大地震生成の過程

1968年十勝沖地震および1963年エトロフ島沖の地震

抄録

On the strong motion seismograms at stations in Hokkaido and northern Honshu for the Tokachi-Oki Earthquake of May 16, 1968, remarkable waves with large amplitude are recorded at about 20-50 seconds after the S phases. The origin time and the hypocentral coordinates of this earthquake are given by JMA as follows; 00^h48^m53.0^s (GMT), 40°44′N, 143°35′E, H=0km.
By mean of the travel time analysis, it is found that these waves can be explained as the S wave radiated at the other place from the hypocenter of the main shock. The time and coordinates of the wave origin estimated by the application of Geiger's method are as follows; 00^h49^m38.3^s±1.3^s(GMT), 40°35′±00′N, 142°20′±00′E, H=0km.
From the strong motion seismogram obtained at Urakawa, Hokkaido, for the Etorofu-Oki Earthquake of Oct. 13, 1963, it is also suggested that the large energy was released at the other place from the origin of the main shock, even though such unique solutions for the time and the coordinates of the origin can not be estimated as above example.