Ex-elevator shaft

Witnessed a group of 4 snowboarders just outside of the Lake Louise boundary line on ex-elevator shaft. They came down one at a time, through the first chute. The 4th boarder through triggered a size 2 slab avalanche, was unaware and thankfully rode out to safety. The slide triggered as the rider came under a rocky/cliff section. Likely a thin spot in the snow pack. The crown was fairly deep, all the storm snow as of late, and ripped to ground and broke around the rock outcropping. Due to the terrain flattening out after the cliff it did not slide far. This picture was taken later in the day, it looks as though a secondary slide had occurred at some point. (Slide to the right of the skier at boundary line). The slide I witnessed was lower.