The First Surface-Rupturing Earthquake in 20 Years on a HERP Active Fault is Not Characteristic: The 2014 Mw 6.2 Nagano Event along the Northern Itoigawa–Shizuoka Tectonic Line

The M w 6.2 (M w 6.7) Nagano-ken-hokubu earthquake (here- after Nagano earthquake) struck northern Nagano, central Ja- pan, on 22 November 2014, destroying more than 100 houses and injuring 46 people. The maximum peak ground accelera- tion of 589 Gal was recorded at K-NET Hakuba station, lo- cated 3.3 km west of the epicenter (National Research Institute for Earth Science and Disaster Prevention, 2014). However, the areas that suffered from severe shaking, Japan Meteorologi- cal Agency (JMA) intensity 6, were limited to a few villages ∼17 km from the epicenter. The most striking feature of this earthquake is that more than 9 km of complex surface faulting occurred on the previously mapped 26-km-long north–northwest-trending Kamishiro fault, one of the seg- ments of the 150-km-long Itoigawa–Shizuoka tectonic line (ISTL) active fault system. This earthquake was the first sur- face-breaking earthquake to occur on one of the 110 major inland active faults prioritized for evaluation by the Head- quarters for Earthquake Research Promotion (HERP) that was launched in 1995 after the 1995 Kobe earthquake. A 14% probability of a large earthquake of M w ∼ 8:3 occurring within the next 30 yrs from the entire ISTL rupture was esti- mated, the second highest among all the major active faults (HERP, 2014). In addition, the anticipated shaking intensities for the regions around the ISTL have been available to the public since 2005, based on simulated strong ground motion with realistic fault dimensions and fault geometry. The 2014 Nagano earthquake, as a surface-rupturing event, gave us an opportunity to see whether our forecast, based on geologic, paleoseismic, and seismic engineering, was appropriate or whether it requires strategic improvement for seismic-hazard assessment in Japan.