Implementation of Basin Effects in Seismic Hazard of the Greater Seattle Region

Date: 4/24/2019

Time: 02:45 PM

Room: Pine

We present a methodology for incorporating basin effects into a seismic hazard assessment for a site located in Seattle. In the Seattle region, there are three major source-types that contribute to the seismic hazard: interface and intraslab subduction events and crustal events. The basin effects are dependent on the source-type. The basin amplification factors (AFs) from the Cascadia interface sources use the University of Washington M9 simulations; intraslab sources use empirical site-specific AFs from the Nisqually earthquake; and crustal sources use basin factors derived for an analogous region.

We show how to incorporate site-specific basin effects from the intraslab and interface sources using the single-station sigma approach as compared to the fully-ergodic approach used in current practice. With the single-station sigma approach, the incorporation of site-specific site-effects provides a more accurate ground-motion estimate than an ergodic model. Consequently, there is a reduction in the aleatory variability as compared to the ergodic model; however, there is epistemic uncertainty in the estimation of the basin AFs which leads to additional branches on the ground motion characterization logic tree. Published values of the reduction of the ergodic standard deviation due to including site-specific terms range between 9 and 16 percent. Use of the single-station sigma approach in practice is not new. Major seismic hazard studies, both in the US and outside the US, have implemented the single-station sigma approach. By using site-specific basin AFs from recorded intraslab events and from the M9 interface simulations into the hazard analysis, we are providing a more informed ground-motion estimate at our site and consequently expect less variability in the ground motion than what would be predicted using the ergodic ground-motion model. As examples, we show a comparison of the uniform hazard spectra for the current approach used in Seattle and the single-station sigma approach for several locations in the Seattle region, both inside and outside of the basin.

Presenting Author: Melanie Walling

Authors