Abstract
An Mj 6.7 earthquake struck the northern part of the Nagano Prefecture, central Japan, at 22:08 JST on November 22, 2014. The earthquake was named 2014 Naganoken-hokubu earthquake, and its coseismic surface rupture has been identified along the northern part of the Kamishiro fault in the northernmost part of the Itoigawa-Shizuoka Tectonic Line active fault system. We have conducted 1) aerial-photograph interpretation, 2) field explorations, 3) photogrammetric analysis, and 4) drilling and pit excavation surveys on the northern part of the Kamishiro fault since 2005, in order to better estimate slip rates and timing of faulting on the fault. At the Shinden site, the Aira-Tanzawa volcanic ash (ca. 30 ka) was identified in the L1 terrace-surface deposit. In addition, the age of L2 terrace surface was estimated at ca. 5 ka. Because the amounts of vertical offset of the L2 terrace surface are 8.0 m and 6.5-7.0 m at the Shinden and Oide sites, vertical slip rates can be calculated to be 1.6 mm/yr and 1.3-1.4 mm/yr, respectively. We also revealed subsurface stratigraphy in the down-thrown side of the Kamishiro fault at the Oide site, and identified three possible paleoseismic events that occurred ca. 1,000-1,300 years ago, 500-600 years ago, and after 300 years ago. Further investigations are needed to better understand paleoseismic activity and long-term seismic risk of the Kamishiro fault.
