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Abstract The regional variability in earthquake ground-motion amplitudes for a given magnitude and distance in western North American environments was examined using ShakeMap data from small-to-moderate events. The abundance of data for small-to-moderate events in California allows average ground-motion levels, as a function of magnitude and distance, to be resolved with a high level of confidence. Ground-motion amplitudes in northern California are lower on average than those for southern California, for events of the same magnitude, at distances in the range from 120 to 250 km, over all frequencies. The observed regional variations could be indicative of regional differences in attenuation effects or site effects. An unexpected result of the study is the finding that ground motions for events of M M M M
Atkinson et al. (Wed,) studied this question.