Great earthquakes in subduction zones occur after stable slip in the proto-seismogenic zone transitions to the unstable slip that characterizes seismogenic zones. Subducted material input to seismogenic zones affects this transition. Material structure, lithology and physical properties change progressively during subduction, and according to current hypotheses, specific material transformations trigger the stable to unstable slip transition.Where accretion dominates a convergent margin, material input is trench sediment that is easily drill-sampled.
However, where erosion dominates a margin, material input is unknown because it originates along the base of the upper plate and alters differently. The depth at which material is eroded lies beyond the sampling capabilities of past scientific ocean drilling, so the protoseismogenic zone transformed material has never been drill-sampled; nor does geophysics resolve its structure, lithology, and physical properties. The Japanese riser drill ship Chikyu in the Integrated Ocean Drilling Program (IODP) overcomes this difficulty. Preparing a site for deep drilling is a much greater task than preparing the shallower sites of past programs, so this is accomplished during workshops.