Annual Meeting - SCEC.org

Poster Abstracts | Group 1 – LAD
measurements of attenuation and the exceptional path coverage that results from the many interstation
measurements, allow us to extend Q estimates to higher frequencies than has previously
been possible using earthquake data. Measurements from paths that cross major sedimentary
basins show both slower wave speeds and lower quality factors than other paths, as expected. Our
results indicate that there is a wealth of information available in the spatial coherency of the
ambient seismic field.
1-128
UPPER MANTLE P VELOCITY STRUCTURE BENEATH SOUTHERN CALIFORNIA
FROM TELESEISMIC TOMOGRAPHY USING LOCAL AND REGIONAL ARRAYS
Schmandt B, and Humphreys ED
Strong seismic heterogeneity in the upper mantle beneath Southern California has been revealed by
several tomographic studies. Previous models require complex seismic structure on a variety of
length-scales, and include high amplitude velocity anomalies at depths ranging from near Moho to
greater than 200 km. Resolution of such structures is limited by the depth distribution of crossing
rays, and modeling of finite frequency and ray bending effects on travel times. We use broadband
data from USArray, permanent regional networks, and temporary deployments spanning 1997
through 2008 to create a new model of upper mantle P velocity. Our tomographic imaging method
uses ray-theoretical travel time sensitivity kernels, and we are currently working on iterative 3-D
ray tracing to mitigate ray bending effects. In the future we plan to model S velocity and
incorporate new 3-D crustal velocity models in our analysis.
1-129
BASAL CRUSTAL ANISOTROPY BENEATH THE SAN GABRIEL MOUNTAINS AND
ADJACENT INNER BORDERLAND: A FOSSIL REGIONAL DETACHMENT? Zandt G,
Porter RC, and Ozacar AA
Receiver functions calculated for long-operating seismic stations in southern California located in
the San Gabriel Mountains (MWC, CHF) and adjacent Inner Borderland (RPV) exhibit converted
phases from the mid- to lower-crust and Moho with large variations in amplitude and polarity
reversals on both the radial and transverse components. These data characteristics are similar to
those observed at station PKD located in the Salinian terrane near Parkfield, central California. The
large amplitudes and small move-out of the phases, and the broad similarity of data patterns on
widely separated stations support an origin primarily from a sub-horizontal layer of hexagonal
anisotropy with a dipping symmetry axis, rather than planar dipping interfaces. However, in some
cases, localized dip or offsets on the interfaces may contribute to complexity in the data (Yan and
Clayton, 2007).
Neighborhood algorithm searches for depth and dip of interfaces, and trend and plunge of
anisotropy symmetry axis have been completed for station PKD and are in progress for the other
stations. At PKD, the best fitting models require a 6-km-thick, high Vp/Vs layer at the base of the
crust with slow axis hexagonal anisotropy > 15% and a slow axis orientation consistent with ENE
dipping (~35 degree) rock fabric. A very similar anisotropy model is recovered from the
preliminary MWC data inversion and is anticipated for the other stations with similar data
characteristics.
The orientation of the anisotropy is consistent with a fossilized fabric created from top-to-the-west
sense of shear that existed along the length of coastal California during past subduction. Under
MWC the top of the anisotropic layer is at ~20 km, the approximate depth of the San Gabriel Bright
Spot (SGBS) observed in LARSE I reflection data ~20 km to the east, and modeled by a 500-m-thick,
136 | Southern California Earthquake Center