三宅島の側火口分布と地殻応力場の性質

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Relation between the ambient crustal stress field and the distribution of flank eruption sites of an arc front volcano, Miyake-jima is studied. Compared with the adjacent arc volcanoes such as the northerly located Oshima volcano, two characteristic features are observed. 1. the strike of the flank eruptive fissures shows a remarkable bent towards either NW or SE as was the case in 1962 and 1983 fissures. 2. flank erutption sites are widely distributed in Miyake-jima in nearly all the azimuthal directions around the summit crater, whereas they are confined in a narrow straight zone passing through the summit on the northerly located Oshima and Hakone volcanoes. These features indicate the crustal stress field having, 1. northwest-southeast trending Sh_<max> and 2. horizontal deviatoric stress smaller than in the northern area where the above-mentioned two volcanoes are located. The earthquakes in the region of the present concern are of mostly strike-slip type. The above volcano-inferred stress field is consistent with the earthquake studies. Similar orientation of the Sh_<max> (P) and Sh_<min> (T) is indicated by the fault plane solutions of earthquakes. Smaller derivatoric stress in the Miyake-jima area than in the northern area is implied by the existence of normal fault component in the fault plane solutions and by temporarily variable principal stress axes of the quakes. The above nature of the crustal stress field, as inferred from both volcanoes and earthquakes, is understood with the plate tectonic setting of this delta-shaped region which is the northernmost portion of the Philippine Sea plate. The Philippine Sea plate subducts along the Suruga and the Sagami troughs which limit the northwestern and northeastern edge of the delta, respectively. The northern apex of the delta marks the site of the collision to which is attributed the origin of horizontal P axis of the quakes in this region (strike-slip type) rather than horizontal B axis (normal fault type) as is normal for bending earthquakes associated with subduction. Therefore the Sh_<max> (P) is expected to decrease southward where it is farther from the origin of stronger lateral constraints. Sh_<min> (T) is also expected larger in the southern Miyake-jima area than in the north because of the larger distance from the troughs where the Sh_<min> should be the smallest. Thus, the horizontal deviatorie stress (P-T) would be smaller in the south. Inflation and deflation of the entire edifice of Miyake-jima volcano appear to have occurred at least associated with the 1940 and 1962 eruptions. No data are available for the 1983 event, however. Deformation study at Miyake-jima will be useful and necessary for understanding the process of magma storage beneath summit crater and for prediction both for the timing of cessation of the once started eruption and for the preestimate of the amount of the eruptible magma.
特定非営利活動法人日本火山学会の論文
1984-12-28

三宅島の側火口分布と地殻応力場の性質

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