RISK POSED BY CASCADIA SUBDUCTION ZONE EARTHQUAKES: AN ANALYSIS OF THE ASSUMPTIONS ON RUPTURE CHARACTERISTICS, RECURRENCE AND TIME-DEPENDENCE

The US Geological Survey (USGS) recently updated the US national seismic hazard maps with a revision of the Cascadia Subduction Zone (CSZ) hazard compared to the previous 2008 version. All aspects of the CSZ model were modified and especially rupture geometry underwent significant change. Differing perspectives on time-dependent and rupture behavior were incorporated in a weighted logic tree approach. The Geological Survey of Canada (GSC) implemented the CSZ in a deterministic manner in their 2005 hazard maps, but moved on to a probabilistic model in their upcoming hazard map version.

Main standard components that build up a probabilistic seismic risk model are hazard, building response, and a financial loss model. In this paper we investigate the impact of changes in the new USGS hazard model and the differences between the GSC and most recent USGS model on seismic risk. We’ll quantify in terms of risk metrics the impact of difference in assumptions on recurrence, time-dependent rupture behavior, characteristic rupture length and variable magnitude-frequency along the zone from north to south. We implement newly gained insights into generation (segmentation), maximum magnitude and impact (ground motions) of mega-thrust earthquakes. To understand the difference in handling the intraslab earthquake activity we implement these deep events in a step-wise manner, similarly to what is proposed for Cascadia by the USGS, thus producing shallower events closer to the trench and deeper events under the mainland.

Considered risk metrics include simple scenario calculations, where we model regional damage and loss due to one worst-case scenario earthquake and several, more probable earthquakes with higher return periods. We compare the metric average annual loss, an annualized expected loss level used by insurers to set their annual premium rates, and the loss exceedance probability curve used by insurers to address their solvency and manage their portfolio risk. We analyze risk profile changes in areas with large population density, Vancouver, Seattle and Portland, and compare the earthquake risk of different types of structures, like well-built high-rise buildings typical for the central business districts, older unreinforced masonry buildings and typical residential wood buildings for these cities.