Fault plane solutions for microearthquakes induced at the Fenton Hill hot dry rock Geothermal site: Implications for the state of stress near a quaternary volcanic center

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Abstract
Fault plane solutions of earthquakes induced during attempts to stimulate two hot dry rock reservoirs at Fenton Hill have significantly different patterns of first motions. The fault plane solution for the lower reservoir indicates strike slip, either left lateral strike slip on a N‐S vertical plane or right lateral slip on an E‐W vertical plane. In contrast, the solution for the upper zone includes largely vertical slip on a N‐S nearly vertical plane, or oblique slip on a nearly horizontal plane. Because the N‐S nodal plane is common in both solutions we infer that this represents the true fracture plane. Faulting thus seems to occur on a series of parallel faults or joints that intersect both reservoirs but a change in the slip vector indicates a major change in the state of stress between the upper and lower reservoirs. This latter conclusion is surprising because the two reservoirs are separated by less than 1 km. We suggest that this rapid change in the stress field may be related to the structure and subsidence of the nearby Valles Caldera.