Study on Ground Amplification Characteristics Based on the Microtremor Observation Database in Japan

The National Research Institute for Earth Science and Disaster Resilience (NIED) has created a shallow and deep integrated ground structure model (SD model) covering approximately one-third of Japan's land area for broadband strong ground motion evaluation. A portion of this model was released by Hedquater Eerthquake Reserch Promotion(HERP) and J-SHIS in March 2021. To create this ground model, over one million borehole data points have been collected and organized nationwide to date. Microtremor array observations and analysis have been conducted at approximately 60,000 locations spaced about 1 km apart (grid cells). Ground amplification coefficients based on AVS30, used to estimate ground amplification within Japan, are primarily calculated from PS logging results at seismic observation points operated by NIED, namely K-NET and KiK-net. However, K-NET has not conducted borehole surveys deeper than 20m in practice, and while KiK-net performs PS logging at depths exceeding 100m, the accuracy of near-surface stratigraphic data is not high. This study aims to verify the accuracy of AVS30-based amplification coefficients using the ground model and to achieve high-precision estimation of amplification coefficients without PS logging. To achieve this objective, microtremor array surveys (small-scale arrays) were conducted at approximately 2,500 sites distributed across K-NET, KiK-net, Japan Meteorological Agency, and municipal seismic observation points. Based on the S-wave velocity structure obtained from microtremor array observations at the aforementioned 60,000 sites, the relationship between AVS30 and ground amplification coefficients was evaluated.