Mystery tremors bring new twist to quake prediction

· Cumulative effect takes on new significance
· 'Separate phenomena' seen as the same thing

Mysterious tremors deep in the Earth's crust could provide a way to predict future catastrophic earthquakes, according to scientists.

Weak "non-volcanic tremors", first discovered five years ago near Shikoku in Japan, pose no dangers in themselves and have previously been dismissed as insignificant by many scientists. But a new study shows that they are related to low-frequency earthquakes (LFEs), slow-moving seismic activity deep underground which can potentially build up enough force over time to cause a major earthquake at the surface.

Gregory Beroza, a geophysicist at Stanford University, who led the new study, said: "Some people believe that LFEs and tremor are separate phenomena, but what we've shown in this paper is that they are actually the same thing. Tremor is simply a swarm of low-frequency earthquakes, but rather than happening quickly and impulsively like ordinary earthquakes, tremor shakes the Earth for hours, days or even weeks at a time."

The findings, published today in Nature, could help experts predict when a deadly earthquake is due to strike.

Both non-volcanic tremor and LFEs are found mainly in subduction zones, where the Earth's tectonic plates meet and one plate is pushed underneath the other. The most destructive earthquakes ever recorded have occurred in subduction zones, such as the earthquake near Sumatra on Boxing Day 2004, when a magnitude 9.2 quake triggered tsunamis killing more than 200,000 people.

These violent events happen once every 100 to 600 years but scientists think that they are often preceded by LFEs which can last days, months or years without being felt at the surface.

"Shikoku experiences a big earthquake every 100 years or so," said David Shelly of Stanford University, who also took part in the study.

"The last one happened in 1946, a magnitude 8.1 event that killed 1,330 people, and the next big one could strike in less than 40 years."

Seismologists think that since the 1946 earthquake Shikoku has experienced a series of slow earthquakes, on average every six months. By looking at hundreds of seismograms of the Shikoku region recorded between 2002 and 2005, Dr Shelly found that tremor events were made up of swarms of thousands of LFEs.

"We now understand tremor is generated directly by slip on the deep extension of the fault. Combining this understanding with our new ability to locate tremor precisely in time and space, we can track the details of how slip evolves during a week-long slow-slip event," he said. "This could potentially lead to an improved ability to forecast a major earthquake there."

The discovery in Japan has important implications for other parts of the world - non-volcanic tremors have also been found under California's San Andreas Fault, and in the Cascadia subduction zone, which stretches from northern California to British Columbia.