2019 Volume 128 Issue 3 Pages 453-464
The Itoigawa–Shizuoka Tectonic Line (ISTL) fault zone is one of the longest and most active fault zones in Japan, extending for approximately 158 km from Otari Village to Hayakawa Town. The southern part of the ISTL fault zone is a ∼48 km long, east-vergent reverse fault zone, and has the potential for causing earthquakes larger than M 7. Slip rates need to be estimated, as well as the timing of faulting and amounts of coseismic displacement, to better understand seismic risk at the southern part of the ISTL fault zone. Geomorphic surveys and tephra analyses were conducted for the middle terrace surface in the Tsukuyama area, Minami-Alps City, Yamanashi Prefecture, to estimate the long-term slip rate. Based on the data obtained, the terrace-surface deposit is at least ∼40 m thick, and is directly covered by Ontake Daiichi tephra (On-Pm1), whose age is ∼100 ka. Thus, it is inferred: (1) the middle terrace in the Tsukuyama area is one of the depositional terrace surfaces developed during the 5d of the marine isotope stage (MIS); (2) dissection of the terrace surface started during the transition period from 5d to 5c of the MIS (∼100-110 ka); and (3) the vertical slip rate of the southern part of the ISTL fault zone is at least 0.9-1.0 mm/yr, because the vertical displacement is estimated at ∼100 m or larger.