High-Frequency Nonlinear Simulations of Southern San Andreas Earthquake Scenarios

Daniel Roten, Kim B. Olsen, Steven M. Day, & Yifeng Cui

Published August 15, 2016, SCEC Contribution #6716, 2016 SCEC Annual Meeting Poster #267

The Southern San Andreas fault (SAF) is particularly likely to host a large earthquake in southern California, with a 19% chance of producing at least one M 6.7 earthquake during the next 30 years (UCERF3). Large-scale computational efforts to simulate large (M 7.5+) earthquakes on the SAF have shown that predicted ground motions are sensitive to 3D basin effects, dynamics of the seismic source (rupture speed, directivity) and nonlinear behavior in the fault zone and on soft sedimentary fills. However, as previous simulations of large San Andreas earthquake scenarios were typically limited to frequencies of up to 1 or 2 Hz, synthetic ground motions were only relevant for structures vulnerable to long-period ground motions, in particular high-rise buildings.

Here, we simulate M 7.7 earthquakes on the southern San Andreas fault with a spatial resolution of 25 m, which allows us to resolve frequencies up to 4 Hz using a minimum shear-wave velocity of 500 m/s. We use a two-step method: First, dynamic rupture on a planar, vertical fault roughly following the surface trace of the SAF between Indio and Lake Hughes is simulated for a compact computational domain using AWP-ODC CPU. Second, seismic waves propagating from this source are simulated within a larger box size including the densely populated Los Angeles basin (LAB) using AWP-ODC GPU. Both simulation steps are performed for a linear medium and a non-linear medium governed by Drucker-Prager plasticity. Because plasticity absorbs part of the seismic moment in the fault zone, we use a fault boundary condition to impose near-fault particle velocities obtained during dynamic simulation (step 1) in the kinematic simulation (step 2). The mesh is defined by SCEC CVM-S4, including a geotechnical layer, small-scale heterogeneities and frequency-dependent quality factors.

Spectral accelerations at 2s (2s-SAs) frequently exceed 0.5g near the fault and in the Los Angeles basin in the linear case. Nonlinearity reduces long-period ground motions in these locations by 25 to 50%. At higher frequencies, plastic effects are most pronounced within ~20 km from the fault, where 1s-SAs are reduced by up to 50% and 0.5s-SAs by up to 75% with respect to linear simulations. However, accounting for plasticity brings synthetic 0.5s-SAs in agreement with ground motion prediction equations (GMPEs) at these fault distances, while linear 0.5s-SAs exceed GMPEs by more than one standard deviation.

Citation
Roten, D., Olsen, K. B., Day, S. M., & Cui, Y. (2016, 08). High-Frequency Nonlinear Simulations of Southern San Andreas Earthquake Scenarios. Poster Presentation at 2016 SCEC Annual Meeting.

Related Projects & Working Groups
Ground Motion Prediction (GMP)