Focal Mechanisms of the Earthquake Swarm Southeast of Mt. Ontake, Central Honshu, Japan

HORI SADAKI, AOKI HARUMI, OOIDA TOORU

doi:10.4294/zisin1948.35.2_161

Recently, the seismic activity of the earthquake swarm southeast of Mt. Ontake has been studied by many researchers, especially after the volcanic eruption on October 28, 1979 because of the proximity of both events in time and space. This paper deals with the focal mechanism solutions with the aid of temporary stations installed after the eruption.<br>Directions of the P-axes are separated into two groups. One is in N70°W-N80°W directions though T-axes are distributed in girdle (Strike-slip, oblique-slip and dip-slip types are equally observed in the earthquake swarm). Directions of N70°W-N80°W are coincident with the pressure direction in the inland of Chubu District, central Honshu and they are supposed to be the pressure directions of the tectonic stresses in this focal region.<br>The other group is in N40°W direction, which is apparently inconsistent with the tectonic stresses. Earthquakes belonging to this group are strike-slip types and are occurring in the two limited regions. One is in the east of Mt. Ontake and the other is in the vicinity of Kuzoo (central part of this focal region) and is aligned across the earthquake swarm region. Such an activity indicates the existence of an active fault inconsistent with the present tectonic stresses.<br>It becomes clear from the present study of focal mechanisms that small amplitudes of S-waves observed at Maze (to the west of Mt. Ontake) are not accounted for the attenuation of S-waves but for the focal mechanisms of the earthquake swarm.

Focal Mechanisms of the Earthquake Swarm Southeast of Mt. Ontake, Central Honshu, Japan

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