The Effect of Fault Geometry and Minimum Shear Wavespeed on 3D Ground-Motion Simulations for an Mw 6.5 Hayward Fault Scenario Earthquake, San Francisco Bay Area, Northern California

Abstract

We investigated the effects of fault geometry and assumed minimum shear wavespeed (VSmin) on 3D ground-motion simulations (0–2.5 Hz) in general, using a moment magnitude (Mw) 6.5 earthquake on the Hayward fault (HF). Simulations of large earthquakes on the northeast-dipping HF using the U.S. Geological Survey (USGS) 3D seismic model have shown intensity asymmetry with stronger shaking for the Great Valley Sequence east of the HF (hanging wall) relative to the Franciscan Complex to the west (footwall). We performed simulations with three fault geometries in both plane-layered (1D) and 3D models. Results show that the nonvertical fault geometries result in larger motions on the hanging wall relative to the vertical fault for the same Earth model with up to 50% amplifications in single-component peak ground velocity (PGV) within 10 km of the rupture. Near-fault motions on the footwall are reduced for the nonvertical faults, but less than they are increased on the hanging wall. Simulations assuming VSmin values of 500 and 250 m/s reveal that PGVs are on average 25% higher west of the HF when using the lower VSmin, with some locations amplified by a factor of 3. Increasing frequency content from 2.5 to 5 Hz increases PGV values. Spectral ratios of these two VSmin cases show average amplifications of 2–4 (0.5–1.5 Hz) for the lower VSmin west of the fault. Large differences (up to 2×) in PGV across the HF from previous studies persist even for the case with a vertical fault or VSmin of 250 m/s. We conclude that assuming a VSmin of 500 m/s underestimates intensities west of the HF for frequencies above 0.5 Hz, and that low upper crustal (depth <10 km) shear wavespeeds defined in the 3D model contribute most to higher intensities east of the HF.

Variability of near‐fault seismic risk to reinforced concrete buildings based on high‐resolution physics‐based ground motion simulations

“…The inclusion of rupture scenarios with distinct large-slip patches is thus an important consideration in structural demand simulations. Further details about the rupture model can be found in Graves and Pitarka, 25 and validations of the generated ground motions are available in Pitarka et al 34 and Rodgers et al 26 The computational domain spans 100 km × 40 km × 30 km, with a minimum grid spacing of 8 m, and incorporates 3D geological features and a flat earth surface. The area north of the fault contains a shallow sedimentary basin (600 m deep with an approximately 90-degree basin-edge dip angle) and that south of the fault is rock, as shown in Figure 2(A) and (B).…”

“…Further details about the rupture model can be found in Graves and Pitarka, 25 and validations of the generated ground motions are available in Pitarka et al 34 . and Rodgers et al 26 …”

“…We utilize the highly dense simulated ground motion dataset to characterize the regional‐scale variation of risk to low‐ and mid‐rise modern reinforced concrete (RC) buildings subjected to shallow crustal earthquakes. The ground motion simulations are generated as part of the EQSIM (Earthquake Simulation Framework for Physics‐Based Fault‐to‐Structure Simulations) application development under the U.S. Department of Energy Exascale Computing Project, 26,27 and employ HPC to span a 100‐km 40‐km regional‐scale domain and resolve frequencies up to 5 Hz. The structural models of the buildings are capable of simulating the expected nonlinear behavior, including strength and stiffness degradation, of reinforced concrete frames up to collapse.…”

“…In this study, we compute broadband (0-5 Hz) ground motions for a canonical 3D basin model using a fully deterministic wave propagation modeling approach, and the kinematic rupture generator of Graves and Pitarka. 25 We utilize the highly dense simulated ground motion dataset to characterize the regional-scale variation of risk to low-and mid-rise modern reinforced concrete (RC) 26,27 and employ HPC to span a 100-km × 40-km regionalscale domain and resolve frequencies up to 5 Hz. The structural models of the buildings are capable of simulating the expected nonlinear behavior, including strength and stiffness degradation, of reinforced concrete frames up to collapse.…”

Exascale applications: skin in the game

“…Second, as the ability to compute at higher frequencies progresses, better characterization of subsurface geologic structure at finer and finer scales is needed, thus a companion schema for representing fine-scale geologic heterogeneities in massive computational models must be developed. For the purpose of evaluating regional-scale simulations and assessing progress, a representative large regional-scale model of the US San Francisco Bay Area is being targeted that includes all necessary geophysics modelling features (three-dimensional geology, earth surface topography, material attenuation, nonreflecting boundaries, fault rupture models) [50]. For a 10 Hz simulation, the computational domain requires 200-300B grid points as a basis for testing and evaluating advanced physics algorithms and implementations.…”