Modeling and Inversion of Site Effects Using Physical Properties of Layered Subsurface

Session: Near-Surface Effects: Advances in Site Response Estimation and Its Applications [Poster]
Type: Poster
Date: 4/28/2020
Time: 08:00 AM
Room: Ballroom
Description: 

In order to achieve accurate seismic characterization of the near surface, it is necessary to identify the set of physical properties that determine wave propagation and resonances. Knowledge of physical properties is particularly important for measuring or predicting the effects of topography, buildings and other structures on seismic shaking. The properties responsible for near-surface seismic phenomena are density, bulk and shear elasticity, anisotropy and “attenuation”. Although the first four of these properties are generally well understood, “attenuation” is only described in terms of empirical attributes such as the spectral decay parameter kappa or damping ratio ksi. These parameters are usually explained by their relations to the quality factor Q. However, upon close consideration, the Q is also a non-physical, empirical quantity, which becomes particularly difficult to justify in the (commonly observed) case when it is frequency-dependent. Thus, in order to quantitatively model the inelasticity of the subsurface and predict the kappa, Q and ksi values, rigorous equations of physics are still needed. Here, we examine a set of such equations combining viscoelasticity, Biot’s poroelasticity and models of “wave-induced fluid-flow” effects. To achieve the simplest parameterization, we only consider effective solid viscosities of the layers. A layered model is constructed with variable elastic and viscous properties, which are inverted for by fitting the horizontal to vertical-component spectral ratios (HVSR) from several seismic stations in Zagros area in Iran. The rigorous physical model allows investigation of the dependence of HVSR on arrival types, angles of incidence and on recording within coda windows. The model also predicts the kappa, Q, ksi, transfer functions and response spectra of the layered subsurface.

Presenting Author: Igor Morozov

Authors