ALBERT

Description: We compute ground motions for the Beroza (1991) and Wald et al. (1991) source models of the 1989 magnitude 6.9 Loma Prieta earthquake using four different wave-propagation codes and recently developed 3D geologic and seismic velocity models. In preparation for modeling the 1906 San Francisco earthquake, we use this well-recorded earthquake to characterize how well our ground-motion simulations reproduce the observed shaking intensities and amplitude and durations of recorded motions throughout the San Francisco Bay Area. All of the simulations generate ground motions consistent with the large-scale spatial variations in shaking associated with rupture directivity and the geologic structure. We attribute the small variations among the synthetics to the minimum shear-wave speed permitted in the simulations and how they accommodate topography. Our long-period simulations, on average, under predict shaking intensities by about one-half modified Mercalli intensity (MMI) units (25%-35% in peak velocity), while our broadband simulations, on average, under predict the shaking intensities by one-fourth MMI units (16% in peak velocity). Discrepancies with observations arise due to errors in the source models and geologic structure. The consistency in the synthetic waveforms across the wave-propagation codes for a given source model suggests the uncertainty in the source parameters tends to exceed the uncertainty in the seismic velocity structure. In agreement with earlier studies, we find that a source model with slip more evenly distributed northwest and southeast of the hypocenter would be preferable to both the Beroza and Wald source models. Although the new 3D seismic velocity model improves upon previous velocity models, we identify two areas needing improvement. Nevertheless, we find that the seismic velocity model and the wave-propagation codes are suitable for modeling the 1906 earthquake and scenario events in the San Francisco Bay Area.

Description: We estimate the ground motions produced by the 1906 San Francisco earthquake making use of the recently developed Song et al. (2008) source model that combines the available geodetic and seismic observations and recently constructed 3D geologic and seismic velocity models. Our estimates of the ground motions for the 1906 earthquake are consistent across five ground-motion modeling groups employing different wave propagation codes and simulation domains. The simulations successfully reproduce the main features of the Boatwright and Bundock (2005) ShakeMap, but tend to over predict the intensity of shaking by 0.1-0.5 modified Mercalli intensity (MMI) units. Velocity waveforms at sites throughout the San Francisco Bay Area exhibit characteristics consistent with rupture directivity, local geologic conditions (e.g., sedimentary basins), and the large size of the event (e.g., durations of strong shaking lasting tens of seconds). We also compute ground motions for seven hypothetical scenarios rupturing the same extent of the northern San Andreas fault, considering three additional hypocenters and an additional, random distribution of slip. Rupture directivity exerts the strongest influence on the variations in shaking, although sedimentary basins do consistently contribute to the response in some locations, such as Santa Rosa, Livermore, and San Jose. These scenarios suggest that future large earthquakes on the northern San Andreas fault may subject the current San Francisco Bay urban area to stronger shaking than a repeat of the 1906 earthquake. Ruptures propagating southward towards San Francisco appear to expose more of the urban area to a given intensity level than do ruptures propagating northward.

Description: We simulate long-period (T〉1.0-2.0 s) and broadband (T〉0.1 s) ground motions for 39 scenario earthquakes (M (sub 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 (sub w) 4.18 Oakland earthquake and the 2007 M (sub 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.

Description: We performed 3D ground-motion simulations for 10 recent small to moderate earthquakes (M (sub w) 4.1-5.4) in the San Francisco Bay area to evaluate two versions of the USGS 3D velocity model (Brocher, 2005; Jachens et al., 2006; Brocher, 2008). Comparisons were made in terms of modeling phase arrival timing, peak ground-motion amplitudes, and the seismic waveforms. In the simulations we assumed the source parameters reported in the Berkeley Seismological Laboratory (BSL) Moment Tensor Catalog. Broadband seismic data from the Berkeley Digital Seismic Network (BDSN), and strong motion data from the USGS and the California Geological Survey (CGS) strong motion arrays were used in the analysis. The comparison of peak ground velocity (PGV) for both models reveals that both 3D models predict the observed PGV well over four orders of magnitude, and P- and S-wave timing and pseudospectral acceleration (PSA) are well modeled by the 3D structure. While the revised model (model 8.3.0) significantly improved the timing of the first arrival, and the waveform fit is generally good, there remain discrepancies in estimated amplitudes and durations that require improvements to the structure. Nevertheless, from our low-frequency (0.5 Hz) analysis we found that the 3D model is suitable for the simulation of PGV to assess the strong shaking hazard of future large earthquakes, because earthquakes larger than M 6 have PGV carried by waves of 1 to several seconds period.

Description: There are many engineering applications that require an understanding of the nature of strong ground motions adjacent to and spanning across faults. Unfortunately, such near-field observations at distances less than 100 m of fault rupture are few and incomplete. In this study a 3D finite-difference method is used to simulate strong ground motions for a hypothetical M (sub w) 6.5 earthquake at sites within a few tens of meters of the fault to document the nature of strong ground motion at pairs of sites across the fault as a first step toward providing ground-motion input for engineering design applications. We employ several distributed slip kinematic models to examine ground-motion variability. We also examine the ground motions for fault scenarios ranging from vertical strike-slip to low-angle thrust faulting. The results show that the motions have two primary components: (1) far-field waves that undergo focusing and amplification due to finite-source rupture directivity and (2) near-field waves that are sensitive to the tectonic rebound, or fling, of the closest section of the fault to the recording stations. Both the far-field and near-field controlled motions result in nonstationary pulse-like velocity waveforms that have many implications for the design of engineered structures located close to or spanning faults.