THE PLATE BOUNDARY THRUST OF THE 2011 MW 9.0 TOHOKU EARTHQUAKE: ORIGIN OF THE SCALY PELAGIC CLAY THAT ALLOWED SHALLOW SLIP PROPAGATION AND TSUNAMI GENERATION

The unprecedented 50 m slip of the 2011 Mw 9.0 Tohoku earthquake occurred along a very fine-grained red-brown smectitic clay. This very weak “scaly” clay comprises the plate boundary fault at Site C0019 and correlates with similar ­­­pelagic clay cored seaward of the trench at Site 436. Correlative pelagic clays occur throughout the NW Pacific Basin. Backtracking of Sites C0019 and 436 indicates 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 about 100 Ma. Then, they entered the realm of pelagic clay deposition where they remained until about 15 Ma. After 15 Ma Sites C0019 and 436 accumulated sediments recording the transition to a continental margin sedimentary environment. Overall the vertical sequence predicted by translation through adjacent sedimentary facies fits well with that cored at Site 436 and C0019, after accounting for structural complications. This sedimentary sequence formed on normally subsiding oceanic crust and is widely distributed on the smooth seafloor NE of the Tohoku rupture zone. The weak pelagic clay and smooth seafloor may explain the numerous tsunami and tsunamigenic earthquakes in this area. In contrast, S and SE of the Tohoku rupture zone, a seafloor of seamounts rising above the normal oceanic crust accumulated sequences of calcareous sediments as their crests remained above the CCD for most of their history. Here, a sea floor including pelagic clay between carbonate-capped seamounts, has produced no instrumentally-recorded tsunami nor large tsunamigenic earthquakes during subduction. Apparently such earthquakes are suppressed by carbonate-capped seamounts.