10:00 AM - 10:15 AM
[SSS14-05] Past activity of Shionohira Fault estimated by micro tremor observation, Iwaki City, North-East Japan
Keywords:microtremor obsevation, Shionohira Fault, Tsunagi Basin, Iwaki City
The 2011 Fukushima Hamadori Earthquake (M7.0) caused a surface earthquake fault in Iwaki City. The main activity was on the west side of the Itosawa fault (Shionohira fault), with a vertical displacement of 1.9 m appearing at Shionohira. The Shionohira fault was active in 2011 up to an area not previously known to be an active fault, and a surface earthquake fault with a vertical displacement of about 60 cm appeared at Tsunagi, the northern end. The authors have previously conducted microtremor observation on transects sandwiching fault lines at the Miyanohara fault in Tsunan Town, Niigata Prefecture, and the Kamishiro fault in Hakuba Village, Nagano Prefecture, and have estimated the displacement of bedrock and terrace gravel layers by determining the underground S-wave velocity structure, and have examined past fault activity. In this research, the past fault activity was estimated using microtremor observation in the Shionohira area and the Tsunagi Basin.
In the Shionohira area, observation was taken at two locations on either side of the fault line. Assuming that the depth at which the S-wave velocity changes dramatically, around 300-400 m/s, is the bedrock depth, a down shift of about 6 m on the west side was confirmed. As there was a vertical displacement of just under 2 m in 2011, it is thought that there may have been three accumulated displacements since the river was formed. The results of the microtremor observation on the upstream side confirmed that soft ground, not reaching 200 m/s, reached a depth of just under 4 m, suggesting that a thick layer of soft ground had accumulated as a result of the upstream drop due to multiple seismic activities.
Observation was taken at 20 points within the Tsunagi Basin. Comparing the Avs30 values, the western edge of the Tsunagi Basin has a relatively smaller value. A surface earthquake fault with a 60cm westward drop has appeared on the southern side of the basin, and observation around this area was compared. If the S-wave velocity reaching the basement depth is set at 300m/s, a basement down shift of about 130cm to the west was confirmed on the fault boundary. It is possible that two displacements were recorded.
The results of the microtremor observation at Shionohira and Tsunagi suggest that activity similar to that of the 2011 earthquake has been accumulating in the past. However, while the fault at Tsunagi is thought to have shifted about twice, it is possible that it shifted over three times at Shionohira, suggesting that part of the past activity may not have reached Tsunagi, and may have only moved around Shionohira. In addition, if the Shionohira Fault is considered to have been involved in the formation of the Tsunagi Basin, this is inconsistent because the fault movement is lowering to the west. But the Avs30 value within the Tsunagi Basin is relatively large, so the basement depth is not deep. This is consistent with the fact that the basin has not been lowered much due to the fault movement lowering to the west and the sedimentary layer is thin.
This study is based on a collaborative research agreement between Ibaraki Univ. and NIED, and is part of the results of Mr. Kaneko's master's research and part of the results of Prof. Koarai's research as a visiting researcher at GSI.
In the Shionohira area, observation was taken at two locations on either side of the fault line. Assuming that the depth at which the S-wave velocity changes dramatically, around 300-400 m/s, is the bedrock depth, a down shift of about 6 m on the west side was confirmed. As there was a vertical displacement of just under 2 m in 2011, it is thought that there may have been three accumulated displacements since the river was formed. The results of the microtremor observation on the upstream side confirmed that soft ground, not reaching 200 m/s, reached a depth of just under 4 m, suggesting that a thick layer of soft ground had accumulated as a result of the upstream drop due to multiple seismic activities.
Observation was taken at 20 points within the Tsunagi Basin. Comparing the Avs30 values, the western edge of the Tsunagi Basin has a relatively smaller value. A surface earthquake fault with a 60cm westward drop has appeared on the southern side of the basin, and observation around this area was compared. If the S-wave velocity reaching the basement depth is set at 300m/s, a basement down shift of about 130cm to the west was confirmed on the fault boundary. It is possible that two displacements were recorded.
The results of the microtremor observation at Shionohira and Tsunagi suggest that activity similar to that of the 2011 earthquake has been accumulating in the past. However, while the fault at Tsunagi is thought to have shifted about twice, it is possible that it shifted over three times at Shionohira, suggesting that part of the past activity may not have reached Tsunagi, and may have only moved around Shionohira. In addition, if the Shionohira Fault is considered to have been involved in the formation of the Tsunagi Basin, this is inconsistent because the fault movement is lowering to the west. But the Avs30 value within the Tsunagi Basin is relatively large, so the basement depth is not deep. This is consistent with the fact that the basin has not been lowered much due to the fault movement lowering to the west and the sedimentary layer is thin.
This study is based on a collaborative research agreement between Ibaraki Univ. and NIED, and is part of the results of Mr. Kaneko's master's research and part of the results of Prof. Koarai's research as a visiting researcher at GSI.