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Estimating earthquake location and magnitude from seismic intensity data

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

Abstract

Analysis of Modified Mercalli intensity (MMI) observations for a training set of 22 California earthquakes suggests a strategy for bounding the epicentral region and moment magnitude M from MMI observations only. We define an intensity magnitude M_I that is calibrated to be equal in the mean to M. M_I = mean (M_i), where M_i = (MMI_i + 3.29 + 0.0206 * _i)/1.68 and _i is the epicentral distance (km) of observation MMI_i. The epicentral region is bounded by contours of rms [M_I] = rms (M_I – M_i – rms₀ (M_I – M_i), where rms is the root mean square, rms₀ (M_I – M_i) is the minimum rms over a grid of assumed epicenters, and empirical site corrections and a distance weighting function are used. Empirical contour values for bounding the epicenter location and empirical bounds for M estimated from M_I appropriate for different levels of confidence and different quantities of intensity observations are tabulated. The epicentral region bounds and M_I obtained for an independent test set of western California earthquakes are consistent with the instrumental epicenters and moment magnitudes of these earthquakes. The analysis strategy is particularly appropriate for the evaluation of pre-1900 earthquakes for which the only available data are a sparse set of intensity observations.