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Deep Earthquakes Happen, But Why?


Minerals Are Key To Earthquakes Deep in the Earth

UC Riverside Team is Published in the Journal “Nature”

(March 31, 2004)

RIVERSIDE, Calif. (www.ucr.edu) -- A team of UC Riverside geologists can tell you more about earthquakes in “Middle Earth” than the whole trilogy of “The Lord of the Rings.”

Specifically, why do earthquakes happen at intermediate depth in the earth, when current wisdom suggests they should not?

Three researchers at the Institute of Geophysics and Planetary Physics and Department of Earth Sciences at the University of California, Riverside — Haemyeong Jung, Harry W. Green II and Larissa Dobrzhinetskaya — think they have found part of the answer, and they have published a paper in this week’s edition of the scientific journal Nature.

The paper, appearing in the Letters to Nature section of the April 1 issue, is titled, “Intermediate-Depth Earthquakes by Dehydration Embrittlement with Negative Volume Change.”

It essentially says that the minerals found deep in the earth can react in ways that trigger earthquakes at depths where they would not be expected to occur, a finding that might eventually help scientists understand the triggers for more shallow earthquakes.

"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’s 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."

The paper points 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 km depth, earthquakes do occur continuously in subduction zones to depths approaching 700 km.

“What is the explanation of this paradox?” Green asks. “For the last 15 years, a principal thrust of our research has been directed to understanding this problem.”

Pressure increases with depth, Green explains, and pressure severely inhibits fractures. Also, temperature increases with depth and makes it easier for rocks to flow instead of break. So, it would be logical that with higher pressure and temperatures, no rocks would break. Yet surprisingly, earthquakes, which are clear evidence of breaking rock, still happen at those great depths.

“Our contribution to this problem has been to identify new ways, new physics, that can explain this conundrum,” he said. Before the work of my laboratory, it was essentially a dark mystery as to how such earthquakes could exist. Our work shows why they are there and why they stop exactly where they stop -- at 700 kilometers.”

Green’s earlier research discovered a new high-pressure faulting mechanism that could only operate at depths between 400 km and 700 km. Green and his scientific colleagues have now focused on developing additional evidence concerning another potential earthquake mechanism, previously discovered by other researchers but little studied in detail.

The mechanism called “dehydration embrittlement,” involves the breakdown of 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 the Nature paper, 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 this pressure range the change in 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, but that is contrary to the present consensus,” Green said. “Our hope for the future 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.”

“These observations move us along on the difficult road toward better understanding of earthquakes,” Green said.

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The University of California, Riverside (www.ucr.edu) is a doctoral research university, a living laboratory for groundbreaking exploration of issues critical to Inland Southern California, the state and communities around the world. Reflecting California's diverse culture, UCR's enrollment has exceeded 21,000 students. The campus opened a medical school in 2013 and has reached the heart of the Coachella Valley by way of the UCR Palm Desert Center. The campus has an annual statewide economic impact of more than $1 billion.

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