相模湾西部, 初島の完新世海成段丘と地殻上下変動

Abstract

Uplifted Holocene marine terraces of Hatsu-shima Island (0.44km2) in the western part of Sagami Bay on the Pacific coast of central Japan have been investigated with special reference to the late Quaternary seismic crustal movement of the island. Hatsu-shima Island, which is located at the top of the western steep scarp of the northernmost part of the Sagami trough, was uplifted almost uniformly by around 2m at the time of the 1923 Kanto earthquake (Ms=8.2).The whole surface of the island consists of a flight of marine terraces developed on Pliocene volcanics. These terraces are clearly classified into three groups; I, II, and III. Terrace I, the highest (about 50m asl), is distributed only in the southeastern small part of the island. Terrace II, 25-40m asl, is the widest in the island and tilting, generally, northwestward. Terraces I and II have been correlated to Obaradai Terrace of about 80, 000yrs B. P. in age and to Misaki Terrace of about 60, 000yrs B. P. in age, respectively, by SUGIHARA (1980) based on marker tephras covering the terraces.Terraces III, rather narrow but well defined especially in the northern and western parts of the island, is distributed along the present shoreline at the foot of a former sea cliff as high as about 20m surrounding Terrace II (or I in the southeastern part). In this paper they are subdivided into three; IIIa, IIIb, and IIIc. Terrace IIIa, the highest and widest among group III, is rather flat with about 10m asl inner margin. Planned excavation at three localities on this terrace has revealed that terrace deposits composed of marine gravel and sand of 1.5-2m thickness rest unconformably on rather weathered bed rock. Fossil shells that were obtained at the base of terrace deposits at Excavation B, which are judged to be in situ and to represent the terrace's age, have been dated at 6, 730±190 14C yrs B. P. (GaK-9080). From this 14C date, as well as from the topographical features, Terrace IIIa can be interpreted as the highest Holocene terrace formed about 6, 000 years ago (so-called Numa Terrace). The height of former shoreline is estimated at around 9m asl. Terraces IIIb and IIIc are narrow emergent shingle beach, whose inner margins are 6-7m asl and about 4m asl, respectively. IIIc includes the emergent beach of 1923 in its lowest part. Although the topographical separation between IIIb and IIIc is not necessarily clear, they can be distinguished from each other as IIIb surface is covered by dense vegetation including many pine trees which have been growing since before the 1923 coseismic uplift, whereas IIIc surface has no vegetation. Therefore, it is almost certain that at least two times of intermittent emergence had occurred between the formation of Terrace IIIa and the 1923 coseismic uplift. However, the dates of emergence have remained undefined, because 14C dates of fossil shells which were collected from Terraces IIIb and IIIc are so much scattered implying that the materials are not in situ. In the northern part of the island there is a prominent flat surface higher than Terrace IIIa, 10-15m asl, where Hatsu-shima village is situated. In this paper it is named IIIa' Surface and considered a secondary surface modified from Terrace IIIa by landslides of a back scarp and artificial alteration based on its sedimentological features, surrounding topographical features, and rather young 14C dates of materials obtained from this surface.