1872年浜田地震による石見畳ヶ浦の隆起

——離水生物遺骸群集と地形データによる検証——

抄録

This paper describes that Iwami-Tatamigaura was uplifted during the 1872 Hamada earthquake based on the analysis of the height and ages of emerged sessile assemblage, and topographic data. In two sea caves located in the northern part of the survey area, we observed the emerged sessile assemblage, which consists mainly of calcareous tubeworm, Pomatoleios kraussii, and collected samples for ¹⁴C dating and measured their altitude referring against the Tokyo Peil (T.P.). Two levels of assemblage distributed at 1.29 to 1.78 m (T.P.) in the upper and 0.90 to 1.28 m (T.P.) in the lower can be recognized in the East Cave. Analyzing the calibrated ¹⁴C age data of the lower assemblage, their height relative to the vertical living range of the present assemblage, and adjacent tide gauge data, it is inferred that the northern part of Iwami-Tatamigaura has 0.8-1.1 m uplifted during the 1872 Hamada earthquake. Assuming a 0.8 m coseismic uplift in the whole of survey area, most of the central and the southern parts of wave-cut-bench would be submerged below the low tide before the earthquake, based on the height distribution represented by Digital Surface Model created from the Unmanned Aerial Vehicle survey. This result shows a discrepancy with the fact that the most part of the wave-cut-bench had already emerged in the historical picture “Tokonoura-Ezu” painted in 1817. To explain the landscape of the picture, the amount of coseismic uplift in the central and the southern part must be 0.3-0.4 m or less. These data suggest that the regional difference in the amount of uplift occurred between the northern part and the central to southern part. Because no recent surface rupture can be identified between the two areas, such crustal deformation was probably caused by tilting movements toward the south or the southwest. We also propose the other emergence event before the 1872 Hamada earthquake from the upper assemblage in the East Cave. Calibrated ¹⁴C age data and height of them suggest a relative sea-level fall of at least 0.4 m around the 14th century or later. This result seems that coseismic uplift event has occurred with an interval of about 500 years, but there are the other several candidates for the cause of this phenomenon, such as intermittent small uplifts with short intervals, aseismic uplift, and eustatic sea-level fall.