Massive earthquakes came as surprise

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CORVALLIS, Ore., Jan. 23 (UPI) -- Scientists say some recent massive earthquakes surprised them because the Pacific locations weren't thought capable of producing earthquakes of their magnitude.

The massive Tohoku, Japan, earthquake in 2011 and the Sumatra-Andaman superquake in 2004 occurred in regions scientists had thought incapable of producing a megathrust earthquake with a magnitude exceeding 8.4.

Researchers said those two temblors have led them to question if existing predictive models looking at maximum earthquake size are valid.

Past global estimates of earthquake potential were constrained by short historical records and even shorter instrumental records, they said, and scientists need to investigate longer paleoseismic records.

"Once you start examining the paleoseismic and geodetic records, it becomes apparent that there had been the kind of long-term plate deformation required by a giant earthquake such as the one that struck Japan in 2011," Oregon State University researcher Chris Goldfinger said. "Paleoseismic work has confirmed several likely predecessors to Tohoku, at about 1,000-year intervals."

"Since the 1970s, scientists have divided the world into plate boundaries that can generate 9.0 earthquakes versus those that cannot," he said. "Those models were already being called into question when Sumatra drove one stake through their heart, and Tohoku drove the second one."

Both the Tohoku and Sumatra regions had been described in textbooks as not having the potential for a major earthquake, Goldfinger said.

"Now we have no models that work," he said, "and we may not have for decades. We have to assume, however, that the potential for 9.0 subduction zone earthquakes is much more widespread than originally thought."