"Permeability governs how fluid flows through rocks, whether it's water or oil, so this has practical implications for oil extraction," says Emily Brodskyof the University of California, Santa Cruz. Brodsky, along with Jean Elkhoury of the University of California, Los Angeles, and Duncan Agnew of the University of California, San Diego, report their findings today in Nature.
The scientists came to their conclusions by looking at 20 years worth of data collected at the Pi¿on Flat Observatory in southern California, where researchers from UCSD's Scripps Institution of Oceanography maintain an extensive geophysical observatory. On this site more than two decades ago, Agnew drilled two wells about seven centimeters in diameters and about 200 meters deep and fitted them with gauges to measure water level and pressure.
When the team analyzed this data, they saw a long-term pattern of rising and falling water level and pressure due to the same gravitational pull of the Moon that causes tides to ebb and flow. The effect squeezes the porous rock that makes up the surrounding aquifer and in the 20-year period analyzed, forced regular changes of up to three centimeters in the level of well water. When the tidal pull abated, the water flowed back into the aquifer.
The time it took the water to move between the aquifer and the well and back again provided an indication of how porous the surrounding rock was. Long delays between these phases meant the surrounding rock was not very permeable; short delays meant it was more permeable. When the scientists looked at the delays following seven earthquakes events nearby, they saw time delays shortened by a factor of two and three. This meant that "the water was flowing with much more ease through the aquifer during the time of the earthquake," Elkhoury says. Why this happens is still unknown, but the two most popular theories, according to Elkhoury, suggest that the earthquake either induces fracturing, which connects existing pores, or shakes free areas clogged with fluid.
The team is now working to pinpoint the amplitude and frequencies that best wring fluid out of rock. The information could be used by engineers in the oil industry to induce artificial quakes--something they already do to generate seismic waves in order to glimpse rocky layers beneath Earth's crust. If they knew the amplitude and frequency that best squeezes the oil out of porous rock, they would have another tool for extracting the fossil fuel.