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

定常観測及び臨時観測データによる中部地方北西部の地震活動

伊藤 潔*; 和田 博夫*; 大見 士朗*; 平野 憲雄*; 上野 友岳*

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

京都大学上宝観測所で30年間以上にわたって続けられてきた高感度地震観測及び陸域活断層フロンティア計画による臨時観測データに気象庁のデータを加えて、中部地方北西部の地震活動、特に跡津川断層帯付近の地震断層について詳細な調査を行った。その結果、新潟-神戸ひずみ集中帯の一部とされる跡津川断層帯付近を境に、地震が北西側では南西側に比べて深くなることがわかった。これはひずみ集中帯を境に地下構造又は応力が異なることを示している。また、断層帯の中でも跡津川断層と茂住祐延断層の地震はそれぞれ、地震発生層の下部15kmまでは、独立の断層面を有することがわかった。

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.