1995 Volume 44 Issue 12 Pages 856-864
Core samples from borehole OS-2 were taken from the seafloor in the Shiribeshi trough after the 1993 offshore earthquake, southwest of Hokkaido. The physical properties, chemical composition and radioactivity of these core samples were measured. Measurements indicate that the water content, sand content and grain size changed abruptly at a depth of 9 cm from the top of the core. These changes are interpreted to indicate an incursion of a turbidite at this depth. The results of 137Cs radioactivity measurements of core samples indicate that this turbidite was a result of submarine slope failure caused by the 1993 earthquake off southwestern Hokkaido. Sediments from 11 to 23 cm depth in the core are also interpreted to be turbidites. The 210Pb activity variation in the core samples can be used to estimate the time interval of sedimentation between the uppermost and the older turbidites, supposing that the turbidites were composed of young surface sediments at that time.