[S-CG48] Ocean Floor Geoscience

[SCG48-11] Possible evidence of the 17th century great earthquake off Hokkaido: Marine paleoseismology in the landward slope of the Kuril trench

*Toshiya Kanamatsu¹, KanHsi Hsiung¹, RYO NAKANISHI², Kaoru Kubota¹, Ken Ikehara³, Yu-Chun Chang¹, Toshiya Fujiwara¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Kyoto Univesity, 3.National Institute of Advanced Industrial Science and Technology)

Keywords:Kuril Trench, Huge earthquake, 17th Century, Paleoseismology

Huge earthquakes have occurred at intervals of several hundred years along the Kuril Trench, as evidenced by tsunami deposits. The most recent huge earthquake occurred in the 17th century great earthquake off Hokkaido. To explain the distribution of tsunami deposits in 17th-century, it is considered that multiple segments were interconnected, causing shallow slip within the plate boundary and amplifying the large tsunami, similar to the 2011 off the Pacific coast of Tohoku Earthquake. On the other hand, despite the occurrence of significant tsunamis during the 17th century great earthquake off Hokkaido, there is no evidence of tsunamis observed along the Tohoku coast. This inconsistency might indicate a potential confusion between the 17th century great earthquake off Hokkaido and the Keicho Sanriku earthquake of 1611, both of which occurred around the same period. This confusion is likely due to the insufficient information available to understand the 17th century earthquake. Since the information for this earthquake have been explored yet in marine archives, a research project aiming to obtain paleoseismologic information of the 17th century great earthquake along marine Kuril Trench was planed.
Two research cruises were conducted in March and June 2022 offshore Hidaka, Tokachi, and Nemuro. Sediment cores obtained from several locations include thick event layers. Some of these layers reveal clear slump structures, which are confirmed by x-ray computed tomographic images and magnetic fabrics of core samples. Based on the correlation of volcanic ashes involved, some of these layers potentially correspond to tsunami deposits on land in 17th-century. The distribution of slump layers was observed over a wide range extending from offshore Hidaka to the western side of the Kushiro submarine canyon. To obtain continuous depth-age information of the core samples, dense radiocarbon dating using bulk organic matter were carried out and the result shows that the ages of slump layers are 1,000 – 2,000 years older than the upper and lower hemipelagic intervals, indicating that the remobilization of old sediment occurred during earthquakes. On the other hand, the ages of turbidite layers in the other intervals do not show a significant difference from the overlying and underlying hemipelagic layers (about a few hundred years), indicating that the turbidite layer was formed by the remobilization of surface sediments. We consider that the significant older age of slump layers can be a key to understand the ground motion induced by the great earthquakes off Hokkaido. Further verification of ages has been conducted by applying additional chronological information to confirm the connection between tsunami deposits on land and marine slump layer.