新潟県粟島の海成段丘からみた地震隆起の累積に関する検討

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Awashima is a small (7 × 2 km) island on the continental shelf along the wetsern coast of northern Honshu, and uplifted with northwestward tilt associated with the 1964 Niigata earthquake (Figs. 1 and 5). NAKAMURA et al. (1964) proposed a cumulative character of the coseismic uplift since the middle Miocene, of the island, based on the comparison of terrace profile and geologic structure of Tertiary sedimentary rocks with the coseismic tilt by the Niigata earthquake. This paper intends to establish such a progressive tilt of the island, on the basis of the detailed investigation of marine terraces. Marine terraces of Awashima are divided into H, M1, M2 and L terraces (Fig. 2). They are abrasion platforms and no tephra and terrace deposits are found, except thick deposits of L terrace at Uchiura area. The most extensive terrace, M1, can be regarded to be formed at the time of the last interglacial maximum (ca. 125 ka). The height of the former shoreline of M1 terrace ranges from ca. 75 in to the northeast to ca. 50 m to the southwest, indicating a notable northwestward tilt, which is similar to that of the 1964 earthquake, but having much steeper gradient. The gradient of the northwestward tilt of M1 terrace (20 × 10-3) is about 67 times of that of the coseismic tilt at the time of 1964 earthquake (0.3 × 10-3). It suggests that the similar coseismic uplift has repeated since the last interglacial maximum with interval of about 1900 years. Estimated resultant coseismic uplift is about 1.1 m at the northeastern part of the island, where M1 terrace is 75 m high, which is approximately same to the height estimated by the amount of coseismic resultant uplift of 67 times. Thus, average recurrence interval of the major earthquake resulting in the coseismic uplift of Awashima is estimated to be about 1900 years from the comparison of amount of both tilt and uplift. However, no significant difference is observed in terrace profiles of Ml, between A-B, parallel to strike of the tilt, and C?D or E?F, normal to it (Fig. 4). Therefore, terrace profile seems to be an unsuitable indicator for the detection of tectonic deformation, unless much steeper tilt has occured. Distribution of H and M2 terraces is too limited to discuss the deformation pattern. L terrace at Uchiura is underlain by thick marine deposits attaining ca 60 m thick, suggesting that it was formed in association with the postglacial sea level rise. No datable material was found despite examination of many borehole data and our excavation works. L terrace is subdivided into L1, L2 and L3 terraces. Episodic or intermittent emergence, probably coseismic, should have occurred at least 3 times since ca. 6000 yBP. It is consistent with the result obtained by Mi terrace. The height of L1 terrace is ca. 11 m above mean sea level, which means average uplift rate is ca. 1.5 m/ka, it is much larger than 0.7 m/ka for Mi terrace. Northwestward tilt of the lower terraces, however, is not confirmed by this study, owing to the limited distribution.
社団法人東京地学協会の論文

新潟県粟島の海成段丘からみた地震隆起の累積に関する検討

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