Ground Motions for Earthquakes in Southwestern British Columbia and Northwestern Washington: Crustal, In-Slab, and Offshore Events

Regional ground-motion generation and propagation must be characterized to adequately assess seismic hazard. In the Cascadia region of southwestern British Columbia and northwestern Washington, the ground-motion issues are particularly complex because of the contributions to hazard from five distinct types of events, all of which behave differently in terms of their ground-motion propagation characteristics: (1) shallow earthquakes occurring in the continental crust; (2) shallow earthquakes occurring offshore in oceanic crust; (3) earthquakes occurring within the subducting Juan de Fuca slab beneath the continent (e.g., Puget Sound); (4) earthquakes occurring within the subducting Juan de Fuca slab at the edge of the continent (e.g., transitional events along the west coast of Vancouver Island); and (5) great subduction earthquakes on the interface between the subducting Juan de Fuca plate and the overriding North American plate. In this study, empirical data recorded within the Cascadia region are used to evaluate the source and attenuation characteristics of ground-motion amplitudes from the first four of these event types (crustal, offshore, in-slab, and transitional) and examine their implications for regional groundmotion relations. For crustal earthquakes in Cascadia, a simple application of the hybrid-empirical approach is used to suggest appropriate regional ground-motion relations. The ground motions are obtained by multiplying California ground-motion relations by a frequency-dependent factor to account for regional differences in crustal amplification. The developed ground-motion relations for Cascadia earthquakes are in reasonable agreement with recorded ground motions in the region. The proposed relations are the first region-specific ground-motion relations to be developed for the Cascadia region, and they provide a useful alternative to California relations for use in seismic hazard analysis.