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Are There Unique Parameters and Proxies for Predicting Site Response? Examples From Selected Borehole Strong Motion Arrays

Description: 

Although the physics governing site response are well understood as the increase of shear-wave impedance (ρVs) with depth, scientists and engineers have yet to reach a consensus on the most important parameters and proxies to quantify or characterize site response. For example, there is no agreement on whether the fundamental frequency (f₀) or peak frequency (f_p) is a better parameter. It has also been found that Vs30 is not an appropriate proxy despite widespread use. In this study, we collected weak (PGA ≤ 0.05g) and limited strong (PGA > 0.05g) ground motions and Vs profiles from 54 borehole arrays penetrating bedrock from the United States and Japan. We derived empirical transfer functions (TFs) from the ground motions and linear theoretical TFs from 1D site-response analyses. We also characterized the Vs profiles using the impedance ratio (IR) and velocity ratio (VR). Our results show that site response is dependent not only on the overall velocity structure of the sediments and bedrock, but also the details of sediment velocity structure in terms of IR or VR distribution, the maximum impedance (IR_max) or velocity ratio (VR_max) in particular. Our results also show that site response is nonlinear: f₀/A₀ and f_p/A_p change with increase of ground-motion level. Due to data limitations, we only extracted the linear site parameters, including the fundamental-mode (i.e., base mode) frequency (f₀) and its associated amplification (A₀), and peak-mode frequency (f_p) and its associated amplification (A_p), from the empirical and theoretical TFs, and site proxies, including IR_max, VR_max, the depth to the interface with IR_max (Z_max), Vs30, Z1.0, and Z2.5 from the Vs profiles. We compared these parameters and proxies and showed that there is no single parameter or proxy which can be used uniquely to quantify or characterize linear site response. Our results demonstrated that the complexity of site response is determined by the complexity of site conditions and nonlinearity.

Session: Site-specific Modeling of Seismic Ground Response: Are We Quantitative Enough to Predict?

Type: Oral

Date: 4/19/2023

Presentation Time: 10:45 AM (local time)

Presenting Author: Zhenming Wang

Student Presenter: No

Invited Presentation: 

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