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[SSS12-06] Relationship between ground characteristics around Miyanohara fault estimated by microtremor observation and damage to houses in the 2011 Northern Nagano Earthquake
Keywords:the 2011 Northern Nagano Earthquake, Damage to houses, microtremor observation, Miyanohara Fault
The magnitude 6.7 earthquake that occurred in the early morning of March 12, 2011, in northern Nagano Prefecture caused significant damage in Sakae Village, Nagano Prefecture and Tsunan Town, Niigata Prefecture. A concentration of landslides and gravity deformation of fill along the fault line was observed around the Miyanohara fault located in Tsunan Town (Nakano et al., 2013). In Sakae Village, extensive damage was concentrated in the Mori, Aokura, and Yokokura districts, with localized areas of severe damage. Takami et al. (2020) conducted landforms classification and microtremor observation and found that damage to houses was greater in areas that had been back marsh before the terraces separated llagefrom flooding and where AVS30 was small and prone to shaking. However, a similar study has not been conducted in Tsunan Town. Therefore, the relationship between the landforms, surface geology, the age of the houses, and the damage to the houses was investigated by estimating the shallow ground structure through microtremor observation in the western part of Tsunan Town. In addition, the effect of the Miyanohara fault on damage to houses was also examined.
The damaged houses data caused by the 2011 Northern Nagano Earthquake was provided by the Tsunan Town Office, and borehole data was obtained from “Kunijiban” site. The age of the houses and their topography were surveyed and classified by aerial photo interpretation. Outcrop survey, topography measurement, and microtremor observation were carried out as field survey. The results of these surveys were overlaid using a Geographic Information System (GIS) to examine the relationship between the damage to houses.
It was found that houses built before about 1980 were more likely to suffer damage. From the results of the microtremor observation, the AVS30 of the low terrace in Tsunan Town is about 300~600m/s, and the base depth is shallower than 5m, indicating that the ground is better than that in Sakae Village (AVS30 is about 250~600m/s; the base depth is about 10m). However, the AVS30 around the Miyanohara fault was relatively small, around 250 m/s, and the ground was not as good as in other Tsunan areas. Since the outcrop of the fracture zone was confirmed in the stream at the extension of the fault line, the influence of the fracture zone may be responsible for the vulnerable ground condition around the Miyanohara fault. In addition, the Miyanohara fault is a reverse fault that tilts down to the north, and the reason why complete collapse damage of houses were concentrated on the downward (south) side of the fault was because the ground of this area was soft with the AVS30 around 200 m/s. This is thought to be because soft sediments were easily deposited on the downward side of the fault; that is, the geomorphic development process caused by the fault activity is possible to have affected the damage to houses.
The S wave velocity at the upper end of the terrace gravel layer and the basement rock are estimated to be around 300 m/s and around 500 m/s respectively, based on the combination of the outcrop observation results of the Shinano riverbed and the microtremor observation results. The average displacement velocity of the Miyanohara Fault was estimated to be 0.118 and 0.164 [m/10^3 years] using the former results and the observations of microtremors on the upper and lower side of the fault, which is consistent with the average displacement velocity of the Miyanohara Fault reported by the Active Fault Research Group (1991).
The damaged houses data caused by the 2011 Northern Nagano Earthquake was provided by the Tsunan Town Office, and borehole data was obtained from “Kunijiban” site. The age of the houses and their topography were surveyed and classified by aerial photo interpretation. Outcrop survey, topography measurement, and microtremor observation were carried out as field survey. The results of these surveys were overlaid using a Geographic Information System (GIS) to examine the relationship between the damage to houses.
It was found that houses built before about 1980 were more likely to suffer damage. From the results of the microtremor observation, the AVS30 of the low terrace in Tsunan Town is about 300~600m/s, and the base depth is shallower than 5m, indicating that the ground is better than that in Sakae Village (AVS30 is about 250~600m/s; the base depth is about 10m). However, the AVS30 around the Miyanohara fault was relatively small, around 250 m/s, and the ground was not as good as in other Tsunan areas. Since the outcrop of the fracture zone was confirmed in the stream at the extension of the fault line, the influence of the fracture zone may be responsible for the vulnerable ground condition around the Miyanohara fault. In addition, the Miyanohara fault is a reverse fault that tilts down to the north, and the reason why complete collapse damage of houses were concentrated on the downward (south) side of the fault was because the ground of this area was soft with the AVS30 around 200 m/s. This is thought to be because soft sediments were easily deposited on the downward side of the fault; that is, the geomorphic development process caused by the fault activity is possible to have affected the damage to houses.
The S wave velocity at the upper end of the terrace gravel layer and the basement rock are estimated to be around 300 m/s and around 500 m/s respectively, based on the combination of the outcrop observation results of the Shinano riverbed and the microtremor observation results. The average displacement velocity of the Miyanohara Fault was estimated to be 0.118 and 0.164 [m/10^3 years] using the former results and the observations of microtremors on the upper and lower side of the fault, which is consistent with the average displacement velocity of the Miyanohara Fault reported by the Active Fault Research Group (1991).