Telemetered Microseismic Study of the Greater Tongonan Geothermal Field

Abstract

A microearthquake telemetry monitoring network was installed in Tongonan geothermal field and operated from February, 1996 to July, 1997 to detect and locate small earthquakes occurring in the area. In the 18 month period, more than six hundredfih events were detected. The hypocentral depths rangedfrom <I km to 12 km with over 40 % deeper than 5 km. The present stu@ conjirms the results of earlier microearthquake studies, that seismicity of the area is relatively high and believe to be tectonic events 6lated to the Phirippine Fault. It also showed that induced seismic occurred during initial injection at wells 5R3D/5R5D/5R7D and MG-2RD while conducting Malitbog power plant test and hydraulic pacturing of MG-2RD.

A three-dimensional velocity structure was obtained porn local earthquake tomography. A low velocity zone, generally oriented in NW-SE direction coincide with the structural trends on the surface. At 1.5 km depth, the low velocity zones coincide with the hot fluids upwelling at Mahiao and outflowing at -1000 msl along the unconformable contact of Mahiao Sedimentary Complex (ABC) and Mamban Formation w). Remarkably, the high velocity zones correlate very well with the impermeable pre-Tertiary serpentinitic basement rocks and Mahanagdong Claystone.

Fault plane solutions were obtained for 89 events. Based on FPS, the most frequent focal mechanisms are of two types: strike-slip (left lateral) and dip-slip (normal faulting) mechanisms with strike oriented N46OW. f i e strike of the source fault plane follows the average trend of the major faults which are principally