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Poster Abstracts
inversion that operates once per second. The forward approach runs a grid search over a suite of possible 2-D Gaussian slip
distributions. In all three approaches we are able to roughly characterize all three earthquakes using about a 2-3 minutes of
data, greatly enhancing the time to obtain fault slip and moment release during medium-to-large earthquakes by almost an
order of magnitude. We investigate gains made through combined analysis of GPS and seismic instruments (accelerometers)
within rapid modeling versus GPS-only modeling with respect to the aforementioned methodologies as well as peak ground
displacement scaling relationships.
DETECTING MISSING EARTHQUAKES ON THE PARKFIELD SECTION OF THE SAN ANDREAS FAULT
FOLLOWING THE 2003 MW6.5 SAN SIMEON EARTHQUAKE (B-069)
X. Meng, Z. Peng, and J.L. Hardebeck
Large shallow earthquakes are typically followed by increased seismic activity, known as “aftershocks”. However, whether
aftershocks are triggered by static or dynamic stress changes is still in debate. Previous studies on aftershock triggering mostly
utilize earthquakes listed in earthquake catalogs, which could be incomplete immediately after moderate to large earthquakes.
In this study, we apply the recently developed matched filter technique to detect missing microearthquakes along the
Parkfield section of the San Andreas Fault (SAF) around the occurrence time of the 2003 Mw6.5 San Simeon earthquake.
Previous studies have found the San Simeon mainshock induced ~10 kPa positive Coulomb stress changes on the SAF, which
is inconsistent with the observation of a decrease in seismicity rate around Parkfield immediately after the mainshock
according to Northern California Seismic Network (NCSN) catalog. Here we use waveforms of ~3000 earthquakes recorded
by 12 High Resolution Seismic Network (NRSN) stations around Parkfield as templates, and scan through the continuous data
48 hours before and 30 hours after the San Simeon mainshock. We band-pass filtered waveforms of 10-25 Hz to depress the
effects of large aftershocks from the San Simeon rupture. A total of 158 events are detected, of which only 8 are listed in the
NCSN catalog. The seismicity rate from the newly detected events shows a clear increase around Parkfield immediately after
the San Simeon mainshock. In comparison, swarm-like activity at south of Gold Hill started about 2 days before and turned off
immediately ~6 hours before the mainshock, which resulted in a decrease of seismicity rate. No detections are found further
north in the creeping section of the SAF either before or after the mainshock, despite the fact that there are many templates in
this region. Our observations suggest that the SAF near Parkfield was positively loaded by the San Simeon mainshock. This is
consistent with the Coulomb stress calculation and triggered right-lateral creep observed by the USGS creepmeters, although
we cannot rule out the possibility of dynamic triggering at this stage.
DIFFERENTIATING STATIC AND DYNAMIC TRIGGERING NEAR SALTON SEA FOLLOWING THE 2010
MW7.2 EL MAYOR-CUCAPAH EARTHQUAKE (B-070)
X. Meng, Z. Peng, and P. Zhao
Whether static or dynamic triggering is the dominant triggering mechanism in near field is currently under heated debate.
Previous studies on earthquake triggering mostly examined seismicity rate changes around the occurrence time of large
earthquakes based on existing earthquake catalogs. However, such catalogs could be incomplete immediately after the
mainshock, which may cause apparent seismicity rate changes that are unrelated to stress changes. In this study, we focus on
Salton Sea geothermal region following the 2010 Mw7.2 El Mayor-Cucapah earthquake, mainly because of its abundant
background seismicity, dense network coverage and being in the stress shadow of the mainshock, which is the key factor to
differentiate static and dynamic triggering. According to the Southern California Seismic Network (SCSN) catalog, the
seismicity rate near Salton Sea increased immediately after the mainshock. It dropped below the pre-mainshock level within a
few days and remained low for another few months. To check whether such patterns are caused by catalog incompleteness,
we are currently applying a waveform-based matched filter technique to detect possible missing events around Salton Sea.
With more completed catalog, we can identify the genuine seismicity rate changes for better comparison with the stress
changes. Static and dynamic stress changes could also affect seismic velocity in the upper crust in the way similar to seismicity
rate. Thus, monitoring seismic velocity can further help differentiate the triggering mechanisms. In our preliminary study, we
apply the ambient noise cross-correlation technique to monitor the seismic velocity near Salton Sea 10 days before and after
the mainshock. We find that immediately after the mainshock, seismic velocity reduced up to 0.4% and followed by a fast
recovery in the next three days. From 5 to 10 days after the mainshock, the velocity changes remained ~0.2% lower than the
pre-shock level. Such co-seismic decrease was most likely caused by the widespread damages induced by strong ground
motion, suggesting that dynamic stress changes were dominant in the short term. We are currently applying the same
technique at later times to check whether the velocity change returns back to the pre-mainshock or not.
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