HR: 0800h
AN: S51A-0233    [Abstracts]
TI: Using ShakeMap to Map MMI Intensity for the 1868 Hayward, the 1898 Mendocino, and the 1906 San Francisco Earthquakes
AU: * Bundock, H
EM: bundock@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: Boatwright, J
EM: boat@usgs.gov
AF: U.S. Geological Survey, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: The utility of the ShakeMap format for depicting ground motion and shaking intensity in recent well-recorded earthquakes has led to its use in mapping MMI intensity for large historic earthquakes. But while the mechanics of incorporating intensity sites as ground motion data in ShakeMap are trivial, the constraints of ShakeMap's native MMI scale and the hazards of ShakeMap's interpolation scheme can make implementation for historic earthquakes difficult. We illustrate these difficulties using the MMI ShakeMaps for the 1868 Hayward, the 1898 Mendocino, and the 1906 San Francisco earthquakes as case studies. To compare historic earthquakes with modern earthquakes, it is critical for ShakeMap to be fixed to a single intensity scale. Wald et al. (1997) base ShakeMap intensities on Stover and Coffman's (1993) revision of the MMI scale. Thus, intensities determined by Lawson (1908) and Toppozada et al. (1981) for these historic earthquakes must be re-evaluated before they can be incorporated in ShakeMap. To interpolate intensities in areas without stations, ShakeMap interposes "phantom" stations where the ground motions are estimated from Boore et al.'s (1997) attenuation relations. This hybrid interpolation leads to two problems. First, the ground motions are only regressed for a restricted distance from the fault (r_JB ≤ 70 km, where r_JB is the Joyner-Boore distance). To extend these relations to r_JB ≤ 300 km, we modify these ground motion predictions using the exponential falloff exp(-0.0035 r_JB). Surprisingly, this term fits the variation of intensity with distance for all three earthquakes. Second, the source strength can vary significantly along the fault for large earthquakes: interposing an "average" value often produces an artificial variation of the intensity. We damp these variations by minimizing the number of phantom stations, increasing both the threshold distance for the phantom stations and the total number of intensity sites in these areas by using all the available newspaper reports and historical narratives that describe shaking effects. We have added approximately 160 sites for the 1906 earthquake, and 30 sites for the 1868 earthquake. Locating these sites in backwoods areas, 10 to 70 km from the Mendocino coast, is particularly critical for the ShakeMaps of the 1898 and 1906 earthquakes.
DE: 7212 Earthquake ground motions and engineering seismology
DE: 7215 Earthquake source observations (1240)
SC: Seismology [S]
MN: 2007 Fall Meeting