11:00 AM - 11:15 AM
[SSS11-07] Seismic hazard risk of Miyanohara fault estimated by micro tremor observation
Keywords:Miyanohara fault , micro tremor observation, 2011 Northern Nagano Prefecture Earthquake, seismic hazard
The 2011 Northern Nagano Prefecture Earthquake caused the extensive damages such as building damages and slope failures concentrated along the Shinano River Tectonic Zone. These damages were concentrated on the upside of reverse fault that caused the earthquake. Concerning building damage in Tsunan Town, Niigata Pref., building damages were concentrated around the Miyanohara Fault, which was not the source fault of this earthquake. The field surveys also showed that there was a surface deformation were concentrated along the Miyanohara Fault (Nakano et al., 2013). This shows the importance of evaluating the seismic risk of geological faults in the vicinity when a large-scale active fault earthquake occurs. In this study, the earthquake disaster risk of the Miyanohara Fault from the perspective of the underground structure by microtremor observations. AS the AIST conducted group boring in a direction perpendicular to the Miyanohara Fault line, the microtremors observation of this study were done in the same area at a higher density than by boring.
Geological surveys along the Shinano River revealed that the basement rock of the terrace deposits was volcanic clastic deposits consisting mainly of volcanic breccia. The depth at the top of the terrace gravel layer roughly corresponded to the depth at which the S-wave velocity suddenly increased around 300 m/s. Comparison results with AIST's borings show that the depth of the terrace gravel layer corresponds to the depth at which the S-wave velocity rapidly increases when the S-wave velocity is around 300 m/s. Therefore, in this study, the depth at which the S-wave velocity reaches 300 m/s was defined as the basement depth, which was estimated to be the top of the terrace gravel layer. The area around the Miyanohara Fault has a smaller AVS30 and deeper basement depth than other measurement points in Tsunan Town, which clearly indicates that the ground strength is not good. The reason why AVS30 is small all around the Miyanohara fault is thought to be due to the influence of the bedrock. In fact, a geological survey near the Miyanohara Fault confirmed a fracture zone, and rock strength measurements using a Schmidt hammer was low around the fracture zone of the Miyanohara Fault.
From the results of microtremor observation, the clear displacement of top of terrace gravel layer was confirmed at two locations. The displacement on the north side is approximately 5m, and the location roughly matches the fault scarp. The displacement on the south side is approximately 7m, and no large displacement is recognized topographically. If the Kaisaka Terrace was formed about 50,000 years ago, the average displacement rate of the north side fault will be 0.11 m/1,000 years, and the south side will be 0.14 m/1,000 years. The AIST (2016) estimates the average displacement rate to be 0.13 to 0.14 m/1,000 years, which roughly matches the displacement rate only on the north side. As the microtremor observation confirmed the new displacement of the top of the gravel layer on the south side, it is necessary to estimate the average displacement rate higher. Additionally, there were several completely destroyed buildings on the south side (lowering side) of the Miyanohara fault. At the southernmost measurement point, there is a 10m or shallower layer with an S-wave velocity of less than 200m/s, so the ground is extremely soft. This suggests a ground disaster risk where a thick layer of soft sediment accumulates on the slide down side of the fault.
Geological surveys along the Shinano River revealed that the basement rock of the terrace deposits was volcanic clastic deposits consisting mainly of volcanic breccia. The depth at the top of the terrace gravel layer roughly corresponded to the depth at which the S-wave velocity suddenly increased around 300 m/s. Comparison results with AIST's borings show that the depth of the terrace gravel layer corresponds to the depth at which the S-wave velocity rapidly increases when the S-wave velocity is around 300 m/s. Therefore, in this study, the depth at which the S-wave velocity reaches 300 m/s was defined as the basement depth, which was estimated to be the top of the terrace gravel layer. The area around the Miyanohara Fault has a smaller AVS30 and deeper basement depth than other measurement points in Tsunan Town, which clearly indicates that the ground strength is not good. The reason why AVS30 is small all around the Miyanohara fault is thought to be due to the influence of the bedrock. In fact, a geological survey near the Miyanohara Fault confirmed a fracture zone, and rock strength measurements using a Schmidt hammer was low around the fracture zone of the Miyanohara Fault.
From the results of microtremor observation, the clear displacement of top of terrace gravel layer was confirmed at two locations. The displacement on the north side is approximately 5m, and the location roughly matches the fault scarp. The displacement on the south side is approximately 7m, and no large displacement is recognized topographically. If the Kaisaka Terrace was formed about 50,000 years ago, the average displacement rate of the north side fault will be 0.11 m/1,000 years, and the south side will be 0.14 m/1,000 years. The AIST (2016) estimates the average displacement rate to be 0.13 to 0.14 m/1,000 years, which roughly matches the displacement rate only on the north side. As the microtremor observation confirmed the new displacement of the top of the gravel layer on the south side, it is necessary to estimate the average displacement rate higher. Additionally, there were several completely destroyed buildings on the south side (lowering side) of the Miyanohara fault. At the southernmost measurement point, there is a 10m or shallower layer with an S-wave velocity of less than 200m/s, so the ground is extremely soft. This suggests a ground disaster risk where a thick layer of soft sediment accumulates on the slide down side of the fault.