14 DAY TRIAL // The big earthquakes are triggered by the high-frequency seismic energy stored within a certain time length in a seismic zone and can turn into sand castles the strongest buildings in just a few seconds. But the recently discovered slow earthquakes can last for months, not seconds, and give off little or no seismic energy at all. They take place in zones affected by regular strong quakes.
Now scientists have detected a connection between the slow earth movements and big quakes that could improve the forecasting of the last ones.
"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.
The deep tremors, low-frequency and silent earthquakes could have been observed only in the past two decades, with the development of more sophisticated technologies like the Global Positioning System (GPS) that enabled the researchers to study them as disparate events.
Japanese and American researchers believe these phenomena can be linked to "slow earthquakes" if they take place in the same area at about the same time interval.
"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.
"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 Japanese-American team made its researches in the Nankai area (western Japan), but the same type of phenomena have been also found elsewhere, like the Cascadia subduction zone (the North American Pacific coast).
"The last slow earthquake observed in Japan was on Shikoku island in 2002," said Ide.
This slow quake of 6.8 magnitude had a three months duration and did not cause any damage.