Observations of Ground Motion Amplification in the Seattle and Tacoma Sedimentary Basins from Local and Regional Earthquakes

Session: Forthcoming Updates of the USGS NSHMs: Hawaii, Conterminous U.S. and Alaska [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

Deep sedimentary basins are known to amplify long-period ground shaking, thus increasing seismic hazard for sites located on such basins. The Puget Sound region in Washington State is characterized by multiple deep sedimentary basins, including the Seattle Basin (7-10 km deep) and Tacoma Basin (6-7 km deep). While ground motion amplification in the Seattle Basin has been well characterized, seismic instrumentation has been largely absent in the Tacoma Basin precluding robust observations of ground motion amplification. We take advantage of concurrent deployments of seismic instrumentation in and surrounding the Seattle and Tacoma Basins during July-October 2019 to measure ground motion amplification from well-recorded earthquakes, including a local M4.6 earthquake in Monroe, Washington; regional ~M6 earthquakes on the Blanco Fracture Zone offshore Oregon; and the M7.1 Ridgecrest, California earthquake. To quantify basin amplification, we take Fourier spectral ratios of recorded ground motions inside the basins compared to outside the basins. For the M4.6 Monroe earthquake, we observe significant amplification at sites above the deepest part of the Tacoma Basin. At 3 s period, the Tacoma Basin exhibits amplification up to a factor of ~3 with respect to a rock site in the Puget Lowland (SP2), comparable to observed ground motion amplification in the Seattle Basin. In the future, comprehensive analyses of basin amplification such as this may allow for regionally specific information to be incorporated into local and national seismic hazard maps.

Presenting Author: Erin Wirth

Authors