Imaging hydraulic fractures in a geothermal reservoir

[1] An injection experiment at the Coso geothermal field in eastern California in March 2005 caused a swarm of microearthquakes that was recorded by a local network of three-component digital seismometers. High-resolution relative hypocenter locations propagated upward and northward on a 700 × 600 m plane striking N 20°E and dipping 75° to the WNW. This plane is a pre-existing fault, whose surface projection coincides with an active scarp. The earthquakes have similar non-double-couple mechanisms that involve volume increases, and the fault plane bisects their dilatational fields, implying a process dominated by tensile failure. The source types require the additional involvement of subsidiary shear faulting, however. Events before and after the swarm have variable orientations and volume changes of both signs. Similar tensile-shear failure is observed in some natural microearthquake swarms, for example at Long Valley caldera, California. Its occurrence under low fluid pressure may imply a heterogeneous stress field or the induction of thermal stresses by introduction of cold fluid.