PROCESSES GOVERNING GIANT SUBDUCTION EARTHQUAKES IODP Drilling to Sample and Instrument Subduction Zone Megathrusts

Scientific ocean drilling from 2007 through 2018 has played a major role in an ongoing revolution in the understanding of plate boundary fault zone mechanics, structure, and associated megathrust earthquake processes and the tsunamis they create. Major efforts at the Nankai, Costa Rica, Sumatra, and Japan Trench subduction zones that have employed both the riser Japanese drillship Chikyu and the riserless US drillship JOIDES Resolution have sampled main plate boundary faults (decollements), associated splay faults, and incoming plate sediments and basement rocks that develop into the fault system. Research on these rocks and in the boreholes shows that great earthquake ruptures not only can slip all the way to the tip of the megathrust at the seafloor in some events but may well do so typically. One location on a plate boundary fault can apparently also exhibit a range of behaviors over the course of a seismic cycle, from slow slip and tremor to rapid coseismic slip, depending on state of stress, pore pressure, and acceleration interacting with intrinsic lithologic properties. Scientific ocean drilling has provided data and samples for laboratory tests of frictional mechanics, for numerical modeling of fault processes, and for testing new hypotheses on megathrust fault processes, thus playing a central role in the modern pursuit of the grand challenge of understanding how faults that are capable of generating giant subduction earthquakes work.