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Poster Abstracts
events than reverse faulting events while the opposite holds for M>2.5 events. A comparison of 23,000 common earthquakes
shows our results generally agree with the focal mechanism catalog obtained by Hardebeck and Shearer (2003). Using 211
moment tensor solutions in Tape et al (2010) as benchmarks, we compare the focal plane rotation angles of common events in
the catalog. 69% of common earthquakes in both catalogs match well, with rotation angles less than 35°. The common events
with relatively large rotation angles are either located around the edge of the SCSN network or poorly recorded.
PRELIMINARY RESULTS ON THE ANALYSIS OF THE DEVIATORIC STRESS FIELD FROM FOCAL
MECHANISMS AND SLIP-LINE FIELD FROM CONJUGATE LINEAR SEISMICITY CLUSTERS IN
SOUTHERN CALIFORNIA (B-139)
W. Yang and E. Hauksson
We analyze the spatial and temporal variations of the deviatoric stress field and the geometric properties of conjugate linear
seismicity clusters in the southern California region using earthquake data recorded by the SCSN from 1981 to 2011. Using a
data set with about 179,000 high quality focal mechanisms that were determined from P first motions and S/P amplitude
ratios, we invert for the variation in the stress field in time and space using the SATSI method. The inversion results match
with results from prior studies with a predominant NS to NNE orientation of maximum horizontal stress, but our results
reveal the stress field at higher level of resolution because we use a larger data set than was available before. The results show
that the stress field exhibits minimal regional temporal variations, but some variations exist in small areas close to large
mainshocks. Similarly, localized depth variations in the stress field suggest the possible existence of vertical strain partitioning.
We determine a data set with approximate 8,000 seismicity clusters using waveform cross-correlation and a clustering method.
We calculate the geometrical properties of clusters. Pairs of conjugated linear clusters exist across southern California. The
angles between conjugate linear clusters in the direction of maximum shortening range from 80 [deg] to 160 [deg], with a
median value of 120 [deg] and a mode value of 115 [deg]. The bisection of conjugate linear clusters in the direction of
maximum shortening generally match with the maximum horizontal stress orientation, which implies a relation between
orientation of the stress field and the orientation of clusters. We infer that the variation of conjugate angles is associated with
brittle and ductile deformation, and apply slip-line field theory to interpret such observations. Overall, our observations
provide new understanding of the kinematic crustal process in southern California.
COMPLEX RUPTURE MODES OF THE 2011 MW 9.0 TOHOKU-OKI EARTHQUAKE INFERRED FROM
TELESEISMIC AND LOCAL WAVEFORMS (B-037)
H. Yao
The 2011 Mw 9.0 Tohoku-Oki earthquake occurred in the offshore Tohoku region in Japan, the Western Pacific subduction
zone. It had very large coseismic slip (30-40 m) in the up-dip region close to the trench and tens of thousands of aftershocks
dominantly in the down-dip region close to the coast. We have analyzed broadband teleseismic P-wave data from about 500
stations in the central and western US as well as local strong-motion waveforms from F-net and KiK-net in Japan to infer the
rupture characteristics. We apply the newly developed time-domain iterative back-projection (IBP) method (with subevent
waveform stripping and subevent relocation) to investigate the spatiotemporal distribution of subevents (P-wave energy
radiation) in the source region in two frequency bands (0.2-1 and 0.05-0.2Hz). We also use a new frequency-domain method
for sparse source inversion, compressive sensing (CS), to study the spatiotemporal distribution of P-wave energy radiation in
four frequency bands between 0.05 and 1 Hz. In addition, we resolve slip details from finite-fault slip inversion of strong
motion data from F-net and KiK-net in the low frequency band (0.005-0.05 Hz). Our results suggest a complicated rupture
process, which is not well described by a constant velocity rupture from the hypocenter. Our results suggest that most highfrequency
seismic energy was radiated near the epicenter for at least the first 80 s. Although the rupture was bilateral,
significant southward rupture mainly occurred after 100 s, as seen from the spatiotemporal distribution of subevents or
energy radiation in both low and high-frequency bands. Our analyses show that the earthquake has frequency-dependent
rupture modes. Radiated high-frequency energy mainly came from the down-dip region, which is similar to the aftershock
distribution pattern. However, radiated low-frequency energy was dominant in the up-dip (offshore) region, which is
consistent with much larger slip in the up-dip region from the low-frequency seismic slip inversion. This frequency-dependent
rupture may be caused by differences in rupture behavior (more intermittently at high frequencies and more continuous at
low frequencies) at the plate interface due to heterogeneous friction properties.
STRAIN PARTITIONING IN LOS ANGELES (A-123)
R.S. Yeats and D. Verdugo
256 | Southern California Earthquake Center