1. 地殻歪の指示者としての火山 : 火山のテクトニクス例(I. 序論)

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A model is presented which explains the temporal relation between an eruption and a succeeding earthquake, taking a basaltic stratovolcano, Izu-Oshima volcano, as an example. In the model, volcano is assumed to consist of an underground reservoir and a long pipe connecting the reservoir to the surface. As the compressional crustal strain is gradually stored toward the earthquakes to occur, the volcano, located near the potential fault, is also deformed and contracts to some degree. Then the magma in the reservoir is squeezed up through the pipe. The rise of the magmatic head above a certain level in the pipe causes an eruption, which, once started, may proceed as a self-moving machine. Later, when the earthquake occurs, the strain that squeezed up the magma is released. And the head of the magma falls off resulting in the end of the eruption, in case it has still continued. The bottom of the summit crater of Oshima volcano showed remarkable rise and fall in this century amounting to some 400 meters. The bottom can be regarded as the head of the magma column, since red hot glow was frequently observed during the period. There were two maxima of the height of the bottom, January 1923 and June 1951. Shortly after each of the maxima, occurred great earthquakes with magnitude larger than 8, September 1923 and November 1953 along the Sagami trough which runs some 20km northeast of the volcano toward northwest, branching off from the Japan trench. The area including the volcano has been under compressional tectonic stress with the maximum pressure axis in a horizontal N30°W direction, during at least these hundreds of thousand years. On the other hand, recent fault-model studies of the 1923 earthquake indicate that the fault trace of the earthquake almost coincides with the Sagami trough and that the slip vector of the southwestern block, in which the volcano is located, is toward northwest almost horizontal with slight down going component. This tectonic situation implies that the strain which had been accumulated prior to the occurrence of the great earthquakes along the Sagami trough was caused by the same origin, probably the motion of the Philippine sea plate against the Japanese plate, with what has produced the compressive stress field of the volcanic area. The model appears to be successfully applied for the interpretation of the relation between the eruption of Akita-Komagatake volcano which started on September 17, 1970 and the October 16 earthquake with the magnitude of 6.2 at the epicentral distance of 55km. The frequency of explosion discontinuously dropped down to one half or lower level, three days after the earthquake together with the cessation of Strombolian type of eruption. The preliminary mechanism study of the earthquake showed that there is some component of thrust motion indicating the accumulation of contractional strain prior to the earthquake. The volcano to which the proposed model is applied is thus able to be regarded as a sensitive natural indicator of tectonic crustal strain, and also at the same time as being in a near critical condition ready to erupt.
1971-12-01

1. 地殻歪の指示者としての火山 : 火山のテクトニクス例(I. 序論)

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