Annual Meeting - SCEC.org

Poster Abstracts
extend to about 600 m below the surface, and well data shows evidence of faulting at ~4 km, dipping towards the NIFZ. The
southeastern 5 km of this section of the fault, between the LA River and Long Beach Blvd, corresponds to the deepest part of
the LA Basin, as contoured in Wright, 1991, and truncates reflectors to within 200 m of the surface, exhibiting the greatest
deformation along the CLA and defining perhaps oldest section of the fault. Newly relocated seismicity by Yang and
Hauksson reveal primarily reverse focal solutions below this section of the CLA. South of the LA River, seismic lines and well
data reveal truncated reflectors up to depths of 5-1.5 km and gently warped basinal stratigraphy, exhibiting less apparent
vertical separation than observed on the northwestern section of CLA. Newly relocated seismicity shows the fault coincides
with southwest-dipping strike-slip foci. This southeastern section of the CLA also corresponds to the easternmost extent of
uplift measured by Gilluly and Grant after the 1933 Long Beach earthquake. Repeated evidence of faulting in seismic lines and
well data suggests the CLA is a significant fault. Its proximity to the eastern boundary of uplift measured in the Long Beach
earthquake by Gilluly and Grant and oblique strike-slip focal mechanisms calculated by Yang and Hauksson suggest a
structural connection between the southeastern extent of the CLA and NIFZ, implying a strain-partitioning relationship
between the two faults.
SEISMIC ANISOTROPY OF THE PELONA SCHIST FROM ELECTRON BACKSCATTER DIFFRACTION
(EBSD) MEASUREMENTS OF CRYSTAL PREFERRED ORIENTATIONS (CPOS) (B-136)
K.C. Wagner, S. Brownlee, and B. Hacker
Seismic Anisotropy of the mid to lower crust is difficult to measure in the field by seismologists. Using a scanning electron
microscope (SEM) to obtain crystal preferred orientations (CPO’s) of mineral grains from samples taken from this region
allows us to calculate seismic anisotropy theoretically using published mineral elastic constants. Calculations are made
according to methods described in Mainprice et. al. (1990). Polished thin sections from the Pelona Schist were analyzed using
electron back scatter diffraction (EBSD) in an SEM. The slide was tilted 70 degrees from horizontal. The samples included: 4
mica schists that were cut parallel to foliation, 3 corresponding mica schists cut perpendicular to foliation, and 2 xenoliths with
high amphibole content. The 4 mica schists had max Vp's that ranged from 6.44 to 7.26 km/s; Vp anisotropies that ranged
from 4.8% to 29.2%; and max Vs anisotropies from 7.0% to 42.6%. The 3 corresponding mica schists had max Vp's that ranged
from 6.26 to 6.61 km/s; Vp anisotropies from 3.1% to 7.9%; and Vs anisotropies from 4.9% to 6.61%. In samples that were cut
orthogonal to foliation biotite is difficult to index using the automated indexing routine and had to be found manually to
improve calculations. This may account for significant differences in anisotropy between corresponding samples. The
xenoliths had max Vp's of 6.14 and 6.48 km/s; their Vp anisotropies were both 6.5%; and their Vs anisotropies were 8.35 and
8.37. Our calculations give slightly higher maximum Vp than the average Vp reported in Pellerin and Christensen (1997) for
similar rock types from the area. Calculated anisotropies are comparable to Pellerin and Christensen (1997). Samples with
large amounts of mica had the highest anisotropy. On average, the calculated velocities have approximately hexagonal
symmetry with a unique slow axis. In contrast, one sample contained significant plagioclase that caused velocities to be fast
perpendicular to foliation. An assumption of hexagonal symmetry with a unique slow axis is a valid assumption for the
Pelona Schist.
PBO SOUTHWEST REGION: NETWORK OPERATIONS AND BAJA EARTHQUAKE RESPONSE (A-068)
C. Walls, A. Basset, D. Mann, S. Lawrence, C. Jarvis, J. Sklar, K. Feaux
M. Jackson
The SW region of the Plate Boundary Observatory consists of 455 continuously operating GPS stations located principally
along the transform system of the San Andreas fault and Eastern California Shear Zone. In the past year network uptime
exceeded an average of 97% with greater than 99% data acquisition. Communications range from CDMA modem (307), radio
(92), Vsat (30), DSL/T1/other (25) to manual downloads (1). Sixty-three stations stream 1 Hz data over the VRS3Net typically
with