Satellites give new view of quakes

Radar detects how faults slip

By Dr David Whitehouse BBC News Online science editor

Data obtained from satellites is providing scientists with a new way to analyse the movements in the Earth's crust that cause earthquakes.

Previously they had to rely on a series of spot measurements across an earthquake zone.

The new data on rock movements comes from satellites that bounce radar signals across a wide swathe of the Earth's surface.

The new insight may provide a way to study earthquake zones and predict when and where quakes may strike.

Conventional wisdom overturned

The earthquake that provided the new information took place on 16 October 1999 in California.

The quake, of magnitude 7.1, caused only a little destruction and no injuries because it occurred in an area with a sparse population and development.

Strain builds up before a quake

Satellite observations of it produced information about never-before documented features of faults. These include the first evidence that faults move backwards, contrary to conventional wisdom.

Yuri Fialko of the Scripps Institution of Oceanography, US, says the research provides a new way to identify potentially active faults, and perhaps a better understanding of the earthquake process.

Dr Fialko says the earthquake was the first one to be comprehensively imaged using so-called synthetic aperture radar that reveals minute details of the fault's movements.

According to a paper in the journal Science the fresh data has given researchers a new window into earthquake processes.

The most surprising finding was the first evidence that faults can move backwards. Prior to an earthquake, faults are locked in position by friction.

"Even small stress perturbations from distant earthquakes can cause faults to move a little bit, but it's only been known to cause this motion in a forward sense," says Dr Fialko.

"Here we observed the faults slipping backwards due to relatively small stress changes, which is really quite unusual."

Soft rock

The data also suggests that rocks in fault zones are more pliable that the rocks that surround them.

"The material within the faults is mechanically distinct from the material surrounding the faults," says Dr Fialko. "The rocks within the faults appear to be softer."

Yuri Fialko says the earthquake data is valuable

He believes that fault zones become strained during periods of stress and begin to act like a soft, sponge-like material.

According to Dr Fialko, the results will guide new seismic studies to areas with contrasting fault material, such as that seen in the Eastern California Shear Zone. They can then be used as a way of identifying potentially active faults.

"Measurements of changes in the mechanical properties of faults may yield valuable information about the earthquake cycle.

For example, we might be able to say how long it was before the fault experienced an earthquake and how long it takes to heal," he says.