Avalanche was triggered remotely from the ridge. Slab failure was actually about 500' wide. Slab collapsed and cracked beneath the ridge where the riders were located and the slope angle was around 30 degrees. Failure propagated north along the ridge line for about 200' before taking in a sub ridge. At this point the slope steepened to 37 degrees and the avalanche occurred. Weak layer was the December facets above the October crust. Because of the relatively shallow depth, suspect a repeat runner.