Geomorphological and Geological verification for documented paleo-seismic activity occurred in Nankai trough

点击翻译按钮获取中文摘要

The main puipose of the research project is to verify huge earthquakes which were identified through historical documents in geological or geomoiphological method. Sedimentologic analyses were conducted on Holocene limestones that occur along the mast from Muroto-misaki to Meoto-iwa, located ~13km north of the cape. The limestones are limited to less than 9.2 in in elevation. The limestones are up to 4.4 min mean diameter, up to 0.5 in in thickness, and consist mainly of ossilized sessile organisms, including annelids, corals, biyozoans, encrusting foraminifers, barnacles, nongeniculate coralline algae, and, to a lesser extent, molluscs and peyssonneliacean algae. Acicular and equant cements are minor components. Acicular cements are found in semiclosed spaces between coralline algal crusts and their substrates. The modal composition of limestones was determined by a point-cointing technique. Based on the biotic composition, the Holocene limestones can be classified into six types(Types I to VI) A comparison of the vertical distribution of these rock types with that of modem sessile organisms indicates that the top of Type I limestone, which is characterized by the occurrence of hennatypic corals, corresponds approximately to the mean low water springs when the limestones formed. A difference in the highest occurrence of Type I limestone between two sites may represent the variation in the total amount of uplift over the last 1000 to 1500 years, which resulted in an apparent northward decline of palemnean low water springs at a rate of ~10cm/km. Therefore, the Holocene limestones are a good paleobathymeter to reconstruct past seismic activities in this area. This study shows thatwann temperate carbonate deposits are as excellent recorders of geologic events, such as the timing and scale of repeated coseismic uplifts and elevation of sea levels, as those in the tropics.