An international research team studied the geological structure of the Japan Trench, where the Pacific plate boundary subducts beneath the Asian plate, to understand why the 2011 earthquake was so devastating.[1][2] Using the research vessel Chikyū, they drilled to a record depth of nearly eight kilometers below the sea floor and obtained sediment samples directly from the plate boundary.[2][4] They found that the main fault zone tapers into a thin layer of very weak, clayey material located just below the seafloor, ranging from a few meters to 30 meters in thickness.[2][4] This weak clay has very low friction and acts as a "lubricant" for the fault, which allowed the 2011 magnitude 9.1 earthquake to spread to the sea floor and cause displacement of 50 to 70 meters.[1][2][6] This shift of the seabed displaced a huge amount of water and caused a devastating tsunami.[2] The same clay extends along much of the Japan Trench, meaning that similar potential for large shallow earthquakes exists in other areas of this plate boundary.[2]