  Multiple line outages that occur together show a variety of spatial patterns
in the power transmission network. Some of these spatial patterns form network
contingency motifs, which we define as the patterns of multiple outages that
occur much more frequently than multiple outages chosen randomly from the
network. We show that choosing N-k contingencies from these commonly occurring
contingency motifs accounts for most of the probability of multiple initiating
line outages. This result is demonstrated using historical outage data for two
transmission systems. It enables N-k contingency lists that are much more
efficient in accounting for the likely multiple initiating outages than
exhaustive listing or random selection. The N-k contingency lists constructed
from motifs can improve risk estimation in cascading outage simulations and
help to confirm utility contingency selection.
