Abstract
The Mw = 6.2 2014 northern Nagano earthquake ruptured the northern and middle strands of the NNEtrending Kamishiro fault located in the northern part of the Itoigawa-Shizuoka tectonic line (ISTL) active fault system. The ISTL is located between the NE and SW Japan arcs, and it is one of the most major tectonic lines in Japan. Although the northern part of the ISTL active fault system had been assessed as a M = 7.5–8.5 seismic source to cause surface ruptures which heights are over several meters, the maximum vertical displacement on the 2014 surface ruptures was only 0.8–0.9 m which was observed at the Shiojima site in Hakuba village, the Nagano prefecture. To investigate shallow subsurface structures and cumulative deformation of the surface ruptures located along the northern part of the Kamishiro fault, we conducted ground-penetrating radar (GPR) profiling across the ruptures. The GPR survey lines were located at the Oide site, where about 0.5 m coseismic vertical displacement was observed, and Shiojima site. These line lengths were about 10–40 m. The GPR data were collected by common-offset modes using 100 MHz GPR system (pulseEKKO PRO made by Sensors and Software Inc.) We acquired also common mid-point (CMP) ensembles at the all survey lines to estimate the electromagnetic wave velocity. The depth-converted GPR sections after careful data processing show subsurface structures above a maximum depth of about 10 m. The shallow subsurface fault angle in the interpreted GPR section at the Oide site is lower than those at the Shiojima site. The dip-slip component of the coseismic displacement calculated from the fault angle at the Oide site was close to that calculated at Shiojima site. The interpreted GPR sections suggested also that the past several coseismic vertical displacements on the Kamishiro fault has been accumulated in the study area and that about 0.5–1.0 m vertical displacement was probably caused during each seismic event.
