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Velocity Structure Identification at a Rock Site in Japan Based on Diffuse Field Concept for Earthquake

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

Type: Oral

Date: 4/21/2021

Presentation Time: 05:30 PM Pacific

Description: 

Many approaches have been proposed to estimate the site amplification factor, but the estimation of the detailed site amplification factor at the rigid rock site is still difficult because the amplification features at such a rock site does not clearly appear on the observed data such as the dispersion curve in the array microtremor measurements. Based on the diffuse field concept for earthquake (DFCe, Kawase et al., 2011) the Horizontal-to-Vertical spectral Ratio of earthquake (EHVR) is interpreted as the absolute solution of the ratio of the horizontal and vertical amplification factors between the ground surface and the seismic bedrock. If the ground motion is not amplified in both horizontal and vertical directions at a rock site, the theoretical EHVR should be flat, therefore we can identify the ground structure of the rock site as the one with no impedance contrast. We applied DFCe to the EHVR observed at a KiK-net site in Japan named as YMGH01 which is installed on the stiff rock. The observed EHVR of YMGH01 does not have any large peaks between 0.08 Hz and 20 Hz, but have two small (~2) peaks around 0.12 Hz and 0.4 Hz, as well as small fluctuations in the higher frequency range. These peaks at 0.12 Hz and 0.4 Hz are also measured at a K-NET site named as YMG013 which is located on the same deep ground structure at a distance of 3.2 km from YMGH01, so we concluded that these small peaks reflected the amplification generated at the deep ground structure in this region. We identified the velocity structure shallower than the seismic bedrock based on DFCe using the EHVR in the wide frequency range of 0.08 Hz to 20 Hz. The obtained velocity structure reproduces the observed EHVR well, especially the small peaks at 0.12 Hz and 0.4 Hz. This means that we can identify the velocity structure at a rock site which explains the whole shape of the EHVR, not only its amplification characteristics but also their detailed features.

Presenting Author: Fumiaki Nagashima

Student Presenter: No

Authors