Geologists reported Friday they may have learned what powers deep earthquakes, concluding the release of water trapped in rocks serves as lubricant to overcome the great pressure holding the rocks together.
In a study prepared for a meeting of the American Geophysical Union, a team from the University of California at Berkeley claimed the first direct experimental evidence for what drives "deep focus" quakes, disturbances ranging from about 60 miles to 400 miles below the Earth's surface.The quakes, which occur primarily along the coasts of Japan, Indonesia and western South America, account for about 10 percent of all quakes. About one each day has been recorded for the last two decades. The more common "surface" quake usually occurs within the top 10 miles of Earth's crust, powered by the sudden motion of large rocks sliding past one another across fault lines.
Geologists have puzzled for years about how the fracturing that occurs in surface earthquakes could happen deep below the surface, noting that immense pressure would be expected to restrict movement along faults.
Charles Meade, a UC-Berkeley geophysicist, said experiments indicate the extreme pressure and high temperature deep beneath Earth's mantle may free mineral-bound water in deep quake regions. Such water would act as a necessary lubricant for rock cracking and sliding at great depths.
"Water provides the lubrication and forces the cracks apart allowing them to slide," Meade explained. The geophysicist said experiments currently are the only way to figure out what is going on deep within the Earth.
"We can't drill and we can't go down there. Besides (recording and analyzing) seismological findings, there is nothing we can do but recreate," he said.
A spokeswoman for the American Geophysical Union said people can feel deep focus quakes and they are virtually indistinguishable from surface quakes. The surface variety generally causes more destruction than deep counterparts.
The Berkeley team experimentally recreated the materials and conditions in deep quakes by using two diamonds to compress a piece of sea-floor mineral, which contained water in its structure, to pressures of about 200,000 times those at the Earth's surface.
The geophysicists then observed the catastrophic crushing of the rock and recorded the seismic waves from the tiny sample.