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

John Rekoske, Morgan P. Moschetti, & Eric M. Thompson

Published August 14, 2020, SCEC Contribution #10627, 2020 SCEC Annual Meeting Poster #223 (PDF)

The U.S. Pacific Northwest has relatively high seismic hazard due to various types of earthquakes (e.g., shallow crustal, interface, and intraslab earthquakes), as well as the presence of deep sedimentary basins that amplify and prolong ground shaking. To better understand basin and site effects, we compile a database of strong-motion and broadband recordings of earthquakes in the Cascadia subduction zone of the Pacific Northwest. We process over 6,000 records from crustal, interface, and intraslab earthquakes with magnitudes ranging from 3.5 to 6.8 and hypocentral depths ranging from 0 to 62 km. For each processed record, we compute the smoothed Fourier amplitude spectrum (FAS) of ground acceleration for frequencies of 0.1 to 20 Hz. Due to a lack of FAS ground motion models (GMMs) specific to subduction zones, we compute ground motion residuals relative to the BA18 GMM which was developed for shallow crustal earthquakes in California. Using the residuals, we perform a linear, crossed, mixed-effects regression to estimate the bias, event, and site terms which allow us to examine the applicability of the BA18 GMM to the Cascadia subduction zone. We investigate the scaling of the site terms with respect to VS30 (average shear-wave velocity from the surface to a depth of 30 meters), as well as basin depth indicators Z1.0 and Z2.5 (depths to the 1.0 km/s and 2.5 km/s shear wave velocity horizons). Although the site term scaling with respect to VS30, Z1.0, and Z2.5 generally shows consistency with the BA18 model, we find that the differences in sites are better characterized when comparing stations within the Puget Lowlands vs. outside the Puget Lowlands. Based on preliminary analysis, we find that for frequencies less than 5 Hz, the Puget Lowlands stations on average exhibit 1.2 – 2.2 times amplification of ground motions, relative to non-Puget Lowlands stations, with maximum amplification occurring around 0.6 Hz. At frequencies greater than 5 Hz, we observe de-amplification among the Puget Lowlands stations, with the smallest average amplification factor of 0.65 occurring at 10.0 Hz.

Citation
Rekoske, J., Moschetti, M. P., & Thompson, E. M. (2020, 08). Basin and Site Effects in the U.S. Pacific Northwest Estimated from Small-Magnitude Earthquakes. Poster Presentation at 2020 SCEC Annual Meeting.

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