This proposal seeks support for a two-year project to investigate the complex but systematic variation of earthquake activities near plate interfaces in Wadati-Benioff zones. In particular, we plan to characterize the nature of a notable number of predominantly normal faulting events that cannot be accounted for by slip on the interplate thrust zone. The depth and mechanism of approximately 100 moderate- sized earthquakes that are shallower than 100 km in major subduction zones will be precisely determined by inverting the long-period P and SH waves at teleseismic distances and by identification of surface reflections from short-period seismograms. We anticipate delineating the position of the plate interface and the seismicity near it in some detail. The results will then be combined with a compilation of source parameters for events in the outer-rise region and intraplate seismicity downdip from the interface. We seek to clarify the possible association of thrust and normal faulting events near the plate interface, the nature of the transition from interplate to intraplate seismicity at intermediate depths, and the connection between intraplate seismicity in the outer-rise region, beneath the plate interface, and beyond the interface in the downdip direction. The ultimate goal of the synthesis is to identify constraints that can be placed upon the state of stress both on the plate interfaceand the subducted lithosphere by the source parameters of earthquakes. We believe that the proposed study would provide enough data to at least qualitatively address several important questions. For instance, if the occurrence and mechanism of events below the plate interface can be correlated with plate tectonic parameters that are believed to control the seismic coupling of the interplate zone, then these events might provide information about the zone both occur, the transition of the strain field within the slab from lithospheric bending seaward of trenches to unbending at intermediate depths will be investigated.
