THE GREAT EARTHQUAKE OF S. E. JAPAiV 165 



The regions of these earthquakes are represented by the letters J, A, 

 C, P and T according as the earthquakes took place in or near 

 Japan, the Aleutian Islands and Alaska, California, the Philippine 

 Islands, and Tonga Deep respectively. The table serves to show how 

 the centres of seismic activity a^jproachod gradually from distant 

 localities to Japan. Indeed, there were live Japan earthquakes among 

 seven which took place during the year previous to the great shock 

 on Sept. 1, the most noteworthy having been that of June 1. This 

 earthquake was accompanied by a dozen fore-shocks, was so big that 

 the double amplitude of the earth-movement at Hongo was as large 

 as (d.Q cm., and originated at a distance of 250 km. from the seat of 

 the last destructive shock. 



No remarkable fore-shock seems to have preceded the last cata- 

 strophe. Only a few shocks were registered in August at Hongo as 

 follows : two unfelt on the 7ih, one unfelt on the 17th, and one slight 

 one on the 24th. All these originated in or near Sagami Bay. It is 

 said that rumblings like earthquake-sounds were heard in the Miura 

 Peninsula before the great shock. 



The gradual upheaval of the land was very definitely shown by 

 the tide gauge register at Aburatubo in Mi^aki. Here the annual 

 mean sea-level iiad gradually rii^en during recent years until in 1920 

 it abruptly began to fall, while those at Ayukawa, Wazima, Kusimoto 

 and others were moving during the latter epoch in the opposite sense 

 and parallel to one another. This means that the land of Aburatubo 

 was notwithstanding the fact that it had previously been gradually 

 falling, elevated as much as the difference of the levels of the two 

 categories during the last three years ; and that is equal to 8 cm. 

 (PI. XXII.). Further, the monthly mean in 1923 showed the same tend- 

 ency before the earthquake, indicating at Aburatubo an upheaval of 

 about 10 cm. during the two months before the catastrophe. This 

 topographical change is doubtless an evidence of the extniordinary 

 stress which had bec;n in action. It was the must striking antecedent 

 phenomenon, and jnay ^lerhaps throw light on the study of pi-edicting 

 destructive earthquakes. 



21. After-shocks. Hitherto the after-shocks which have been 

 sttidied by difhnvnt investigators have related, so far as I am aware, 

 to those in which the fundamc-ntal shock was simple, such that its 

 after-shocks originated within dehnite limits and in a narrow space. 



