Next Great Quake?

Subduction zones in blue; tectonic plate boundaries in brown. Credit: Robert McCaffrey

Friday, March 23rd, 2007--

In December of 2004, when the earth below the Indian Ocean buckled, producing the Sumatra-Andaman earthquake and one of the deadliest tsunamis in recorded history, researchers were surprised. The fault that produced the earthquake had been judged one of the least likely faults to rupture.

Earthquakes as powerful as the Sumatra-Andaman quake—a Magnitude 9 (M9)—mostly occur at “subduction boundaries,” places where two plates run up against each other, with one sliding underneath. As the plates converge, they stick to each other—neither budges—and pressure builds up. Eventually the force is too much for the fault line, and one plate slips below the other, producing an earthquake.

The magnitude of the earthquake depends on the size of the slip and the length of the boundary. Rob McCaffrey, a researcher at Rensselaer Polytechnic Institute in Troy, New York and author of a paper in the journal Science this week on earthquake prediction says: “Earthquakes come in all sizes. So if you have a Magnitude 7, maybe it moves a half a meter on a fault that may be fifty kilometers long." An M9 is produced by a 20 meter slip along a 1000 kilometer fault. "That's Boston down to Florida—that’s the whole eastern seaboard of the U.S,” says McCaffrey.

Forty thousand kilometers of subduction boundaries exist, most of which are located beneath the ocean. So far, our ability to predict where and when M9s will happen has been limited. Researchers have used age of boundary, angle of subduction and plate speed to try to predict earthquakes. But these models didn’t explain the Sumatra earthquake. Temperature could explain the Sumatra-Andaman earthquake—high temperatures below the fault could heat the subduction zone and make the plates less brittle and less likely to cause a large earthquake. But temperature doesn't explain other M9s earthquakes on record.

Rate of convergence of the plates should also be factored in, McCaffrey says. As plates converge, there is “slip build-up” at the boundary, which eventually gives out causing the earthquake. So the faster the plates move, the more build up of tension, and the more magnitude 9 earthquakes. But, again, the historical record doesn’t always match. While the tension on the boundary is “building up linearly through time,” says McCaffrey, “The slip itself doesn’t follow such a simple relationship.”

The take home point, McCaffrey writes, is that “every subduction zone is potentially locked, loaded and dangerous.” So what do we do? Focus on education, McCaffrey says: “Put it in the history books and have kids at school study it—make it part of the curriculum.”

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--Flora Lichtman