Reuters AlertNet Lite - Deep tremors may hold key to predicting big quakes

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Deep tremors may hold key to predicting big quakes

02 May 2007 17:00:13 GMT
Source: Reuters

HONG KONG, May 3 (Reuters) - Researchers have identified a link between a series of recently discovered earth movements that they believe may hold the key to better forecasting major earthquakes.

Instances of deep tremors, low-frequency and silent earthquakes have only been observed in the past two decades, with the advent of equipment like the Global Positioning System (GPS), and researchers have been studying them as disparate events.

But scientists in Japan and the United States say these events may be symptoms of what is known as a "slow earthquake", if they occur in the same place and around the same time.

"They can be thought of as different manifestations of the same phenomena and they comprise a new earthquake category," they wrote in the latest issue of the journal Nature.

Big earthquakes are a result of high-frequency seismic energy and can cause massive damage in a matter of seconds.

But slow earthquakes, which can last for months, give off little or no seismic energy. The researchers said slow earthquakes occur in places where there are regular quakes.

"Slow earthquakes occur very close to areas of regular earthquakes. Although slow earthquakes don't radiate seismic waves, they increase the stress in areas of regular earthquakes," Satoshi Ide at the University of Tokyo told Reuters.

"If we know more of these slow earthquakes, we can estimate how much stress is accumulated in regular earthquake zones. We can assess the probability of (damaging) earthquakes happening; we can tell how high the risk is."

The researchers made their observations in the Nankai trough in western Japan. Similar phenomena were also detected elsewhere, such as the Cascadia subduction zone along the North American Pacific coast.

The last slow earthquake observed in Japan was on Shikoku island in 2002, Ide said. The slow quake of 6.8 magnitude lasted three months and resulted in no visible damage.

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Last updated:Wed May 2 17:02:10 2007