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Comparison of Depth-to-Bedrock with VS30 and Terrain Classes, Including Implications for Predicting Dominate Site Frequencies (fd)

Elizabeth Brown, & Alan Yong

Published August 5, 2021, SCEC Contribution #11180, 2021 SCEC Annual Meeting Poster #030

Local geologic conditions can strongly influence seismic responses that are due to subsurface stiffness and resonance effects of a site. Depth-to-Bedrock (DtB)—known to directly affect site resonance frequencies—is an important site parameter but is often difficult to constrain. Recently available high-resolution global DtB models may be useful for a first-order constraint of DtB but have not been tested for seismological applications. For the San Francisco Bay Area (SFBA) and Los Angeles Basin (LAB), we investigate the correlation of the Shangguan et al. (2016) geomorphic global DtB model with two site amplification proxies: the time-averaged shear wave velocity to a depth of 30 m from the surface (VS30) (McPhillips et al., 2020) and dominant site frequency (fd). Preliminary results suggest a poor fit of VS30 with DtB in the SFBA (R^2 = 0.152) and LAB (R^2 = 0.001). Constraining DtB and VS30 slightly improves fit. Observations of VS30 may be biased by their locations within these coastal urban basins, where the confluence of fluvial and coastal depositional environments can affect near surface seismic properties; thus, additional VS30 data may improve the relationship of DtB and VS30. We also investigate the fit of the global DtB model with the Iwahashi et al. (2021) geomorphic terrain classification model. The fit of terrain classes to DtB also appears to be poor for the SFBA (R^2 = 0.020) and LAB (R^2 = 0.015). However, patterns in the DtB across terrain classes for the SFBA and LAB suggest the terrain classes may need to be grouped by geomorphic parameters to improve fit. We test hybrid groupings of terrain classes and identify five that comprise the SFBA and LAB; then, we average all DtB (DtBave) in each class. We find DtBave decreases across each grouping from the basin center to the basin edges, which conforms to the overall trend of DtB in the SFBA and LAB. Although the change in DtBave is small, these findings demonstrate a strong fit of DtBave with these groupings in the SFBA (R^2 = 0.724) and LAB (R^2 = 0.836). Future work includes constraining DtB with terrain classes and comparing these new hybridized models to locations with measurements of fd, which can have implications for predicting fd using the hybridized models where observed fd data are sparse or not readily available.

Brown, E., & Yong, A. (2021, 08). Comparison of Depth-to-Bedrock with VS30 and Terrain Classes, Including Implications for Predicting Dominate Site Frequencies (fd). Poster Presentation at 2021 SCEC Annual Meeting.

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