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Development and Application of Near-Surface Soil Velocity Models for Seismic Hazard in the Pacific Northwest

Session: Earthquake Science, Hazards and Policy in Cascadia II

Type: Oral

Date: 4/20/2021

Presentation Time: 05:45 PM Pacific

Description: 

Site response is a critical component of ground motion propagation and can significantly impact the amplitude and frequency content of surficial shaking. To improve estimates of earthquake shaking in the Pacific Northwest (Oregon and Washington, as well as British Columbia, Canada), we developed region-specific soil velocity models to compute site response. A regional shear wave velocity model was derived from 580 measured velocity profiles across the Pacific Northwest (Ahdi et al., 2017), to capture generic regional conditions as well as sub-regional models for basin/non-basin and specific geologic settings. These soil velocity profiles were modeled as a function of site condition (Vs30) and depth, to facilitate regional application with limited inputs. Representative Pacific Northwest soil velocity profiles were found to be more constant with depth in the near-surface (< 100 m) than generic profiles from elsewhere (e.g., California), likely due to the glacial origin of surficial deposits across the region. These Pacific Northwest representative soil profiles were then used to compute site adjustment factors for incident synthetic earthquake ground motions. Using maps of estimated Vs30 of the region, we then developed site-corrected ensemble ShakeMaps for a M9.0 earthquake along the Cascadia Subduction Zone (CSZ) from a suite of 30 simulated earthquakes. New ensemble ShakeMaps are generally in agreement with GMM-based existing ShakeMaps but show significant increases in shaking intensity within basins, where 3D ground motion simulations better capture low-frequency amplification and some smaller-scale changes in high-frequency response due to site amplifications. Ensemble ShakeMaps developed in this work are being adopted for national and state-level emergency planning exercises. Ongoing work will incorporate these soil velocity profiles in the generation of high-frequency ground motion content for next-generation crustal earthquake simulations in the Pacific Northwest.

Presenting Author: Alex Grant

Student Presenter: No

Authors