Seismic monitoring of the Chipilapa geothermal area (El Salvador)

Journal of Volcanology and Geothermal Research, 43 (1990) 311-320

311

Elsevier Science Publishers B.V., Amsterdam

Seismic monitoring of the Chipilapa geothermal area (El Salvador) H . F A B R I O L , A. B E A U C E

a n d D. L E M A S N E

Bureau de Recherches G~ologiques et Mini~res (BRGM), Institut Mixte de Recherches G~othermiques (IMRG), Avenue de Concyr - BP 6009 - 45060 Orleans Cedex 02, France

(Received September 21, 1989; revised and accepted March 21, 1990)

Abstract Fabriol, H., Beauce, A. and Le Masne, D., 1990. Seismic monitoring of the Chipilapa geothermal area (El Salvador). J. Volcanol. Geotherm. Res., 43: 311-320. Microearthquake monitoring of the Chipilapa geothermal area (El Salvador) was carried out between 26 August, 1988 and 7 November, 1988 in order to determine the background level of the seismicity before any exploitation of the field. More than 500 local events were recorded, as well as 500 regional ones related to the subduction of the Cocos plate under the Caribbean plate. The local microseismicity is rather continuous, with small swarms every 10-15 days. Duration magnitudes do not exceed 2.3, and the b parameter has a value of 1.02, slightly higher than the normal value for tectonic events. The seismicity is mainly located at the southern edge of the recording network, outside the geothermal area. Six clusters of hypocenters were defined: One very shallow which overlaps the southeastern boundary of the network and could be related to the geothermal field itself. Four clusters are sited beneath faults or volcanic centers visible at the surface, and consequently could be associated with the recharge of the geothermal field. The sixth cluster is located at the north of the area and is irrelevant to the Chipilapa zone.

Introduction T h e C h i p i l a p a g e o t h e r m a l field is sited n e a r t h e A h u a c h a p a n g e o t h e r m a l field, 80 k m west of S a n Salvador, capital of E1 Salvador, a n d 15 k m east of t h e G u a t e m a l a b o r d e r (Fig. 1). Exploitation of A h u a c h a p a n s t a r t e d in t h e e a r l y seventies, 32 wells h a v e b e e n drilled a n d a power p l a n t produces more t h a n 60 MW, w h i c h is 25% to 35% of the n a t i o n a l electric consumption. T h e C h i p i l a p a project covers a 2 5 - k i n 2 a r e a located 3 k m east of the A h u a c h a p a n p r o d u c t i o n area, t h e h e a t source b e i n g p r o b a b l y t h e same for

b o t h reservoirs. T h e e x p l o r a t i o n phase is concluded and the first e x p l o r a t o r y well, C H - 7 , r e a c h e d the r e s e r v o i r s t a r t i n g at 6 5 0 - m depth, w i t h t e m p e r a t u r e s g r e a t e r t h a n 190°C at the b o t t o m of the drillhole (1500 m). T h e g e o t h e r m a l a r e a s lie in the n o r t h e r n foothills of the volcanic coastal r a n g e of Pleistocene age, which e x t e n d from the G u a t e m a l a b o r d e r at t h e west to the G u l f of F o n s e c a at the east. The volcanic a c t i v i t y is r e l a t e d to t h e s u b d u c t i o n of t h e Cocos P l a t e b e n e a t h the C a r i b b e a n plate, a n d it is locally c h a r a c t e r i z e d by m a n y y o u n g volcanoes (e.g.

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Cerro Laguna Verde, 1699 m a.s.l., Cerro Las Ninfas, 1774 m a.s.1.) and by impressive fumarolic areas and solfatara (Fig. 2). North of the volcanic range lies the main E - W structure of the Central Graben. The geothermal reservoirs belong to the volcanic unit although spatially linked to the E - W faulting of the Graben.

Our study of the local microseismicity started in August 1988 with a 2-month effective monitoring period that had two objectives: (1) A better definition of the reservoir and its recharge and the associated structures. (2) Evaluation of the main characteristics of the local seismic activity to define the initial state of the seismicity before any geothermal

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production. For the 1988 survey, the network of seismometers was designed to cover the supposed production area with instruments sensitive enough to detect any event of low magnitude. A monitoring of microseismicity in the Ahuachapan area took place between December 1969 and August 1970 (Ward and Jacob, 1971). The number of regional (T s - Tp > 8s) events recorded at that time was rather high (350) and the number of local ( T s - T p > 8s) events lower (150). Regional and local event rates observed during the 1988 survey were higher. This paper deals with a qualitative description of the seismicity in a preliminary state and with a first interpretation of the spatial distribution of the hypocenters according to geology.

The network includes 6 vertical geophones and 3 three-component sites, all linked by wires to the central station (Finca Los Angeles), where the data are digitized at 200 Hz and preliminary event detection and location of events are carried out in real time. The system is permanently on watch and triggers the data storage from the 15 channels, as soon as a seismic event is detected on three presetected stations. For each detected event, the computer picks the arrival times of Pwave (6 out of the 9 vertical geophones) and Swave onsets (horizontal geophones) and com~ putes the hypocenter coordinates. The resulting epicenter and the seismograms used for detection are automatically plotted on the console. Back in the BRGM laboratory the data were carefully analysed and events classified into three classes: (a) Local events (T s - Tp < 3s); (b) Regional events (3s < Ts-- Tp < 30si; (c) Teleseismic events (Ts- Tp > 30s). Figure 3 shows a chronological histogram fbr local events. The daily rate of local seismicity

T h e n e t w o r k a n d the m a i n c h a r a c t e r i s t i c s o f the s u r v e y

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SEISMIC MONITORING OF THE CHIPILAPA GEOTHERMAL AREA

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ranges from 5 to 10 microearthquakes per day, with small crises every 1 0 - 1 5 days, corresponding to swarms of events clustered in space and time. This is rather different from the results of the 1 9 6 9 - 1 9 7 0 survey, probably because of the lower sensitivity of the earlier network. The regional seismicity was very high, in particular between 13 and 19 October, a period during which 6 deep earthquakes were reported by the United States Department of Interior (1988), near the Guatemala coast. Another major regional crisis was recorded between 3 and 5 November with two offshore earthquakes of magnitude 5.4 and 5.1, strongly felt in Chipilapa. In the absence of any specific magnitude formula for E1 Salvador or Central America we used the formula given by Lee et al. (1972)for California to evaluate duration magnitudes in Chipilapa: M d = - 0.97 + 2 log(d) + 0.00325 5 where 5 = epicentral distance (in km) and

d = duration of the event (in s) Figure 4 shows the magnitudes of 377 selected local events. The continuous character of the seismicity is clear. There are no duration magnitudes higher than 2.3, but magnitudes around 2 are regularly occurring. Negative magnitudes are observed after October 10, when the rainy season was over and the background seismic noise dropped, increasing the detection threshold. The value of the b parameter in the frequencymagnitude relation is around 1.02, which is in agreement with the values higher than 0.8 usually found in volcanic or geothermal areas (e.g. Majer and Mc Evilly, 1979; Batini et al., 1984). It means that earthquake sources tend to produce a larger number of small-magnitude events, which can be related to a low stress state at the source (Scholz, 1968) or to high temperatures in case of a volcanic origin (Hunt and Latter, 1982).

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H y p o c e n t e r s distribution in the Chipilapa area The hypocenters of 125 microearthquakes with clear P- and S-wave onsets were determined with the computer program HYPO71 (Lee and Lahr, 1975). A 5-layer model was defined, using

409

three calibration dynamite shots in shallow holes and the known geology and geophysics (Table 1). The common value of 1.732 was used for Vp/V s ratio, since there was no information on the S-wave velocities. Figure 5 shows the location map of the epicenters together with two vertical projections of hypocenters on N - S and

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317

SEISMICMONITORINGOFTHECHIPILAPAGEOTHERMALAREA TABLE 1 P-wave velocity model used by HYPO71 for the deterruination of hypocenters Layer no.

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Res.P (Res.S): Mean residual error in P-wave (S-wave) arrival times for a given station. Num-P (Num-S): Number of events used to evaluate the mean P-residual (S-residual) for this station.

E - W planes. T h e s e i s m i c i t y is m a i n l y distribu t e d s o u t h of t h e n e t w o r k , u n d e r t h e v o l c a n o e s at d e p t h s less t h a n 6 k m . Six s p a t i a l c l u s t e r s w e r e defined a n d can be s e e n in F i g u r e 6. T h e m e a n location e r r o r s e s t i m a t e d b y H Y P O 7 1 for 125 e v e n t s a r e as follows: - h o r i z o n t a l error: 0.32 kin; - v e r t i c a l error: 1.18 k m ; - R M S e r r o r in a r r i v a l times: 28.7 ms.

T a b l e 2 p r e s e n t s t h e m e a n r e s i d u a l e r r o r s for e a c h station. It c a n be seen t h a t P - w a v e r e s i d u a l e r r o r s a r e r a t h e r low for all s t a t i o n s except 8 a n d 11. S t a t i o n 8 is t h e m o s t d i s t a n t f r o m m o s t of t h e e v e n t s a n d s t a t i o n 11 m u s t be considered as anomalous, which had been observed during the c a l i b r a t i o n shots. F o r S-wave r e s i d u a l s , s t a t i o n s 3,7 a n d 11 c a n only be considered, since S arr i v a l s a r e a c c u r a t e l y picked on the 3 - c o m p o n e n t s e i s m o m e t e r s only. H e r e a g a i n s t a t i o n 11 is a n o m a l o u s , c o m p a r e d w i t h s t a t i o n s 3 a n d 7. To t e s t t h e v a l i d i t y of t h e location of t h e h y p o c e n t e r s , we r a n H Y P O 7 1 for t h e s a m e 125 m i c r o e a r t h q u a k e s a l r e a d y selected w i t h o u t stat i o n 11. T h e g e n e r a l t r e n d s a r e p r e s e r v e d , b u t t h e e v e n t s of e a c h g r o u p a r e less c l u s t e r e d a n d t h e positions of t h e d i f f e r e n t g r o u p s a r e app r e c i a b l y m o v e d in space ( b e t w e e n 0.5 a n d 1.5 k m in t h e h o r i z o n t a l p l a n e a n d b e t w e e n 0.5 a n d 3 k m in t h e v e r t i c a l plane). T h e m o r e i m p o r t a n t shift occurs for g r o u p 5, w h i c h is m o v e d 3 k m downwards.

Relations between seismicity, tectonics and geothermal activity T h e 6 g r o u p s of e v e n t s of F i g u r e 6 c a n be interp r e t e d as follows, b a s e d on t h e s t r u c t u r a l f r a m e w o r k of t h e a r e a (CEL, 1988): (a) G r o u p I b a s i c a l l y includes 16 e v e n t s occurring d u r i n g two s m a l l s w a r m s a n d is s u r r o u n d e d b y m o s t of t h e e v e n t s t h a t c a n n o t be c l u s t e r e d in a specific group. T h e volcanic axis of C e r r o L a s Ninfas, Hoyo Cuajuste and Cerro Laguna Verde a p p e a r s to be a n a c t i v e zone, w i t h e v e n t s occurring d u r i n g s w a r m s or all o v e r t h e survey. T h i s volcanic a r e a is k n o w n to be t h e r e c h a r g e zone of t h e A h u a c h a p a n g e o t h e r m a l field a n d of t h e C h i p i l a p a field too. H o w e v e r , we c a n n o t c o n f i r m a r e l a t i o n b e t w e e n t h e m i c r o e a r t h q u a k e s of g r o u p 1 a n d g e o t h e r m a l activity, since t h e f o r m e r a r e a t 2.5 to 3.5 k m depth. T h e diffuse activity, a t s h a l l o w e r d e p t h s a n d a r o u n d g r o u p 1, is p r o b a b l y l i n k e d to t h e f r a c t u r e s t h r o u g h w h i c h t h e g e o t h e r m a l fluid percolates.

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Fig. 6. Interpretation map of microearthquake groups in relation with the structural framework.

(b) Group 2 is the shallowest and closest to the network and the Chipilapa geothermal area. It corresponds to 2 small neighbouring swarms t h a t occurred in less t h a n 24 hours with magnitudes r an g in g from I to 2.2. The vicinity of the very active fumaroles of Cerro C uyanaus ul and the fact t h a t parallel faults, r u n n i n g along

N - S and N W - S E directions, intersect the group 2 area, indicate t h a t this activity may be related either to the flashing of geothermal fluid near the surface or to a tectonic movement. In the first case the relation with the geothermal reservoir would be straightforward.

319

SEISMIC MONITORING OF THE CHIPILAPA GEOTHERMAL AREA

(c) Group 3 is roughly aligned along a N W - S E vertical plane, between 3.5 and 5 k m depth. The 13 low-magnitude earthquakes (0.3-1.4) were observed over the two months of monitoring, which is characteristic for the continuous seismic activity located SW of Cerro Cuyanausul. (d) Group 4 consists of seven events clustered in space and time. Their epicenters coincide with the intersection of two faults, NNE and SSW respectively, visible east of the network. (e) Group 5 lies close to Cerro Cuyanausul, and concerns a small swarm whose first four events have magnitudes higher t h a n 2. It seems to define an E - W plane steeply dipping towards the north, which may be related with the prolongation of a W N W - E S E fault shown on the tectonic map, close to Hoyo de Cuajuste. (f) Group 6 is the only one outside the volcanic area, lying far north of the network and the geothermal area. Depths are larger t h a n for the other groups ( 7 - 9 kin) and, without more informarion about the deep structures in the graben, this group cannot be interpreted more precisely.

Conclusion A first monitoring of the geothermal area of Chipilapa has shown important microseismic activity, mainly located outside the supposed geothermal reservoir, beneath the volcanic structures south of the area. The depths of the hypocenters do not exceed 6 k m for the microearthquakes located in the south, and 9 km for the group 6, which may be related to deeper structures in the Central Graben to the north. The relation between seismicity and the geothermal reservoir is not straightforward, except for group 2, which is quite shallow and close to station 11. The seismicity is possibly linked to the faults t h a t control the recharge of the reservoir, i.e. the descent of meteoric waters down to

the heat source and the ascent of geothermal fluids up to the reservoir. A longer period of monitoring with a larger network will be necessary to get better information, in particular for focal mechanisms determination. One of the objectives would be to constrain the relations between seismicity and tectonics and/or volcanism, as well as between seismicity and the geothermal reservoir. At present, one fact stands out, which is the lack of seismicity in the supposed geothermal area. It will be important to continue the monitoring during production and reinjection tests: if some seismicity occurs at t h a t time, it should be carefully analysed to determine whether it is induced by exploitation or not.

Acknowledgements This work was carried out by BRGM as subcontractor for the Compagnie Fran~aise de G~othermie under Contract C E L - 1 6 8 4 with the Comision Ejecutiva Hidroelectrica del Rio Lempa.

References Batini, F,, Console, R. and Luongo, G., 1984. Seismological study ofLarderello-Travale geothermal area. Seminar on utilization of geothermal energy for electric power production and space heating. Florence (Italy). CEL, Comision Ejecutiva Hidroelectrica del Rio Lempa, 1988. Proyecto desarollo acelerado del campo geotermico de Chipilapa, Estudios geocientificos, Informe de evaluacion de la informacion existente. 58 pp. 88 CFG 19. Hunt, T.M. and Latter, J.H., 1982. A survey of seismic activity near Wairakei geothermal field, New Zealand. J. Volcanol. Geotherm. Res., 14: 319334. Lee, W.H.K. and Lahr, J.C., 1975. HYPO 71 (revised): a computer program for determining hypocenter, magnitude and first motion pattern of local earthquake. U.S. Geol, Surv., Open File Rep., pp. 75-311. Lee, W.H.K., Bennelt, R.E. and Meagher, K.L., 1972. A method for estimating magnitude of local earthquakes from signal duration. U.S. Geol. Surv., Open File Rep., 37 pp.

320 Majer, E.L. and Mc Evilly, T.V., 1979. Seismological investigation at The Geysers geothermal field. Geophysics, 44: 246-269. Schoh, C.H., 1968. The frequency-magnitude relation of microfracturing in rock and its relation to earthquakes. Bull. Seismol. Soc. Am., 58: 399-415.

tl FAP;RI[)I, ET A i

U.S. Department of Interior, Geological Survey, 1988. Preliminary Determination of epicenters. 35-88 to 45-88 (Sept 29 to Dec 1, 1988). Ward, F.L. and Jacob, K.H., 1971. Microearthquakes in the Ahuachapan geothermal field, E1 Salvador~ Centro America. Science, 173: 328-330.