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タイトル: 5. 深溝斷層
その他のタイトル: 5. The Fukozu Fault. A Remarkable Earthquake Fault formed during the Mikawa Earthquake o f January 13, 1945
著者: 津屋, 弘逵
著者(別言語): Tuya, Hiromichi
発行日: 1948年6月30日
出版者: 東京大学地震研究所
掲載誌情報: 東京大學地震研究所彙報. 第24冊第1/4号, 1948.6.30, pp.59-75
抄録: During the Mikawa Earthquake of Jan. 13, 1945, a remarkable earthquake-fault named the Fukozu Fault, after the village lying in the middle of the fault-line, was formed in the portion of the meizoseismal area adjoining the west of the Tokaido Railway Line between Okazaki and Gamagori, Aichi Prefecfure. As shown by the thick continuous line of Fig.2, the fault begins towards the north near the pass between the vitlages, Kiriyama and Miyahasama, and from this point it has been followed without interruption as far as Katanoharamachi on the coast of Atsumi Bay, a total distance of a little more than 9km, and it is not impossible that it may continue still farther to the south and to the floor of the bay. In spite of its relatively small length, the fault is very remarkable in the fact that, although in most cases the general course of earthquake-fault has been either straight or a smooth uniform carve, in this case it is bent at nearly righ : angle at its middle portion. Thus, the fault makes a great turn at Fukozu from the E-W course of its northern wing to the NNW-SSE course of its southern wing. Moreover, the fault-line shows a small sigmoidal bending at the middle portion of the southern wing. The fault is of a reverse type or a thrust with the southwest side as the hanging wall, and at the surface it has manifested itself as relative depression amounting up to about 2m. of the northeast side. The horizontal displacement along the fault is small compared with that in a vertical direction. Thus the northern wing of the fault shows a relative eastward shift of less than 1m. of the south side, and the southern wing a rela ive northward shift of less than 50cm. of the west side. But wherever the horizontal shifts appear in larger amounts they seem to be rather local or only superficial, and it is inferred that the actual horizontal shift, if there was any, didnot exceed a few cen imeters, the shift being probably due to a northeastward upthrusting of the land on the southwest side of the fault. The appea ance of the fault varies with the nature of the surface-rock. In hard gr and the fault forms usually steep overhanging scarps ; in loose soil i appears either as a simple flexure of the ground or as a crumbled-down slope formed by the drag of the soil over the fault-rupture. The faultplane dips 50°~70° to the south in the south wing and to the west in the south wing, as measured directly on the scars cut by the fault. But in soft and loose ground composed either of the Quaternary deposits or of the subsoil over the older rocks, it shows sometimes a. gentler dip(15°~20°) of the over-thrust type. That the fault is not geologically a new one is at once evident from both the geological structure and topographical features of the zone along that. The northern wing of the fault follows a chain of small valleys which, trending E-W, represents a geological fault-line separating the structurally different parts of the Upper Palaeozoic metamorphics (mica-schist and quartz-schist on both sides. The southern wing of the fault passes rightly through the boundary between the metamorphics on th west and the more or less schistose hornblende-biotite-granite on the east, which also represents and ubtedly an old fault. There can therefore be little doubt that the present earthquake-fault has been form d by accentuated and renewed crust-movements along the pre-existing geological faults. From the appearance of the fault alone, it is difficult to determine which side moved in the neighbourhood of the fault, whether the southwest side was upthrusted to the northeast, or the northeast side downthrusted to the southwest, or whether both sides moved at once. But the area of earthquake-damages to human structures were virtually confined to the southwest side of the fault, while none of important damages occurred in the northeast side, except for a narrow zone along the fault. Thus, for example, the Tokaido Railway Line suffered no damage by the earthquake, although it is only 150m distant from the fault at Fukozu. Furthermore, observations made in those parts of the coast intersected by the fault furnished definite evidences of the upheaval of the land on the west side of the fault. The upheaval was as much as 1m on the coast near the fault, and lessened gradually westwards, until it vanished on the mast of Nishi-Hazu, about 5km west of the fanlt. On the other hand, the relative level of land and sea on the east side was nearly the same after the earthquake as before. Although the land on that side was Said to have been slightly depressed relative to the sea, the evi ence did not amount to proof. Accordingly, it may be fairly inferred that, at the very moment of the earthquake, the Sangane-san mountain-block bounded with the fault on its northern and eastern sides was upthrusted forward in a northeasterly direction.
URI: http://hdl.handle.net/2261/10631
ISSN: 00408972


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