Development of design spectra for long-duration ground motions from Cascadia subduction earthquakes

Abstract

There is now growing evidence that large-magnitude earthquakes have occurred and could occur again along the Cascadia subduction zone located west of Vancouver Island, Bristish Columbia. Numerical simulations indicate that these earthquakes would produce long-duration ground motions and would thus be capable of inducing a large number of reversals of inelastic deformations in engineered structures. Efforts have now been undertaken to account for this damage potential in building codes. In this paper, inelastic design spectra are developed for Cascadia subduction earthquakes for four sites in British Columbia. These spectra are compared with elastic design spectra that have been developed recently for the same sites based on empirical attenuation relationships for Cascadia events. The approach used to develop the inelastic spectra aims at providing the same level of protection against structural failure for both subduction events and crustal or subcrustal earthquakes. Force modification factors are first determined for structures exhibiting various failure modes and ductility levels when subjected to representative crustal and subcrustal earthquake ground motions. Thereafter, design spectra are developed for the same structures to prevent structural collapse under simulated Cascadia subduction ground motions. The study reveals that the elastic spectra do not reflect adequately the damage potential of Cascadia earthquakes. These elastic spectra generally are unconservative for Tofino and Victoria. For Vancouver and Prince George, the elastic spectra overestimate the demand, especially for short-period structures.Key words: collapse, crustal earthquakes, damage index, design spectrum, ductility, duration, ground motion, subduction zone.