A fundamental understanding of the deformation behavior and failure modes of sediments and porous rocks is important in understanding the distribution of seismicity in a number of geological settings. This is particularly true of the deepest, arcward ends of accretionary wedges, the aseismic masses of accreted sediments beneath the landward slopes of many convergent plate boundaries. This problem is particularly important in accretionary wedges because of their aseismicity, and because of the importance of understanding the nature of the 'seismic front', which marks the trenchward limit of plate-boundary thrust earthquakes at or near the back of the wedge. Although the lower, thermally controlled, boundary of the thrusting seismogenic zone is reasonably well understood, the shallow limit has not been properly explained. Because of the very small dip of the plate boundary at such shallow depths, this leads to a great deal of uncertainty about the total width of the seismogenic zone, upon which the maximum possible earthquake size depends very strongly.
