Ht. Wu et Jm. Lees, Three-dimensional P and S wave velocity structures of the Coso Geothermal Area, California, from microseismic travel time data, J GEO R-SOL, 104(B6), 1999, pp. 13217-13233
High precision P and S wave travel times for 2104 microearthquakes with foc
us <6 km are used in a non-linear inversion to derive high-resolution three
-dimensional compressional and shear velocity structures at the Coso Geothe
rmal Area in eastern California. Block size for the inversion is 0.2 km hor
izontally and 0.5 km vertically and inversions are investigated in the uppe
r 5 km of the geothermal area. Spatial resolution, calculated by synthetic
modeling of a cross model at critical locations, is estimated to be 0.35 km
for V-p and 0.5 km for V-s. Model uncertainties are estimated by a jackkni
fe approach and simulation of random and associated picking errors. Low-vel
ocity zones for both P and S waves are identified at geothermal production
depths (1-3 km). A large, low V-p (-6%) zone is found at depth 2-2.5 km 2 k
m southwest of Sugarloaf Mountain where high attenuation has been previousl
y reported. However, a general high-V-p zone is seen under Coso Hot Springs
with a slightly low V-s zone, which is characteristic of fluid saturation.
The overall distributions of V-p and V-s perturbations do not correlate. A
n isolated high-V-s (+9%) feature, about 2 km in diameter, is unambiguously
seen 2 km due west of Sugarloaf extending from surface to depth. This feat
ure is surrounded by a circular, low-V-s belt of similar to 1 km width. The
surrounding belt is probably the cracked, high-porosity reservoir/conduit
of geothermal fluid flow. In the 2 km southwest Sugarloaf region, we found
low V-p and high V-s at geothermal production depths from 1 to 2.5 km. Comb
ined with attenuation results, this may represent a hot, fluid-depleted cen
ter of magmatic activity.