The existence of a major right-lateral transform fault, Boso transform fault, is proposed. This Boso transform fault is the result of shear partitioning due to oblique subduction and has delimited a Boso sliver for 2 Myr. It is proposed that the slip on the Boso transform fault was responsible for two large (M=7.0 and 7.5) aftershocks which occurred on the second day after the 1923 great Kanto earthquake.