60-9 Site-Specific Seismic Risk Analysis Using Distributed Acoustic Sensing Derived Vs30 Measurements from Port Angeles, Washington

Session: 2YC and 4YCU Geoscience Student Research Poster Showcase

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Abstract:

Port Angeles, Washington, is located above the Cascadia subduction zone, a region susceptible to significant seismic activity. Accurate seismic hazard assessments are critical for preparing for earthquakes and mitigating their impacts. A key parameter in such assessments is Vs30, the average shear wave velocity in the top 30 meters of soil or rock, which reflects ground stiffness and influences ground motion during earthquakes. The USGS provides estimates of Vs30 across the U.S. using a topographic proxy map. However, these estimates are low resolution, and site specific surveys provide more robust values. One site-specific Vs30 measurement in Port Angeles is available through the USGS. However, by utilizing Vs30 measurements extrapolated from surveys along a Distributed Acoustic Sensing (DAS) array in Port Angeles, we obtained denser, site-specific Vs30 estimates. The DAS-derived Vs30 values averaged 587 m/s (NEHRP soil class C), higher than the USGS map values, which averaged 339 m/s (soil class D), indicating stiffer soil and rock conditions near the DAS array. The USGS proxy map estimates Vs30 values at our site to range from 360-490 m/s, while the DAS-derived range was higher, from 513–671 m/s. We integrated these measurements into ground motion models to assess how earthquake ground acceleration estimates differ. Predicted ground motions estimate shaking during earthquakes and help evaluate hazards and risks to infrastructure. We utilized both the USGS Earthquake Hazard Toolbox and the American Society of Civil Engineers (ASCE) Hazard Toolbox to generate seismic hazard reports. The USGS Toolbox compares median peak ground acceleration (PGA) with distance and magnitude, informing seismic risk assessment. The ASCE Toolbox estimates spectral acceleration (SA), further evaluating structural risk. For a magnitude 8 Cascadia Subduction Zone earthquake 200 km from Port Angeles, the median PGA using DAS-derived Vs30 averaged across all locations is 1.1 × 10⁻2 g, compared to the USGS topographic proxy map which has a modeled median PGA range of 1.3 x 10-2 to 1.6 x 10-2 g. Both tools showed that incorporating higher DAS-derived Vs30 values resulted in consistently lower PGA and SA estimates. These findings suggest that prior assessments relying on generalized Vs30 data may overestimate shaking and seismic risk in Port Angeles. DAS-derived Vs30 values improve hazard assessment and enhance understanding of infrastructure vulnerability.

Geological Society of America Abstracts with Program. Vol. 57, No. 6, 2025

doi: 10.1130/abs/2025AM-7171

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