Annual Meeting - SCEC.org

Poster Abstracts | Group 1 – EFP
range of magnitudes, in particular what conditions allow small earthquakes to grow into larger
ones and whether accelerating seismicity at a smaller scale presages larger events. Because we
employ a realistic geometry, use rate and state friction, and use a quasi-static approach to the
behavior during rapid slip, we hope that the behavior of the simulated sequence of events will be
as similar to that of real earthquakes as possible. Because a detailed time-space history of
microseismicity and larger events at Parkfield exists, it will be possible to make comparisons
between the properties of the simulated history of events and those of the actual events. The
smallest elements in the multiscale grid are 7 m in dimension, values small enough to represent a
continuum with laboratory values of Dc and the other constitutive parameters. The multiscale grid
is used so that only the areas having experienced earthquakes are represented by the smallest
elements. The largest elements are used in areas where microearthquakes do not occur, and these
are 200 m in dimension. A total model area 47 km along the San Andreas by 15 km deep that is
based on the observed distribution of 4966 microearthquakes has 1,464,433 elements. Running this
entire model for sufficient time steps, even with the Fast Multipole approach, is probably beyond
the range of existing computers. Consequently we are starting with subsets of the total area having
a range of sizes and numbers of actual earthquakes, to gain experience with the behavior of the
simulations. Note that although the distribution of constitutive parameters and of the smallest
elements may restrict the simulated earthquakes in the model to be spatially similar to actual
earthquakes at Parkfield, the time histories of the simulated earthquakes will occur spontaneously.
The space-time distribution of simulated seismicity is the output of the simulation that we plan to
analyze to see if detectable patterns of activity premonitory to larger events can be detected.
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COMPARISON OF A MULTI-CYCLE EARTHQUAKE SIMULATOR WITH A
DYNAMIC FINITE ELEMENT METHOD FOR A HETEROGENEOUS PLANAR FAULT
Stevens JA, Dieterich JH, Oglesby DD, and Richards-Dinger KB
RSQSim is an earthquake simulator that produces fast simulations of numerous earthquake
ruptures, up to 10^6 events in a simulation, in complex fault systems under the effects of rate- and
state-dependent friction. As part of an effort to validate the quasi-dynamic rupture propagation
aspects of RSQSim, we compare rupture propagation on a variable-strength planar fault in RSQSim
to that on a similar fault in DYNA3D (a fully dynamic finite element mode employing slipweakening
friction). Validation of the final slip and stress change distributions are important to
ensure that subsequent events in RSQSim will inherit the proper stress fields. In addition, if the
time evolution of ruptures in RSQSim is realistic, they may be useful as kinematic sources for
ground motion calculations.
By using fairly consistent values of physical variables in both codes, previous comparisons for
ruptures with uniform fault properties found that both methods produced very similar results. Our
asperity model consists of multiple rectangular zones of increased normal stress of varying size
and amplitude. In both modeling methods the heterogeneities produce complex ruptures - as the
rupture front encounters a barrier, it tends to wrap itself around the barrier and create a burst of
energy once it propagates across the barrier. Both codes allow rupture propagation over significant
zones of negative stress drop in these asperity regions. Rupture durations, average rupture
propagation speeds and overall slip pattern are quite similar with both methods. However,
ruptures with the fully dynamic method (DYNA3D) propagate more rapidly through the barriers,
and generate less high-frequency variations of slip than ruptures with the quasi-dynamic method
(RSQSim). Regardless, the qualitative agreement of these two very different methods is remarkably
good and may improve more with further tuning of quasi-dynamic computational parameters.
116 | Southern California Earthquake Center