- 火山. 第2集 (ISSN:04534360)
- vol.14, no.1, pp.8-20, 1969-04-01
Examples are presented in which arrangement of lateral and post-caldera cones indicates the probable regional stress field in the late Quaternary. Izu-Oshima, Hakone and Fuji are among the larger stratovolcanoes to the southwest of Tokyo, the former two having collapse calderas on their summit. Parasitic and post-caldera cones and craters of the three volcanoes are following a trend of similar direction, some of them being produced by fissure eruptions on their flank. The zones trend in the direction of about N 35° W at Fuji, N 45° W at Hakone and N 30° W at Oshima. Dikes are also found in these zones running mostly parallel to them. The trend of fissures of fissure eruptions on the flanks of Fuji and Oshima is also similar to the zones of recent activity. Because sites of flank eruption are regarded as points where radially formed dikes around the central magma column have penetrated the flank, the above described distribution of craters in these volcanoes would indicate a concentration of radial dikes in a specific direction at the three adjacent volcanoes. Considering that dikes are fossil tension cracks formed perpendicularly to the axis of the minimum principal compressional stress, the concentration of parasitic craters can be explained by the stress field caused by the pressure increase in the magma column superimposed on the preexisting regional field with the maximum principal stress axis in a NW direction. The nearly identical, preexisting stress field in the three adjacent volcanoes suggests that the field is part of a more regional one including the area of the volcanoes. This suggestion is strongly supported by the presence of active, conjugate strike-slip faults in the same general area, i.e. on the Izu-peninsula. The maximum compressonal axis indicated by the faults is again in a direction of about N 30° W and oriented horizontally. The last movement of the fault system was observed in 1930, at the time of the Kita-Izu earthquake, whose magnitude was 7.0.