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Poster Abstracts | Group 2 – Seismology
of the Kanamori and Rivera scaling parameter e is between 0 and 0.3 indicating no resolvable
dynamical differences between magnitude 3 and magnitude 7 crustal earthquakes
2-112
FINITE FAULT MODELING FOR MODERATE-SIZED EARTHQUAKES (M ~ 5) IN
SOUTHERN CALIFORNIA Shao G, Ji C, and Hauksson E
We develop a finite fault inverse system to routinely study the rupture process of moderate-sized
earthquakes within the southern California. The slip model is constrained by matching the body
waves recorded at local stations. We calculate the earth response with the 1D smoothing SoCal
model but correct the 3D structure effects on the body waves by adding time shifts and frequency
dependent amplitude corrections [Tan and Helmberger, 2007]. We have constructed a table of the
3D time-correction for each stations based on Hauksson's update 3D Southern California model,
using a 3D finite difference algorithm [Vidale, 1990]. We construct the amplitude corrections based
on the analysis of a magnitude ~ 4 aftershock with known source mechanism. We test the method
using recent Mw 5.4 Chino Hills earthquake.
2-113
EARTHQUAKE STRESS DROPS AND INFERRED FAULT STRENGTH ON THE
HAYWARD FAULT Hardebeck JL, and Aron A
The Hayward Fault and other faults of the East Bay provide an opportunity to study variations in
earthquake stress drop with depth, faulting regime, creeping versus locked behavior, and the
strength of the wall rocks on either side of the fault. We use the displacement spectra from borehole
seismic recordings of 529 M1.0-4.2 earthquakes in the East Bay to estimate stress drop using an
empirical Green's function method (Shearer et al., 2006). The median stress drop is 8.7 MPa, and
50% of stress drops are between 3.2 MPa and 25 MPa. Several lines of evidence indicate that stress
drop is controlled by the applied shear stress, even though the median stress drop values are
significantly less than the theoretical shear stress assuming strong faults (Byerlee's law) and
hydrostatic pore pressure. There is a trend of increasing stress drop with depth, with median stress
drop of about 5 MPa for 1-7 km depth, about 10 MPa for 7-13 km depth, and about 50 MPa deeper
than 13 km. Higher stress drops are observed for a deep cluster of thrust-faulting earthquakes near
Livermore than for a deep cluster of strike-slip events on the Calaveras Fault. The changes in stress
drops with depth and faulting regime imply that stress drop is related to the applied shear stress.
We compare the spatial distribution of stress drops on the Hayward Fault to models of creeping
versus locked behavior of the fault, and find that high stress drops are concentrated around the
major locked patch near Oakland. This also suggests a connection between stress drop and applied
shear stress, because the locked patch might be expected to experience higher shear stress as a
result of either the difference in cumulative slip or the presence of higher-strength material.
Comparison of stress drops with the wall-rock geology at depth does not show a correlation
between stress drop and rock strength, suggesting that the fault strength is not directly related to
the strength of the wall rock.
2-114
OVER TWO ORDERS OF MAGNITUDE VARIATION ARE DETERMINED SOLELY
BY SLIP FOR A GROUP OF SMALL EARTHQUAKES ON THE SAN ANDREAS
FAULT NEAR PARKFIELD, CA Harrington RM, and Brodsky EE
Ordinarily, earthquake magnitude is controlled by both rupture length and slip variation. Here we
show that a special population of earthquakes has a constant rupture length, but varying slip. We
compare the source time function pulse widths of 25 earthquakes on the San Andreas Fault, and 11
202 | Southern California Earthquake Center