4 17.9 — 2.4 3 2.2 Yes TDSN27 11 5 2000 12:07:36.5 39.209 −0.606 12.5 9 — 2.5 3.1 2.3 No TDSN

Notes. Instituto Geografico Nacional (IGN; National Geographical Institute Seismic Network, Spain); Red Sısmica de Andalucıa (RSA; Andalusian SeismicNetwork); Red Sısmica El Cabril (RSC; El Cabril Seismic Network); Tous Dam Seismic Network (TDSN). 23∗: it is the same earthquake as 23, but locatedby the IGN.

Hence, we conclude that maximum principal stress is vertical andmaximum horizontal principal stress is subequal to vertical stress(Cornet et al. 1997). Zones with induced seismicity are not zonesof high fluid flow but, rather, zones of high fluid pressure (Cornet& Yin 1995).

In order to offer some speculation on the causative mechanismsof the microearthquakes, it is very important to correlate changesin reservoir water level, reservoir volume and the occurrence ofinduced seismic activity (Gupta 1992; Talwani 1997).

The initial cause of seismicity might have been the rapid increasein reservoir lake level during the last months of 1998 and the firsthalf of 1999. It appears that the dam and reservoir may have passedthrough an initial phase of induced seismicity caused mainly byperturbation of the stress field and rapid increases in elastic stress(Simpson et al. 1988) as a result of reservoir water load. Also promi-nent is the important influence of the rapid draining of the Tousreservoir during the summer of 1999. Presumably, the considerableincrease in induced seismic activity was caused by the very rapiddecrease in load stress.

Induced seismic activity recorded during 2000 seems moreclosely related to the filling of the reservoir and its influence ap-pears to have reached a distance of 9–23 km from the dam. Thismay be a result of diffusion of the pressure wave caused by the wa-ter load, leading to an increase in pore fluid pressure and consequentdecrease in friction across pre-existing faults and cracks.

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Table 5b. Data of the microearthquakes registered around the Tous New Dam (Spain) betweeny 1999 January and the first half of 2000 May.

MagnitudesOrigin time, Latitude Longitude Depth � to dam Focal SeismicNo. Day Month Year or first P time (◦N) (◦E) (km) (km) mb Md Md Md mechanism network

(IGN) (RSA) (IGN) (RSC) available

28 5 10 2000 07:22:54.2 — — — 10.83 — 1.7 2.3 1.7 No TDSN29 5 10 2000 12:06:42.0 — — — 8.55 — 2.6 3.3 2.4 No TDSN30 6 10 2000 11:46:02.5 — — — 10.83 — 2.6 3.3 2.4 No TDSN31 8 10 2000 00:57:54.40 39.148 −0.119 16.7 45.8 4 3.2 3.8 2.8 No IGN32 16 10 2000 11:54:13.0 — — — 22.23 — 1.4 2 1.5 No TDSN33 16 10 2000 11:54:33.5 — — — 22.23 — 2 2.5 1.9 No TDSN34 16 10 2000 20:34:05.31 39.129 −0.916 — 22.23 2.3 2.1 2.7 2 No IGN35 17 10 2000 12:16:00.0 — — — 11.4 — 1.4 2 1.5 No TDSN36 30 10 2000 22:18:00.0 — — — 6.84 — 1.9 2.5 1.9 No TDSN37 14 11 2000 13:04:00.0 — — — 7.41 — 1.3 1.9 1.5 No TDSN38 11 12 2000 08:59:15.0 — — — 5.7 — 1.7 2.3 1.8 No TDSN39 17 2 2001 19:05:00.0 — — — 5.7 — 0.1 0.7 0.6 No TDSN40 23 2 2001 09:58:35.07 38.732 −0.682 9.1 38.19 3.4 2.9 3.5 2.6 No IGN41 7 3 2001 10:38:29.11 39.331 −0.55 — 25.08 2 1.8 2.3 1.8 No IGN42 9 3 2001 20:35:00.0 — — — 23.37 — 1.6 2.2 1.6 No TDSN43 9 3 2001 21:20:56.16 39.376 −0.705 — 24.51 1.9 1.7 2.3 1.7 No IGN44 10 3 2001 00:04:15.89 39.391 −0.702 4 21.09 2.2 1.7 2.3 1.7 No IGN45 10 3 2001 00:40:30.90 39.441 −0.689 — 18.81 1.8 1.5 2.1 1.6 No IGN46 12 3 2001 13:17:22.8 — — — 15.96 — 2.2 2.8 2.1 No TDSN47 12 3 2001 13:48:00.0 — — — 15.39 — 1.8 2.4 1.8 No TDSN48 2 4 2001 11:00:00.0 — — — 14.82 — 1.9 2.5 1.9 No TDSN49 17 4 2001 22:43:58.84 39.394 −0.902 10.4 22.8 2.7 1.9 2.5 1.9 No IGN50 26 7 2001 14:41:11.24 39.346 −0.211 — 37.62 2 1.3 1.8 1.1 No IGN51 27 7 2001 12:01:20.99 38.867 −0.458 15.6 33.63 3.4 — — — No IGN52 17 10 2001 06:50:23.09 39.11 −0.575 10.9 6.9 3.2 — — — No IGN53 24 11 2001 19:56:01.44 39.434 −0.387 — 25.08 2.1 1.9 2.5 1.9 No IGN54 31 12 2001 00:52:09.47 39.447 −0.585 7.6 35.2 2 — — — No IGN55 13 1 2002 05:08:34.76 39.355 −0.787 10.4 22.23 2.7 2.1 2.7 2 No IGN56 13 1 2002 20:04:14:11 39.341 −0.742 — 19.95 2.3 1.6 2.2 1.7 No IGN57 21 1 2002 09:26:09.5 — — — 19.95 — 1.3 1.9 1.4 No TDSN58 23 1 2002 09:11:36.35 38.852 −0.666 10.6 26.79 3.6 2.4 3 2.2 No IGN59 23 1 2002 13:05:24.0 — — — 25.65 — 0.9 1.5 1.2 No TDSN60 25 1 2002 21:22:00.0 — — — 2.85 — 1.1 1.7 1.3 No TDSN61 1 3 2002 10:14:24.5 — — — 11.4 — 1.6 2.2 1.7 No TDSN62 3 3 2002 19:44:37.66 39.354 −0.763 10 15.39 3.2 1.7 2.3 1.7 No IGN63 3 4 2002 12:41:00.0 — — — 20.52 — 2.1 2.7 2 No TDSN64 4 4 2002 10:52:00.0 — — — 19.95 — 1.7 2.3 1.7 No TDSN65 4 4 2002 11:43:00.0 — — — 7.41 — 2 2.6 2 No TDSN66 19 4 2002 10:03:10.0 — — — 15.39 — 2.1 2.7 2 No TDSN67 19 4 2002 19:04:59.77 38.958 −0.637 11.5 15.39 3.1 2.2 2.8 2.1 No IGN68 9 6 2002 05:49:00.37 38.866 −0.891 0.9 31.92 2.9 2.2 2.8 2.1 No IGN69 4 7 2002 05:34:12.9 — — — 7.98 — 1.3 1.5 No TDSN70 11 7 2002 07:51:46.6 — — — 9.69 — 1 1.6 1.2 No TDSN71 30 7 2002 20:54:10.19 39.011 −0.745 11.2 14.82 3.3 2.2 2.8 2.1 No IGN72 17 8 2002 04:07:33.87 39.008 −0.593 11.2 17.1 3.3 2.1 2.7 2 No IGN73 26 8 2002 01:41:07.58 39.411 −0.868 9.8 24.51 2.1 2.2 2.8 2.1 No IGN74 23 9 2002 08:11:00.0 — — — 7.98 — 0.4 1 0.8 No TDSN75 23 9 2002 09:33:14.6 — — — 9.69 — 1.1 1.7 1.3 No TDSN76 23 9 2002 09:33:56.2 — — — 12.54 — 2 2.6 2 No TDSN77 16 9 2002 00:56:49.0 — — — 6.27 — 0.5 1.1 0.9 No TDSN78 16 9 2002 21:51:19.29 38.97 −0.395 10.3 28.5 2.2 0.4 1 0.8 No IGN79 1 11 2002 06:07:46.1 — — — 3.99 — 0.7 1.3 1 No TDSN80 1 11 2002 18:39:43.19 39.13 −0.447 10.9 9.69 1.9 2.4 3 2.2 No IGN81 16 11 2002 00:33:50.46 39.413 −0.705 — 18.81 2.3 1.5 2.1 1.6 No IGN

Notes. Instituto Geografico Nacional (IGN; National Geographical Institute Seismic Network, Spain); Red Sısmica de Andalucıa (RSA; Andalusian SeismicNetwork); Red Sısmica El Cabril (RSC; El Cabril Seismic Network); Tous New Dam Seismic Network (TDSN).

6 C O N C L U S I O N S

The TDSN, including six outstations and a central station locatedinside the dam, has been operational since 1999 January. This hasprovided us with a catalogue index of all events within a 50 km

radius of the dam. We recorded 27 microearthquakes in the areaprior to 2000 May.

On the basis of the spatial 2D–3D and temporal distribution ofthis seismicity, we conclude with some certainty that 24 of theearthquakes–microearthquakes registered by the TDSN might have

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Table 6a. Seismogenic zones around the Tous Dam (1999 January to firsthalf 2000 May).

Name Location with Distance to the dam Earthquakes enclosedregard to the dam (km)

A NE Over 50 13, 14 and 21B ENE 21–23 23 and 24C E 10–20 25 and 26D SSE 12–40 2, 4, 11, 12, 16 and 19E Downriver About 6 km 3, 5, and 17

throughout the tailF NNW Area of the reservoir 6, 7, 8, 9, 10, 15, 18 and

and up to 12 km 27

been induced by the Tous reservoir. In addition to the geographicalproximity of the epicentres to the Tous New Dam and reservoir,there are correlations between substantial changes (in water level)and rapid changes (in time) in the reservoir conditions and seismicactivity.

The first induced microearthquakes correlate to the increase inload as a result of the rapid increase in reservoir water level, prob-ably modifying the pre-existing tectonic stress field. Some monthslater, it seems clear that reservoir unloading produced by rapid

Figure 6. (a) Epicentral distribution map for the earthquakes and microearthquakes registered by the Tous Dam Seismic Network, around the Tous Dam andreservoir, between 1999 and 2000 May. In this figure coordinates are UTM zone 30 eastings and northings, in metres. Three kinds of data are displayed: theepicentres for the earthquakes located with accuracy and certainty are displayed as closed circles (•); the epicentres for earthquakes located with a lower degreeof accuracy and certainty are represented as open circles (◦); for two microearthquakes shown as circles centred on VTOU, only the epicentral distance fromVTOU is known. Also some epicentral zones have been highlighted. Note that the border lines of these zones are not delimited in a precise way, and theyonly group epicentres whose seismic events have analogue characteristics. (b) Epicentral distribution map for earthquakes and microearthquakes registered bythe Tous New Dam Seismic Network, around the Tous Dam and reservoir between 1999 and 2002. In this figure coordinates are UTM zone 30 eastings andnorthings, in metres. Three kinds of data are displayed: epicentres for earthquakes located with accuracy and certainty are represented by closed circles (•);epicentres for earthquakes located with a lower degree of accuracy and certainty are represented by open circles (◦); for two microearthquakes shown as circlescentred on VTOU, only the epicentral distance from VTOU is known.

Table 6b. Seismogenic zones around the Tous Dam (1999 January to 2002December).

Name Location with Distance to the dam Earthquakes enclosedregard to the dam (km)

A NE over 50 13, 14, 21, 51 and 54B ENE 21–23 23 and 24C E 10–20 25, 26 and 53

2, 4, 11, 12, 16, 19, 20,D SSE 12–40 28, 41, 52, 59, 68, 69, 72,

73, 79E Downriver About 6 km 3, 5 and 17

throughout the tailF NNW area of the reservoir 6, 7, 8, 9, 10, 15, 18 and

and up to 12 km 27G N–NNW >25 km 42, 44, 45, 46, 50, 55, 56,

57, 63, 74 and 82

draining during the summer correlates to the considerable increasein induced seismic activity, probably as a consequence of the veryrapid decrease in load stress. Later, induced seismic activity seemsto be more closely related to the the refilling of the reservoir, reach-ing a radius of influence between 9 and 23 km from the dam.

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Figure 6. (Continued.)

It is difficult to predict the possible duration of induced seismic-ity around the Tous New Dam and reservoir. Seismicity reportedhere appears to be in an initial phase. Observations at other sitesand dams show that the duration of seismic activity after the mainearthquake can be considerable, extending over some 10 yr (Zhanget al. 1997). Furthermore, protracted seismicity can appear as lateas three decades after initial reservoir impoundment.

Finally, we concur with Kafka & Driscoll (1999) in suggest-ing that continued instrumental monitoring in the TDSN is nec-essary and might yield data necessary to unravel the pattern ofearthquake activity in the area and discern the relationship be-tween that seismicity and the bedrock geology and/or reservoir waterlevel.

A C K N O W L E D G M E N T S

We are very grateful to all staff of the Confederacion Hidrograficadel Jucar (Spain), who operate the Tous New Dam, especially ChiefEngineer Jose Valero, all the technicians who maintain the TousNew Dam and Reservoir Seismic Network, and the engineers JoseLuis Utrillas and Jose Sanchez, for placing their trust in our effortsand work in this study. Our special thanks to the Editors ProfessorGunther Boek and Dr J. Russ Evans for their inestimable help andpatience, Dr Harsh K. Gupta in his role as referee and to anotheranonymous referee, for their excellent contributions and patience inimproving this paper. Thanks to philologist Bryan Robinson for hiscontribution to the proofreading and the improvement of the style ofthe text. We sincerely wish to dedicate this paper to Editor Professor

Gunther Bock, who died in a plane crash while he was managingthis paper in the initial stages.

N O T E A D D E D I N P RO O F

While this article was being revised, more data have been analysed(Torcal 2003). The data set now extends to 2002 December. As aresult we have been able to update the following tables and figures.

(i) Table 3b. Basic statistics about the several types of seismicevents registered by the Tous New Dam (Spain) Seismic Networkbetween 1999 January and 2002 December.

(ii) Table 5b. Data of the microearthquakes registered around theTous New Dam (Spain) between 2000 May and 2002 December.

(iii) Figs 6(b) to 11(b), inclusive.(iv) From new data, we have identified a new zone with some

seismic activity to the N of the Tous New Dam. Figure 11(b) showsthe relationship between the occurrence of the earthquakes and thelake level evolution over time between 1998 September and 2002December. In addition to what has been described previously inthe main text, a cluster of seismic activity occurred between 1999December and 2000 June, related to well-defined changes in thelake level during that period. The two earthquakes that occurred on1999 December 19 seem to be related to a relatively rapid decreaseof approximately 59 cm in the lake level, occurring between 1999November 23 and December 7 and the subsequent increase of 1.49 min the lake level. Subsequent earthquakes happened between 2000February 1 and May 5, simultaneous with substantial changes in theevolution of the lake level. Later earthquakes occurred mainly:

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Figure 7. (a) 3-D distribution of seismicity recorded around the Tous NewDam by the Tous New Dam Seismic Network between 1999 January andthe first half of 2000 May, in the form of pairs of figures showing epicentraldistribution (top) and depth distribution (bottom). As viewed towards N–S,N45◦E, E–W and N135◦E, respectively. (b) 3-D distribution of seismicityrecorded around the Tous New Dam by the Tous Dam Seismic Networkbetween 1999 January and 2002 December, in the form of pairs of figuresshowing epicentral distribution (top) and depth distribution (bottom). Asviewed towards N–S, N45◦E, E–W and N135◦E, respectively.

(a) between 2000 October and 2001 January, after a substantialdecrease in the lake level from 2000 May;

(b) between 2001 February 17 and April 2, after a major increaseof 10.47 m in lake level between 2000 September 21 and 2001March 28;

(c) 2001 October 17, during the annual minimum lake level pe-riod and a net decrease of 14.58 m in lake level;

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(d) between 2002 January and April, after an impoundment ofthe lake and a net increase of 10.66 m in lake level; and

(e) between 2002 June and November, while the lake level fell by19.56 m, with other smaller-scale fluctuations in the lake levelmeantime.

In conclusion, we can say earthquakes appear to occur mainlyafter periods of a rapid decrease in lake level, and to a lesser extentat the end of the periods of a rapid increase in lake level of theTous New Dam. So, it seems this seismicity is closely associatedwith the periods of filling and draining of the reservoir, and can beconsidered as seismicity induced by changes in the lake level of theTous New Dam.

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Figure 8. (a) 3-D distribution of seismicity recorded around the Tous NewDam by the Tous New Dam Seismic Network between 1999 January andthe first half of 2000 May, in the form of pairs of figures showing epicentraldistribution (top) and depth distribution (bottom). As viewed towards N–S,N45◦E, E–W and N135◦E, respectively. (b) 3-D distribution of seismicityrecorded around the Tous New Dam by the Tous Dam Seismic Networkbetween 1999 January and 2002 December, in the form of pairs of figuresshowing epicentral distribution (top) and depth distribution (bottom). Asviewed towards N–S, N45◦E, E–W and N135◦E, respectively.

R E F E R E N C E S

Bernal, A., Lopez de Alda, F.J., Torcal, F. & Serrano, I., 1992a. SegundoEstudio de Microsismicidad en el Area de El Cabril: Informe de es-pecificaciones de operacion e interpretacion de los datos de la Red deVigilancia Sısmica de El Cabril (Cordoba) restricted report for EN-

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RESA, Empresa Nacional de Residuos Radiactivos, Madrid, Spain, p. 147(in spanish).

Bernal, A., Vidal, F., De Miguel, F., Torcal, F., Serrano, I. & MartınBourgon, P., 1992b. Crustal model and local magnitude at El Cabril(Spain). In: X World Conference on Earthquake Engineering (WCEE),Madrid, Spain. June, 1992, Vol. 1, pp. 321–325, A.A. Balkema, Rotter-dam, ISBN 90 5410 060 5.

Cornet, F.H. & Yin, J., 1995. Analysis of induced seismicity for stress fielddetermination and pore pressure mapping, Pure appl. Geophys., 45(3/4),677–700.

Cornet, F.H., Helm, J., Poitrenaud, H. & Etchecopar, A., 1997. Seismic andaseismic slips by large-scale fluid injections, Pure appl. Geophys., 150,563–583.

De Miguel, F., Alguacil, G. & Vidal, F., 1988. Una escala de magnitud a partirde la duracion para terremotos del sur de Espana (In Spanish), Revista deGeofısica, 44, 75–86.

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Figure 9. (a) 3-D distribution of seismicity recorded around the Tous NewDam by the Tous Dam Seismic Network between 1999 January and thefirst half of 2000 May, in the form of pairs of figures showing epicentraldistribution (top) and depth distribution (bottom). As viewed towards N–S,N45◦E, E–W and N135◦E, respectively. (b) 3-D distribution of seismicityrecorded around the Tous New Dam by the Tous Dam Seismic Networkbetween 1999 January and 2002 December, in the form of pairs of figuresshowing epicentral distribution (top) and depth distribution (bottom). Asviewed towards N–S, N45◦E, E–W and N135◦E, respectively.

Fontbote, J.M., Guimera, J., Roca, E., Sabat, F., Santanach, P. & Fernandez-Ortigosa, F., 1990. The Cenozoic geodynamic evolution of the Valen-cia trough (western Mediterranean), Rev. Soc. Geol. Esp., 3(3–4), 249–259.

Gelabert, B., Sabat, F. & Rodrıguez-Perea, A., 1992. A structural outline ofthe Serra de Tramuntana of Mallorca (Balearic Islands), Tectonophysics,203, 167–183.

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Guimera, J., 1984. Palaegene evolution of deformation in the northeasternIberian Peninsula, Geol. Mag., 121(5), 413–420.

Guimera, J. & Alvaro, M., 1990. Structure et evolution de la compressionalpine dans la Chaıne iberique et la Chaıne cotiere catalane (Espagne),Bull. Soc. Geol. Fr., 6(2), 339–348.

Gupta, H.K., 1992. Reservoir induced earthquakes, Elsevier, New York.Gupta, H.K. & Rastogi, B.K., 1976. Dams and earthquakes, Elsevier,

Amsterdam.Julivert, M., Fontbote, J.M., Ribeiro, A. & Conde, L., 1977. Mapa tectonico

de la Peninsula Iberica y Baleares (Tectonic map of the Iberian Peninsulaand Balearic Islands), Instituto Geologico y Minero de Espana, Madrid,Spain.

Kafka, A.L. & Driscoll, E.K., 1999. Earthquakes in the area surroundingthe Quabbin Reservoir in Central Massachusets, Seism. Res. Lett., 70(4),446–460.

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Figure 10. (a) 3-D distribution of seismicity recorded around the TousNew Dam by the Tous Dam Seismic Network between 1999 January andthe first half of 2000 May, in the form of pairs of figures showing epicentraldistribution (top) and depth distribution (bottom). As viewed towards N–S,N45◦E, E–W and N135◦E, respectively. (b) 3-D distribution of seismicityrecorded around the Tous New Dam by the Tous Dam Seismic Networkbetween 1999 January and 2002 December, in the form of pairs of figuresshowing epicentral distribution (top) and depth distribution (bottom). Asviewed towards N–S, N45◦E, E–W and N135◦E, respectively.

Kitov, I.O., Murphy, J.R., Kusnetsov, O.P., Barker, B.W. & Nedoshivin, N.I.,1997. An analysis of seismic and acoustic signals measured from a seriesof atmospheric and near-surface explosions, Bull. Seism. Soc. Am., 87(6),1553–1562.

Moissenet, E., 1985. Les depressions tarditectoniques des Chaınes Iberiquesmeridionales: distension, diapirisme et depots neogenes associes, C. R.Acad. Sci. Paris, 300(11), 523–528.

Ott d’Estevou, Ph., Montenat, Ch., Ladure, F. & Pierson d’Autrey, L., 1988.

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Evolution tectono-sedimentaire du domaine prebetique oriental (Espagne)au Miocene, C. R. Acad. Sci. Paris, 307, 789–796.

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Pomar, L., Marzo, M. & Baron, A., 1983. El Terciario de Mallorca (in Span-ish). In: eds Pomar, L., et al., El Terciario de las Baleares (Mallorca-Menorca), Libro guıa de las excursiones al X Congreso Nacional de Sed-imentologıa, Menorca, pp. 21–42, GES, Mallorca, Spain.

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Rastogi, B.K., Mandal, P. & Kumar, N., 1997. Seismicity around DhamniDam, Maharashtra, India, Pure appl. Geophys., 150, 493–509.

Real, C.B. & Teng, T.L., 1973. Local Richter magnitude and total signalduration in Southern California, Bull. seism. Soc. Am., 63, 1809–1827.

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Table 7. Focal mechanisms parameter calculated for several earthquakes–microearthquakes located around the Tous Dam and reservoir (1999 Januaryto first half 2000 May).

Earthquake No. Element Azimuth (◦N) Dip (◦) Rake

3 1st plane 194–200 45 NW 134–1252nd plane 320–335 60–55 NE 55–60

T axis 178–187 59–65 —P axis 74–86 8–5 —

4 1st plane 45–60 30–40 SE 146–1612nd plane 165 74–78 SW 64–52

T axis 44–38 54–44 —P axis 275–284 25–23 —

6 1st plane 60–82 25–29 SE −104–(−71)2nd plane 255–240 66–63 NW −84–(−101)

T axis 340–338 21–17 —P axis 177–128 69–70 —

23 1st plane 0–13 45–41 ESE 85–1342nd plane 112–184 70–49 WSW 49–84

T axis 338–51 49–84 —P axis 231–278 15–4 —

24 1st plane 105–135 65–85 SSW 90–1052nd plane 285–315 25–45 N 50–60

T axis 15–45 70–50 —P axis 195–225 20–40 —

26 1st plane 125 45 SW −1412nd plane 5 63 E −52

T axis 69 11 —P axis 324 55 —

Reamer, S.K., Hinzen, K.-G. & Stump, B.W., 1992. Near-source character-ization of the seismic wavefield radiated from quarry blasts, Geophys. J.Int., 110, 435–450.

Simpson, D.W., 1976. Seismicity changes associated with reservoir loading.,Eng. Geol., 10, 123–150.

Simpson, D.W., Leith, W.S. & Scholz, C.H., 1988. Two types of reservoirinduced seismicity, Bull. seism. Soc. Am., 78(6), 2025–2040.

Figure 11. (a) Comparison of Tous reservoir lake level (line) and seismicity recorded by day (bars) from September 1998 to first half of May 2000. The firstday of every month is displayed in the middle of the label of the month. Over three bars, the number indicates the number of earthquakes on that day, showingthe scale for the other bars. (b) Comparison of Tous reservoir lake level (line) and seismicity recorded by day (bars) from 1998 September to 2002 December.The first day of every month is displayed in the middle of the label of the month. Over three bars, the number indicates the number of earthquakes on thatday, showing the scale for the other bars. Also the number of earthquakes recorded and the increase(+)/decrease(−) of the lake level ratio is shown in eachcharacteristic segment of lake level evolution over time.

Talwani, P., 1997. On the nature of reservoir-induced seismicity, Pure appl.Geophys., 150, 473–492.

Torcal, F., 1999. Estudio de la microsismicidad inducida por el Embalse deTous (Valencia), Primer informe: Recopilacion, analisis e interpretacionde los registros sısmicos obtenidos por la Red de Microsismicidad delEmbalse de Tous, durante el perıodo comprendido entre el comienzo desu funcionamiento regular en Enero de 1999 y hasta el mes de Junio de1999, restricted report for the CHJ, Confederacion Hidrografica del Jucar,Granada, Spain, p. 338 (in Spanish).

Torcal, F., 2000a. Estudio de la microsismicidad inducida por el Embalsede Tous (Valencia), Segundo informe: Analisis e interpretacion de losregistros sısmicos obtenidos por la Red de Microsismicidad del Embalsede Tous, durante los meses de Julio y Agosto de 1999, restricted report forthe CHJ, Confederacion Hidrografica del Jucar, Granada, Spain, p. 158(in Spanish).

Torcal, F., 2000b. Estudio de la microsismicidad inducida por el Embalse deTous (Valencia), Tercer informe: Analisis e interpretacion de los registrossısmicos obtenidos por la Red de Microsismicidad del Embalse de Tous,durante los meses de Septiembre y Octubre de 1999, restricted report forthe CHJ, Confederacion Hidrografica del Jucar, Granada, Spain, p. 207(in Spanish).

Torcal, F., 2000c. Estudio de la microsismicidad inducida por el Embalse deTous (Valencia), Cuarto informe: Analisis e interpretacion de los registrossısmicos obtenidos por la Red de Microsismicidad del Embalse de Tous,durante los meses de Noviembre y Diciembre de 1999, restricted report forthe CHJ, Confederacion Hidrografica del Jucar, Granada, Spain, p. 178(in Spanish).

Torcal, F., 2000d. Estudio de la microsismicidad inducida por el Embalse deTous (Valencia), Quinto informe: Analisis e interpretacion de los registrossısmicos obtenidos por la Red de Microsismicidad del Embalse de Tous,durante los meses de Enero y Febrero de 2000, restricted report for theCHJ, Confederacion Hidrografica del Jucar, Granada, Spain, p. 205 (inSpanish).

Torcal, F., 2000e. Estudio de la microsismicidad inducida por el Embalse deTous (Valencia), Sexto informe: Analisis e interpretacion de los registrossısmicos obtenidos por la Red de Microsismicidad del Embalse de Tous,durante los meses de Marzo y Abril y primera mitad de Mayo de 2000.Recopilacion e interpretacion de la actividad sısmica registrada entre

C© 2005 RAS, GJI, 160, 144–160



Enero de 1999 y Mayo de 2000, restricted report for the CHJ, Confed-eracion Hidrografica del Jucar, Granada, Spain, p. 346 (in Spanish).

Torcal, F., 2003. Estudio de la microsismicidad inducida por el Embalse deTous (Valencia), Septimo informe: Analisis e interpretacion de los reg-istros sısmicos obtenidos por la Red de Microsismicidad del Embalsede Tous, entre la segunda quincena de Mayo de 2000 y Diciembre de2002, restricted report for the CHJ, Confederacion Hidrografica del Jucar,Granada, Spain, p. 1742 (in Spanish).

Torcal, F. & Serrano, I., 1995. Ubicacion de las estaciones de la Red deMicrosismicidad del Embalse de Tous restricted report for the CHJ, Con-federacion Hidrografica del Jucar, Granada, Spain, p. 32 (in Spanish).

Torcal, F. & Serrano, I., 1999. Modelo de corteza preliminar para el area delEmbalse de Tous (Valencia), restricted report for the CHJ, ConfederacionHidrografica del Jucar, Granada, Spain, p. 193.

Zhong, Y.Z., Gao, C. & Yun, B., 1997. Induced Seismicity in LiaoningProvince, China, Pure appl. Geophys., 150, 461–472.

C© 2005 RAS, GJI, 160, 144–160

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