Annual Meeting - SCEC.org

Poster Abstracts
little pre-shaking such as the 1896 Sanriku earthquake. The well recorded Tohoku-Oki earthquake sheds some new light on
these issues. Here we document the history of the rupture from the inversion of accelerometric waveform data and high-rate
GPS data. Our results demonstrate that the flooding was caused by the shallow ruptured (Mw~9) portion close to the trench
while the shaking came from ruptures at depth >30km with a relatively small magnitude (Mw~8.5). Several deep pockets of
high frequency energy release can be resolved, which correspond to locations predicted from back-projection. The Peak
Ground Velocity (PGV) recorded by over 1400 strong motions and ~400 high rate GPS stations is nicely correlated with
geological structure (topography). Separation of the wavefield into upgoing and downgoing energy indicates that the shaking
on the island is mainly controlled by upgoing energy. Forward calculation indicates that the shallow basin structure could
amplify the ground shaking and some of the high frequency arrivals could be explained by using a modified source time
function which suggests a pulse like rupture.
SENSITIVITY OF THE SOUTHERN SAN ANDREAS FAULT SYSTEM TO TECTONIC BOUNDARY
CONDITIONS (B-141)
J.W. Herbert and M.L. Cooke
The southern Big Bend of the San Andreas fault incorporates numerous non-vertical, non-planar, and intersecting active
surfaces. Our three-dimensional Boundary Element Models incorporate complex fault geometries from the Southern
California Earthquake Center’s Community Fault Model and a range of tectonic boundary conditions from Global Positioning
System studies of the region (45-50mm/yr and 320˚-325˚). We also test the sensitivity of fault slip rates to the magnitude and
orientation of plate velocity applied to the model. Consequently our models provide a range of fault slip rates associated with
present-day uncertainties on plate motion rather than a single slip rate. Model slip rates match most of the available geologic
slip rates and discrepancies may owe to inaccurate fault geometries. More northerly plate velocity (325˚) produces greater
transpression along the San Andreas Fault system. This is associated with greater uplift of the San Bernardino Mountains,
greater reverse-slip along range bounding reverse thrust faults, lower strike-slip rates along the San Andreas Fault and greater
strike-slip rates along the Eastern California Shear Zone and Garlock fault. These results suggest that the degree of regional
transpression controls the partitioning of deformation between local uplift, slip along the San Andreas system and slip along
the ECSZ. A northerly shift in plate velocity orientation could account for the abandonment of the Mill Creek strand of the San
Andreas fault ~120 ka.
FREQUENCY-MAGNITUDE DISTRIBUTIONS OF TRIGGERED AND NON-TRIGGERED POPULATIONS OF
SEISMICITY (B-108)
S. Hernandez and E.E. Brodsky
Large dynamic strains carried by seismic waves are known to trigger seismicity far from their source region. This effect has
been attributed to several mechanisms including Coulomb frictional failure and induced changes in fault strength and pore
pressures in the far field. As a harbinger of seismicity rate change however, it is as yet unconfirmed whether the passage of
dynamic surface waves can also induce societally significant earthquake changes. To test for such changes, we utilize the interevent
time ratio R to identify populations of seismicity that contain larger than average proportions of triggered earthquakes.
The inter-event time ratio, or R-ratio, is computed in California for the time period 1 Jan 1984 - 15 Apr 2011 and utilizes the
ANSS catalogue, nominally complete down to M2.1. In particular, R-ratios for spatial grids of 0.1x0.1 degrees are calculated
with respect to each potential far field trigger, defined as any event with imposed strain greater than 1e-8 and epicentral
distance greater than 800 km from source. The bias introduced by finite catalog effects is corrected using 2 techniques: 1)
directly estimating the deviation from homogenous (i.e., Poissonian) behaviour using randomized simulations of local
seismicity catalogs, and 2) restricting local seismicity catalogs to symmetric windows of 1, 2, and 5 years around each potential
far field trigger. We then measure the Gutenberg-Richter (GR) frequency-magnitude distributions for 2 broad classes of
seismicity: those that include many triggered events (R < 0.484, or ~10% rate change or greater) and those that include very
few or possibly negative rate change (R > 0.484). In general, we find that the distribution of b-values for both types of datasets
show significant overlap and persists when both techniques for eliminating our time-dependent finite catalog bias are applied.
For populations with low R, GR b-values are 1.13 +/- 0.15. For high R populations, GR b-values are 1.09 +/- 0.09. Additionally,
neither b-values nor mean maximum magnitude show any correlation with respect to calculated peak dynamic strain. In
terms of hazard estimates, our results indicate an inability to resolve societally significant induced changes in Gutenberg-
Richter frequency-magnitude distributions within California at various temporal scales.
STOCHASTIC SOURCE MODEL FOR SEISMIC HAZARD ESTIMATION (B-113)
176 | Southern California Earthquake Center