Two short (4 and 6 day) recording periods at The Geysers geothermal field provided useful data on two large refraction explosions and numerous microearthquakes. The vapor-dominated reservoir appears to be characterized by regionally anomalous high P- and S-wave velocities and low attenuation, but the anomaly seems to decrease, possibly reversing, with depth. Microearthquakes occur in a diffuse pattern, with no indication of dominant throughgoing faults and an absence of activity in the main production zone. Mechanisms are generally consistent with northeast-southwest compression. Occurrence rates indicate a slightly high incidence of smaller magnitude shocks. It is possible that the microearthquake activity is related to an expanding steam zone. While the present anomalies appear to delineate the reservoir, it is not certain that they would have been detectable during exploration prior to large-scale exploitation of the field.

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