April 1, 2004
Specifically, how do earthquakes happen in Earth's tightly squeezed middle layers where pressure is far too great to allow any shifting of the rock? According to a paper published in the April 1 issue of the journal Nature, breakdown of the mineral serpentine provides enough wiggle room to trigger an earthquake. The report suggests a new mechanism to explain how quakes can occur at such depths.
"This exciting work addresses the central question of how large earthquakes can be generated in deep subduction zones," said Robin Reichlin, program director in the National Science Foundation (NSF) division of earth sciences, which funded the research. "This has been a much-debated topic, and this work goes a long way toward showing that dehydration of minerals plays an important role in this process."
Haemyeong Jung, Harry W. Green II and Larissa Dobrzhinetskaya of the University of California at Riverside, point out that while it is impossible to break anything by normal brittle fracture at pressures higher than those found at only a few 10s of kilometers (km) deep, earthquakes occur continuously at depths close to 700 km.
What is the explanation of this paradox?
A mechanism called "dehydration embrittlement" breaks down the mineral serpentine, to form the mineral olivine, accompanied by the release of water. That water can assist brittle failure at high pressure, but how? Green explains that before now, scientists have expected faulting instability only if the volume change during serpentine breakdown is positive.
In their article, the team reports experiments conducted between 10,000 and 60,000 times the pressure of the atmosphere at sea level, corresponding to depths in the earth of 30-190 km. Over that pressure range, the volume upon dehydration of serpentine changes from strongly positive to markedly negative, yet the faulting instability remains.
The microstructures preserved in the rocks after faulting provide insight into why this is so. The results confirm that earthquakes can be triggered by serpentine breakdown down to depths of as much as 250 km.
"I am becoming more and more convinced that mineral reactions also are involved in triggering shallow earthquakes such as those that threaten California," Green said. "Our hope is that we learn more about the thing we know least about, the initiation part of these earthquakes, how they get started. This is what we are trying to understand."
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