Reconstruction of paleoseismic history in the Nankai Trough Zone by analyzing Holocene raised limestones

利用全新世滨海隆起灰岩对南开海槽带古地震史进行了研究。沉积学研究是对全新世石灰岩进行的，这些石灰岩分布在从Muroto角到Meoto-iwa海岸沿线，位于海角以北约13公里处。石灰石的分布仅限于海拔<10米。石灰岩平均直径可达4.4米，厚度小于0.5米，主要由环节动物、苔藓虫、珊瑚、包壳有孔虫、珊瑚藻等无根生物的化石组成。相关成分包括藤壶、介形虫、软体动物、棘刺类、底栖有孔虫、水藻、非钙质碎屑和颗粒。水泥是一种次要成分，存在于珊瑚藻结壳和底物之间的半封闭空间中。用点计数法测定了石灰石的模态组成。结果表明，根据主要的无根生物化石，可将灰岩划分为6类。主要成分为：ⅰ型珊瑚和珊瑚藻；第II型珊瑚藻；第三类珊瑚藻、环节动物及藤壶；IV型珊瑚藻和环节动物；V型包覆有孔虫和苔藓虫；与现代无底生物的垂直分布比较表明，第一类生物在某一露头处的最高海拔与石灰岩形成时的平均低海平面（MLSL）相对应。因此，全新世石灰岩被认为是分析相对海平面变化的理想选择。

This study aims to elucidate paleoseismic history in the Nankai Trough Zone by using Holocene raised limestones attached to coastal rocks. Sedimentological study was conducted on Holocene limestone that occurs along the coast from Cape Muroto to Meoto-iwa located about 13 km to the north of the cape. Distribution of the limestone is limited to <10 m in elevation. The limestone is up to 4.4 meter in mean diameter and less than 0.5 m in thickness, and consists mainly of fossilized sessile organisms such as annelids, bryozoans, corals, encrusting foraminifera, and coralline algae. Associated components include barnacles, ostracods, molluscs, echinoids, benthic foraminifera, peyssonneliacean algae, and non-calcareous clasts and grains. Cement is a minor component and found in a semi-closed space between coralline algal crusts and their substrates. Modal composition of limestone was determined by a point counting technique. The results show that the limestone can be classified into 6 types on the basis of predominant fossilized sessile organisms. The dominant components are : corals and coralline algae in Type I ; coralline algae in Type II ; coralline algae, annelids and barnacles in Type III ; coralline algae and annelids in Type IV ; encrusting foraminifera and encrusting bryozoans in Type V ; and molluscs in Type VI. Comparison of vertical distribution of the six types with those of modern sessile organisms indicates the highest elevation of Type I at a particular outcrop corresponds to mean low sea level (MLSL) when the limestone formed. Therefore the Holocene limestone is considered ideal for the analysis of relative sea-level changes.