A COMPARISON OF EARTHQUAKE DAMAGE AREAS AS A FUNCTION OF MAGNITUDE ACROSS THE UNITED-STATES

This study investigates the relationship between earthquake magnitude and the size of damage areas in the eastern and western United States. To quantify damage area as a function of moment magnitude (M), 149 MM I VI and VII areas for 109 earthquakes (88 in the western United State s, 21 in the eastern United States and Canada) were measured. Regressi on of isoseismal areas versus M indicated that areas in the East were larger than those in the West, at both intensity levels, by an average 5 x in the M 4.5 to 7.5 range. In terms of radii for circles of equiv alent area, these results indicate that damaging ground motion from sh ocks of the same magnitude extend 2 x the epicentral distance in easte rn North America compared to the West. To determine source and site pa rameters consistent with the above results, response spectral levels f or eastern North America were stochastically simulated and compared wi th response spectral ordinates derived from recorded strong ground mot ion data in the western United States. Stress-drop values of 200 bars, combined with a surficial 2-km-thick low velocity ''sedimentary'' lay er over rock basement, produced results that are compatible with the i ntensity observations, i.e., similar response spectral levels in the e ast at approximately twice their epicentral distance in the western U. S. distance. These results suggest that ground motion modeling in east ern North America may need to incorporate source and site parameters d ifferent from those presently in general use. The results are also of importance to eastern U.S. hazard assessments as they require allowanc e for the larger damage areas in preparedness and mitigation programs.