The May 16, 1968, Tokachi-Oki (Mw =8.2) and the August 11, 1969, Kurile Islands (Mw =8.2) shallow thrust earthquakes occurred along the Japan-Kurile arc about 500 km apart and possess similar static fault parameters. Their dynamic source processes were investigated using the observable directivity in the source time functions deconvolved from long-period P waves. The rupture processes of both earthquakes can be understood in terms of a combination of the end-member asperity and barrier models of earthquake rupture. The 1969 Kurile Islands earthquake ruptured in the sense of the asperity model; one region of the fault plane radiated most of the seismic moment (i.e., the asperity) and is surrounded by the rupture zones of large precursor events. The rupture process of the 1968 Tokachi-Oki earthquake contains elements of both end-member rupture models. A dominant asperity is bounded to the south by a region of the fault plane ruptured by a large precursor event and is bounded to the north by a geometric barrier where the plate margin changes direction. This geometric feature is responsible for the abrupt termination of the rupture front. It appears that the tectonic setting plays a vital role in determining the rupture mode. Earthquakes that occur along linear trenches, where no geometric or topographic complications exist, seem to rupture in the sense of the asperity model, whereas earthquakes that occur in the vicinity of such complications are more likely to possess rupture processes intermediate between the two end-member models.