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San Andreas fault begins to yield its secrets

SEISMIC REFLECTION imaging is being used to better understand properties of rocks near the surface of the San Andreas fault in California, USA.

Researchers are using the computer-based technique - developed by the petroleum industry to locate oil pooled along steep, vertical sides of salt domes - to collect and process energy reflections.

This reveals, among other things, that the rock immediately west of the fault has low seismic velocities. Earlier studies revealed this rock to have high electrical conductivity.

'Most likely, the rock has a lot of spaces - 10% to 30% porosity - filled with salt water, ' explained Professor John Hole of Virginia Tech in the journal Science .

Imaging is being used to help plan a proposed borehole through the fault at a site halfway between Los Angeles and San Francisco.

'We are looking down about a kilometre, ' said Hole. 'Our primary goal is to look at near-surface structures to help plan the drilling. We also want to image and better understand the fault.' Scientists know where the fault is at the surface, because it is moving by about 3cm a year.

In 1966, scientists predicted a major earthquake in 1988, give or take five years, based on six previous major quakes that were all about 22 years apart.

As a result, a great deal of monitoring equipment has been placed at the borehole site.

The predicted earthquake did not happen - the major Colinga earthquake of 1985 may have released pressure.

Because the site has been so well instrumented, there is excellent documentation of several small, very shallow earthquakes that repeat every one or two years.

'That is why this site was selected for a proposal to drill through a fault and record what happens in a borehole, ' said Hole.

The plan is to drill 2km down from 1.5km away then angle toward the fault through the small, repeating quake site.

'Researchers will measure samples along the path of the drill to understand the physics of the site, ' he said.

The reflection image contains high-resolution vertical lines corresponding perfectly with the fault.

To the researchers' surprise they found the reflector to be vertical in the upper half kilometre, then to slope steeply to the south west. This is interesting because the location of faults in the subsurface are not precisely known.

Actual misalignment of the surface with the deeper fault seems unlikely as the surface rock is too weak to generate an earthquake and should break vertically above the stronger, deep fault, he added.

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