The world's largest and some of its most destructive earthquakes occur on the interface between subducting and overriding plates, with the degree of interplate coupling playing an important role in their seismogenesis. Updip from this interface, the outer rise comprises an upwarping of the oceanic lithosphere just before it descends into the trench. Isacks and Molnar (1969, 1971), Isacks et al. (1969) and Oike (1971) established the characteristics of the stress regime within the subducting lithosphere and suggested that intraplate earthquakes serve as stress gauges for the large-scale deformation involved in subduction zones. Polet (2005) recently compiled a new catalog of outer rise seismicity (here defined as earthquakes located seawards from the trench), partly confirming earlier results (Christensen and Ruff, 1983; Lay et al., 1989) by showing that normal faulting events occur preferentially after large interplate thrust events. Compressional outer rise events were found to show a similar temporal pattern, in contrast to earlier findings suggesting they occurred more frequently prior to large interface earthquakes, which hints that a more complex physical mechanism may be at work than simple elastic plate bending (Ward, 1983; Liu and McNally, 1993). An inelastic analysis of lithospheric stress distributions by Mueller et al. (1996) predicts seismic behavior similar to that observed by Polet (2005). From these temporal and spatial relationships, it is clear that the state of stress in the subducting plate may change in response to interplate coupling. The seismicity record should reflect this and thus shed light on the physical processes at work. The P.I. of this project is constructing a complete catalog of intraslab seismicity, extending the analysis of Polet (2005) to events landwards of the trench, greater depths (150 km) and lowering the magnitude threshold from 6.0 to 5.0. The next step of the research is to apply a teleseismic P-wave modeling technique to refine the depths of the intraplate events to a higher precision and homogeneity. Subsequently a source spectral analysis of the Mw>=6.5 earthquakes can be carried out to determine dynamic rupture parameters. These investigations will provide us with unique insight into a myriad of issues, including: the failure mechanism of intraplate and outer rise earthquakes, the state of stress in the subducting lithosphere, the origin of the hydration of the subducting plate and the mode of deformation of the outer rise. Bathymetric and seismic profiles across subduction-related trenches commonly show distinctive patterns of normal faulting on the outer trench wall (Masson, 1991). Hilde (1983) reviewed the occurrence of outer trench wall faulting and found it to be essentially ubiquitous. The relationship between this type of faulting and the structure and geometry of the subduction zone is still not clear (Hilde, 1983; Aubouin et al, 1984; Scholl et al., 1982). Obvious factors that may control the strike of these faults include the strike of the trench and any weakness in the subducting plate, such as the fabric resulting from oceanic spreading. These and similar observations from bathymetric and seismic trench profiles can be related to intraplate earthquake occurrences and seismic moment release. Based on the catalog currently in development, the P.I. is investigating whether the asymmetry in the fault system previously found for the source mechanisms of tensional outer rise events persists to greater depths, testing the hypothesis that reactivation of preexisting weak zones, created prior to subduction, is responsible for these events. The main interest of this research is in improving our understanding of the role of intraplate seismicity in the earthquake cycle, the mode of outer rise deformation, the relationship between seismic coupling and intraplate seismicity and the role of pre-existing weak zones and dehydration in the generation of intraplate earthquakes. Improved knowledge of the temporal and spatial character of intraplate seismicity will also be an important first step towards the development of a new generation of models of subduction zone dynamics. The behavior of intraplate events in time, and their relationship to the earthquake cycle, may also be significant for intermediate term earthquake hazard assessment.
