Getting inside an earthquake

The fault extends almost the full length of California
(Image by SPL)

By Molly Bentley in Parkfield, California

Geologists in the US have broken new ground in earthquake research.

In the central California town of Parkfield, the self-professed "earthquake capital of the world", a research team led by the United States Geological Survey (USGS) has begun drilling a two kilometre pilot hole at the San Andreas Fault.

It's like using a stethoscope and listening very, very carefully

Dr Mark Zoback

If all goes well, engineers will drill a second hole to penetrate the fault and establish the first earthquake observatory in an active fault zone.

The drilling is part of a major earthquake project called the San Andreas Fault Observatory at Depth (Safod), designed to collect unique data on the fundamental processes that cause earthquakes.

Currently all seismic instruments sit on the Earth's surface or in shallow boreholes. By placing instruments in the heart of a fault zone, Stanford geologist and Safod principal investigator Mark Zoback says scientists can monitor earthquake activity up-close.

Better prediction

"It's like using a stethoscope," said Dr Zoback, "and listening very, very carefully."

He and other scientists hope to determine what forces act on a fault before, during and after earthquakes. This includes chemical changes that might precipitate an earthquake, as well as physical stress.

They would also like to know whether earthquakes are predictable. Currently, quake prediction is shaky.

"There is no way reliably to provide short-term warning of earthquakes," Safod principal investigator and USGS geologist Stephen Hickman told BBC News Online.

"There have been many more failures than successes." Currently, geologists can only provide 30-year "hazard estimations" of an earthquake occurring on a particular fault in a particular area, he added.

Theory test

So Safod scientists are slowly digging for answers. Hard granite limits the drilling to four or five metres an hour.

When engineers finish the hole - as is expected by the month's end - they will collect data to guide another drilling into the fault itself.

The drilling is awaiting further funding

The second drilling awaits Congressional budget approval of Safod, expected in the autumn.

If they get funding, geologists will penetrate the San Andreas Fault four kilometres beneath the Earth's surface and establish a subterranean seismic observatory.

A range of seismometers will record the force of earthquake shake, while meters and sensors measure ground deformation and fluid pressure. Some scientists theorise that changes in fluid pressure may trigger earthquake slip.

Hang on

If true, there may be no better place than Parkfield to discover it. It has been an outpost of the USGS since geologists first bolted down seismometers there more than 20 years ago.

The project could lead to better prediction

The location is ideal. Parkfield sits on a particularly active section of the San Andreas Fault where the plates continuously "creep". As a result, Parkfield experiences small to moderate-sized quakes at regular intervals, which make for a "natural laboratory," according to Dr Hickman.

But even in the best laboratories, serendipity plays a role. Parkfield's latest magnitude six earthquake, for which the town is famous, is overdue by about 10 years.

The magnitude six quakes have occurred, on average, every 22 years since 1857. The next one was anticipated to take place within the time frame 1988 to 1993.

Scientists hope the next one holds off a bit longer - until they have instruments in the ground to catch it.