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<5.5 in California attenuate more rapidly with distance than predicted by the recent Pacific Earthquake Engineering Research Center–Next Generation Attenuation (PEER–NGA) ground-motion prediction equations for shallow crustal earthquakes in active tectonic regions (Abrahamson and Silva, 2008; Boore and Atkinson, 2008; Campbell and Bozorgnia, 2008; Chiou and Youngs, 2008). It appears that the limited M<5.5 data used in the PEER-NGA study are not representative of ground motions from M<5.5 events in California in general. (At larger magnitudes, however, the California ShakeMap observations converge with expectations based on the PEER-NGA equations.) By contrast, the California ground motions for M<5.5 at distances less than 100 km appear consistent with those that would be predicted by the ground-motion model of Atkinson and Boore (2006) for eastern North America, if an adjustment is made for a factor of 2 difference in stress drop between eastern and western North America. This attests to the need to include a broad range of magnitudes and distances in the development of comprehensive ground-motion prediction models.

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