The great Rat Islands underthrusting earthquake (Mw=8.7), of February 4, 1965, represents subduction of the Pacific plate beneath the North American plate along a 600-km segment of the western end of the Aleutian Islands. Body wave inversion techniques are used to determine the spatial and temporal heterogeneities associated with the Rat Islands earthquake. We have deconvolved World-Wide Standard Seismograph Network long-period teleseismic P wave seismograms to obtain source time functions. Directivity associated with the three major pulses of moment release in the source time functions indicates a total source duration of 160 s, unilateral rupture in the direction 300¿, fault length of 420 km, and average rupture velocity of 2.5 km/s. The three pulses of moment release are located along the fault, and these regions of high moment release are interpreted as asperities. The first asperity extends from the epicenter to 100 km to the WNW. This is the largest asperity and corresponds to a smooth pulse of moment release in the source time function with a duration of 50 s. The second pulse of moment release is very jagged, is less coherent between stations, and is centered ~200 km WNW of the epicenter. The third pulse of moment release extends from 360 to 420 km WNW of the epicenter. Although the aftershock area is ~600 km in length, we can not resolve any moment release from the P waves beyond ~420 km WNW of the epicenter.

The Rat Islands event was closely followed by a large tensional outer-rise event on March 30, 1965, (Ms=7.5), which is located oceanward of the largest moment release associated with the Rat Islands mainshock rupture. Detailed analysis of the P waves for this large outer-rise event confine the depth extent to the upper 30--35 km of the crust. The spatial and temporal association between the February 4 mainshock and the March 30 tensional outer-rise event suggests the tensional event may have been triggered by the large displacement near the mainshock epicenter. The overriding plate along the western Aleutian subduction zone is laterally segmented into a series of rigid tectonic blocks separated by fault controlled canyons and extensional basins (Geist et al., 1988). We suggest that the central undeformed parts of the blocks have the strongest coupling with the down-going plate and hence are the sites of the largest moment release during an underthrusting earthquake. The three asperities determined from the P waves correspond to the Rat, Buldir, and Near tectonic blocks respectively. Hence the P wave seismic moment release of the Rat Islands earthquake is controlled by the lateral segmentation of the overriding plate. ¿1991 American Geophysical Union