9:00 AM - 9:15 AM
[SSS31-01] Faulting history and segmentation of the Tokamachi Fault Zone in Niigata Prefecture, Central Japan
Keywords:active fault, Tohkamachi Fault Zone, faulting history, segmentation of fault, trenching survey, Niigata Prefecture
On Tsunan fault, we excavated all core borings and a pit along two lines across the fault scarplet at Himizo, Tokamachi city. Fault cuts Holocene fluvial terrace with direction of NE-SW and produces east-facing-scarplet with height between 2-5 meters. We excavated boring of depth with 4-5 meters at 4 sites along the southern line and 3 sites along the north line. A pit with 7 meters in length and 2 meters in depth, was excavated on the north line.
On Miyanohara fault, which is located on the late Pleistocene fluvial terrace near the prefecture boundary between Niigata and Nagano, we excavated all core boring at Kameoka, Tsunan town. Borings with depth of 10 meters were excavated at 5 sites across the south-facing fault scarp with 5 m height and an extra-boring with depth of 21 meters on the down-thrown side of the fault scarp.
We also conducted trenching surveys on the eastern segment of Tokamachi Fault Zone at 2 sites, Otajima and Baba. One of results from Otajima will be presented by another presentation in this session (Taniguchi et al., 2016). About another result at Baba, we could not find any faults on the trench walls, meaning the scarplet produced not by faulting but by erosion of a blanch stream of the Shinano river.
In our presentation, we will present the history of the fault activities and slip-rates of these faults by using the results of 14 dating and tephra analysis. And, we will discuss the segmentation model of Tokamachi Fault Zone, based on comparison with those data in Tokamachi basin area and difference of geological structure both side of basin.
The contents of this presentation is a part of the result of the Complementary Survey Project of Active Fault by HERP in 2015 FY.