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Group 2 – Tectonic Geodesy | Poster Abstracts
longer distance and reached the maximum slip of 8.5 m about 130 km northeast of the epicenter. It
was presumably the cause of huge damage at nearby Beichuan city. Northeast of Beichuan city, the
rupture on Beichuan fault significantly reduced, accompanying with the rupture of a large blind
thrust on Pengguan fault. The later apparently triggered the strike-slip motion on the last segement
of Beichuan fault at 180 km northeast of the epicenter at about 75 sec.
2-026
MODELING AND DETECTION OF SEISMIC SWARMS TRIGGERED BY ASEISMIC
TRANSIENTS Llenos AL, McGuire JJ, and Ogata Y
The rate of earthquake occurrence varies by many orders of magnitude in a given region due to
variations in the stress state of the crust. Of particular interest are variations in seismicity rate
triggered by transient aseismic processes such as fluid flow, fault creep or magma intrusion. While
these processes have been shown to trigger earthquakes, implementing an inversion algorithm that
can map seismicity variations into estimates of stress rate variations has been challenging.
Essentially aftershock sequences can obscure changes in the background seismicity rate resulting
from aseismic processes. Two common approaches for estimating the time dependence of the
underlying driving mechanisms are the stochastic Epidemic Type Aftershock Sequence model
(ETAS) (Ogata, 1988) and a physical approach based on the rate-and state-model of fault friction
(Dieterich, 1994). The models have different strengths that could be combined to allow more
quantitative studies of earthquake triggering. To accomplish this, we identify the parameters that
relate to one another in the two models and examine their dependence on stressing rate. A
particular conflict arises because the rate-state model predicts that aftershock productivity scales
with stressing rate while the ETAS model assumes that it is time independent. To resolve this issue,
we estimate triggering parameters for 4 earthquake swarms associated with geodetically observed
deformation transients in various tectonic environments. Our results suggest that stressing rate
transients increase the background seismicity rate without affecting aftershock productivity.
We can then specify a combined model for seismicity rate variations that can be used in a data
assimilation algorithm to invert seismicity catalogs for variations in aseismic stressing rates. For a
given earthquake catalog, we produce maximum likelihood estimates of the ETAS parameters and
use an extended Kalman filter to estimate the evolution of underlying state variables (background
stress rate, aseismic stress rate, and \gamma of the rate-state model). We have tested our algorithm
on the 2005 Obsidian Buttes earthquake swarm, which was triggered by geodetically-observed
shallow aseismic creep (Lohman and McGuire, 2007). Our algorithm successfully detects the
swarm and produces an estimate of the aseismic stressing rate transient that triggered it. Our
method therefore has the potential to be a highly sensitive detector of transient deformation.
2-027
COMBINING GPS AND METEOROLOGICAL DATA TO MITIGATE ATMOSPHERIC
PHASE IN INTERFEROGRAMS: THE SAN GABRIEL VALLEY, CALIFORNIA Funning
GJ, Houlié N, and Burgmann R
GPS and InSAR data both independently sample the troposphere state at the time of observation.
Given the recent proliferation of continuous GPS sites, the use of this redundant information to
characterise and remove tropospheric signals from InSAR data is becoming increasingly viable.
This capability is critical if transient deformation signals are to be identified with confidence.
Here we focus on data from southern California, where a dense continuous GPS network and a
diverse set of tectonic and nontectonic deformation sources make for an excellent test site. We
focus on an uplift transient that occurred in the San Gabriel valley, approximately 30 km NE of Los
2008 SCEC Annual Meeting | 151