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Inherent Limitations of One-Dimensional Ground Response Analyses

Description: 

The first computational model of elastic wave propagation in horizontally layered media was proposed 70 years ago by Haskell and Thomson. Later, ground motion modelers further simplified the horizontally layered model for vertical wave propagation by assuming deep seismic sources and low-velocity near-surface layers. This simplification reduces the governing 3D vector wave equation, which involves diffraction, reflection-transmission, and conversion of P and S body and surface waves, to a 1D scalar wave equation that only contains the reflection-transmission of S waves. Such 1D models have been utilized as “reasonable” tools for ground response analyses because of the higher computational cost of 2D/3D models and the lack of adequate subsurface data. Recent advancements in computational science and ambient noise subsurface inversion, however, have alleviated the limitations posed by computational cost and insufficient subsurface data and made true ground response analyses more feasible. In this study, we model seismic wave propagation in the Treasure Island Downhole Array site using both 1D and 2D representative soil profiles and discuss the inherent limitations of 1D models. We use the finite volume code FLAC, which combines the simplicity and robustness of finite difference method with the geometric flexibility of finite element method. The velocity model shows a bedrock layer in the form of an embedded single slope. We use high-frequency excitations to “decouple” various components of the scattered wavefield and reveal their interactions in forming the observed ground response. Part of the vertically propagating shear wave is diffracted at the tip and toe of the single slope bedrock and further propagates toward the ground surface as cylindrical P and S body waves. These obliquely incident body waves will then generate surface P and S waves that travel in the downhill direction. While these 2D surface waves give rise to a better match between field recordings and numerical simulations, we still need to consider 3D out-of-plane effects and non-vertical excitations to adequately capture the true response.

Session: Ground Truthing Multidimensional Site Response Analyses at Borehole Array Sites

Type: Oral

Date: 4/20/2023

Presentation Time: 04:45 PM (local time)

Presenting Author: Kami Mohammadi

Student Presenter: No

Invited Presentation: 

Authors