  Observed transmission line outage data is grouped into successive generations
of events. The empirical distribution of the number of generations in the
cascades follows a Zipf distribution that implies the increasing propagation as
cascades progress. The slope of the Zipf distribution gives a System Event
Propagation Slope Index (SEPSI). This new metric quantifies the cascade
propagation, varies as expected, and determines the probabilities of small,
medium, and large cascades.
