[H-CG23] 混濁流:発生源から堆積物・地形形成まで

[HCG23-P08] 船上サブボトムプロファイラーを用いた青森県東方沖～茨城県沖の海溝軸近傍の表層浅部イベント層序の試み

*Kioka A.¹、Schwestermann T.¹、Moernaut J.¹、Ikehara K.²、Kanamatsu T.³、Strasser M.^1,4 (1.インスブルック大学・地質学専攻、2.産業技術総合研究所・地質調査総合センター、3.海洋研究開発機構・地震津波海域観測研究開発センター、4.ブレーメン大学・海洋環境科学センター)

キーワード：日本海溝、イベント堆積物、2011年東北地方太平洋沖地震、貞観地震、イベント層序、サブボトムプロファイラー

The giant M9 Tohoku-oki earthquake in 2011 has been documented to remobilize a wide area of surface sediment, redistributing from the slope into the >7 km deep Japan Trench forming distinct event deposits in hadal terminal basins. Other than the 2011 earthquake, similarly large earthquakes are historically known along the Japan Trench, and respective deposits have been described in recent studies from cores retrieved from the Japan Trench. Yet, little is understood about (i) the temporal and spatial distribution of event deposits along the entire trench axis, (ii) how the spatial extend may link to historically-reported earthquakes, and (iii) whether older deposits may indicate prehistoric large earthquakes along the Japan Trench. We herein study dm-scale vertical resolution subbottom profiler data acquired during 2012–2016 using PARASOUND and TOPAS recording systems installed on R/Vs Sonne and Shinsei-Maru over the entire Japan Trench area (36.0°–40.5°N), in order to investigate the event-stratigraphic distribution of acoustically-transparent layers that represent sediment remobilization event deposits. The subbottom profiler data generally record well up to ~40 m below the seafloor. Transparent bodies of the upper ~10 m well correlate to event layers reported from sediment cores linked to major historic earthquake such as the AD 2011 Tohoku-oki, AD 1454 Kyotoku, and AD 869 Jogan events. Mapping of these event deposits in the spatially-extensive subbottom profiler data allows to study the spatial distribution of these events and its comparison to their reconstructed rupture areas. Furthermore, the lower part of the hitherto only acoustically-imaged stratigraphic succession of isolated trench-floor basins along the Japan Trench also documents several thick acoustically-transparent bodies. Preliminary interpretation suggests several of the bodies could be event deposits triggered by old large earthquakes. While ground-truth of this interpretation and absolute dating requires long coring efforts in the ultra-deep Japan Trench, our study yields relative stratigraphic information and quantitative constraints of along strike variation of sediment volumes redistributed by episodic events along the Tohoku seismogenic margin.