Abstract
A seismic reflection/refraction survey across the San Andreas fault
near Parkfield, California, has refined our knowledge of the upper
crustal structure of the central California Coast Ranges at the San
Andreas Fault Observatory at Depth (SAFOD). The survey consisted
of a 46-km-long line of seismographs (25-50 m spacing) and 63 explosions
(25-200 kg; nominal spacing of 500 m, with some gaps). The traveltimes
of refracted P and S waves from the explosions constitute independent
data sets of relatively high quality that were inverted to produce
P- and S-wave velocity models (Vp, Vs) along the profile, extending
to as much as 5 km depth. The Vp and Vs models show a prominent lateral
drop in velocities a few hundred metres northeast of SAFOD, between
the drill hole and the San Andreas fault. The Vp model shows particularly
well a southwest-dipping velocity inversion beneath SAFOD, the top
of which correlates with a fault penetrated by the drill hole that
separates granitic rocks above from sedimentary rocks below. In addition
to Vp and Vs models, a Vp/Vs model was derived. A Vp/Vs ratio lower
than 1.73 is seen only at depth, in a narrow zone beginning at the
target earthquakes for SAFOD and extending downward and northeastward
into the North America Plate. Clusters in the parameter space spanned
by Vp/Vs ratios and Vp can be identified by two different methods,
one more intuitive analytical method and one more abstract method
based on neural network techniques. These clusters are correlated
to different rock types, based on laboratory and in situ data. These
clusters are remapped back into x-z plane along the profile. Prominent
features mapped this way include Salinian granitic rocks beneath
and west of SAFOD, and a body of sedimentary rocks faulted beneath
these granitic rocks along what we and others interpret to be a branch
of the Buzzard Canyon Fault (BCF) system. These sedimentary rocks
extend from this fault to the San Andreas fault system. Unfortunately,
our cluster analysis shows no significant discontinuity at the San
Andreas fault, owing presumably to the fact that the San Andreas
fault is located within sedimentary rocks having similar elastic
properties. This paper is an attempt to 'downward' continue a geological
map by geophysical means based on elastic properties of rock samples
from the region.
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