Towa OYA

Elucidation of earthquake damage in the Budokubo district during the Niigata Chuetsu earthquake, focusing on ground characteristics

Takaaki IKEDA

During the Chuetsu earthquake that occurred in Niigata Prefecture on October 23, 2004, a maximum seismic intensity of 7 was observed, causing significant damage at many observation points, and a lot of damage occurred in the former Kawaguchi town in Niigata Prefecture, which also observed a seismic intensity of 7. However, it was reported that the situation of house damage in the Budokubo area varied by area, despite being within a narrow range of less than 1 km² in one settlement, and it was pointed out that the ground characteristics may have affected the earthquake damage. In previous studies, detailed analysis of the damage situation of houses in the area was conducted, and the damage information of the houses was updated. Also, in the evaluation of ground characteristics using constant microtremor exploration, the predominant vibration number of the ground at multiple points in the area was calculated, and it was confirmed that in the area with large damage, it was predominant below 2.5Hz, and in the area with small damage, it was predominant above 3.0Hz, suggesting a correlation between the predominant period of the surface ground and the distribution of earthquake damage. However, although a rough correlation between large and small damage was shown, the results were unclear about the relationship between moderate damage and the predominant vibration number. Takada, with the aim of continuous evaluation of ground characteristics, which was a challenge in Asama, estimated the ground structure using a surface wave exploration method that can grasp continuous ground characteristics, and evaluated the relationship between the ground structure and the damage distribution using the results. As a challenge, it was pointed out that the analysis results of surface wave exploration were improved, and that no clear difference was found between large and medium damage, and that it was a qualitative evaluation. Surface wave exploration greatly changes the ground structure obtained depending on the measurement data and analysis method, so in this study, we performed a re-analysis considering the removal of high-order modes of the dispersion curve and the continuity of the ground. After that, generally, the results of surface wave exploration are corrected using other ground data. However, since there is no ground data including drilling in the area, in this study, we conducted constant microtremor exploration at 20m intervals on the survey line of surface wave exploration, and used the results to correct the surface wave exploration. The observed H/V spectrum obtained by constant microtremor exploration and the theoretical H/V spectrum calculated theoretically from the ground structure obtained from surface wave exploration at that point were compared, and the S-wave velocity of the ground structure of surface wave exploration was changed for the ground where the shape of the H/V spectrum roughly matches, and the correction was made by matching the predominant vibration number of the theoretical H/V spectrum within 10% of the observed H/V spectrum. A two-dimensional seismic response analysis was performed using the corrected ground structure obtained by superFLUSH. 
From the above, it was found that in the areas with large and medium damage, layers with low S-wave velocity were thickly deposited, and in areas with small damage, layers with high S-wave velocity were thickly deposited. It was found that these layers with low S-wave velocity in the surface ground significantly amplified the seismic motion, and the difference in S-wave velocity structure caused a significant difference in the seismic motion on the ground surface. Therefore, it is believed that the difference in the thickness of the weak layer of the surface ground caused a significant difference in the damage to wooden houses.