Ground-Motion Modeling of Hayward Fault Scenario Earthquakes, Part II: Simulation of Long-Period and Broadband Ground Motions

Abstract

We simulate long-period (T > 1:0-2:0 s) and broadband (T > 0:1 s) ground motions for 39 scenario earthquakes (M w 6.7-7.2) involving the Hayward, Calaveras, and Rodgers Creek faults. For rupture on the Hayward fault, we consider the effects of creep on coseismic slip using two different approaches, both of which reduce the ground motions, compared with neglecting the influence of creep. Nevertheless, the scenario earthquakes generate strong shaking throughout the San Francisco Bay area, with about 50% of the urban area experiencing modified Mercalli intensity VII or greater for the magnitude 7.0 scenario events. Long-period simulations of the 2007 M w 4.18 Oakland earthquake and the 2007 M w 5.45 Alum Rock earthquake show that the U.S. Geological Survey's Bay Area Velocity Model version 08.3.0 permits simulation of the amplitude and duration of shaking throughout the San Francisco Bay area for Hayward fault earthquakes, with the greatest accuracy in the Santa Clara Valley (San Jose area). The ground motions for the suite of scenarios exhibit a strong sensitivity to the rupture length (or magnitude), hypocenter (or rupture directivity), and slip distribution. The ground motions display a much weaker sensitivity to the rise time and rupture speed. Peak velocities, peak accelerations, and spectral accelerations from the synthetic broadband ground motions are, on average, slightly higher than the Next Generation Attenuation (NGA) ground-motion prediction equations. We attribute much of this difference to the seismic velocity structure in the San Francisco Bay area and how the NGA models account for basin amplification; the NGA relations may underpredict amplification in shallow sedimentary basins. The simulations also suggest that the Spudich and Chiou (2008) directivity corrections to the NGA relations could be improved by increasing the areal extent of rupture directivity with period. Online Material: Comparison of ground-motion intensities from Hayward synthetics to NGA results.

The 2014 Mw 6.1 South Napa Earthquake: A Unilateral Rupture with Shallow Asperity and Rapid Afterslip

“…1). Thus, we extract site-specific 1D profiles from the 08.3.0 3D Bay area velocity model (Aagaard et al, 2010) to approximate the paths from the hypocenter to each of the various near-fault stations (Fig. 2a).…”

The Distribution of Red and Yellow Tags in the City of Napa

“…Thus, the lack of tags on the older Pleistocene fan to the north fits the expected amplification, but the lack of tags along the Napa River and on the west side of the valley does not. Figure 4 plots the red and yellow tags over the depth to bedrock inferred in the USGS Bay area 3D seismic-velocity model (Aagaard et al, 2010). The seismic-velocity model contains a sedimentary basin that underlies the City of Napa.…”

“…We compare these zones with the pre-1950 development of Napa, the recent alluvial geology of the Napa Valley (Witter et al, 2006), and the depth of the underlying sedimentary basin, as inferred from the Bouguer gravity anomaly (Jachens et al, 2006;Aagaard et al, 2010). The extent of the central damage zone is strongly correlated with the pre-1950 buildings and with the underlying sedimentary basin, but poorly correlated with the alluvial geology.…”

Serpentine: Finite Difference Methods for Wave Propagation in Second Order Formulation

“…This code has shown excellent scaling on up to 32,768 processors and has enabled seismic wave calculations with up to 26 Billion grid points. The WPP calculations have resulted in several publications in the field of computational seismology, e.g., [28,2,1,29].…”