Induced Seismicity Mechanism at The Geysers, California

Abstract

Induced microearthquake activity at the Geysers geothermal reservoir is observed in the vicinity of eight geothermal steam power units. The earthquakes do no align with mapped faults but occur adjacent to steam wells. The sense of motion as deduced from focal mechanisms is strike slip to reverse in the upper 1 km of the reservoir and changes to strike slip to oblique normal slip at greater depth because of eh increased lithostatic load. Below 1 km the reservoir is undergoing horizontal extension with the maximum and intermediate principal stress axes nearly equal in magnitude. Alignment of P and T axes with the regional stress field suggests that contraction of the reservoir contributes the incremental stress perturb action to the regional stress field and caused microearthquake. The reverse faulting near the surface is likely caused by subsidence from contraction, and tectonic extension provides an explanation for magmatic intrusion in the region.