Rapid Rock Changes Deep Under Fault Lines Can Trigger Repeated Earthquakes

The hypothesis, developed by Drs. Jay J. Ague, Jeffrey Park, and Danny M. Rye, will be published in the November 15 issue of Geophysical Research Letters, as "Regional Metamorphic Dehydration and Seismic Hazard." Geophysical Research Letters is published by the American Geophysical Union.

The researchers have analyzed chemical reactions in Earth's middle crust, some 10-20 kilometers (6-12 miles) beneath the surface. At the high pressures and temperatures found there, minerals in rock recrystallize into other minerals by releasing water entrapped in their crystalline structures.

The Yale model shows that relatively small increases in temperature beyond chemical equilibrium conditions can rapidly accelerate the process of fluid release. Such changes can occur within certain earthquake fault zones, such as California's San Andreas Fault, when downward fault motion transports "hydrated," or water bearing, rock to depths where higher temperatures encourage dehydration of rock.

Dehydration of serpentinite, a rock found along many faults, results in water release as metamorphism transforms it into water-poor minerals such as olivine. Rapid release of this water can produce high internal fluid pressures, decrease rock strength, and induce rock failure. By modeling the rates of metamorphic water release, Ague, Park, and Rye show that repeated rock failure may be possible on human, rather than geologic, time scales in the middle crust.

In the middle crust, rocks undergo transition from brittle to ductile behavior and, thus, the escape of overpressured fluids is impeded. As a result, rocks may break apart under internal stresses due to hydrofracturing. Such rock failure could trigger an earthquake if it occurs near an active fault.

According to Ague, the study may provide a link between regional metamorphism and earthquake generation. Further, metamorphism-induced rock failures leading to repeated quakes can occur within a human time scale, not millions of years of geologic time.

Park notes that the model can cause repeated earthquakes over millions of years if fresh, hydrated minerals are 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, he says, 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.