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Poster Abstracts
COMPOUND EARTHQUAKES ON A BIMATERIAL INTERFACE AND IMPLICATIONS FOR RUPTURE
MECHANICS (B-040)
E. Wang and A.M. Rubin
Rubin and Ampuero [2007] simulated slip-weakening ruptures on a 2-D (line fault) bimaterial interface and observed
differences in the timescales for the two edges to experience their peak stress after being slowed by barriers. The barrier on the
"negative" side reached its peak stress when the P-wave stopping phase arrives from the opposite end, which takes ~20 ms for
a 100 m event. This may be long enough for a potential secondary rupture to be observed as a distinct subevent. In contrast,
the same timescale for a barrier at the "positive" front is nearly instantaneous, possibly making a secondary event there
indistinguishable from the main rupture. Rubin and Gillard [2000] observed that 5 of a family of 72 similar earthquakes along
the San Andreas fault in Northern California are compound and in all cases the second event was located on the negative
(NW) side of the main event. Based on their simulations, Rubin and Ampuero interpreted this as being due to the abovementioned
asymmetry in the dynamic stressing-rate history on the two sides of a rupture on a bimaterial interface.
In this study, we search more systematically for secondary arrivals within 0.15 s of the first P arrival for microearthquakes on
the San Andreas. Using an iterative deconvolution method described in Kikuchi & Kanamori [1982], we deconvolve the first
0.64 s following the P arrival of a target event using a nearby Empirical Green's Function (EGF). When the EGF is a simple
earthquake and the target is compound, the deconvolution result is expected to show two positive spikes, corresponding to
the main and secondary events. A subevent is considered robust only when the difference between the waveforms of the
target event and the aligned and scaled EGF is similar enough to the EGF at multiple stations. The azimuthal consistency of
delays between the main and secondary arrivals is more convincing evidence that the target is a compound event.
Using these criteria we temporarily identified ~70 compound events out of ~8300 in our catalog. Future work will include
improving the quality of the inter-event delay time by using Monte Carlo simulations to allow the amplitudes and arrival
times of both spikes (as opposed to just the second spike) to vary. Accurate relative locations and times can improve our
understanding of the triggering mechanism of the subevents and perhaps the longer-timescale aftershock asymmetry
observed in this region as well.
AUTOMATIC DETECTION OF SHALLOW FAULT CREEP IN INSAR DATA (A-065)
M. Wei, D. Sandwell, and J. McGuire
Fault creep in Southern California mostly occurs at the shallow part of a fault with clear surface breakage. Monitoring and
observing these shallow fault creep is important for regional seismic hazard assessment and improving our understanding of
fault mechanics. Many fault creep occurred in remote area where no other observation instruments nearby. Even on faults
with creepmeters, partial creep on sections that do not coincide with the creepmeter becomes undetectable. InSAR has been
used to study fault creep in this region. However, as the volume of InSAR data has increased dramatically, over ~1000 scenes
in California, and will continue to increase as future missions such as DESDynI start, searching for fault creep manually in
InSAR data becomes time consuming and unrealistic. An automatic detector of fault creep in InSAR data is called for.
Here, we developed an automatic detector using pattern recognition algorithm developed for road detection in SAR data to
find shallow fault creep in InSAR data. The main workflow includes: process InSAR data, make gradient image of the phase
image, detect linear features, and report possible fault creep. We tested this detector in Salton Trough with ERS, ENVISAT and
ALOS data. Our method successfully detected most fault creep, mainly on the Superstition Hills Fault and San Andreas Fault.
Limitations of the method include: prefer flat area with little sharp topography change; only be able to detect displacement
larger than ~1 cm; can't detect deep fault creep. Nevertheless, this detector is a useful tool for monitoring shallow fault creep.
REGIONAL BROADBAND WAVEFORM MODELING USING UPGOING AND DOWNGOING WAVE FIELDS
(A-116)
S. Wei and D. Helmberger
3D crustal structure usually complicates the modeling of regional waveforms. The Cut and Paste (CAP) waveform inversion
method, on the other hand, is much less sensitive to velocity models and lateral crustal variation, because it breaks the
waveforms into Pnl and Surface waves and inverts them separately, allowing different time shifts to align the data and the
synthetics for different portions of the waveform. These time shifts represent the difference between the 1D structure and the
real earth. Similar idea is applied to the upgoing and downgoing wave fields, where we use different time adjustments for
upgoing and downgoing waveforms to improve the broadband waveform fits. Furthermore, different source-time-functions
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