Potential for 'Superquakes' Underestimated

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Superquakes and supercycles

Goldfinger said scientists' failure to recognize that faults could store energy comes from a lack of data. Historic earthquake records go back only 100 years, he noted. Geologists are only now getting histories that reach back thousands of years, via techniques that decode evidence of past earthquakes in sediments.

"What is happening on a short-term timescale is actually imposed on a long-term cycle," he said.

Goldfinger calls these long-term histories supercycles, and the unusually large and rare earthquakes that discharge the battery are superquakes. The sequence, size and location of quakes vary from one supercycle to the next, he said.

Seismologist Marco Cisternas first proposed that faults could store energy in 2005, with a study showing that the magnitude 9.5 Chile earthquake in 1960, the largest on record, released more energy than had been stored since its most recent quake, in 1837. Tsunami deposits in Chile indicate the last superquake occurred in 1575, and smaller quakes since then had only partly released the strain built up on the fault, his study found.

In Sumatra, south of the Andaman region, analyses of corals uplifted and killed during earthquakes also indicated that the subduction zone undergoes supercycles, according to a 2008 study led by geologists at the Earth Observatory Institute in Singapore. Each series of quakes in the region lasts between 30 and 100 years, according to the study. The supercycles unfold every 200 years or so.

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Forecasting the future

Goldfinger and his colleagues have evidence that the Cascadia Subduction Zone, which stretches from Northern California to British Columbia, is also in the middle of an earthquake supercycle.

Over the past 10,000 years, 19 superquakes and four supercycles have occurred along the zone, Goldfinger said.

"These would typically be of a magnitude from about 8.7 to 9.2, really huge earthquakes," Goldfinger said. "We've also determined that there have been 22 additional earthquakes that involved just the southern end of the fault. We are assuming that these are slightly smaller, more like 8.0, but not necessarily. They were still very large earthquakes that if they happened today could have a devastating impact," he said.

The present cycle seems like it's gently ratcheting downward, Goldfinger said. "This would suggest that we're not due for a giant anytime soon, but the model has no predictive value," he said.

The battery model of earthquake energy storage and discharge makes it difficult for scientists to forecast future earthquakes, as there's no explanation yet for why faults would behave this way, Goldfinger said. Plus, it's hard to say how much energy a fault's battery stores. "We haven't yet figured out how to effectively put a voltmeter on a fault and say how charged it is," Goldfinger said.

But with more detailed records of past earthquakes, such as those in Sumatra and Cascadia, Goldfinger believes scientists can give better estimates of seismic hazards, and prevent surprises like Sumatra and Tohoku.

"The long records are revealing very useful things," he said. "We're not sure what's driving the long-term cycling, but at least we can tell people what to prepare for," Goldfinger said.

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