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Vs30 Site Characterization in the City of St. Helena, Napa County, California Using Active-Source Refraction Tomography

Description: 

As part of the USGS’ efforts to characterize strong-motion recording sites in northern California, on August 8, 2022, we conducted a shallow-depth, active-source seismic survey to evaluate time-averaged shear-wave velocities in the upper 30 m (Vs₃₀) near strong-motion seismometer NP-1764 in the city of St. Helena, Napa County, California. Shear wave measurements are used to determine soil densities based on National Earthquake Reduction Program (NEHRP) site characterization chart. We recorded seismic data using 47 SmartSolo three-component nodal seismometers, placed along a 92-m-long (2-m spacing) linear array. We used a hammer and plate combination to generate seismic sources at each seismometer. Compressional and Rayleigh waves were generated from vertical strikes on an aluminum plate, and shear and Love waves were generated from horizontal strikes on an aluminum block, anchored to the ground with a metal rod. We developed two-dimensional P- and S-wave refraction tomography velocity (Vp and Vs, respectively) models. Vp ranges from 1000 m/s to 4500 m/s along the profile, and Vs ranges from 420 m/s to 800 m/s. We evaluated Vs₃₀ at each seismometer location and found that Vs₃₀ ranges from 521 m/s to 660 m/s along the profile, with the mean average velocity at 580 m/s. For the seismometer closest to strong-motion station NP-1764, we determined Vs₃₀ to be 541 m/s, indicating a NEHRP site classification of C (dense soil, soft rock). Geologic mapping shows the site to consist of surficial Quaternary alluvial deposits, underlain by Tertiary basement volcanic rocks. The 1500 m/s velocity contour in our Vp model suggests that the groundwater table is about ~11 m deep. We also developed Vp/Vs ratio and Poisson’s ratio models for the profile, with Vp/Vs ratios (2 to 3) and Poisson’s ratios (0.3 to 0.4) being highest in the upper 3 m along the profile and at depth near the west end of the profile, between meter 1 and 20.

Session: From Sensors and Networks to Site Characterization and Site Response: Coming Full Circle [Poster]

Type: Poster

Date: 4/18/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Delton A. Samuel

Student Presenter: Yes

Invited Presentation: 

Authors