"We've never been inside the fault zone before," said William Ellsworth, a principal investigator on the project for the US Geological Survey. "The goal has been to understand the basic mechanics of faults," he said.

Geologists who have been analysing earthquakes along the San Andreas fault near Parkfield in central California told a scientific conference they plan to install a number of monitoring devices within meters (yards) of one of the world's most notorious fault zones.

Preliminary monitoring is set to begin in January.

"The instruments will be detecting what is occurring exactly in the fault zone before, during and after" an earthquake, Zoback said.

The San Andreas fault, which triggered the 1906 San Francisco earthquake and the deadly fires that followed, runs from the Southern California desert to northern California and marks the boundary of the Pacific and North American tectonic plates.

The monitoring instruments, part of a five-year, $20-million project by the university and the USGS, will be installed two to three km (1.2 to 1.9 miles) underground in a portion of the San Andreas fault zone that triggered several moderate quakes of about magnitude 6 from 1857 to 1966. Drilling for the project began in 2004.

"This is not an earthquake prediction instrument," Zoback said. "We are testing whether these types of earthquakes on this particular fault are predictable."