Abstract
Three-dimensional Q-1 variations in the aftershock region of Loma
Prieta are derived by tomographic inversion. The data set consists
of over 4000 aftershock recordings at 22 PASSCAL (Program for Array
Seismic Studies of the Continental Lithosphere) stations deployed
after the Loma Prieta mainshock of 1989. Estimates of attenuation
are determined from nonlinear least squares best fits to the Fourier
amplitude spectrum of P and S wave arrivals. The linear attenuation
inversion is accomplished by using three-dimensional velocity variations
derived previously in nonlinear velocity inversions. Low Q is observed
near the surface and Q generally increases with depth. The southwest
side of the San Andreas fault exhibits lower Q than does the northeast
side and this feature apparently extends to approximately 7 km depth.
The fault zone, as determined by the dipping plane of aftershock
activity, is characterized by slightly higher Qp and lower Qs, compared
to regions immediately adjacent to the fault. These correlate with
high-velocity anomalies associated with seismicity at depth. The
results are in agreement with earlier observations regarding the
association of high-velocity anomalies, seismicity, and fault zone
asperities.
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