抄録
Since August 1965, lots of earthquakes have been occurring at Matsushiro and its vicinity, northern part of central Honshu. In order to elucidate the cause of the peculiar phenomenon various kinds of observations and surveys have been done in the earthquake occurrence area.
On the basis of data obtained by the permanent seismological stations of the Japan Meteorological Agency and 5 temporary stations established in the area, the mechanism of earthquakes which occurred during the period from October 1965 to February 1967 was analysed.
The distribution of the initial motion of P waves for the earthquakes is separated by two straight nodal lines, which are perpendicular to each other, and lie in the directions of N35°-55°W and N35°-55°E. This suggests that the maximum pressure of stresses generating the earthquakes exists in the azimuth ranging from N80°W to N100°W. The number of nodal plane solutions obtained is more than 300.
Results of the statistical studies on the solutions and on the relationship between the focal mechanism and the geodetic surveys are summarized as follows:
1) The predominant directions of the pressures corresponding to the major earthquakes systematically changed by 10°to 20°on December 1965 and April 1966.
2) There exist systematic variations in the predominat directions of pressures in each earthquake occurrence area. This may be caused by the variation of local geology in the area.
3) The strike direction of N55°W for one of the two nodal lines agrees well with the strike of fault produced by the earthquake swarm in the central part of the earthquake occurrence area.
4) The predominant pressure directions are in good harmony with the axes of contraction of horizontal strains computed on the basis of data obtained by the geodetic survey carried out during the period from November 1965 to August 1966 at the central part of the earthquake occurrence area.
