3:30 PM - 5:00 PM
[HDS09-P01] Tectonic geomorphological study on Byobuyama-Enasan fault zone and Sanageyama fault zone
Keywords:Central Japan, Active fault, Tectonic Geomorphology
1. Introduction
“Research and observation in the Byobuyama-Enasan fault zone and the Sanageyama fault zone (Enayasan-Sanageyama-north fault zone)” was implemented. This fault zone extends over 100 km from the Ena Mountains to the Chita Peninsula. The purpose of this project is to improve the long-term evaluation of earthquake occurrence and the prediction of strong ground motions for the northern part of the fault zone. In previous long-term evaluations, the Enasan-Sanageyama-north fault zone was ranked A*, the Byobuyama fault zone was ranked A, and the Sanage-Takahama fault zone was ranked Z. However, data on activity history and fault structure were scarce. There are also discrepancies in the identification of active faults in previous active fault maps, and the continuity between active faults were not clear. Sub-themes were set as 1) tectonic geomorphological survey, 2) paleoseismicity and geological survey, 3) underground fault structure survey, 4) prediction of strong ground motion, and 5) cooperation with local communities aiming at appropriate handling of prediction information. This presentation will deal with the result of 1) tectonic geomorphological survey.
2. Detailed location and displacement survey for seismic source fault scenario evaluation
In FY2020, we prepared high-resolution LiDAR data with a resolution of 1m or less as basic data for the survey. Since the large amount of data available, mainly for civil engineering, some areas were re-measured. The accuracy of the conventional data was confirmed, and defective points were subject to remeasurement. Areas with dense vegetation were measured by helicopter. In addition, the amount of displacement was measured mainly for lateral displacement.
In 2021, we used LiDAR data to interpret the fault deformation and examined the continuity of fault lines. We focused on the following four points. (1) The western half of the Byobuyama fault where activity is unclear, (2) The relationship between the Byobuyama and the Tegano faults, (3) The Enasan fault and the eastern edge of the Byobuyama fault (relationship with the Atera fault), (4) ) The western end of the Enasan fault approaching the Sanageyama fault.
In FY2022, we created a geomorphological classification map and examined the time-space distribution of fault displacement. Drilling survey, volcanic ash analysis of terrace deposits, and dating were carried out. We estimated the age of the geormorphological surface and calculated the slip rate mainly for the vertical displacement. Because chronology of strike-slip terrain was difficult, we decided to consider the vertical/horizontal ratio of displacement using the slip vector data clarified in Subtheme 2.
3. Main results
As of February 2023, there are some unconfirmed parts, however the results are summarized as follows. A quantitative discussion of the slip rate will be supplemented in the presentation.
(1) The western part of the Byobuyama fault is also considered to be active because there is some clear cut in the geomorphological surfaces. In particular, a displaced landform was confirmed near Dachi, Toki City.
(2) The western edge of the Tegano fault shows a shape that diverges from the Byobuyama fault. Therefore, the Byobuyama fault is judged to continue to Nakatsugawa.
(3) A distinct right-lateral strike-slip topography was newly found at the eastern end of the Enasan. It was found that the fault extends further east than before. However, the Byobuyama and Enasan faults do not reach the Atera fault.
(4) South-upward right-lateral strike-slip displacement was confirmed at the western edge of the Enasan fault. It shows that the western end is be continuous with the Sanageyama-north fault.
(5) On the other hand, no recent displacement topography is found in the western part of the Enasan fault near Sue-cho, Mizunami city. Faulting activity may be interrupted here.
(6) The Sanage-Sakaigawa fault continues to the Sanageyama-north fault at its northern end. The Sanage-Sakaigawa fault shows a shape that diverges from the Sanageyama-north fault.
“Research and observation in the Byobuyama-Enasan fault zone and the Sanageyama fault zone (Enayasan-Sanageyama-north fault zone)” was implemented. This fault zone extends over 100 km from the Ena Mountains to the Chita Peninsula. The purpose of this project is to improve the long-term evaluation of earthquake occurrence and the prediction of strong ground motions for the northern part of the fault zone. In previous long-term evaluations, the Enasan-Sanageyama-north fault zone was ranked A*, the Byobuyama fault zone was ranked A, and the Sanage-Takahama fault zone was ranked Z. However, data on activity history and fault structure were scarce. There are also discrepancies in the identification of active faults in previous active fault maps, and the continuity between active faults were not clear. Sub-themes were set as 1) tectonic geomorphological survey, 2) paleoseismicity and geological survey, 3) underground fault structure survey, 4) prediction of strong ground motion, and 5) cooperation with local communities aiming at appropriate handling of prediction information. This presentation will deal with the result of 1) tectonic geomorphological survey.
2. Detailed location and displacement survey for seismic source fault scenario evaluation
In FY2020, we prepared high-resolution LiDAR data with a resolution of 1m or less as basic data for the survey. Since the large amount of data available, mainly for civil engineering, some areas were re-measured. The accuracy of the conventional data was confirmed, and defective points were subject to remeasurement. Areas with dense vegetation were measured by helicopter. In addition, the amount of displacement was measured mainly for lateral displacement.
In 2021, we used LiDAR data to interpret the fault deformation and examined the continuity of fault lines. We focused on the following four points. (1) The western half of the Byobuyama fault where activity is unclear, (2) The relationship between the Byobuyama and the Tegano faults, (3) The Enasan fault and the eastern edge of the Byobuyama fault (relationship with the Atera fault), (4) ) The western end of the Enasan fault approaching the Sanageyama fault.
In FY2022, we created a geomorphological classification map and examined the time-space distribution of fault displacement. Drilling survey, volcanic ash analysis of terrace deposits, and dating were carried out. We estimated the age of the geormorphological surface and calculated the slip rate mainly for the vertical displacement. Because chronology of strike-slip terrain was difficult, we decided to consider the vertical/horizontal ratio of displacement using the slip vector data clarified in Subtheme 2.
3. Main results
As of February 2023, there are some unconfirmed parts, however the results are summarized as follows. A quantitative discussion of the slip rate will be supplemented in the presentation.
(1) The western part of the Byobuyama fault is also considered to be active because there is some clear cut in the geomorphological surfaces. In particular, a displaced landform was confirmed near Dachi, Toki City.
(2) The western edge of the Tegano fault shows a shape that diverges from the Byobuyama fault. Therefore, the Byobuyama fault is judged to continue to Nakatsugawa.
(3) A distinct right-lateral strike-slip topography was newly found at the eastern end of the Enasan. It was found that the fault extends further east than before. However, the Byobuyama and Enasan faults do not reach the Atera fault.
(4) South-upward right-lateral strike-slip displacement was confirmed at the western edge of the Enasan fault. It shows that the western end is be continuous with the Sanageyama-north fault.
(5) On the other hand, no recent displacement topography is found in the western part of the Enasan fault near Sue-cho, Mizunami city. Faulting activity may be interrupted here.
(6) The Sanage-Sakaigawa fault continues to the Sanageyama-north fault at its northern end. The Sanage-Sakaigawa fault shows a shape that diverges from the Sanageyama-north fault.