著者

伊倉 久美子 太田 陽子

出版者

Tokyo Geographical Society

雑誌

地学雑誌 (ISSN:0022135X)

巻号頁・発行日

vol.112, no.3, pp.394-405, 2003-06-25 (Released:2009-11-12)

参考文献数

23

被引用文献数

3 3

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Well-defined Holocene marine terraces are present continuously along the west coast of the Asahi Mountains. This area faces the Sea of Japan and is located east of the epicenter of the 1964 Niigata earthquake, which caused an uplift with considerable westward tilting of Awashima Island, located west of the epicenter. In contrast, the study area subsided at the time of the destructive 1964 earthquake, although the presence of Holocene and stage 5e or other marine terraces record the long-term uplift during the late Quaternary of this area (Ota, 1971). This paper describes the nature and the age of Holocene terraces, which had not been studied in detail, and discusses the uplift pattern and its tectonic significance.The Holocene marine terrace here is usually wave-cut platform with few beach deposits. At large river mouths, however, there are relatively wide and flat valley bottom plains, burying drowned valleys, and separated by sand dunes from the present beach. Radiocarbon age from the bottom of terrestrial deposits on marine deposits is 6.4 ka or slightly younger, and indicates that the emergence of Holocene terrace took place ca. 6 ka. The former shoreline height of the Holocene terrace ranges from 8 m to 4 m. Thus, the maximum uplift rate reaches 1.3 m/ka, which is larger than the uplift rate deduced from the Ml (stage 5e) terrace. The Holocene terrace is subdivided into two on the southern part of the Budo Mountains, where the uplift rate of Ml terrace is the maximum. The late Quaternary uplift of this area, deduced from marine terraces, is discordant with coseismic subsidence caused by the 1964 Niigata earthquake. This means that the uplift of this study area is not caused by seismogenetic fault for the 1964 earthquake. We cannot find the specific onshore active fault within or at both sides of the Budo Mountains and the Maya Mountains that was responsible for the coastal uplift. To interpret such a long-term uplift of the coastal area, we need to assume that offshore reverse active fault dipping eastward is repeatedly activated. Establishing the exact location, nature, and length of offshore faults is important to interpret the coastal uplift of this area.