| Abstract |
Microearthquake (MEQ) data from three (3) surveys at Darajat show different seismic characteristics. Two surveys were run in 1997 and the last survey was in 2003. The surveys were conducted under differing conditions including survey duration, number and location of injector wells, different injection rates, stimulation through fluid injection and steam production, with or without start-up of power plants and fixed seismic station versus mobile seismic station configurations. The microearthquakes from the first survey, induced by additional fluid injection, showed good correlation between events and location of injectors, formed organized swarms, with a relatively high number of events per day, and the event distribution was consistent with known structural trends. The second MEQ survey, with no additional injection program, resulted in a scattered event distribution, lower number of events per day and no clear swarm patterns. The 2003 survey showed a new seismic swarm near a new injector location and other events consistent with known structural trends. Overpressures in the reservoir related to the shutdown of Unit I plant generated a sharp response in the NW and central parts of the field. Instead, the opening of DRJ-21 (the largest producing well) at different wellhead pressures induced only a few earthquakes near the well. The most effective microearthquake triggering mechanism in Darajat is related to the increase of pore pressure in the reservoir either through fluid injection or generation of plant shutdown overpressures. The moment tensor analysis on selected swarms showed that the dominant mode of failure is left-lateral strike slip on N to NE-orientated sub-vertical faults. The analysis of the moment tensor (including the isotropic components of deformation) suggests predominance of double-couple mechanisms with minor volumetric components. This is consistent with high tectonic stress regimes and triggering of active faults. In particular, the lack of predominant implosive components of the moment tensor, typical of heavily exploited geothermal fields, indicates that Darajat may still be significantly underexploited. |