HR: 1340h
AN: S13A-1035    [Abstracts]
TI: Distribution of seismicity perpendicular to strike-slip faults in Southern California
AU: * Powers, P M
EM: pmpowers@usc.edu
AF: University of Southern California, Zumberge Hall of Science 3651 Trousdale Pkwy., Los Angeles, CA 90089-0740 United States
AU: Chen, P
EM: pochen@usc.edu
AF: University of Southern California, Zumberge Hall of Science 3651 Trousdale Pkwy., Los Angeles, CA 90089-0740 United States
AU: Jordan, T H
EM: tjordan@usc.edu
AF: University of Southern California, Zumberge Hall of Science 3651 Trousdale Pkwy., Los Angeles, CA 90089-0740 United States
AB: We examine the spatial variability of frequency-magnitude distributions transverse to near-vertical strike-slip faults in Southern California. To calibrate the location precision in this third dimension, we compare three catalogs: the raw Southern California Seismic Network (SCSN) catalog, one relocated by Hauksson, and one relocated by Shearer. We denote the catalogs as C, H, and S respectively, and define the intercatalog variation $\sigma_{A-B}$ to be the RMS of the differences in focal locations between catalogs A and B after adjustment to a common mean location. The computed values for the intercatalog triplet [$\sigma_{C-H}$,$\sigma_{H-S}$,$\sigma_{S-C}$] are [3.0, 2.2, 2.6] km with most of the variation in focal depth. Subcatalogs were constructed for 14 near-vertical fault segments by selecting hypocenters up to 20 km on either side of the segment trace. The perpendicular distance from the fault plane to each epicenter was estimated as the distance from a smoothed version of the fault segment as represented in the new SCEC Community Fault Model, with the distance being measured perpendicular to a best fit plane of the fault segment. As expected, the intercatalog variations of the perpendicular distances depend on the location of the faults within the SCSN; the smallest values are located near the center of the network and the greatest values on its periphery. Although these intercatalog variations measure only the uncorrelated location errors among the three catalogs, they can be used to approximate the relative uncertainties within each fault-segment catalog. Based on our error analysis, we constructed frequency-distance histograms, shifted seismicity peaks to zero-distance from the faults, and stacked the data for all 14 segments to obtain a frequency-magnitude-distance histogram. We used maximum-likelihood to test the significance of frequency-magnitude variations as a function of the perpendicular distance from the fault plane, and reached two principal conclusions: (1) the upper magnitude cutoff decreases with distance from the fault plane, and (2) for small events (M$<$2.5), the slope of the frequency magnitude curve increases with distance from the fault plane. The first simply reflects the strain localization on the fault plane. We interpret the second as a depletion of small earthquakes in the damage zone of the main fault. Two disjunct hypotheses that could explain this depletion are (a) catalog bias associated with near-fault attenuation or detection problems, and (b) a decrease in minimum cutoff magnitude due to a decrease in critical slip $D_{c}$ with distance from the fault plane.
DE: 7230 Seismicity and seismotectonics
SC: Seismology [S]
MN: 2004 AGU Fall Meeting