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Ground-Motion Modeling of the 1906 San Francisco Earthquake, Part I: Validation Using the 1989 Loma Prieta Earthquake

U.S. Geological Survey, MS977, 345 Middlefield Road, Menlo Park, California 94025

David Dolenc^* and Douglas Dreger

Seismological Laboratory, University of California, Berkeley, 215 McCone Hall, Berkeley, California 94720

Robert W. Graves

URS Corporation, 566 El Dorado Strett, Pasadena, California 91101

Stephen Harmsen and Stephen Hartzell

U.S. Geological Survey, MS966, P.O. Box 25046, Golden, Colorado 80225

Shawn Larsen

Lawrence Livermore National Laboratory, Box 808, L-103, Livermore, California 94551-0808

Mary Lou Zoback

U.S. Geological Survey, MS977 345 Middlefield Rd., Menlo Park, California 94025

^* Present address: Large Lakes Observatory, University of Minnesota, Duluth 109 RLB, 2205 E. 5th Street, Duluth, Minnesota 55812.

Online Material: Modified Mercalli intensities and velocity waveforms, and a movie of simulated wave propagation.

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.

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