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Poster Abstracts
movies of the propagation of the seismic waves across the region. Deviation from the concentric propagation pattern expected
in a 1D velocity model highlights abrupt structural transitions, such as known and concealed faults, dipping layers, or lateral
changes in velocity. Also shown are the arrivals and relative amplitudes of various seismic phases as they propagate across
the network of seismometers. The movies will ultimately guide the development of the 3D seismic velocity model by
identifying the locations and possible types of geological structural disturbances.
EARTHQUAKE MOTIONS RECORDED ON A DENSE 5000-STATION NETWORK IN LONG BEACH, CA (B-
048)
M. Barba, R.W. Clayton, and D. Hollis
We present detailed maps and movies of small local earthquakes recorded by a dense seismic network in Long Beach,
California. The network consists of approximately 5,000 short-period, vertical sensors spaced an average 100m apart in a 7 by
10 km area. The autonomous network recorded continuously (24 hours/day) for a period of 6 months. The results show local
basins and faults, such as the Newport-Inglewood Fault (NIF), strongly focusing and diffracting wave fields, suggesting the
existence of fault strands diverging from NIF. The apparent amplification varies significantly over the region. It is also clear
that the coda has waves travelling in a wide variety of directions away from the direct path to the epicenter. By combining the
maps of peak acceleration from a number of earthquakes, a detailed map of ground amplification can be constructed.
A PHYSICAL MODEL OF THE MW 6 EARTHQUAKE CYCLE AT PARKFIELD (A-096)
S. Barbot, N. Lapusta, and J.P. Avouac
Advances in geophysical monitoring now provide an extensive set of observations of all aspects of the earthquake cycle. Yet,
unifying physical models that connect these observations into a coherent picture are lacking. At the same time, laboratory
experiments and theoretical developments provide an increasingly detailed understanding of the fault physics, offering the
basis for an extrapolation to natural conditions.
In this study, we bridge the gap between observations and fault physics by developing the first model of the full earthquake
cycle that explains a number of interesting and robust observations of the crustal dynamics at Parkfield. Despite the
similarities between the repeating Mw 6 earthquakes and their short recurrence times (from 12 to 32 years for 5 events until
1966), the latest rupture of 2004 defied the odds by taking place a decade later than anticipated and initiating at the south end
to propagate northward, contrarily to all previous events.
We build our model of fault friction using the spatial patterns of microseismicity, the time series of GPS displacements in the
1999-2010 period, the InSAR data, and the GPS offsets of the 2004 earthquake. We also consider the slip distribution of the
1966 event and the historical catalog of recurrence times and hypocenter locations. We show the special role of microseismicity,
which marks the transition between stable and unstable friction and circumscribes the seismogenic zone. We use the program
BICYLE (Boundary Integral Cycles of Earthquakes) of Lapusta & Liu (2009) to solve the elasto-dynamic equations that govern
the fault slip evolution. We obtain a sequence of Mw~6 earthquakes that can explain the observed variability of hypocenters
and reproduce the geodetic observations of surface deformation in the co- and postseismic periods associated with the 2004
event. The change of hypocenter between 1966 and 2004 and the delay of the latest event is consistent with the occurrence of a
swarm of smaller-magnitude earthquakes during the 1992-1994 period and these two locations being close to the boundary of
the seismogenic zone. Our study introduces a methodology capable of integrating seismological and geodetic observations
into a coherent physical model of the earthquake cycle. Our approach can serve as an important tool to investigate the effect of
other components of earthquake physics and to help understand and mitigate seismic hazards around active faults.
CO-LOCATED PORE PRESSURE AND VOLUMETRIC STRAIN AT PLATE BOUNDARY OBSERVATORY
BOREHOLES (A-070)
A.J. Barbour and D.C. Agnew
To establish the viability of pore-pressure instrumentation as a measurement of volumetric deformation in the solid rock
matrix, we present initial studies, using Plate Boundary Observatory data, of the relationship between apparent areal strain
and pore pressure in the same borehole. The areal strains are given by Gladwin borehole tensor strainmeters (BSM) (and in
one case by longbase laser strainmeters). The pore-pressure is measured in a partially sealed, sand-packed region in an
uncased section of the borehole containing the BSM. We find parameters relating pore pressure to atmospheric loading and
tidal strains. Although these forces explain much of the pressure signal, residual pressures not associated with earthquakes
have significant other changes (daily rms signal on the order of 1.5 hPa) attributable to a delay between stress and strain. As
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