飛騨地方北部の地震活動, 地震メカニズム及びテクトニクス

Abstract

Seismicity and focal mechanism of earthquakes in the northern Hida region, central Japan, are closely investigated, and seismo-tectonic features of this region are discussed, mainly on the basis of five years' data obtained at seismic network stations of the Kamitakara Geophysical Observatory in cooperation with some other observatories. More than 5, 800 earthquakes with magnitudes greater than 0.5 have been located. It was found that high seismicity is concentrated along the Atotsugawa fault and around the Ushikubi and Mozumi faults, all of which are typical, Quaternary active faults in this region, and just beneath the Hida mountain range, which is highest in the Japan Islands. Also noticeable is relatively high seismicity around conjugate sets of minor faults northeast of the Atera fault, and in the southwest part of the Toyama plain. Most of larger earthquakes along and around the Atotsugawa fault indicate strike-slip type mechanisms consistent with right-lateral slip motion that had been continued since Quaternary ages. Earthquakes at shallow depths beneath the northern part of the Hida mountains also show strike-slip mechanisms with a nodal plane nearly parallel to the axis of the mountain range, and sometimes with a small component of normal faulting. In the southern part of the mountains southeast of Mt. Ontake, however, two different types of earthquakes, one with strike-slip, another with reverse faulting, are concentrated in a limited area, suggesting complex structures there. All earthquakes northeast of the Atera fault indicate strike-slip mechanisms.