Abstract
We refined the three-dimensional (3-D) Vp, Vs and Vp/Vs models around
the San Andreas Fault Observatory at Depth (SAFOD) site using a new
double-difference (DD) seismic tomography code (tomoDDPS) that simultaneously
solves for earthquake locations and all three velocity models using
both absolute and differential P, S, and S-P times. This new method
is able to provide a more robust Vp/Vs model than that from the original
DD tomography code (tomoDD), obtained simply by dividing Vp by Vs.
For the new inversion, waveform cross-correlation times for earthquakes
from 2001 to 2002 were also used, in addition to arrival times from
earthquakes and explosions in the region. The Vp values extracted
from the model along the SAFOD trajectory match well with the borehole
log data, providing in situ confirmation of our results. Similar
to previous tomographic studies, the 3-D structure around Parkfield
is dominated by the velocity contrast across the San Andreas Fault
(SAF). In both the Vp and Vs models, there is a clear low-velocity
zone as deep as 7 km along the SAF trace, compatible with the findings
from fault zone guided waves. There is a high Vp/Vs anomaly zone
on the southwest side of the SAF trace that is about 1-2 km wide
and extends as deep as 4 km, which is interpreted to be due to fluids
and fractures in the package of sedimentary rocks abutting the Salinian
basement rock to the southwest. The relocated earthquakes align beneath
the northeast edge of this high Vp/Vs zone. We carried out a 2-D
correlation analysis for an existing resistivity model and the corresponding
profiles through our model, yielding a classification that distinguishes
several major lithologies.
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