(17) Dynamics of the turbidity current generated by the 2011 Tohoku-Oki earthquake and tsunami
Kazuno Arai1, Hajime Naruse2, Kiichiro Kawamura3, Tomohisa Irino4, Ken Ikehara5, Yu Saitoh6, Masafumi Murayama6, Ryo Miura7, Ryota Hino8, Yoshihiro Ito8, Daisuke Inazu9 , Miwa Yokokawa10, Norihiro Izumi4
1: Chiba Univ., 2: Kyoto Univ., 3: Yamaguchi Univ., 4: Hokkaido Univ., 5: AIST, 6: Kochi Univ., 7: Nippon Marine Enterprises, Ltd., 8: Tohoku Univ., 9: NIED, 10: Osaka Inst.
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We examine the dynamics of the tsunamigenic turbidity current associated with the 2011 Tohoku-Oki earthquake and tsunami, using the observed data and the numerical model of turbidity currents. This study is important for understanding the generation and behavior of the tsunamigenic turbidity currents, and for establishing a method to reconstruct the recurrence interval of large paleoearthquakes and paleotsunamis on the basis of the geologic record.
The OBPs and OBSs recorded the anomalous event occurred on the sea floor off Tohoku coasts at about 3 hours after the main shock of Tohoku-Oki Earthquake. It is considered that this anomalous event was affected by the turbidity current run from shallow marine. The average head velocity of the turbidity current based on the observed data is estimated at least 2.4 – 7.1 m/s. 16 sediment core samples were collected using R/V Mirai and R/V Tansei-maru over range of water depth 170 – 1700 m off Miyagi Prefecture in March and May, 2012. As a result, event deposit layers (new sediment layer) were observed at the top of 9 core samples. The event deposits, which are 2-4 cm thick, can be interpreted as deposits associated with the main shock on the basis of radioisotope data.
From the observed data described above, it is considered that the suspension cloud stirred up by the tsunami at shallower depths grew into the turbidity current. We then estimated the condition for generating tsunamigenic turbidity current by comparison between observation (estimated velocity and sediment distribution) and numerical simulation of the unsteady tsunamigenic turbidity current. Consequently, it was suggested that seafloor sediment at shallow marine should have been eroded at least 1.4 cm in thickness (in case of the porosity 50%) by the tsunami to generate the tsunamigenic turbidity current. In our future study, we will attempt to conduct inverse analysis of the flow condition of the turbidity current associated with Tohoku-Oki events using the thickness data of the event deposits as constrains.