A Proposal of An Index Based on Microtremor Measurements To Discriminate Irregularity of Subsurface Boundary

Abstract eng:
The subject of this article is to see if a spatial variation of horizontal to vertical spectral ratios (HVSRs) of microtremors can be regarded as a proxy of an irregularity effect of sediment-basement interface in order to readily discriminate a flat layer site from an irregular layer site. We performed 3 investigations: 1) we evaluated coefficients of variation (CVs) of HVSRs’ peak periods at 4 sites based on densely mobile microtremor measurements, 2) evaluated sensitivity for CVs by numerical simulations for wave propagation with complex media, and 3) compared power spectral density estimated from CVs with that calculated from subsurface structure model. We found that CVs were 1) obviously different between flat layer sites and irregular layer sites in observed microtremors, and 2) sensitive to the slope angle of sediment interfaces. We also found that 3) CVs were related to the irregularity of the basement interfaces. As a result, we propose that CVs of 0.1 is a threshold to sort out flat layer sites where amplification factor can be approximately calculated assuming stratified media.

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