Response of Idealized Structural Systems to Simulated M9 Cascadia Subduction Zone Earthquakes Considering Local Soil Conditions

Abstract

A research team from the United States Geological Survey and the University of Washington developed thirty full-rupture scenarios of magnitude-9 (M9) Cascadia Subduction Zone earthquakes. The simulated M9 motions account for the varying geology in western Washington and include the effects of basin amplification for periods above 1s. The hazard level considered by the current Washington State bridge design provisions is based on the 2014 National Seismic Hazard Maps, which do not take into consideration the amplification of long-period ground motions by the deep sedimentary basins that underlie much of the Puget Sound region. As a result, the bridge design spectral accelerations for cities located on sedimentary basins are lower for long periods than the spectral accelerations for simulated M9 motions. The response of three types of nonlinear, single-degree-of-freedom oscillators to an M9 event indicate that an event of this magnitude could inflict significant damage to short-period structures located near the Pacific Coast and to long-period structures located on the deep sedimentary basins that underlie much of the Puget Sound region. Furthermore, older structures designed to lower standards are likely to suffer damage for all periods.