Seismic activity from routine and temporary observations of earthquakes in the northwest Chubu district, central Honshu, Japan

Ito, Kiyoshi*; Wada, Hiroo*; Omi, Shiro*; Hirano, Norio*; Ueno, Tomotake*

The seismicity in the Atotsugawa fault area is studied in detail from the data of dense network stations, together with temporary station data. Since the observation network has been improved by adding new stations, the accuracy of the hypocenters has been accordingly raised by using the dense network data of temporary observations conducted by the Frontier Project. As a result, the distribution of hypocenters has greatly improved to reveal regional changes in focal depths along the fault. Besides, the characteristic features of seismicity along the Atotsugawa fault system are well derived from the high frequency and high-gain observations for about the last 30 years. Each distribution of focal depths along the Atotsugawa and Mozumi-Sukenobe faults is found to be vertical in the seismogenic zone of 15 km in depth from the detailed analyses of hypocenters. The distance between the two faults is 8 km at most. hypocenters are relatively deeper in the northwestern side of the fault zone compared to those in the southeast of the fault. These observation results, together with those of the GPS observations, lead to a model of the strain concentration along the fault zone. The model contains a detachment dipping towards the northwest in the middle or in the lower crust, where no earthquakes occur. In the Hida Mountains, shallow seismicity is well correlated with volcanoes. The seismicity in the mountains is different from those of the tectonic event for their focal depth and a maximum event size of M5.5. Detailed surveys in the Tateyama volcano area show that focal depths of earthquakes become shallower stepwise from the Atotsugawa fault system to the Hida Mountains. This suggests that the Atotsugawa fault ends at the foot of the Tateyama volcano.