Himaru NISHIO

Ground Motion Amplification Characteristics on Surface Ground in The Chuetsu Region of Niigata Prefecture

Takaaki IKEDA, Masataka SHIGA

Japan is one of most seismically active regions in the world experiencing may large and small earthquakes annually. Some of them cause serious damage to infrastructure (roads, bridges, buildings) and human life. To mitigate the disaster risk, it is essential to understand why certain areas experience more severe damage than others.
This research is about seismic intensity distribution of the Noto Peninsula Earthquake on January 1, 2024. Maximum intensity around the epicenter (Ishikawa prefecture) was recorded as 7. However, Niigata prefecture being several kilometers away from epicenter experienced strong intensity of about 6-lower. Earthquake intensity of 6-lower was observed at one site in Nakanoshima in Nagaoka City. This area experienced stronger intensity than its surrounding stations even being at relatively same distance from epicenter. This area recorded intensity levels higher than those observed at surrounding stations in multiple past earthquakes.
This observation of larger intensity in Nakanoshima is likely due to site effects. Site effect refers to the amplification of earthquake wave by the surface ground layers. The study first compared observed intensity with the surface amplification ratio (ARV) published by J-SHIS. Generally, sites with larger ARV show larger intensity even being at larger distances. But in several sites in Niigata, including Nakanoshima, no clear relationship is observed between ARV and larger epicentral distance. Further investigation is needed to understand the deviation of general trend of ARV with epicentral distance incorporating field work and other important factors.
This research further developed ground models using borehole data and performed seismic analyses using equivalent-linear methods for Nakanoshima and surrounding sites. It also used microtremor surveys and compared H/V spectra with transfer functions from the models. The results suggest that the engineering bedrock at Nakanoshima may be deeper than the borehole-based model. Using Earthquake observation records, the engineering bedrock depth was re-estimated, and a deeper bedrock model was developed. Updated models generated engineering bedrock waveforms with improved correspondence to the surrounding sites. Therefore, the larger intensity at Nakanoshima may be caused not only by shallow layers, but also by deeper ground structure that was not recorded by the previous boreholes.