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Averaged S-Wave Site Amplification Factors for Sites With Vs30≧760 M/s Derived From Git Analysis of K-Net and Kik-Net Ground Motions and Its Consequence for Ground Motion Modeling

Description: 

We have been studying horizontal site amplification factors (HSAFs) from the observed strong motions by K-NET and KiK-net of the National Institute of Earthquake Research and Disaster Resilience by using the so-called generalized spectral inversion technique (GIT, Nakano, et al., BSSA, 2015). We have used a reference site where we extracted the site amplification at a KiK-net site using a theoretical transfer function to get the outcrop motion with the S-wave velocity (Vs) of 3.45 km/s. Thanks to this reference site’s condition, our separated HSAFs are the largest and our separated acceleration source spectra of events (ASSs) are the smallest in amplitude among GIT studies conducted for a similar dataset. The choice of the reference site condition determines the relative contribution between HSAFs and ASSs, however, the importance of the choice seems to be underestimated in the past. The reference site condition is important because any HSAF at the reference site will be automatically mapped onto all the ASSs in GIT. The situation is the same for GMPE analyses. In this report, we showed the averaged HSAFs of our GIT results as a function of Vs30 at K-NET and KiK-net sites. The purpose of this analysis is not to recommend the use of the binned averages as a substitute for a site-specific HSAF, but to investigate quantitatively the risk of a reference site or sites that is(are) not considered to be site-effect free. We calculated geometrical averages of HSAFs with Vs30 in the ranges of Vs30<180 m/s (Class E), 180≦Vs30<360 m/s (Class D), 360≦Vs30<760 m/s (Class C), 760≦Vs30<1,000 m/s (Class B2), and 1,000 m/s≦Vs30 (Class B1 to A). We found that the average HSAF for Class B2 has 2.0 at 1 Hz and more than 3.0 at 10 Hz. Therefore, if we use a site as hard as Class B2 as a reference, this level of amplification will be mapped onto the ASSs and will result in a higher stress-drop estimate because of higher high-frequency amplitude in ASSs.

Session: Future Directions in Physics-based Ground-motion Modeling in Preparation for the Fall 2023 Meeting

Type: Oral

Date: 4/18/2023

Presentation Time: 05:00 PM (local time)

Presenting Author: Hiroshi Kawase

Student Presenter: No

Invited Presentation: 

Authors