A seminal 2009 article in Nature magazine described yet another statistical pattern common to terrorism: its “burstiness.” Prolonged quiet periods are followed by frantic and deadly activity. The researchers suggested that this may be the result of competition for media attention. Terrorism, after all, is so called because its goal is to instill terror rather than kill the maximum number of people or blow up the most buildings. One group, or even a lone fanatic, gets terrorism back into the media’s sights, and other groups or individuals jump on the bandwagon to boost the effect.
As far as the physics of terrorism is concerned, Boston was not a severe event. In a 2012 article, Aaron Clauset and Ryan Woodard set the threshold at 10 casualties — a category that included about 8.3 percent of all terrorist attacks from 1998 to 2007. After correcting for Iraq and Afghanistan, which have caused a dramatic increase in the number of deadly events in recent years, the share of severe events remains fairly constant — another sign that global terrorism obeys consistent statistical laws.
If attacks come in bursts, it is conceivable that insurgents could follow with larger acts, seeking to take advantage of people’s frayed nerves after the Boston event. According to Clauset and Woodard, there is a 19 percent to 46 percent chance of a “catastrophic” event on the Sept. 11 scale occurring in 2012-2021. The forecast is as imprecise as any estimate based on historical data. The point is that the probability is too high to ignore.
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