The Next Great Earthquake

The 2004 Sumatra-Andaman earthquake and resulting tsunami are now infamous for the damage they caused, but at the time many scientists believed this area was unlikely tocreate a quake of such magnitude.

In the March 23 issue of the journal Science, a geophysicist from Rensselaer Polytechnic Institute urges the public and policy makers to consider all subduction-type tectonic boundaries to be "locked, loaded, and dangerous."

"Seismologists have long tried to determine which subduction boundaries are more likely than others to break," says Robert McCaffrey, professor ofearth and environmental sciences at Rensselaer. "Yet, the great earthquake of 2004 ruptured a segment that was thought to be among the least likely to go."

On Dec. 26, 2004, the earth beneath the Indian Ocean buckled and ruptured, unleashing one of the largest earthquakes in recorded history. Shockwaves from the magnitude 9.2 (M9) quake created a wall of rushing water that devastated communities up to 1,000 miles away.

M9 earthquakes typically occur at a specific type of tectonic boundary called a subduction zone, where one plate is gently slipping underneath another plate, which causes friction, cracking, and lifting of the plates. An M9 earthquake can be created by only 20 meters of slip between two converging plates -- less then the length of an 18-wheeler truck -- but its effects can be global in their impact.

Slips of this length only occur every 200 to 1,000 years or more at aparticular boundary, leaving no reliable historic records to track their frequency, McCaffrey notes. Complete records are only available going back 100 years. Scientists had widely accepted that the age and speed of the subducting plate is important in creating M9 earthquakes, based primarily on support from this 100-year historical record.

But this narrow understanding put the Sumatran subduction zone in avery low risk category. McCaffrey suggests that such limited records are incapable of mapping a trend in geological events that could be several centuries or more apart.

Geologists also focused on the temperature of subduction zones, indicating that temperature at the plate convergence region plays a strong role in the strength of a resulting earthquake. These thermal considerations place the Andaman subduction zone in the high-magnitude class, but one pitfall with this type of classification is that it characterizes some subduction zones as being incapable of producing an M9.

"[The day of the quake], Earth gave us a stark reminder of the important difference between improbability and impossibility," McCaffrey says. "Our understanding of where and when the next great earthquake will happen is in its infancy at best. We have not had enough time to decipher M9 behavior."

In creating new public policy, McCaffrey urges officials to consider all subduction zones as lethal. "Several are near densely populated landareas, and the potential impacts of shaking and tsunamis cannot be overstated," he says.

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