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[SSS10-04] Rerationship between earthquake disaster and basement depth due to active faults in Iiyma City, Nagano Prefecture
Keywords:Zenkoji Earthquake, Earthquake disaster, basement depth, micro tremor, Iiyma City, active fault
Iiyama City is located in the Iiyama Basin, which extends northeast-southwest along the Chikuma River in the northeastern part of Nagano Prefecture, and the western edge of the basin is bordered by the Western Edge Fault Zone of the Nagano Basin. The authors conducted leveling surveys and micro tremor survey in the center of Iiyama City area, and examined the relationship between the estimation of the shallow ground structure of Iiyama City area and past earthquake disasters, and the relationship with tectonic landforms and active structures.
Leveling survey was carried out with two survey lines crossing the low earthquake cliff with north-south direction caused by the 1847 Zenkoji Earthquake, and micro tremor survey were carried out at 11 points covering the target area around it. In the micro tremor survey, the depth at which the S wave velocity exceeds 300 m / s was used as the engineering base depth. On survey line 1 on the north side of the survey area, the relative height of the low cliff was about 1 m, and the displacement of engineering basement depth of the two points across the low cliff was about 5 m. On survey line 2 on the south side of the survey area, the relative height of the low cliff was about 80 cm, and the displacement of engineering basement depth of the two points across the low cliff was about 3 m. Assuming that the basement depth is contemporaneous, the displacement of the basement depth is larger than the relative height of the low cliff, which may indicate the accumulation of fault displacements that formed the low cliff.
In the area around Kita-Iiyama Station northwest of the Iiyama Castle Ruins, the basement depth is more than 15m lower than the surrounding area. The area around Kita-Iiyama Station was the area where the building damage caused by the 1847 Zenkoji Earthquake was remarkable. It can be considered that the earthquake damage was remarkable in the area where the soft ground was thickly deposited. The authors considered that the cause of the deep basement depth at the western edge of the Iiyama basin is likely to be due to structural movements such as back thrust associated with active faults.
Leveling survey was carried out with two survey lines crossing the low earthquake cliff with north-south direction caused by the 1847 Zenkoji Earthquake, and micro tremor survey were carried out at 11 points covering the target area around it. In the micro tremor survey, the depth at which the S wave velocity exceeds 300 m / s was used as the engineering base depth. On survey line 1 on the north side of the survey area, the relative height of the low cliff was about 1 m, and the displacement of engineering basement depth of the two points across the low cliff was about 5 m. On survey line 2 on the south side of the survey area, the relative height of the low cliff was about 80 cm, and the displacement of engineering basement depth of the two points across the low cliff was about 3 m. Assuming that the basement depth is contemporaneous, the displacement of the basement depth is larger than the relative height of the low cliff, which may indicate the accumulation of fault displacements that formed the low cliff.
In the area around Kita-Iiyama Station northwest of the Iiyama Castle Ruins, the basement depth is more than 15m lower than the surrounding area. The area around Kita-Iiyama Station was the area where the building damage caused by the 1847 Zenkoji Earthquake was remarkable. It can be considered that the earthquake damage was remarkable in the area where the soft ground was thickly deposited. The authors considered that the cause of the deep basement depth at the western edge of the Iiyama basin is likely to be due to structural movements such as back thrust associated with active faults.