We investigate the spatial and temporal seismicity parameters and the related probabilistic aftershock hazard for the aftershock sequence of the 1999 Mw 7.1 Hector Mine mainshock and compare it with the neighboring 1992 Mw 7.3 Landers sequence. Using a catalog of 11,000 earthquakes, we determine the earthquake size distribution (b-value), the aftershock decay rate (p-value), and the seismic activity rate (a-value). The b-values are high (b > 1.2) within the rupture area, significantly lower (b ≈ 0.7) north of the rupture area, and increase with time since the mainshock. Probabilistic aftershock hazard maps, computed automatically as early as 4 days after the mainshock, identified the northernmost part of the sequence as the highest-hazard region. These maps show a good agreement between the forecasts and the recorded large aftershocks. Based on the asymmetrical b-value and hazard patterns for both the Hector Mine and Landers sequences, we hypothesize that the mainshock rupture directivity and slip distribution influence aftershock hazard. Current static or dynamic stress triggering models cannot resolve this spatial and temporal evolution of the hazard. Stress tensor inversions of 1400 relocated first-motion focal mechanisms show predominantly a strike-slip stress state with a SW–NE trend of the greatest principal stress. The heterogeneity of the stress field is unusually high near the Hector Mine and Landers mainshock ruptures, particularly near patches of large slip.