Directional Dependence of the Horizontal-to-Vertical (H/V) Spectral Ratios of Microtremors due to Lateral Heterogeneity of the Subsurface Structure (in-person presentation)
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Shinichi Matsushima
Disaster Prevention Research Institute (DPRI), Kyoto University
- Date & Time
- Location
- In-person presentation (online via Microsoft Teams)
- Host
- Alan Yong
- Summary
To estimate site effects during strong shaking induced by earthquakes, knowledge about the characteristics of the subsurface structure is essential. Information derived from microtremors (ambient noise) are often used to characterize the site or identify the subsurface structure. Horizontal-to-Vertical (H/V) spectral ratios (HVRs) are used practically to delineate the site characteristics or the subsurface structure. In this presentation, a new interpretation of Microtremor HVRs (MHVRs) and its potential to detect the lateral heterogeneity of the subsurface structure is introduced. MHVRs have been traditionally interpreted as representing either the Rayleigh wave ellipticity for a horizontally layered structure or directly the S wave amplification. However, based on the diffuse field theory (DFT), an alternative theoretical basis has been proposed: MHVR corresponds to the square root of the ratio of the energy density, which can be related to the imaginary part of the Green’s function for the horizontal and vertical components. If this condition holds, the one-dimensional (1D) horizontal layering assumption used in traditional interpretation is no longer necessary. As observational evidence of non 1D MHVRs show, significant directional dependency was found at some sites in Japan. At the Uji campus, the north-south/up-down (NS/UD) spectra of MHVRs have higher peak amplitudes at approximately 0.5 Hz and the east-west/up-down (EW/UD) spectra have slightly higher peak frequencies. At Onahama, there are sites where strong directional dependency are found. Results of numerical analyses that were performed by the Spectral Element Method using a unit load on the surface to calculate Green’s functions to derive numerical MHVRs based on DFT to examine the effect of the two-dimensional (2D) basin structure to the MHVRs is introduced. They show that the directionally dependent MHVRs can be simulated, at least qualitatively, using a 2D velocity structure and considering DFT. Since the 2D velocity structures are known to amplify the ground motion during earthquakes, the effort to investigate if the site is influenced by 2D subsurface structure instead of 1D is very important. Understanding the directional dependence of MHVRs may provide significant advancements toward estimating the effect of lateral heterogeneity in and around the site.