An avalanche that fails in symphony with another adjacent avalanche. 

Credit: Colorado Avalanche Information Center

The sympathetic avalanche fails because the crack propagation from the initial failure (either from a natural or artificial trigger) spreads to an adjacent start zone. With this type of failure, you can have multiple start zones release avalanches at approximately the same time.  

Skier triggered (on the right) and sympathetic (on the left) avalanches released at approximately the same time. Credit: Crested Butte Avalanche Center.

A pair of sympathetic avalanches that released in synchrony with a skier triggered avalanche. Credit: Crested Butte Avalanche Center

A pair of sympathetic natural avalanches. Credit: Crested Butte Avalanche Center