Quantitative Ground‐Mo tion Estimates a

Estimates of ground motion in eastern North America (ENA) are hampered by a lack of data. Because of this, most estimation schemes have been based, at least in part, on data obtained from the more seismically active and better-instrumented western North America (WNA). In view of the different geologic structures and tectonic settings in the two regions, however, it is not clear a priori that data from WNA can be exported to ENA. A theoretical model has been developed in the last few years that does not require WNA data for the estimation of ground motions in ENA. This model constructs the ground motion from filtered random Gaussian noise, for which the filter parameters are determined by a seismological model of both the source and the wave propagation. Many of the parameters in the model can be determined from independent seismological investigations in E N A or in other regions with similar tectonic characteristics. Crucial ingredients of this model are the shape of the radiated energy spectrum and the relation between the seismic moment of the earthquake and the corner frequency of the spectrum (usually referred to as the “scaling law”). In two recent papers, predictions of ENA ground motions using a scaling law characterized by a constant stress parameter were in reasonable agreement with the sparse observed data a t rock sites, most of which come from earthquakes near moment magnitude 4.5. Subsequent to these studies, however, two variable-stress scaling laws have been proposed. All the laws lead to similar motions for moderate earthquakes (M = 4.51, but for larger events the estimates diverge (the range in predicted motions is a factor of 6 for an M = 7.5 earthquake, the estimates from the constant stress scaling being between the other two). The new model can be w e d to gain insight into similarities and differences between ground motions for ENA and WNA earthquakes. The calculations suggest that the motions would be the same at distances within about 60 km for ground motions with frequencies less than about 10 to 15 Hz. At greater distances the differences in regional attenuation of seismic waves leads to larger ground motions in ENA (although these motions would probably not cause much damage), and at higher frequencies the generally more competent rock in E N A will allow higher frequencies to propagate to the site than would be the case for typical rock sites in WNA.