The most obvious manifestation of an earthquake is the shaking from seismic waves that knocks down buildings and rattles people. Now researchers have established a more subtle effect of this shaking - it increases the permeability of rock to groundwater and other fluids. The enhanced permeability caused by seismic shaking could potentially be harnessed to help extract oil from natural reservoirs, says Emily Brodsky, assistant professor of Earth sciences at the University of California, Santa Cruz.
"Permeability governs how fluid flows through rocks, whether it's water or oil, so this has practical implications for oil extraction," Brodsky says.
Brodsky is coauthor of a paper describing the new findings in the June 29 issue of Nature. The first author is Jean Elkhoury, a graduate student who worked with Brodsky at UCLA, and the other coauthor is Duncan Agnew of University of California, San Diego.
The study was based on two decades of data from the Piñon Flat Observatory in southern California, where researchers from University of California, San Diego's Scripps Institution of Oceanography maintain an extensive geophysical observatory.
"It's probably one of the best-monitored pieces of land anywhere on Earth," Brodsky says. The monitoring includes records of fluctuating water levels in wells. The water levels fluctuate in response to tidal effects similar to oceanic tides. In this case, the gravitational effects of the moon on the solid Earth squeeze and stretch the rocks in the crust, forcing water in and out of the wells from the surrounding rocks. The speed of the response in a well depends on the permeability of the surrounding rock.
"We know the tidal strain very well, so we can measure the lag between the imposed tidal strain and the response in the well to get a precise measure of the permeability of the rock," Brodsky says.
The researchers analyzed the data in relation to earthquakes and saw a striking correlation. "Every time there's a big earthquake in southern California, the permeability jumps. We saw this in two different wells for more than seven different earthquakes," Brodsky says.
After an earthquake, the rock surrounding the wells became as much as three times more permeable to ground water, she says. Furthermore, the size of the increase in permeability was proportional to the peak amplitude of the shaking. The changes were transient, with permeability returning to the original level within a few months after an earthquake.
The oil industry might be able to exploit this phenomenon by using "vibroseis" trucks to send seismic waves into the ground. Currently used for seismic imaging studies, vibroseis trucks vibrate at a particular frequency for a prolonged period.
"If we understood the physics of the permeability enhancement well enough, the vibrations could be tuned to increase the flow of oil," Brodsky says.