Rapid Finite-source Analysis and Near-fault Strong Ground Motions: Application to the 2003 Mw 6.5 San Simeon and 2004 Mw 6.0 Parkfield Earthquakes

Abstract

Rapid characterization of near-fault strong ground shaking is essential for assessment of an earthquake's impact, including the scale of damage and loss. ShakeMap (Wald et al. , 1999a) was developed as a tool for this purpose. To generate a ShakeMap, observed peak and spectral ground motion, as well as estimated instrumental intensity (Wald et al. , 1999b), are interpolated with a model of ground-motion attenuation to produce contour maps of the various parameters. In California, the California Integrated Seismic Network (CISN) automatically produces ShakeMaps for moderate to large events within several minutes of their occurrence. ShakeMaps have proven to be a valuable information resource for seismologists, emergency responders, and the general public. The ShakeMap approach works best in regions with dense station coverage. In such cases, the observed ground motions control the contouring of the maps. In poorly instrumented regions or in cases where data telemetry is lost (possibly due to strong shaking), ShakeMap relies on an assumed ground-motion attenuation model to estimate the motions based on the magnitude of the earthquake and the distance of a site to the source. This model is based on published ground-motion attenuation relationships and the distance from site to epicenter in the absence of information regarding the fault orientation and length. Once additional information is available, such as aftershock distribution and surface faulting reports, ShakeMaps are updated to account for rupture finiteness by computing the distance from the site to the closest surface projection of the fault. Rapid finite-source models can also provide the information needed to update ShakeMaps in this manner (Dreger and Kaverina, 2000). For large regions of California, station coverage in outlying areas is insufficient to obtain detailed data-based ShakeMaps. To address the coverage issue, and to have a back-up system should telemetry fail at key near-fault strong-motion stations, we …