Abstract
Tomographic inversion is applied to delay times from local earthquakes
to image three dimensional velocity variations near Parkfield, California.
The 25x20 square km region is represented by nearly cubic blocks
of 0.5 km per side. Arrival times of P waves from 551 local earthquakes,
with depths of 0 to 15 km, were used as source producing 3135 rays
covering the target region. The data were recorded on low-noise downhole
seismographs. A conjugate gradient method is used to invert the resulting
sparse system of simultaneous equations. To diminish the effects
of noisy data, the Laplacian of the model parameters is constrained
to be small within horizontal layers, providing smoothing of the
model. The resolution of the model is estimated by calculating point
spread functions at blocks of interest. Estimates of standard errors
of the model parameters of calculated by the jackknife statistical
procedure. The results of the inversion show correlation with some
of the local geological and geophysical features. Station corrections
removed the long-wavelength anomaly assoicated with the contrast
of the Salinian block southwest of the San Andreas fault versus the
Franciscan to the northeast. A velocity low located a few kilometers
northewest of Parkfield (depth 2.5-3.5 km), appears to lie along
the gradient of the large Bouguer gravity anomaly associated with
the Parkfield syncline. The south-southeastward extension of the
low velocities may relate to reflections observed on the Parkfield
Consortium for Continental Reflection Profiling (COCORP) lines. We
speculate on the geological meaning of these features and interpret
them either as part of the local strike slip tectonics or a shallow
crustal detachment. The correlation of higher-velocity features and
seismic activity may indicate that earthquakes are occurring in more
competent zones while aseismic slip takes place in zones of lower-velocity,
less competent rocks.
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