関東地方の地殻水平変動

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The results of geodetic observations to detect crustal movements in the southern Kanto district, Japan before and after the 1923 Kanto earthquake with magnitude 7.9 show that the land was compressed along the NNW-SSE direction from the ocean to the land both for the preseismic and the postseismic periods, and abruptly reversed at the time of the earth-quake. These facts enable us to presume that the southern Kanto district is mainly compressed by the NNW-SSE drifting of the Philippine Sea plate during the interseismic period and the giant earthquake occurred in the Sagami Bay is nothing but an elastic rebound to the drifting of the oceanic plate. It is remarkable that the seismic upheaval was observed not only along the eastern coast of Sagami Bay, the Syonan and the Miura-Boso peninsula, but also along the western coast of Sagami Bay, the north-eastern part of the Izu peninsula. The above-mentioned facts can not be interpreted by the Ando's fault their model of the 1923 Kanto earthquake in which only the Sagami Bay fault was ruptured but well interpreted by the Ishibashi's model in which both the Sagami Bay fault and the Nishi-Sagami Bay fault were ruptured. Moreovcr, the land upheaval as well as the horizontal movement along the NNW-SSE direction from the land to the ocean were continued even after the 1923 Kanto earthquake. The present authors interpret these postseismic crustal movements by the creep dislocation model in which slow reverse faulting along the deeper extension of the Nishi-Sagami Bay thrust is assumed. It is concluded that the stored strain in the southern Kanto district was released not only by elastic dislocation at the time of the 1923 Kanto earthquake but also by creep dislocation after the earthquake especially along the East-off-Izu tectonic line. At last, the authors pointed out that further studies should be forcussed into investigating the mechanism of the intraplate earthquakes in the Kanto district in order to make clear the tectonic movement. It is important to know whether strain field caused by the drifting of the Pacific plate and the Philippine Sea plate is compatible to the mechanism of the intraplate earthquakes or not.
日本地質学会の論文
1981-03-30

関東地方の地殻水平変動

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