2017 Volume 123 Issue 1 Pages 1-21
On 22 November 2014, the Kamishiro Fault, situated along the northernmost segment of the Itoigawa-Shizuoka Tectonic Line, ruptured, causing the Mw 6.2 Nagano-ken Hokubu earthquake. We mapped a 9-km-long surface rupture along the Kamishiro Fault, monitored earthquake-related deformation, and measured co-seismic displacements at 42 localities. The surface rupture consists of main and additional sections. The maximum vertical displacement is 95±17 cm at Shiojima, and the amount of displacement generally decreases to the south. The main and additional rupture traces are divided by a 1-km-wide step-over. The slip distribution and fault geometry indicate that the earthquake rupture terminated at this structure, preventing it from propagating farther to the south. The length of the surface rupture is shorter than the total length (24 km) of the Kamishiro Fault. The main shock epicenter is located near the northern end of the surface rupture, and the aftershock distribution indicates that the source fault extends farther to the north. The relationship between the maximum net displacement of 2.0 m and the surface rupture, or source fault lengths, show a good fit with empirical data. Historical documents suggest that a similar-sized earthquake occurred in the same region in 1714 AD, indicating they may occur at an interval of ~300 years. The 2014 earthquake was a partial rupture on the Kamishiro Fault, unlike the characteristic earthquakes modeled in long-term predictions. A paleo-seismic reconstruction of displacement along the Kamishiro Fault is necessary to understand the recurrence intervals of both irregular and characteristic earthquakes.
