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A Minimized Site Characterization Method to Estimate VS30 using VR40

Jose R. Gomez, Alan Yong, Koichi Hayashi, Antony Martin, Andrea Di Martino, Tara Nye, Lilah M. Guerra, & Jamison H. Steidl

Submitted September 11, 2022, SCEC Contribution #12275, 2022 SCEC Annual Meeting Poster #041

We estimate VS30, the time-averaged shear wave velocity of the upper 30 m from the ground surface, using a novel minimal site characterization method (MSCM) based on correlations with VR40, the Rayleigh-wave phase velocity at the 40 m wavelength determined from fundamental-mode dispersion curves. Conventional methods calculate VS30 from shear-wave velocity (VS) profiles, which are determined by expensive and invasive borehole-based methods or by noninvasive body- or surface-wave methods (SWMs). Although they are more economical, SWMs still involve field acquisition procedures requiring large multi-channel arrays and multiple methods using active and/or passive sources, as well as necessitate time-consuming iterative inversion calculations. Our goal, while not to eliminate the need for VS profiles necessary for detailed ground response analyses, is to design a method to expedite estimation of VS30 at seismic stations where currently no measured shear-wave velocity information exists. Two linear empirical relations between VS30 and VR40 have been established by Brown et al. (2000) and by Yong et al. (2017). Thus far, our testing of our MSCM is by two means: utilizing decimated datasets originally acquired using conventional SWMs by Yong et al. (2013); and, by field data acquisitions to directly apply our MSCM. We measure VR40 utilizing the fundamental-mode dispersion curve, typically derived from the VR spectrum of various SWMs, and we utilize the empirical relations to directly estimate VS30. We perform these tests on data from 55 seismic stations: 25 stations are tested by means of direct application of our MSCM, while 30 stations are tested using decimated datasets from Yong et al. (2013). Pacific Gas and Electric-operated dam sites are also sites of interest and provide test cases that contribute to verifying the viability of our MSCM. In addition to measurements of VR40, the site characterization approach provides microtremor (noise) recordings for estimations of the dominant site frequency (fd) using the horizontal-to-vertical spectral ratio (mHVSR) method. Comparison of our VR40-based VS30 and conventional VS profile-based VS30 show good agreement, with average differences less than 3.7%, while mHVSR-based fd analyses are ongoing. Further field tests of our MSCM will validate the robustness of the approach across a variety of site conditions.

Gomez, J. R., Yong, A., Hayashi, K., Martin, A., Di Martino, A., Nye, T., Guerra, L. M., & Steidl, J. H. (2022, 09). A Minimized Site Characterization Method to Estimate VS30 using VR40. Poster Presentation at 2022 SCEC Annual Meeting.

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