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Poster Abstracts
triggering subevents around the hypocenter by dynamic and static stress transfer. Resolving the details of this process should
help in understanding what triggered the large shallow slip event.
DYNAMIC TRIGGERING EVENTS IN HIDA REGION BY THE P WAVE FROM THE 2011 TOHOKU
EARTHQUAKE (A-107)
T. Uchide
Seismicity rates in the Hida region of central Japan, located about 400 km west of the source area of the 2011 M9 Tohoku
earthquake, significantly increased at a time coincident with the passage of the Tohoku's P-wave. These local events are clearly
identifiable in 8 Hz high-pass filtered Hi-net data in the region. Here, I systematically search for P-wave triggered events in
the Hida region using a matched filter analysis. I examine data spanning 600 s before to 600 s after the origin time of the
Tohoku mainshock, which nets 887 events, of these 82% occurred on or after the Tohoku P-wave arrival. I hypothesize the
passage of the mainshock generated P-wave played an important role in raising the seismicity rate in the Hida region. Using
50s of waveform data, starting at the time of the mainshock's' P-wave arrival, I estimate the dynamic Coulomb stress change
induced by the M9 mainshock reaches a maximum level of 1 - 10 kPa in the Hida region. This value, although on the low end,
is comparable to other triggering thresholds. The immediacy of remote earthquake triggering at the time of the P-waves
passage indicates secondary triggering processes, such as fluid movement or stress recalibration, are not necessarily required
to remotely triggered earthquakes in this region.
SAN BERNARDINO GPS NETWORK: TIME SERIES THROUGH 2010 (A-049)
E. Upton, S. McGill, R. Bennett, J. Spinler, and A. Torrens-Bonano
As the Pacific plate slides past the North American plate along the San Andreas Fault (SAF) it exerts stress upon the fault.
Although there has been no major earthquake in San Bernardino along the SAF in the past two centuries, the plates are in
constant motion, moving a small amount every year. Yearly Global Positioning System (GPS) monitoring at benchmark sites
around the SAF and other faults allows us to calculate the velocity of the Pacific plate in relation to stable North America.
These calculations allow us to test hypotheses regarding the slip rate of the SAF as well as other faults. This year we
conducted GPS monitoring at 25 locations. The data collected this summer is currently being processed at the University of
Arizona. This poster summarizes the data collected in 2010 and previous years into a set of time series plots for 34 locations
around the San Bernardino area. Our analysis shows the horizontal motion of the benchmarks is bounded by a minimum of
11.6 mm/yr (Site 7211, in the San Bernardino Mountains) and a maximum of 33.8 mm/yr (Site NORC, in Norco). Our results
also show a distinct northwesterly trend of movement for stations in the San Bernardino GPS Network. The benchmarks
display a direction of motion of between N17.6˚W (Site RICU, near Landers) and N56.7˚W (Site LACY, near Hemet).
Furthermore, there is a noticeable increase in plate velocity the farther to the west as one moves from the North American
plate onto the Pacific plate. Although they lie on a stable plate, there are locations on the North American plate are moving in
the direction of the Pacific plate—northwest—due to the deformation of the plate boundaries along the locked SAF. While
sites on the stable North American plate are moving to the northwest, sites located on the Pacific plate are moving to the
northwest at greater velocities. Site NORC, which is the westernmost site of our survey, has a horizontal velocity of 33.8
mm/yr. In comparison, site RICU, located the farthest to the east, shows a horizontal motion of 16.2 mm/yr. Both sites are
moving to the northwest but the locations on the Pacific plate are moving at a greater speed.
AUTO-ACOUSTIC COMPACTION IN STEADY SHEAR FLOWS: EXPERIMENTAL EVIDENCE FOR
SUPPRESSION OF SHEAR DILATANCY BY INTERNAL ACOUSTIC VIBRATION (A-089)
N.J. van der Elst and E.E. Brodsky
Granular shear flows are intrinsic to many geophysical processes, ranging from landslides and debris flows to earthquake
rupture on gouge-filled faults. The rheology of a granular flow depends strongly on the boundary conditions and shear rate.
Earthquake rupture involves a runaway transition from quasi-static to rapid shear rates. Understanding the rheology of the
granular flow in this transition is therefore crucial for understanding the rupture process and the coseismic strength of faults.
Here we explore this transition experimentally using a commercial torsional rheometer. We measure the dilatation of a steadystate
shear flow at velocities ranging between 10 -3 and 10 2 cm/s, and observe that dilatation is suppressed at intermediate
velocities (0.1 - 10 cm/s) for angular particles, but not for smooth glass beads. The maximum reduction in thickness is on the
order of 10% of the active shear zone thickness, and scales with the amplitude of shear-generated acoustic vibration. By
examining the response to externally applied vibration, we confirm that the intermediate-velocity compaction represents a
feedback between internally generated acoustic vibration and flow rheology. We link this phenomenon to that of acoustic
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