We make use of the effects of rupture directivity on waveform shape to analyze the rupture processes of 58 large, deep earthquakes in the Izu-Bonin-Marianas subduction zone. For more than half of the analyzed earthquakes, we determine a best-fitting unilateral rupture direction using teleseismic data. For 20 of these earthquakes, the constraints on the rupture direction allow the fault plane to be identified. Where the subducting slab dips at a moderate angle, near-horizontal fault planes dominate at all studied depths (50-600 km). Within more steeply dipping slabs, fault planes tend to dip toward the south and west. Rotated into the plane of the slab, the poles of the definitively identified faults form a single tight cluster pointing up and toward the surface of the slab. Identified ruptures have a tendency to propagate away from the top surface of the slab between 100 and 300 km depth, but appear to be randomly oriented at greater depths. The occurrence of predominantly near-horizontal faults at intermediate depths agrees with previous observations in several other subduction zones. However, at depths ≥300 km, the results from the Izu-Bonin-Marianas system differ from those previously obtained for Tonga-Kermadec, where both horizontal and vertical faults were identified. We consider various physical mechanisms to explain our observations and conclude that, while pre-existing slab structures may be reactivated if they are favorably oriented, the observed asymmetry in deep fault orientations may instead result from external forces acting on the slab, and resulting changes in slab morphology.