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Basin and Site Effects in the U.S. Pacific Northwest Estimated From Small-Magnitude Earthquakes

Session: Earthquake Science, Hazards and Policy in Cascadia [Poster]

Type: Poster

Date: 4/20/2021

Presentation Time: 11:30 AM Pacific

Description: 

We analyze ground motions recorded in the U.S. Pacific Northwest to better understand basin and regional site effects. Toward this end, we compile a database of recordings from crustal, interface, and intraslab earthquakes with magnitudes ranging from 3.5 to 6.8 and hypocentral depths up to 62 km, resulting in more than 6,000 records. Our analysis begins with the Fourier amplitude spectrum (FAS) of these ground motion acceleration records, and we compute residuals relative to a ground motion model (GMM) developed for shallow crustal events and perform a series of linear, crossed, mixed-effects regressions. We consider a series of models, starting with a simple linear mixed-effects model for event terms, and incrementally add site, region, seismotectonic regime, and interaction terms. The regionalization taxonomy includes three classes: Seattle basin, Puget Lowlands, and non-Puget Lowlands. The scaling of site response, parameterized with respect to the time-averaged shear-wave velocity to a depth of 30 meters and shear-wave velocity horizon depths, is broadly consistent with the GMM. Additionally, the region terms capture significant frequency-dependent spatial patterns of amplification and de-amplification. With respect to the non-Puget Lowlands, we observe that the Seattle basin amplifies ground motions up to a factor of four at low frequencies (f < ~5 Hz), and de-amplifies ground motions at high frequencies (f > ~5 Hz), with the smallest average amplification factor of ~0.65 occurring at 10.0 Hz. The regional-seismotectonic interactions are less pronounced than the other terms in our analysis, but indicate stronger (up to 40%) amplifications from interface and intraslab earthquakes relative to shallow crustal earthquakes.

Presenting Author: John M. Rekoske

Student Presenter: No

Authors