Empirical ground motion relations for earthquakes in the Cascadia region

Empirical relations are developed to describe response spectral amplitudes as a function of earthquake magnitude, focal depth, and distance, for earthquakes in the Cascadia region of southwestern British Columbia and northwestern Washington State. The relations are based on regression of response spectra data from crustal, subcrustal, and subduction earthquakes recorded on rock and soil sites. The relations apply to earthquakes of moment magnitude, M, in the range 4 ≤ M ≤ 8.2 with focal depths from 1 to 60 km, at distances up to several hundred kilometres. Most of the data for moderate-to-large events (4 ≤ M < 7) are from the Cascadia region, while those for very large events (M ≥ 7.5) are from other subduction zones. The ground motion amplitudes for large (M = 7) shallow earthquakes in the Cascadia region are about the same as those for corresponding events in California. By contrast, moderate (M = 5) Cascadia earthquakes have smaller ground motion amplitudes than do moderate California earthquakes. The motions predicted for very large subduction earthquakes (M = 8) are similar to those obtained by Crouse.