Impacts of an M9 Cascadia Subduction Zone Earthquake on Structures Located in Deep Sedimentary Basins

Abstract

Thirty scenarios of magnitude-9 Cascadia subduction interface earthquakes were simulated by a research team from the United States Geological Survey and the University of Washington. These motions were strongly modified by the deep sedimentary basins that underlie much of the Puget Sound region. The simulated motions within the basin had high spectral accelerations at moderate-to-long periods, damaging spectral shapes, long durations, and varied greatly among the scenarios. The simulated effects of the Seattle basin were consistent with those observed for Japanese basins during subduction earthquakes. The response of deteriorating single-degree-of-freedom oscillators suggests that an M9 event could inflict widespread damage across much of the Puget Sound region. Using an improved modeling methodology, the seismic response was estimated for 32 reinforced concrete wall archetypes designed for Seattle. The collapse risk for these structures exceeded values targeted by current codes, particularly when the archetypes designed to barely meet the code requirements. The damage estimates correlated well with a new measure of ground-motion intensity that reflects the contributions from spectral acceleration, spectral shape, and duration.