Deep Slab Avalanche on Martis Peak

We went to Martis Peak to investigate a previous deep slab avalanche that was reported to us this morning.  The avalanche occurred on a N to NE aspect between 8430 ft. to 7885 ft. with a slope angle in the start zones of 37 to 39 degrees.  As we approached this avalanche, it became apparent how large it was.  The crown of the avalanche was 450 ft. wide at the top. By the bottom of the slope, the debris fanned out across an 800 ft. wide area.  The avalanche ran downslope around 550 ft. and went through rocks, gully features, and trees.  

The trigger is unknown. This area had a fair amount of snowmobile traffic on the lower part of the slope and the meadow below.  We were able to see snowmobile tracks that were covered by about 4 inches of settled snow and that were overrun by the avalanche debris indicating that those tracks were there before the avalanche occurred.  We think snowmobile activity was in this area on Tuesday, and then a small amount of snow came in on Tuesday night.  We believe the avalanche occurred on Wednesday and the debris covered up some of the snowmobile tracks.  It is possible that this avalanche was snowmobile triggered from somewhere on the slope or remotely triggered from the slope from below.  We could not see any apparent tracks leading into the crown area.

This deep slab avalanche failed on the facet/crust combination that has existed throughout the forecast region.  Around 4 ft. of settled storm snow sits on top of a series of crusts and weak faceted snow. When the light was right, we could see the shine of the bed surface crust.  The facet layer at this location was well developed.  We dug a snowpit at the avalanche crown today (3/21/2020) 3 days after we think the avalanche occurred. Snowpit tests showed improvement in the strength of this weak layer (see attached snowpit).  No other signs of instability were observed in this area on 3/21/2020.