Seismic monitoring of the Chipilapa geothermal area (El Salvador)

Abstract 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.