Comparison of Observed Earthquake and Microtremor Horizontal-to-Vertical Spectral Ratios and Inversion of Velocity Structures Based on Their Empirical Ratios

Abstract

In order to establish a method to estimate velocity structures using single-station microtremor records, we compared horizontal-to-vertical spectral ratio of earthquakes (EHVR) with that of microtremors (MHVR) at 100 K-NET and KiK-net sites in Japan. We found that they share similarities but have some differences in their shapes, especially after the first peak. This is because MHVR mainly consists of surface waves so that peaks associated with higher modes would not be prominent. Based on this observation, we calculated EHVR-to-MHVR ratios (we called them as EMRs) to investigate if there are statistical difference to find that there are systematic difference in the observed EMRs. Using EMRs averaged over different peak-frequency categories, we converted MHVRs to pseudo EHVRs by multiplying the EMRs to MHVRs, which are found to have higher correlation with EHVRs than MHVRs. Finally, we identified velocity structures by using EHVRs, MHVRs and pseudo EHVRs, and found that velocity structures obtained from pseudo EHVRs were more similar to those obtained from EHVRs than MHVRs. When we use MHVRs directly as a simple substitute of EHVRs, we can obtain velocity structures that can reproduce MHVRs quite well but the structures obtained from MHVRs are not necessarily reflecting the true structures.