Empirical ground-motion relations for ShakeMap applications in southeastern Canada and the northeastern United States

We have developed empirical relations that describe ground-motion amplitudes from earthquakes of 2 < M < 6 at regional distances of 10-500 km in southeastern Canada and the northeastern United States, for ShakeMap applications. The predictive ground-motion parameters are peak ground velocity, peak ground acceleration, and response spectra at frequencies of 1, 2, 5, and 10 Hz. The relations differ from previous relations developed by Atkinson and Boore (1995) in their focus on applicability to the small to moderate regional events important to reliable ShakeMap development. Because of this focus, they are empirical, rather than being based on a stochastic ground-motion model. The stochastic model predictions of Atkinson and Boore (1995) are used at larger magnitudes (M > 4), however, to ensure reasonable values over a broad range of magnitudes (2 to 6). The new relationships are used to estimate ground-motion parameters for ShakeMaps in Ontario. The ShakeMap program combines predicted ground-motion values with recorded ground-motion values to produce an interpolated map of ground-motion amplitudes. It is essential that the predicted ground-motion values be consistent with recorded values in order to generate reliable ShakeMaps. We found that with our new predictive relations, we obtain ground-motion estimates that closely resemble the recorded ground motions for the small events (2 < M < 4) that occur relatively frequently in eastern North America and are often felt. Thus we recommend the use of our predictive relations for ShakeMap applications in southeastern Canada and the northeastern United States. They are not intended for engineering predictions for large rare earthquakes (M > 6).