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Last Updated: Thursday, 22 June 2006, 12:38 GMT 13:38 UK
Southern California quake fears
Image of San Andreas fault made by combining radar data with true-colour satellite imagery   Image: SRTM, JPL, Nasa
The San Andreas fault runs for 1,280km through the western US
The southern part of the San Andreas fault is overdue for a large earthquake, according to a study in the journal Nature.

This end of the fault has not experienced a major rupture for at least 250 years and is now primed for a release of the built-up tension.

The study by geophysicist Yuri Fialko provides the most precise measurements yet of this accumulated stress.

But scientists cannot predict when another quake is likely to strike.

The southern section of the fault is fully loaded for the next big event
Yuri Fialko, Scripps Institution of Oceanography
The San Andreas fault runs for roughly 1,280km (800 miles) through California in the US.

It marks the meeting point of the Pacific and North American tectonic plates.

The southern segment begins near the Salton Sea and runs northward before bending to the west where it meets the San Bernardino Mountains. Los Angeles is the biggest city located near this part of the fault.

Professor Fialko looked at eight years of radar data from European Space Agency satellites that measure in detail how the ground moves and 20 years of global-positioning system (GPS) data.

His analysis suggests the two plates either side of the San Andreas fault are moving past each other at a rate of about 25mm (an inch) each year - the fault's "slip rate".

The bigger the average slip rate along a fault line, the more stress might be expected to accumulate on parts of the fault that remain locked together. In the absence of a sudden rupture to ease the strain, the fault has built up 5.5-7m (18-23ft) of "slip deficit".

Taking the strain

If all the strain was released at once, it would have enough energy to unleash a magnitude eight earthquake - roughly the size of the devastating 1906 quake in San Francisco.

"The southern section of the fault is fully loaded for the next big event," Professor Fialko, of the Scripps Institution of Oceanography in La Jolla, explained.

ERS-2 (Esa)
Europe's ERS mission monitors movements in the Earth's surface
Quakes are predicted to occur on the southern part of the fault every 200-300 years. And according to Professor Fialko, the observed movement on the fault is on a par with the maximum amount of shift the fault has ever experienced between quakes.

Scott Brandenberg, a professor at the University of California, commented: "This is new evidence that tells us the same story that we have known for a while.

"It's a reminder that we need to be ready for it when it happens."

The most recent major earthquakes in the northern and central zones of the San Andreas fault were in 1857 and 1906 respectively.

Deadly history of earthquakes
27 May 06 |  In Depth
San Francisco faces big shaker
17 Apr 06 |  Science/Nature
Preparing for the next 'Big One'
16 Apr 06 |  Science/Nature

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