Direct Evaluation of Horizontal Amplification Factor HHbR From Earthquake HVR and Empirical Vertical Amplification Factor VHbR

Date: 4/26/2019

Time: 09:15 AM

Room: Pine

We have been discussing on how to obtain velocity structures below the target sites and what is the best proxy for site characterization for long time. But what we really need is not the velocity but the horizontal amplification factor (HHbR) relative to the reference bedrock spectrum Hb, since it controls the ground motion characteristics.

As an empirical tool, the Horizontal-to-Vertical ratios of earthquakes (EHVRs) or microtremors (MHVRs) have been utilized to extract ground motion characteristics. The so-called “Nakamura” method assumes that MHVR provides us directly HHbR. However, we need to validate two main assumptions; 1) EHVR and MHVR should be the same and 2) the vertical amplification factor (VHbR) should be unity.

As for the difference of EHVR and MHVR, the diffuse field concept (Sánchez-Sesma et al., 2011; Kawase et al., 2011) lead us to see the systematic difference between EHVRs and MHVRs and therefore we have proposed empirical correction factors, EMRs, the ratios of EHVR to MHVR (Mori et al., 2018). We can use these EMRs to transform MHVR into pseudo EHVR (pEHVR).

As for VHbR we have used generalized spectral inversion technique (Nakano et al., 2015) to separate HHbR and VHbR from S-wave part of weak-motions observed in Japan. We used YMGH01 with small site correction to obtain amplification relative to the seismological bedrock spectrum Hb. We found that VHbR is much similar to each other than HHbR because P-wave velocity contrasts would be smaller than S-wave velocity contrasts in general. We calculated the averaged VHbRs for eight categorized sites with different predominant frequencies in EHVR. Then we can obtain pseudo HHbR as EHVR*VHbR. Since the site-specific variation in VHbR is relatively small, the pseudo HHbR can be conveniently and precisely derived from EHVR in the frequency range from 0.1 to 15 Hz without any velocity structures, as long as we have small numbers (~10) of weak-motion records at the target site. If we have only MHVR, we need first transform it to pEHVR then get pseudo HHbR as EMR*MHVR*VHbR.

Presenting Author: Hiroshi Kawase

Authors