Minimizing Geophysical Site Characterization Procedures to Estimate VS30 through the Use of VR40

Jose R. Gomez, & Alan K. Yong

Published August 14, 2021, SCEC Contribution #11408, 2021 SCEC Annual Meeting Poster #203

VS30, the time-averaged shear wave velocity of the upper 30 m from the ground surface, is typically calculated from shear-wave velocity (VS) profiles. VS profiles are often determined by expensive and invasive borehole-based methods; or most recently, by noninvasive surface-wave methods (SWMs). These SWMs, while both less costly and environmentally restrictive, involve time-consuming iterative inversion calculations, in addition to extensive field acquisition procedures requiring large multi-channel arrays and/or multiple methods involving active- and/or passive-sources. Studies, like that of Brown et al. (2000) and Yong et al. (2017), have established linear empirical relations between the Rayleigh-wave phase velocity at the 40 m wavelength (VR40) and VS30. Furthermore, VS30 estimated by means of VR40 have been shown to correlate strongly (R2 > 0.97) with VS30 obtained by means of VS profiles (Yong et al., 2017). We test a minimal procedure for estimating VS30 by measuring VR40 using standalone passive or active SWMs from data acquired by Yong et al. (2013). Our procedure utilizes the fundamental-mode dispersion curve, typically derived by the VR spectrum of various SWMs, to measure VR40. The purpose—while not to eliminate the need for VS profiles necessary for detailed ground response analyses—is to design a method to efficiently expedite the estimation of VS30. Our procedures emphasize the use of active-source SWMs in rural areas, and passive-source SWMs in urban or high ambient-noise areas where sensors are known to be less sensitive to active-source signals. In addition, our SWM deployment and acquisition procedure requires an hour or less to complete in the field, and with minimal equipment. Analyses show good agreement (average difference of 4%) between our VR40-based VS30, using our minimal 3–5 channel array designs, and traditional VS-profile based VS30, derived from more extensive standard 24–48 channel arrays and/or multiple SWMs. The main motivation for employment of our rapid data collection and VS30 estimation procedures is to quickly advance the supplementation of VS30 data in areas of sparse coverage, such as at seismic stations lacking near surface velocity site characterization data.

Key Words
site characterization, VS30, VR40, surface wave methods

Citation
Gomez, J. R., & Yong, A. K. (2021, 08). Minimizing Geophysical Site Characterization Procedures to Estimate VS30 through the Use of VR40. Poster Presentation at 2021 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology