3-D MICROEARTHQUAKE ATTENUATION TOMOGRAPHY AT THE NORTHWEST GEYSERS GEOTHERMAL REGION, CALIFORNIA

We present the results of the modeling of a 3-D differential attenuati on structure (delta Q(-1)) beneath the Northwest Geysers geothermal fi eld, A set of 480 high-quality microearthquakes distributed evenly thr oughout the field and with a minimum of 10 P-wave arrivals was selecte d for the study. We constructed spectral ratios by dividing each spect rum with a reference spectrum from each observing station, The referen ce spectrum was derived from the average spectrum of all events that w ere recorded at that station to correct for the strong site dependence of the observed spectra. We then estimated the differential attenuati on operator from the slopes of the spectral ratios. The velocity model s and the raypaths for all events are known from a previous velocity i nversion study, The inversion for the differential attenuation structu re was carried out using a modified progressive inversion method, The observed delta Q(-1) structure correlates well with mapped geologic un its. High delta Q(-1) and lower velocities correlate with Franciscan m elange, while lower attenuation and higher velocities correspond to me tagraywacke units. High P-wave delta Q(-1) also underlies the southern region between 2 and 3 km depth where low V-p/V-s values suggest unde rsaturation of the reservoir rocks. Most of the steam entries also occ ur within this region and probably delineate the steam reservoir. Thes e anomalies may be explained by high rock temperatures and the presenc e of steam and other gases.