55. Migration of Active Centres on the Seismic Zone off the Pacific Coast of Japan.

Transcription

1 284 [Vol. 22, 55. Migration of Active Centres on the Seismic Zone off the Pacific Coast of Japan. By Akitune IMAMURA, M. I. A. (Comm. Oct. 12, 1946.) During historic times in Japan, the region off its Pacific coast has been the seat of a number of regional earthquakes which caused loss of lives and destruction of property not only through shakings but alto by the inundations of tremendous tunamis associated with them. Their epifoal areas, though very wide each in itself as was shown in the case of the Sanriku earthquake of 1933(I~ or the Tokaido earthquake of 1944(2), were presumably enclosed within some zonal tracts stretching, broadly speaking, parallel to the general trend of the Japan arc. It could be determined, according as the difference of features manifested by the seismic areas or the, distributions of tunamis, whether the earthquakes were originated off the coast of Nankaido (say, region A), Tokaido (B), the Boso peninsula (C), the Sanriku district (D), or E. Hokkaido (E). Each of these regions may be assumed as separated from the next by a barrier, so to speak, lying between them ; that is to say, A and B by the zone of deep-seated earthquakes running southwards from wakasa Bay via Oomi and Mino, B and C by the volcanic ridge of the Idu Islands, C and D by the seismically stable crustal block off the coast of Iwaki, and D and E by that of S. Hokkaido. The first mentioned, however, is rather vague so that regional earthquakes occurred from A as well as B sometimes simultaneously or within a very short interval of time, as, for example, the Tokaido and Nankaido earthquake of 1707(3), or the Tokaido earthqurke of 1854 on Dec. 23 and the Nankaido earthquake on the next day. It might be expected that the said earthquakes would manifest some characteristic features especially on their distribution in space and time, seeing that they were undoubtedly caused by certain seismogenic forces exerted on a huge scale upon the Japan arc As regards this matter, we can imagine, first of all, an echelon arrangement of the zonal segments, although it is difficult to ascertain it. Be that as it may, it is recognizable that there was (1) N. Miyabe : An Investigation of the Sanriku Tunami based on Mareogram Data ;,Bul. Earthq. Res. Inst., Suppl. Vol. 1 (1934). (2) S. Omote : The Tunamis Associated with the Tokaido Earthquake of 1944 (in Jap.), Prompt Report, Earthq. Res. Inst., 4 (1945). (3) A. Imamura; Proc. Imp. Acad., 19 (1943),

2 No. 9.] Migration of Active Centres on the Seismic Zone off the Pacific Coast. 285 some regular course in the migration of active centres; namely, north easterly direction from A towards E, or an inverse order. In this connection, it would not be worthless to take earthquakes originated in the Inland Sea or Wakasa Bay into consideration, both seats being likely to have depressed as the results of onward crustal movement o the Pacific side of the Japan arc. In these regions, however, destructive earthquake occurred very rarely ; we can exemplify only the Wakasa Bay earthquake of?01, the Bungo Bay earthquake of 1596, and the Inland Sea earthquake of Earthquakes belonging to the system so far mentioned are given in chronological order in the annexed table. It is to be remarked that the Sanriku tunami earthquake of 1933 has been treated here as a mere appendix of the 1896 earthquake on the assumption that a greater part of seismic energy stored up there so far was spent in 1896 and the residual in 1933, in which case it would rot be so absurd to replace such an imaginary event, instead of the actual, as if the total energy was dissipated at some time intervening between themw. It must also be remarked that, since our knowledge concerning the seismic activity of region E goes back barely to the XVIII century, our discussion as regards this region is available only since the said period. On examining the table, it will be seen- that the said seismic activity began from the south-western end with the Nankaidb earthquake of 684; afterwards it migrated north-eastwards to culminate with the S18 and 869 earthquakes respectively in regions C and D, until it was repeated again in (1) Given in the table tentatively as 1908.

3 286 A. IMAMURA. [Vol. 22, Table I.

4 No. 9.] Migration of Active Centres on the Seismic Zone off the Pacific Coast. 287 regions A and B in 887. Then came, eismica11y, an inactive period of as long as about 5 centuries with interruption by two minor shocks in 1096 and In the revival of activity, the same course was taken by the 1361 and 1498 earthquakes respectively ire regions A acid B, and again by the 1605 and 1611 earthquakes, the former in regions A (about 22h, Jan. 31,) and C (about 1h, Feb. 1,) and the latter in D, these having apparently been heralded by the Bungo Bay earthquake of Later, the course of migration of activity was changed into an inverse order. At first occurred the 1677 earthquake in the southern part of D, this being followed by the Kwanto earthquake of 1703 and then by the Tokaido and Nakaido earthquake of Again, after the repose of 136 years, the activity was renewed with the E. Hokkaido earthquake of 1843, which was followed in December of 1854 by three earthquakes that took place successively as follows : 1) The Tokaido earthquake on the 23rd. 2) The Nankaido earthquake on the 24th. 3) The Inland Sea earthquake on the 26th. Similar tendency i~ noticeable also in the recent cycle of activity ; it began with the E. Hokkaido earthquake of 1894, which was followed by the Sanriku earthquake of 1896 with its appendix in 1933, the Kwanto earthqnake of 1923, and the Tokaido earthquake of As regards the nature of migration of active centres, it seems that it takes place in nowise abruptly, the space and time intervening between two such centres being sometimes filled up with a number of minor earthquakes in such a manner as was shown by certain inland seismic zones~l1. The consideration is based on the facts that the region to the north of C became active just before the Kwanto earthquake of 1923, the largest shock having occurred off the coast of Hitati preceding the main shock by three months, that is to say on June 2, the same year. Again, the seat lying between regions B and C became active in 1936, the activity culminating on Dec. 27 with the result of 3 death role and collapse of 35 houses in Niisima and Sikinezima. As I have discussed elsewhere12~, the southern part of Nankaido is undergoing since a few decades of years a characteristic chronic tilt dipping southwards, that is to say, in the sense opposite to the acute tilts experienced in (1) A. Irnamura : On the Northward Movement of Crustal Deformation along the Western Boundary of the Kwanto Plain ; Proc. Imp. Acad., 7 (1931), Further Notes &c.: Jap. Jour. Astro. & Geop., 11 (1934), (2) Jap. Jour. Astro. & Geop., 6, No. 2 (1928); 7, No. 1 (1929); 8, No. 1 (1930).

5 288 A. IMAMURA. [Vol. 22, the cases of the 1707 and 1864 earthquakes. On the other hand, as we have just seen, of the five different regions from A to E, A, the region off the coast of Nankaido, was most active seismically, notwithstanding it remains still inactive in the recent migration of active centres from NE to SW. It thus appears that this region proves to be one deserving of concentrated study, although it is questionable whether the features hitherto manifested will proceed still further in the future.