Frequent turbidite depositions in the Holocene along the Kuril trench, northwestern Pacific: implications for paleoseismic activities

Over 30 turbidite layers during the Holocene are identified in total of 20 gravity and piston core samples along the southwestern Kuril trench, off the Pacific coast of Hokkaido, northern Japan. With a help of tephra layers in the cores, the recurrence intervals of turbidite depositions are estimated to be several decades. Large (> M8) earthquakes, recurring every 50--100 years along the Kuril trench, are the most likely triggering mechanism for generating turbidity currents. Because the Hokkaido's written history is limited in the last two centuries, the large earthquakes before the 19th century are not well known. Therefore, sedimentological analysis of submarine core samples allows us to estimate the earthquake recurrence in a longer time period and the result will be used for a long-term forecast of large earthquakes along the Kuril Trench. The cores were obtained from the water depths between 2000 and 3600 m. Sampling sites were off Nemuro to the east (submarine fan on a flat belt along the trench), bottom of the Kushiro Submarine Canyon, and off Tokachi to the west (small sedimentary basins). Almost all the turbidites are very fine-grained to fine-grained, and intercalated with olive black diatomaceous mud. Several tuffs from known eruption constrain the depositional ages in the cores. Cores from off Nemuro contain zero to two sand layers above volcanic ashes of the 17th century. This indicates that fewer turbidite depositions have occurred than large earthquakes deduced from the historical records. Samples from the bottom of the Kushiro Submarine Canyon also include turbidites and the tephras. Four to five turbidites above the tephra suggest that the earthquakes recurrence interval is 57--85 years off Kushiro, which are similar to the historical ones. Off Tokachi area, although many turbidites are included in the cores, absence of tephras makes it difficult to estimate the recurrence intervals of the turbidite depositions. There is no turbidite deposited by the 2003 Tokachi-oki earthquake on the surface of the cores, off Tokachi.