Submarine paleoseismology in the Japan Trench of northeastern Japan: turbidite stratigraphy and sedimentology using paleomagnetic and rock-magnetic analyses

Abstract

Abstract Previous studies on sediment recovered from the Japan Trench document distinctive turbidite beds induced by huge earthquakes along the Japan Trench and their wide occurrences in area of 37°25’- 38°30’N. We studied two sedimentary cores at 39°N in order to investigate the depositional earthquake record in the further spatio-extened areas of the Japan Trench. We examined specifically the precise stratigraphy of turbidite beds using paleomagnetic secular variation (PSV), and a tephra correlation. Additionally, anisotropy of magnetic susceptibility (AMS) was investigated to understand the depositional conditions of each turbidite bed. The inferred ages of turbidite beds in this study closely approximate their earlier reported, which are correlated to the historical and pre-historical huge earthquakes off Tohoku, northeastern Japan. The paleo current directions during deposition of turbidite are inferred from their grain alignment based on AMS data. The directions of basal part reveal northeastward in the slope-side basin and north-northeast in oceanward basin. The directions of basal and upper thick muddy part of a turbidite bed are not always consistent, which suggests the hydraulic condition in the narrow elongated deep-sea basin. This fact could be essential information to elucidate a unique hydraulic condition during the turbidite deposition in the confined basin in the Japan Trench basin.