Basin Amplification Factors and Probabilistic Seismic Hazard Maps for Seattle, Washington Based on 3D Simulations of Subduction-Zone and Crustal Earthquakes

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska
Type: Oral
Date: 4/28/2020
Time: 11:15 AM
Room: 230 + 235
Description: 

We recently used a large set of 3D ground-motion simulations of magnitude 9 (M9) earthquakes on the Cascadia megathrust to determine the amplification in the Seattle sedimentary basin for response spectral accelerations (Sa) at periods of 1-10 s. These amplification factors of 2-5 are larger than those derived in NGA West 2 ground-motion models from recordings of crustal earthquakes for the same reference site conditions. The M9 simulations consisted of 30 rupture scenarios that varied the hypocenter, slip distribution, sub-event locations and down-dip edge of rupture, as well as 20 simulations used to evaluate sensitivity of ground motions to each rupture property. Observations of an M5 earthquake near the plate interface show similar basin amplifications as the M9 simulations. The M9 simulations are our first step towards producing new probabilistic seismic hazard maps (1-10 s Sa) for Seattle based on 3D simulations and the earthquake recurrence parameters in the National Seismic Hazard Maps. We have started the process of producing new 3D simulations of M6-7 Seattle fault and intraslab earthquakes. Probabilistic seismic hazard maps for Seattle determined from 541 3D simulations were last released in 2007 (Frankel et al., 2007). The maps depicted 1 s Sa with 2, 5 and 10% probabilities of exceedance in 50 years. To construct the maps, 3D simulations were applied to quantify the amplification of the Seattle basin for rupture scenarios of M6-7 earthquakes on the Seattle and Southern Whidbey Island faults, sources on the plate interface and spatially-distributed crustal and intraslab earthquakes. The recent M9 simulations provide a lower bound for single-station standard deviation (sigma) of Sa that could be used in non-ergodic probabilistic seismic hazard analysis. The M9 single-station sigma increases for sites towards the ends of the megathrust and is generally larger for sites in basins. These lower bound estimates of single-station sigma from the M9 simulations typically exceed values reported in the literature from observations of smaller earthquakes.

Presenting Author: Arthur Frankel

Authors