Observed relations between felt area and moment magnitude M for stable continental regions (SCR) and California earthquakes can be explained by a simple model with geometrical spreading, attenuation (Q−1), and earthquake stress drops that remain constant from M 4 to 8. Differences in the slopes of the observed felt area-M relations between SCR and California earthquakes are consistent with differences in Q between these tectonic settings. Fitting the felt area-M data to the model yields an estimate of f/Q, where f is the average frequency of minimum perceptible ground motion. Using fits to the data and instrumentally derived values of Q, I find that f is between about 2 and 4 Hz. I present an estimate of the spectral acceleration level associated with the lower limit of perceptibility.

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