New warning signs identified

2007-03-16 16:41

Paris - Geophysicists poring over an earthquake hotspot beneath southern Japan believe massive temblors may be preceded by slow, barely perceptible quakes that can last for days or weeks.

In a new paper published in the British journal Nature, the scientists say the warning signs are buried in tiny seismic signals caused by a slip deep within a fault.

Their focus is on phenomena that lie far below the threshold of human sensation called low-frequency earthquakes (LFEs) and non-volcanic tremor.

Seismologists have until now mainly viewed non-volcanic tremor as a weak shaking of the Earth, and LFEs as a swarm of small temblors, with a magnitude of just one or two, that can last for weeks or even months at a time.

These low rumblings are typically found in subduction zones - the regions on the Pacific's "Ring of Fire" that have unleashed the mightiest quakes on record, say the US-Japanese team, led by David Shelly of Stanford University, California.

A subduction zone is a meeting of two tectonic plates, where one plate dives under the other. The 9.2-magnitude quake off Sumatra on December 26, 2004, which triggered a tsunami that killed more than 200 000 people, is an example.

Shelly's team looked at hundreds of seismograms recorded by sensors that stud the southern Japanese island of Shikoku, which lies at the interface of the Philippine Sea subduction zone around 35km below the surface.

The researchers found a "perfect correlation" between non-volcanic tremor and LFEs.

What were considered to be separate phenomena are simply, they believe, different manifestations of the same event - a slip in a deep part of the fault.

"(It is) the same mechanism by which regular earthquakes are generated, but with a twist. The slip in deep tremor happens more slowly than in ordinary earthquakes," said co-author Gregory Beroza, a professor of geophysics at Stanford.

- AFP