To explore aftershock-triggering mechanisms for the 2003 Big Bear, California earthquake sequence, we determined differential travel-times and applied the double-difference technique to relocate these events, which formed three clusters. The main cluster coincides with the 3 km long northwest striking sub-vertical mainshock fault. The other two sub-vertical clusters, located at opposite ends of the mainshock rupture, are almost perpendicular to the mainshock fault, contradicting the 60¿ separation angle of conjugate faults as predicted from frictional laws. Allowing for a 30¿ uncertainty in the cataloged strike, dip and rake values about 75% of the aftershocks are strike-slip as determined from first motion and complete waveform moment tensor inversions. We use a mainshock conceptual slip model to derive Coulomb Failure Stress regions, and assess correlations between stress increases and aftershock locations. We conclude that the perpendicular aftershock clusters were triggered by the mainshock static stress perturbation.