1999 Volume 49 Issue 49 Pages 13-21
Large earthquake is a trigger of submarine slope failures, and may form submarine gravity flow deposits such as turbidites and subaqueous debris flow deposits. Therefore, there is a possibility that the recurrence intervals of submarine gravity flow deposits intercalated in hemipelagic mud indicates those of large earthquakes occurred near a site. The recurrence intervals of deep-sea turbidites deposited in the Kumano Trough, a forearc basin along the Nankai Trough, where large and destructive earthquakes occurred periodically, were determined to evaluate the deep-sea sediments as earthquake recorder. Radiocarbon datings using planktonic foraminifers in hemipelagic mud were carried out at 10 horizons. By using the foraminifer 14C ages, depositional age of each turbidite layer could be determined in high accuracy. The reccurrence intervals of turbidites were calculated as 170-680 years. This intervals were 2 or 5 times longer than the recurrence interval of large earthquakes along the Nankai Trough. The largest problem of the age determination is thought to be the reserver effect of radiocarbon in ocean.