While technically an "aftershock" of the Landers earthquake (indeed, the largest aftershock), the Big Bear earthquake occurred over 40 km west of the Landers rupture, on a fault with a different orientation and sense of slip than those involved in the main shock -- an orientation and slip which could be considered "conjugate" to the faults which slipped in the Landers rupture.
The Big Bear earthquake rupture did not break the surface; in fact, no surface trace of a fault with the proper orientation has been found in the area. However, the earthquake produced its own set of aftershocks, and from these, we know the fault geometry -- left-lateral slip on a northeast-trending fault.
Following the Landers mainshock by three hours (it occurred while TV news coverage of the Landers earthquake was being broadcast live from Caltech), the Big Bear earthquake caused a substantial amount of damage in the Big Bear area, but fortunately claimed no lives. Landslides triggered by the jolt blocked roads in the San Bernardino Mountains, however, aggravating the clean-up and rebuilding process.