Release of Water from Rocks Deep Under Fault Lines Can Trigger Repeated Earthquakes, Yale Geologists Theorize

Chemical reactions in the Earth’s middle crust that cause rocks to dehydrate and fracture under pressure may trigger repeated earthquakes along some fault zones, according to research published in the Nov. 15 issue of the Geophysical Research Letters, which is published by the American Geophysical Union.

The finding, based on a computer model of conditions under which rocks release water, eventually could help scientists predict earthquakes in active fault areas, such as along California’s San Andreas fault. The model, developed by three Yale University geologists, also could explain why friction between continental plates so often is linked to increased seismic activity.

The release of water by rocks within fault zones can occur relatively rapidly – over decades or centuries rather than eons – and occurs when downward fault motion transports water-bearing rocks to depths where temperatures are high, the Yale researchers reported.

“At the high pressures and temperatures found in the middle crust some 10-20 kilometers (6-12 miles) beneath the surface, minerals in rock recrystallize into other minerals by releasing water trapped in their crystalline structures,” said Jay J. Ague, a rock specialist who collaborated on the research with seismologist Jeffrey Park and geochemist Danny M. Rye, all of the Yale geology and geophysics department.

Their model could also help explain changes in electromagnetic signals and fluid discharge that have been noted before some earthquakes. These changes could be caused by the release of saline fluid and pressure caused by rock failure preceding complete rupture of the fault zone, the Yale geologists reported.

Their computer model shows that relatively small increases in temperature beyond chemical equilibrium conditions can rapidly accelerate the release of water. Because rocks in these fault zones are not very porous, and the water within them is very difficult to compress, only a small volume of water is necessary to trigger a seismic event at high pressures, Ague noted. Such conditions appear to have contributed to the 1995 earthquake in Kobe, Japan, and the 1989 Loma Prieta earthquake in California, he said.

Dehydration of serpentinite, a rock found along many faults worldwide, results in the release of water as the rock is transformed into olivine and other water-poor minerals. “Rapid release of this water can produce high internal fluid pressures, decrease rock strength and induce rock fissures,” Park said.

In the middle crust, rocks undergo transition from a brittle state to a state where they can be shaped or molded, which slows the escape of fluids under pressure. As a result, rocks may break apart under internal stresses due to “hydrofracturing,” Ague said. Such rock failure could trigger an earthquake if it occurs near an active fault.

Park notes that dehydration can cause repeated earthquakes over millions of years if fresh, water-bearing minerals continue to be transported downward along a fault. This, he says, suggests a feedback mechanism to encourage the thrust faults that build mountain ranges. It also may explain why thrust faults and mountain building are common along a plate boundary in which tectonic plates slide by each other, like the San Andreas Fault.

To test the theory, Park said, researchers would have to drill into the middle crust in fault zones, or examine old deep fault zones that have been exposed by later erosion.

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