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The Shakeout earthquake scenario with plasticity

Daniel Roten, Kim B. Olsen, Steven M. Day, & Donat Fäh

Published September 2013, SCEC Contribution #1891

A major challenge in seismic hazard assessment consists in the prediction of near-source ground motions resulting from large, rare earthquakes, which are not well represented in observed data. Realistic simulation of both the rupture dynamics and wave propagation during such events should be based on rheology models which are able to describe plastic yielding on and off the fault as well as nonlinear damping in soils near the surface. Towards this goal we have implemented the Drucker-Prager yield condition based on the return map algorithm in the highly scalable AWP-ODC finite difference code. We use the Shakeout earthquake scenario, using a kinematic source description (Graves et al., 2008), to study how plasticity on and off the fault and near the surface would affect ground motions during a M 7.8 earthquake on the Southern San Andreas fault.
Because the plastic material parameters required for the yielding model (i.e., friction angle and cohesion) are poorly constrained, we derive them from known geophysical properties defined in the SCEC community velocity model (CVM version 4). We experiment with different cohesion models, partly basing our choice on empirical equations developed for wellbore stability problems, which relate unconfined compressive strength (UCS) in sedimentary rock to Young's modulus. We also consider published laboratory results that characterize the UCS in the Tertiary sedimentary formations and basement rocks underlying the Los Angeles and San Gabriel basins.
We find that long-period ground motions in the downtown Los Angeles area, amplified by a wave guide of interconnected sedimentary basins, could be significantly reduced as compared to visco-elastic solutions, even for conservative values of cohesion. This reduction can mostly be attributed to plasticity on and off the fault. For the frequency range considered in our scenario (0 - 0.5 Hz), plastic yielding in the shallow low-velocity deposits of the Los Angeles basin becomes only important if the sediments are assumed to be nearly cohesionless.
These results indicate that more research is warranted into the dynamics of rupture and wave propagation in nonlinear media. They may also suggest a need to develop community structural models that contain information on plastic parameters based on geotechnical and geophysical data as well as visco-elastic parameters.

Roten, D., Olsen, K. B., Day, S. M., & Fäh, D. (2013, 9). The Shakeout earthquake scenario with plasticity. Poster Presentation at SCEC Annual Meeting 2013.