2006 Volume 2006 Issue 26 Pages 95-104
The western margin of the Nagano basin is bounded by north-to-northeast-trending west-dipping active reverse faults that extend for about 58 km. We conducted drilling surveys at Kusama site, Nakano City, in order to estimate timing of recent surface faulting on the central portion of the faults. At the site, a 6-m-high elongated tectonic bulge has been preserved in the basin on and parallel to the basal part of the 40-m-high southeast-facing warping scarp. We obtained 19 cores with length of 1 to 7 meters from the backlimb portion of the tectonic bulge and constrained its subsurface geologic structures with 12 radiocarbon ages measured by means of AMS, and identified active southeast-dipping reverse faults and related warping in the structures, indicating two surface faulting that had occurred after AD1420 (El) and just after AD690-890 (E2). El would be correlated to the 1847 M7.4 Zenkoji earthquake, during which the whole segments of the faults ruptured. In addition, timing of E2 is consistent with that of the penultimate faulting on the northern portion of the faults, which is estimated to have been between AD560 and AD1160 by compiling and re-interpreting the previous studies. Consequently, we suggest that the penultimate surface faulting on the northern to central portion occurred between AD690 and AD1160.
