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.