The geysers geothermal area, California : tomographic images of the depleted steam reservoir and non-double-couple earthquakes
The Geysers geothermal area, California is the world's largest and most intensively exploited steam field, providing about 6% of California's electrical power. The geothermal area is very active seismically, generating about 140 earthquakes per month with M(_D)≥l.2. Non-DC earthquakes have been routinely detected in other geothermal and volcanic areas such as the Hengill-Grensdalur volcanic complex, Iceland but previously went undetected at The Geysers. The steam field is, however, a likely source of non-DC earthquakes because large volumes of steam are extracted and condensate injected during the course of commercial exploitation which might cause cracks or fractures to open and close. Maps of seismic activity through time show conclusively that earthquakes initiate at the onset of production, continue through it and stop when production ceases. Furthermore the volume of steam extracted and/or condensate injected may directly control the rate of seismicity within the geothermal area. A temporary field experiment in April, 1991 recorded about 4000 high-quality earthquakes on three-component digital sensors. Three-dimensional tomographic models of V(_p) and, for the first time at The Geysers, V(_p)/v(_s) were determined using 3906 P-wave and 944 5-wave arrival times from 185 earthquakes. Variations m lithology, temperature and the pore-fluid phase probably produce the variations in V(_p). A strong low in the V(_p)/v(_s) model defines fluid-deficient areas in the steam reservoir and is surrounded by a "halo" of high-V(_p)/v(_s) anomalies. V(_s)/v(_s) can remotely monitor temporal depletion of liquid reserves in the steam reservoir. Well-constrained moment tensor solutions for 30 earthquakes were determined by inverting the polar' ices and amplitude ratios of P- and 5-wave arrivals. Strong evidence for the existence of non-DC earthquakes at The Geysers geothermal area was found. Explosive and implosive events occur in equal numbers and probably reflect source processes involving opening and closing cracks or cavities. The events form a symmetric pattern on source-type plots extending from the postive dipole-to-negative dipole loci, passing through the DC locus. The association with dipole loci rather than crack loci suggests the source process must also involve a compensating flow of fluids, liquid or steam.