次の南海トラフ巨大地震に備えて 隆起付着生物のＡＭＳ｀１４´Ｃ年代からみた室戸岬の地震性隆起に関する再検討

Abstract

Many great earthquakes occurring along the Nankai Trough, where the Philippine Sea Plate subducts under the Eurasian Plate, have been recorded in historical documents such as ancient Japanese government's and private diaries in Kyoto, Edo (Tokyo) or elsewhere when severe damage was experienced. The Pacific coast of western Japan along the Nankai Trough has been deformed significantly by great earthquakes, which are recorded in geomorphological or geological evidence such as emerged marine terraces and tsunami deposits.There are many fossil calcareous assemblages of raised sessile organisms along the coast of Cape Muroto up to a height of about 9 m. Calcareous assemblages had been grown up to 1 m thick in some places. Sessile organisms in this area consist of Pomatoleios kraussii (phylum ANNELIDA), which lives in the midst of the intertidal zone. The growth process of the assemblages of sessile organisms could suggest the history of relative sea-level change ; that is tectonic uplift since the late Holocene around Cape Muroto. The author collected some cores from these assemblages using a hand-coring system with a single core tube and a diamond bit, and dated them using the AMS14C method. Dating results suggest that relative sea-level was fairly stable, at least during 4500-2800 yrsBP, 2700-1000 yrsBP, and after 900 yrsBP. There should be coseismic uplifts of more than 2 m between these stable phases. Historical records in Japan, however, suggest that inter plate earthquakes along the Nankai Trough have recurred at intervals of 100 to 200 years. The author concluded that there were two types of earthquake that have made this area uplifted. One type is the inter-plate earthquake, which recurred at intervals of 100 to 200 years, and which had scarcely contributed to cumulative uplift due to inter-seismic subsidence. The other type is the intra-plate earthquake, which originated in shallow sea active faults near Shikoku island, and resulted in cumulative uplifts of the cape. The recurrence intervals of intra-plate earthquakes were longer than those of the inter plate type.