3D Geologic ″Map″ of the San Francisco
Bay Area for Seismic Hazard Mitigation
Robert C. Jachens, United States Geologic Survey, Menlo Park, California
May 1999
California has suffered a number of damaging earthquakes during the past few decades, with hundreds of people killed or injured and economic losses measured in billions of dollars. For example, the 1989 Loma Prieta earthquake caused extensive damage in the greater San Francisco Bay area. As with earthquakes worldwide, most of the damage associated with Loma Prieta was caused not by faults breaking the ground surface, but as a direct or indirect result of ground shaking corresponding to the passage of seismic waves. In an effort to reduce losses during future earthquakes, the U.S. Geological Survey, in concert with other earthquake researchers, is striving to improve our understanding of the parameters that control ground shaking and our ability to predict it.
Figure 1
In general the severity of ground shaking decreases with distance from the earthquake′s epicenter, but damage surveys following actual earthquakes indicate that the real world is not so simple. Extensive damage in the Marina District of San Francisco and other locations far removed from the Loma Prieta earthquake epicenter show that other factors are involved. We know that local basins filled with sedimentary deposits can trap seismic waves, causing them to resonate and amplify shaking compared to the shaking at nearby bedrock sites. The fractured rock in fault zones can also trap seismic waves, causing locally amplified shaking. Buried faults and other interfaces where abrupt changes in rock type occur reflect seismic waves and can enhance shaking by focusing seismic energy to local areas. Although the processes that produce locally severe ground shaking are extremely complex, they all result from the influence of geologic materials on the passage of seismic waves. We, therefore, are using EarthVision to build a three-dimensional geologic ″map″ with associated physical properties (seismic wave velocities and attenuations, and densities) of the greater San Francisco Bay area (Fig. 1). The digital 3D physical property ″map″ will be the input for sophisticated computer modeling to predict ground shaking.
Figure 2
Figure 2a
EarthVision plays a critical role in this effort because the geology of the Bay area is extremely complicated, having been assembled over more than 150 million years under the influence of faulting, folding, uplift, erosion, subsidence, and deposition. The gridding capabilities are used to construct surfaces representing the base of the crust and deep crustal interfaces from scattered data derived from seismic refraction and reflection profiling. They also are used to define the shapes of basins (Figs. 2 and 2a) derived from the inversion of gravity data constrained by wells and seismic profiling. An intricate network of young faults (Fig. 3) divides the region into discrete geologic entities whose accurate representation requires the power of the Geologic Structure Builder. Frequent use is made of the Formula Processor in assigning physical properties based on geologic identity, fault block location, and depth. Finally, we use the 3D Viewer not only to visually demonstrate the results, but also to ″walk around″ in the ″map″ because the shear volume of data precludes other methods of examination and evaluation.
Figure 3
The 3D geologic ″map″ of the greater San Francisco Bay area is a work in progress, but preliminary tests on early versions indicate that it will be a valuable tool for predicting amplified ground shaking and, as such, will provide important information for earthquake hazard mitigation. More generally, because we are forced for the first time to define the geology completely in three dimensions, data deficiencies, problems, and questions are coming to light that, when dealt with, will result in improved understanding of the geologic framework of the San Francisco Bay area.
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