Seismic and aseismic plate-boundary slip and the mechanics of the overriding plate at subduction zones: Application to the seismotectonics of Makran

This thesis is a study of the transition in the style of slip along the shallow region of the plate boundary at subduction zones and its implications for the mechanics of the upper plate and for the size of great thrust earthquakes. I investigate the nature of slip along the shallow part of the plate boundary primarily by studying the updip extent of seismic rupture as defined by earthquake locations. I document the occurrence of aseismic zones along the shallowest section of the plate boundaries at a number of subduction zones and discuss the cause of such zones. The transition from the aseismic to the seismogenic zone occurs at depths greater than most marine geophysical studies reach and more shallow than most seismological studies consider, thus this study has called upon results from the disciplines of seismology, rock mechanics, marine geophysics, structural geology, and laboratory and numerical modeling. The transition in slip behavior has implications for the structure and mechanics of the overriding plate. I infer that the onset of seismicity, the seismic front, marks a transition in the material properties of the rock within the overriding plate. Arcward of the seismic front a region of more consolidated rock exists; that rock is known as a backstop. The greater consolidation (and hence greater cohesion and strength) of the backstop has two effects: it produces a transition from stable slip to stick-slip sliding behavior and it allows the backstop to act as a load bearing member within the upper plate. The first effect is manifested by the seismic front and the second effect explains the development of the dominant forearc structures and their relationship to the seismic front. The ideas developed concerning the transition in plate-boundary slip, the structure of the upper plate and the effects of unconsolidated sediment are tested in a detailed analysis of the seismic behavior of the Makran subduction zone of Iran and Pakistan. I find that the aseismic zone occurs along that part of the plate boundary that is dominated by relatively unconsolidated sediments and that the down-dip width of the aseismic zone corresponds to the volume of such sediment. I also observe that the seismic front occurs near the structures at the arcward end of the accreted wedge of relatively unconsolidated sediments, and suggest that both are indicative of a transition to a region of larger shear strength--the backstop--within the upper plate. These results may be applied to map out the extent of the aseismic zone at subduction zones; such an assessment may be particularly useful at subduction zones where the potential for great earthquakes is poorly known.