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Poster Abstracts
damage and alteration as more slip occurs. Spring flow increases post seismic events, and we believe by monitoring fluid
chemistry and comparing seismicity along the faults we will see precursors to and effects from fault motion.
TELESEISMIC P WAVE CODA OF SUBDUCTION EARTHQUAKES FROM WAVE INTERACTION NEAR
TRENCHES (B-059)
W. Wu and S. Ni
Teleseismic P waves are essential for imaging rupture processes of great earthquakes, either in the back projection method or
in finite fault inversion method involving of quantitative waveform modeling. In these studies, P waves are assumed to be
direct P waves generated by localized patches of the ruptured fault. However these methods fail to produce accurate rupture
models when some portion of the P waves are later arrivals generated by structural complexities such as mantle
discontinuities or rapid lateral variation. For ~M7 earthquakes near the 2011 M9 Tohoku-Oki earthquake, we observed strong
long period (10~20s) signals between P and PP on seismic network in eastern and central US. These P wave coda signals show
strong coherence and are frequency dependent. The coda's amplitudes are comparable with those of the direct P wave in the
10-20s band and are much weaker in shorter period. With array analysis, we find that the coda's slowness is very close to that
of the direct P wave, suggesting that they are generated near the source region. There have been reports of the coherent P
wave coda interpreted as the P wave converted or reflected in the upper mantle discontinuities, but their amplitudes should
be much smaller than those of the direct P wave. As the earthquakes occur near the Japan trench featuring rapid variation in
bathymetry, we hypothesize that the coda waves are converted from surface waves at the trench.
To investigate the effects of the bathymetry on P wave coda, we apply ray theory to interface full wave field from spectralelement
simulation to get the teleseismic P waves. With this approach, computation efficiency is greatly improved. The
simulation results demonstrate that the surface wave can be severely scattered by the trench structure and coherent P waves
are generated. We find that the coda's amplitudes are mainly controlled by the focal mechanism and depth. The frequency
dependent feature from our simulation is also consistent with the observation that P coda is strongest in the 10-20s band. Thus,
the coherent long period P codas are confirmed to be generated by the rapid variation of bathymetry near the trenches. Our
study argues that the topography/bathymetry effect should be taken into account when imaging rupture processes of mega
thrust earthquakes with teleseismic P waves.
PROPERTIES OF INELASTIC YIELDING ZONES GENERATED BY IN-PLANE DYNAMIC RUPTURES (A-079)
S. Xu, Y. Ben-Zion, and J.-P. Ampuero
We investigate the spatial distribution and intensity of off-fault yielding, the local orientation of the expected microfractures
inside the yielding zone, and scaling relations or correlations among different measurable quantities. The study employs
simulations of in-plane dynamic ruptures on a frictional fault governed by slip-weakening and rate-and-state friction laws,
with a Mohr-Coulomb type off-fault inelastic response and a possible contrast of elasticity across the fault. The results indicate
in agreement with previous studies that the location and spatial pattern of the yielding zones are strongly affected by the
angle \Psi of the background maximum compressive stress relative to the fault. The yielding extent depends strongly on
rupture mode (crack- or pulse-like), the seismic S ratio representing the initial stress level relative to failure, and the value of
rock cohesion c. For cracks, the yielding zone thickness T linearly increases with the rupture distance L and the ratio of T/L is
inversely proportional to the square of S. For pulses, the local T positively correlates with the maximum slip velocity at the
point, while the average T along-strike approaches a constant for a range of S values (although it can change for other S levels).
The yielding zone thickness for both rupture modes is reduced by increasing c. The magnitude of plastic strain decays
logarithmically with fault normal distance, while its maximum value close to the fault approaches a constant along strike as
the rupture speed is gradually stabilized. The local angle to the fault of the expected microfractures (assumed to be tensile)
inferred from the distributed plastic strain is generally shallower and steeper than \Psi in the compressional and tensional
quadrants, respectively. With a velocity contrast across the fault, ruptures can have preferred propagation direction that is the
same as the slip in the compliant medium (positive direction) or opposite propagation direction, depending on a competition
between dynamic changes of normal stress across the bimaterial interface and generation of off-fault yielding. With large
values of \Psi representing large strike-slip faults, the off-fault yielding enhances the bimaterial effect and promotes
propagation in the positive direction, with larger rupture and slip velocities, and with a more prominent off-fault yielding
zone.
FULL-3D WAVEFORM TOMOGRAPHY FOR NORTHERN CALIFORNIA USING AMBIENT-NOISE CROSS-
CORRELATION GREEN'S-FUNCTIONS (B-042)
254 | Southern California Earthquake Center