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Creating artificial seismic waves, similar to those that occur during an earthquake, could help squeeze more oil from natural reservoirs, scientists said.

They discovered that in addition to sending tremors that knock down buildings, earthquakes greatly increase the permeability of rocks to transmit fluids including oil.

"Shaking increases permeability," Emily Brodsky, an assistant professor of Earth Sciences at the University of California, Santa Cruz, said in an interview yesterday.

"Permeability governs how fluid flows through rock, whether it's water or oil, so this has practical implications for oil extraction," she added.

Brodsky and her colleagues, who reported their research in the journal Nature, also found that the amount of permeability is directly related to the amplitude of the shaking.

"Potentially if you could increase permeability you could greatly increase the available oil you could tap out of a reserve," she explained.

One way of doing that, in principle, is by mimicking the effects of an earthquake but scientists do not understand the physics well enough, or how to tune the vibrations, to increase the flow of oil.

One possibility would be to use vibroseis trucks that shake the ground to take a type of X-ray of the Earth to find out the structure of rocks and where the oil is located.

"If we understood the physics of the permeability enhancement well enough, the vibrations could be tuned to increase with the flow of oil," Brodsky added.

The scientists made their discovery after studying 20 years of data of water seeping in and out of wells during seven earthquakes in an area of California.

They noticed that every time an earthquake occurred the permeability jumped and the surrounding rocks became up to three times more permeable. A few months after the tremor the permeability returned to normal levels.

The scientists are planning more studies to better understand what aspects of the shaking makes the permeability increase.