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Linear and Nonlinear Site Effects at Several Sites in the Noto-hanto Area in Japan and Its Possible Cause as 2D/3D Deep Basin Resonance

Description: 

On January 1, 2024, the Noto-Hanto earthquake of Mw7.6 occurred in the northern part of the Noto Peninsula in Japan. The shaking caused devastating damage to wooden houses in rural cities and towns as well as local roads and expressways in the peninsula. The rupture of the seismogenic fault was extended bilaterally to SSW and NNE directions for about 70 to 80 km from the epicenter. About 20 strong motion stations are operated by NIED and JMA within 60 km of the fault. Before the earthquake, we performed the generalized inversion analysis (GIT) to separate source, path, and linear site terms for both the S-wave portion with the maximum duration of 15 s and the whole duration of motion. The resultant horizontal site amplification factor for the former is named sHSAF, while the latter is wHSAF. Among 20 sites inside the Noto Peninsula, we found three sites with extraordinarily large (80~100 times) sHSAFs and wHSAFs in the frequency range equal to or less than 1 Hz, which we cannot explain by simple 1D structures; we desperately need to model 2D/3D resonance of the deep basins below these sites.

As for the mainshock records, we performed GIT in two steps. The first step is a linear GIT for only far-field records with peak ground acceleration (PGA) less than 200 cm/s². From this first step, we determined source and path terms for the mainshock. In the second step, we determined the bedrock spectra for S-wave portions at the sites with PGA larger than 200 cm/s² and calculated nonlinear horizontal site amplification factors, sHSAF_NL and wHSAF_NL as the ratios of observed spectra with respect to the bedrock spectra at a specific site. We found that the peak frequencies at these sites with extraordinarily large HSAFs were clearly shifted toward the lower frequency side during the mainshock (about 1/2 of the linear one). Still, the peak amplitudes were not so much reduced or even became higher than the linear HSAFs. To account for these peculiar empirical observations, we need to model the deep basin structures and their nonlinear behavior in a quantitative manner as much as we can.

Session: Recent Advances in Modeling Near-source Ground Motions for Seismic Hazard Applications [Poster]

Type: Poster

Date: 4/16/2025

Presentation Time: 08:00 AM (local time)

Presenting Author: Hiroshi

Student Presenter: No

Invited Presentation: 

Poster Number: 53

Authors