Why Do Terrorists Blow Up Shopping Malls?

The brazen terrorist attack by Islamic militants on the Westgate Mall in Nairobi this week was tragic, devastating and emotionally wrenching. It was also, unfortunately, inevitable. No matter how good a job government does in protecting us from terrorists and uncovering nefarious plots, we were due for such an attack to take place. To understand why terrorists blow up shopping malls, however, you do not need to get inside the heads of the Somali Shabaab militants and learn their ideology. All you need to understand is statistics, probability and the inexorable mathematical logic of power law distributions.


In the final chapter of his book The Signal and the Noise – which is probably the single most readable and mainstream explanation of why predictions both succeed and fail – Nate Silver asks a provocative question, “Why don’t terrorists blow up shopping malls?” Until Nairobi, of course, terrorists did not blow up shopping malls, so the easy answer to this question was that there simply aren't all that many terrorists in the world, and they aren't going to waste their time on smaller targets. The more sophisticated answer, however, is that, until now, we've confused the "unfamiliar" with the "improbable." In other words, just because attacks on shopping malls don't occur in the West, doesn't mean that they're improbable.

In fact, in studying the history of terrorist attacks in NATO nations during a 30-year period (1979-2009), Nate Silver found that the size and occurrence of these attacks followed much the same pattern as earthquakes. Once an area becomes seismically active, the presence of smaller magnitude quakes inevitably implies the presence of a larger quake on the way. In the same way, once terrorists infiltrate a region, the presence of smaller-scale attacks implies the future arrival of larger-scale attacks. Over time, you'll see a large number of relatively small-scale events (underwear bombers) and a very tiny number of massive, large-scale events (9/11).

As Silver points out in his book, both terror attacks and earthquakes follow a power law distribution. In short, a very small number of cases cause a very large proportion of the total impact. From this perspective, there is a ruthless mathematical logic to the occurrence of terror attacks like the one in Nairobi:

"Terror attacks behave in something of the same way (as earthquakes). The Lockerbie bombing and Oklahoma City were the equivalent of magnitude 7 earthquakes. While destructive enough on their own, they also implied the potential for something much worse - something like the September 11 attacks, which might be thought of as a Magnitude 8. It was not an outlier but instead part of the broader mathematical pattern."

Once you realize that earthquakes follow a power law distribution, there's actually a general law in seismology for predicting the occurrence and magnitude of earthquakes. This law, known as Gutenberg-Richter, is remarkably accurate:

"Over the long term, the frequency of earthquakes is reduced about 10 times for every one-point increase in magnitude. However, the energy released by earthquakes increases exponentially as a function of magnitude. In particular, for every one-point increase in magnitude, an earthquake's energy release increases by about 32 times. So a magnitude 6 earthquake releases around 32 times as much seismic energy as a magnitude 5, while a magnitude 7 is close to 1,000 times more powerful."

If terror attacks follow the same pattern as earthquakes, this is scary stuff. That means that a Magnitude 9 attack would be 32 times more powerful than the 9/11 attacks, which can be thought of as a Magnitude 8 event. If you run the numbers (as Silver did), that implies that there is a 10% chance of an attack that would kill at least 10,000 people in a NATO country over the next decade.

And that was before Nairobi took place.

The question now should be whether an attack of the scale and magnitude that took place in Nairobi could ever take place in America (or any other NATO nation). Would terrorists attack a mall in a popular suburban neighborhood, or would they go for maximum impact -- by exploding a weapon of mass destruction in a busy urban mall? As additional details about the Nairobi bombing become clear - such as the growing evidence that the attackers actually rented space at the mall months in advance of the killing - we will need to adjust the mathematical odds of whether or not an event like the explosion of chemical or biological weapons over a major urban area is probable or improbable.

So back to our original question -- why do terrorists blow up shopping malls? While mathematics and probability tell the story of terror attacks, the answer is also based on the type of society we choose to live in. The type of society impacts the shape of the final power law distribution. In Israel, as Silver points out, every citizen lives with the reality of terrorists blowing up shopping malls. In exchange, this also negate the chances of any large-scale terror event happening in Israel. It almost sounds cynical to say it, but the high probability of daily everyday violence actually lowers the chances for any spectacular type of 9/11 violence ever happening.

Is this where we're headed in America?

The mall bombing in Nairobi offers a strong signal, just as earlier terrorist attacks against the U.S. overseas were strong signals of the impending 9/11 attacks. If terrorist attacks really do follow the same behavior of earthquakes, this is a signal that we would do best to heed sooner rather than later.

image: Defending the Airports / Shutterstock

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