Propagation of seismic wave in Japan. (Seventh report)

[摘要] It was in 1924 that in this Journal we noticed that the apparent velocity of the seismic waves is difierent in different directions of propagation. At that time, owing to the lack of suitable materials, the investigation was confined to earthquakes which occured in the Kwantô plain and the middle part of Honsyû.Since 1923 i.e. the year of the great Kwantô earthquake, the scismic observations in our country made a decided progress and now a dense net of the seismic observatories enables us to extend our study of abnormal propagation of seismic wave to the earthquakes occurring all over the country. The present note is a summary of the results of our researches.(1) Method of investigation. The isochronous lines of an earthquake are drawn on a map so as to fit the localities having equal time of arrival of the first impulse of the longitudinal wave. 106 quakes of tolerably large intensity have been investigated for the purpose. The general form of those isobhronals is not concentric circles, but somewhat resembles ellipses or ovals.The direction of the major axis of such an isochronal ellipse or oval is that to which the seismic wave propagates with a velocity apparently greater than in the other directions. Fig. 1 to 12 show the isochronous lines of earthquakes which occurred at several districts of our country.The hodographs of those earthquakes are drawn with times of occurrences and epicentral distances. On these hodographs are measured epicentral distances corresponding to every 10 seconds of the time of occurrence. With those epicentral distances as the radii, concentric circles are drawn with the epicentres as their respective centres. Then, those concentric circles may be regarded as the mean isochronals free from the abnormality of the propagation of seismic wave due to locality or to direction.The deviation of the observed isochronals from the mean, determines the amount and direction of abnormal propagation of the seismic wave. In Fig. 13 is shown a specimen of such mean isochronals (full line) and the observed isochronals (dotted line) of the destructive earthquake which occurred at about 11h 20m (J. S. T.) on 21st., Sept., 1931 in Saitama province.(2) The direction of abnormal propagation. Almost all the earthquakes which occurred in our country show the abnormality in their propagation.The direction in which the seismic longitudinal wave propagates with a velocity apparently greater than in other directions is determined by measuring the deviation of observed isochronals from the mean. We call, for simplicity's sake, such a direction as that of abnormal propagation.The directions of abnormal propagation have been determined from the isochronals of 106 earthquakes which occurred in 1927 and 1931. They are shown on a map by arrows from their epicentres. Fig. 14 and 15 show the distribution of the direction of abnormal propagation of seismic wave thus determined.From the direction of abnormal propagation shown in Figs. 14 and 15 we see that they are perpendicular to the arc of the Japanese islands in Kwantô and the middle part of Honsyû district. They show West and East direction in the western part of Japan, and the direction of North and South in Tôhoku and Hokkaidô districts and on the Pacific coast of the eastern Kwantô district.(3) The geologicol nature and the direction of abnormal propagation. It is a well known fact, that the geological structure of our country is very complex so the velocity of seismic wave differs according to the geological nature of the medium in which the wave propagates. On this point, we may calla ttention to an excellent investigation of Prof. Kusakabe on the elastic properties of rocks.

[学科分类] 大气科学