*Hisao Kondo1, Hideki Kurosawa2, Yoko Hirakura2, Keita Takada3, Kohei Abe2, Miwako Ito2, Atsuo Igarashi3, Tetsuya Ikeda3, Atsushi Miwa2
(1.Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, 2.OYO Cooperation, 3.Fukken Co., Ltd)
Keywords:active fault, paleoseismology, ISTL active fault zone
We perfomed paleoseismological invetigations on the un-ruptured fault section along the Kamishiro fault, ISTL active fault zone, central Japan. The un-ruptured section extends for about 15 km, consisting of the southernmost section of the Kamishiro fault. The northern most section have activated during the 2014 Nagano-ken Hokubu earthquake, Mj 6.7. Near Lake Nakatsuna, the Kamishiro fault forms tentonic geomorphic features such as fault scarp on the lucustrin terrace and fluvial terrace, monoclinal scarp on the lowland. Beside the eastern side of Lake Nakatsuna, we obtained two bore holes in order to estimate vertical slip rate based on lucustrin terrace deposite at hanging-wall side and fine sediments at foot-wall side. We estimated about 1.0 mm/yr of vertical slip rate as the minimum value. The fault exposure at the depth of -29 m in the borehole core at the foot-wall side implies that thrust front is located at the west of the borehole site. It indicates that the slip rate at present is the minimum estimatation. 350 meter south of this site, we obtained several Geoslicer across the fault scarp. The scarp hight is about 2 m. Based on geomorphic and geologica section, we identified two paleoseismic events after the depositon of alluvial fun deposite. At this moment, we estimated that the most recent event occurred after 664 A.D. More carbon dating samples will allow us to constrain teh timing of paleoseismic events. Near the sourthern most termination of the Kamishiro fault, south of Lake Kizaki, we re-investigated timing and slip per event during 5-6 paleoseismic events. Here, 2-3 events has been identified based on the shallow geologic section up to 6 m deep revealed by Geoslicer technique. We added four borehole cores to identify thrust structures and older events. At present, fine sediments and carbon dating results yeild 1.0 mm/yr of vertical slip rate after 1.2 ka. More dating results will provide high-resolution chronology of past events and slip per events, that will help us to discuss the occurrence of past multi-segment earthquakes.