A relocated earthquake sequence in the western Aleutian arc lasting from February 1965 through December 1968 is shown to map an episode of subduction of the Pacific plate. The February 4, 1965, Rat Island main shock (Ms=8.1) was a complex rupture involving greatest underthrusting at lock zones beneath the Aleutian arc transverse canyons. The primary tectonic consequences of the main shock rupture were (1) the downslab diffusion of a compressional pulse, (2) the oceanward diffusion of an extensional pulse in the oceanic lithosphere, and (3) the slow rebound of the continental plate. The velocities of these strain pulses (actual plate motions) are determined by the viscosity of the constraining mantle material: observed strain velocites indicate a Newtonian mantle viscosity of 6¿1019 P. The extensional pulse that propagated oceanward is shown to determine the locations and normal-faulting mechanisms of the trench earthquakes of this sequence. It is inferred that the entire subducting oceanic lithosphere contains a sequence of near-vertical faults that strike parallel to the trench axis, reflecting former trench-related normal-faulting earthquakes. This condition is supported by considerable data and suggests a slabbing, escalatorlike descent mechanism for the upper 100 km of the subducting Pacific plate. A working hypothesis is presented that relates the primary features of the typical volcanic arc and interarc basin to episodic downslab compressional pulses. This hypothesis involves a sequence of high-loading-rate mantle compression, viscoelastic rebound, pressure reduction, increased partial melt concentrations, and counterflow in a thin tabular zone above the subducting plate.