Temporal and spatial distribution of local seismicity in the Chipilapa-Ahuachapan geothermal area, El Salvador

Microseismic monitoring of the Chipilapa-Ahuachapan area was carried out during August-November 1988 and October 1991-April 1992. The objective was to use the study of microearthquakes as an exploration tool to investigate the geothermal potential of the Chipilapa area and to evaluate the main characteristics of the seismic activity, prior to and during the exploitation tests. Since 1989, seven wells have been drilled in the area, two of which have encountered three geothermal aquifers that could be exploited for electricity generation by means of binary-cycle technology. The 1988 survey detected important, shallow and low magnitude seismic activity, located mainly south and southwest of the explored area. This activity is possibly related to the recharge zone of the Chipilapa-Ahuachapan geothermal system, located further south, beneath the Pleistocene Pacific Volcanic Chain. The 1991-1992 survey confirmed the existence of seismicity beneath the southern volcanic axis, but other important clusters of activity were recorded northward, related to the deeper structures of the Central Graben, and southwest of the Ahuachapan geothermal field, close to the 1990 hydrothermal eruption of Agua Shuca. Shallow microseismic activity also appeared along the faults limiting the Chipilapa geothermal field to the east. Although it is probable that this seismicity is due to fluid circulation in fractures, no geothermal reservoirs were intercepted by wells CHA and CH8. Moreover, no significant induced seismicity was recorded during production and injection tests. (C) 1997 CNR. Published by Elsevier Science Ltd.