RELATIONSHIP BETWEEN SHALLOW-DEPTH AND INTERMEDIATE-DEPTH SEISMICITY IN THE EASTERN ALEUTIAN SUBDUCTION ZONE

The transition from shallow interplate thrusting to intermediate-depth seismicity is often poorly observed, but critical for understanding die fate of the downgoing slab. In order to better examine the transition, 1448 earthquakes are relocated from data recorded by a regional seismic network in the eastern Aleutian arc, using an improved three-dimensional velocity model and accurate my tracing. Single-event first-motion solutions are determined from these rays for 31 slab events. The interplate thrust zone is a planar fault zone, dipping 10-15-degrees at 25-35 km depth, and is no more than 5-10 km wide. Most intermediate-depth earthquakes are localized to a plane no wider than 5 km near the top of the descending plate. Fault-plane solution orientations for these events vary by several tens of degrees in orientation, although 73% show T axes aligned within 45-degrees of the slab dip. A parallel seismic zone, 20-25 km deeper into the slab, also shows down-dip plunges of T axes for 3 of 5 solutions. The fault-plane solutions are poorly explained by plate bending or unbending about a neutral fiber. Hypocenters show that intermediate-depth events are confined near the subducted oceanic crust, supporting compositional rather than pure thermal control of intermediate-depth seismicity. One explanation is that the upper-plane events are an indirect consequence of phase changes in subducted crust. Perhaps similar processes are important in producing earthquakes in the lower, parallel zone.