Characteristics and distribution of the event deposits induced by the 2011 Tohoku-oki earthquake and tsunami offshore of Sanriku and Sendai, Japan

Abstract

Abstract Deep-sea turbidite deposits are useful tools for submarine paleoseismology. It is fundamental to understand what kind of disturbance occurred owing to the earthquake and which event-induced deposits were formed in a particular area. The 2011 Tohoku-oki earthquake and tsunami were the most destructive geohazards in Japanese history. To understand their influence in the seafloor environment and to characterize the related event deposits, undisturbed sediment cores were collected using a multiple corer along two transects off Sanriku and Sendai, Japan, during July–August 2011. The deposits from the 2011 Tohoku-oki earthquake and tsunami were widely observed along the seafloor, from the outer shelf to the lower trench slope off Sanriku. Their composition indicated multiple sources for the deposited material, reflecting that the seafloor disturbance occurred across a wide area. Two types of sedimentary structures (Types 1 and 2) have been recognized in the event deposits. The Type 1 event deposit is further divided into two subtypes, 1-1 and 1-2. Subtype 1-1 is characterized by a fine-grained turbidite sequence with a sharp basal contact and a parallel and/or cross-laminated basal coarse-grained bed that is a few cm thick, suggesting that sediment particles were transported over relatively long distances by turbidity currents. Relatively short-distance transport by sediment resuspension and settling is hypothesized for subtype 1-2 deposits, which have a thin basal coarse-grained bed without internal structure, and for Type 2, which are composed of homogeneous muddy sediments above a sharp surface but without any basal coarse-grained bed. However, organic geochemical characteristics of the event deposits suggest that long-distance transport was limited even for subtype 1-1 deposits. The wide spatial distribution of the event deposits, which also are evident across a wide range of water depths, reflects the magnitude of the earthquake and size of the tsunami it triggered.