Seismologists fear that existing predictive models looking at maximum earthquake size are no longer valid. They are now going back to the proverbial drawing board, as per findings of latest research in the US.

The massive Tohoku, Japan quake in 2011 and the Sumatra-Andaman superquake in 2004 had stunned them. This is because neither region was thought to be capable of producing a megathrust quake with a magnitude exceeding 8.4.

“Now we have no models that work,” says Chris Goldfinger, leader of a team of scientists at Oregon State University. And we may not have one for decades, he added. “We have to assume that the potential for 9.0 subduction zone earthquakes is much more widespread than originally thought.”

PLATE BOUNDARIES

Both Tohoku and Sumatra were written off in the textbooks as not having the potential for a major earthquake. The newly published analysis acknowledges that scientists historically may have underestimated the number of regions capable of producing major earthquakes.

Since the 1970s, the world has been divided into plate boundaries that can generate 9.0 earthquakes versus those that cannot. Those models are already being called into question when Sumatra delivered the first super shock, and Tohoku the second one.

HIDING QUAKES

The team point to several subduction zone areas that previously had been discounted as potential 9.0 earthquake producers – but may be due for reconsideration.

These include central Chile; Peru; New Zealand; the Kuriles fault between Japan and Russia; western Aleutian Islands; the Philippines; Java; the Antilles Islands; and Makran, Pakistan/Iran.

Onshore faults such as the Himalayan front may also be hiding outsized earthquakes, the researchers add. They said scientists need to investigate longer paleoseismic records to gain a better appreciation for earthquake potential.

SUPER CYCLES

Paleoseismic work has confirmed several likely predecessors to Tohoku, at about 1,000-year intervals, the team said. It also identified long-term ‘super cycles’ of energy within plate boundary faults.

They store energy like a battery for many thousands of years before yielding a giant earthquake and releasing the pressure. At the same time, smaller earthquakes occur that do not dissipate to any great extent the energy stored within the plates.

vinson.kurian@thehindu.co.in

(This article was published on February 3, 2013)
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