Estimating earthquake location and magnitude from seismic intensity data

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 * Δ i )/1.68 and Δ 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 − rms ( M I − M i ), where rms is the root mean square, rms ( 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.

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