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Primary Linear Site Response Parameters From Transfer Functions and Ratios of Response Spectra

Session: How Well Can We Assess Site Effects So Far? IV

Type: Oral

Date: 4/21/2021

Presentation Time: 05:00 PM Pacific

Description: 

Site response is determined by the velocity structure and other dynamic parameters and input ground motion at a site and measured by the soil-to-rock spectral ratio (SSR) or borehole transfer function (TF) in seismology and ratio of response spectra (RRS) in earthquake engineering. Thus, quantification of site response is nonergodic, or site-specific. A response spectrum is a plot of the peak response of a series of a single-degree-of-freedom system with a natural period and damping to an input ground motion; it is fundamentally different from the Fourier spectrum of the input ground motion. In other words, site response is quantified differently in seismology and earthquake engineering. The site response parameters (i.e. base-mode and peak frequencies or periods and corresponding amplifications) are of primary importance and are the subject of this investigation. We used weak ground-motion recordings (PGA < 0.05g) to measure these parameters from empirical TFs and RSSs at 13 U.S. borehole arrays. We also performed 1D linear site-response analyses to obtain theoretical TF and RSS at each site. Our theoretical and empirical results are consistent with each other and show that TFs are significantly different from RRSs. We also observed that 1) the fundamental site period, T_f, measured from TF and from RRS are nearly identical, 2) the maximum RRS, RRS_max, occurs at T_f, and 3) the magnitude of TF at T_f, A₀, is larger than RRS_max at all sites. Furthermore, we found that RRS_max and A₀ are correlated. Therefore, the T_f and RRS_max or T_f and A₀ are of primary important and can be considered as the primary parameters for quantifying linear site response, site resonance in particular, in earthquake engineering. We also compared T_f and RRS_max with Vs30 and found no correlation between them. Therefore, Vs30 is not an appropriate proxy to parameterize site response in engineering design and other applications.

Presenting Author: Zhenming Wang

Student Presenter: No

Authors