Potential for 'Superquakes' Underestimated

//
A village near the coast of Sumatra lays in ruin, Jan. 2, 2005, as a result of the tsunami that struck South East Asia Dec. 26, 2004.
U.S. Navy photo by Petty Officer 2nd Class Philip A. McDaniel

The earthquakes that rocked Tohoku, Japan in 2011, Sumatra in 2004 and Chile in 1960 — all of magnitude 9.0 or greater — should not have happened, according to seismologist's theories of earthquake cycles. And that might mean earthquake prediction needs an overhaul, some researchers say.

All three earthquakes struck along subduction zones, where two of Earth's tectonic plates collide and one dives beneath the other. Earlier earthquakes had released the pent-up strain along Chile's master fault, meaning no big quakes were coming, scientists had thought. Japan and Sumatra both sat above on old oceanic crust, thought to be too stiff for superquakes.

And records of past quakes, combined with measurements of the speed of Earth's tectonic plates, suggested the Tohoku and Sumatra-Andaman regions couldn't make quakes larger than 8.4, almost nine times smaller than a magnitude 9.0 temblor.

PHOTOS: Japan Earthquake and Tsunami: Before and After

"These areas had been written off as places incapable of producing a great earthquake," said Chris Goldfinger, a marine geologist at Oregon State University in Corvallis.

But the events of 1960, 2004 and 2011 showed that these faults were capable of producing some of the most destructive earthquakes in recorded history, suggesting earthquake researchers need to re-think aspects of how they evaluate a fault's earthquake potential.

"It's time to come up with something new," Goldfinger told OurAmazingPlanet.

Faults are like batteries

When two tectonic plates collide, they build up strain where a fault sticks, or locks, together. Earthquakes release this strain, which is a form of energy.

For decades, scientists assumed faults acted like rubber bands, steadily building up strain and then releasing it all at once, Goldfinger said. The longer the time since the last earthquake, the larger the next earthquake would be, the model predicted. (Video: What Does Earthquake 'Magnitude' Mean?)

The problem was researchers failed to recognize that faults can store energy like a battery, Goldfinger said. And just like batteries, they can discharge energy in small amounts, or all at once, he explained.

Goldfinger and other researchers now think if a "small" quake hits, it may not release all of the accumulated energy in a fault. (On a subduction zone, a small quake can still register in the magnitude-8.0 range, which is devastating to nearby cities.)

8.0 Quake Strikes Santa Cruz Islands

Thus, a fault can "borrow" stored energy from previous strain-building cycles, generating larger earthquakes than expected, such as those that hit Sumatra and Tohoku, Goldfinger and his colleagues propose in a study published in the January/February 2013 issue of the journal Seismological Research Letters.

"Those models were already being called into question when Sumatra drove one stake through their heart, and Tohoku drove the second one," said Goldfinger, the lead author of the study.