The ∼50 m slip of the Tohoku earthquake occurred along a very fine grained red-brown smectitic clay horizon subducting in the Japan Trench. This clay, cored in the plate boundary fault at Integrated Ocean Drilling Program Expedition 345, Site C0019, correlates with similar pelagic clay recovered seaward of the trench at Deep Sea Drilling Project Sites 436 and 1149. Comparable clays occur throughout the northwest Pacific Basin. Backtracking of ocean drilling Sites 436, C0019, and 1149 indicates that they formed during the Early Cretaceous at the Kula-Pacific Ridge. These sites traveled northwestward through the equatorial zone, accumulating siliceous and calcareous oozes until ca. 100∼85 Ma. Sites 436, C0019, and 1149 then entered the realm of pelagic clay deposition where they remained until ca. 15 Ma. From ca. 15 Ma to the present, Sites 436, C0019, and 1149 accumulated clays and silty clays with variable amounts of siliceous microfossils and volcanic ash, representing the transition from deep-sea conditions to a continental margin sedimentary environment. The predicted backtracked vertical sequence of sediments fits well with the cores at Sites 436, 1149, and C0019, after accounting for structural complications in the latter.
Pelagic clay occurs in numerous boreholes penetrating the relatively smooth ocean floor of the Pacific plate north and northeast of the Tohoku earthquake. Here the widespread pelagic clay apparently fosters tsunami and tsunamigenic earthquakes. Seamounts rising above the normal oceanic crust accumulated sequences of calcareous sediments as their crests remained above the calcite compensation depth for most of their history. A seafloor including pelagic clay and carbonate-covered seamounts occurs south and southeast of the southern extent of the Tohoku earthquake rupture zone. This area has no historic tsunami or tsunamigenic earthquakes along the Japan and Izu-Bonin Trenches with the possible exception of the poorly located Enpo earthquake of A.D. 1677. We believe that the seamounts incoming on the oceanic plate to the south and southeast of the Tohoku rupture zone interfere with long-distance propagation of slip in the pelagic clay, limiting earthquake magnitude, shallow slip, and tsunami generation.
- Received 1 July 2014.
- Revision received 23 February 2015.
- Accepted 17 March 2015.
- © Geological Society of America