跡津川断層周辺における地殻構造と地震活動

Abstract

The Atotsugawa fault system, which is located in the northern region of Central Honshu, Japan, is composed of the Amidagawara, Atotsugawa, Mannami, Mozumi-Sukenobe, and Ushikubi faults. The Atotsugawa fault in the central part is creeping where seismicity is low. In contrast, the edges of the creeping zone are locked showing relatively high seismicity. In order to study the relationship between the distribution of earthquakes and the fault structure, we analyzed seismic survey data along and across the fault system, and relocated hypocenters using a 1-D velocity structure determined in this study. P-wave velocity structures around the Atotsugawa fault system are determined by comparing first arrival travel time data from explosion surveys with the travel times from forward calculations by a ray tracing method. On the basis of the P-wave velocity structures, we have found that the crust around the Atotsugawa fault system consists of three layers, an upper crust which includes a surface layer, a middle crust and a lower crust. Furthermore, two distinct reflectors are located at depths of about 11km and 20km below the Atotsugawa fault system. The depth of the shallower reflector is close to that of low resistivity layer. Comparing the relocated hypocenters with the depths of the reflectors, the shallower reflector is roughly coincident with the base of the seismogenic layer and the second reflector is several kilometers deeper. In addition, seismicity is concentrated in the upper crust (with velocities of 5.9-6.2km/s) and only a few earthquakes occur at the bottom of the middle crust. The difference in seismic structures between the creeping zone and the other regions is not clear. This is probably because the creeping zone does not have a significantly different velocity structure that is detectable from these data.