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Deep Earthquakes, Take Two


Second Paper on “Deep Earthquakes” Appears in Nature

Three UC Riverside Scientists Study the Impact of Minerals on Earthquakes Deep in the Earth.

(April 15, 2004)

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RIVERSIDE, Calif. (www.ucr.edu) -- Three researchers at the University of California, Riverside - Junfeng Zhang, Harry W. Green II and Krassimir Bozhilovhave — have joined with colleague Zhenmin Jim, of China University of Geosciences, Wuhan, to publish a paper in the scientific journal Nature.

The paper, appearing in the Letters to Nature section of the April 8 issue, is titled, “Faulting Induced by Precipitation of Water at Grain Boundaries in Hot Subducting Oceanic Crust.” This is the second paper published on the subject this month in this journal.

“We discovered a new way that fluid can be produced in rocks and initiate failure,” said Green, of UC Riverside's Institute of Geophysics and Planetary Physics. “We found that the very small amounts of H20 - hundreds to a few thousand parts per million - can be dissolved in minerals that are otherwise anhydrous and can come out of solution and trigger very small amounts of melting, followed by faulting at high temperature.”

Previous studies, Green said, showed that such mechanisms worked at only relatively low temperatures. With this mechanism working at high temperatures, a subset of earthquakes occurring 100 to 250 kilometers deep can now be explained.

“Another very important part of this study is that it showed only very small amounts of fluid are needed to trigger faulting, so even rare minerals that can carry small amounts of water deep into the earth may be involved in causing earthquakes,” Green said.

The principle reason for geophysicists to be interested is that faulting at high pressure is impossible without the generation of a fluid, and this is a new and unexpected way for such a fluid to be produced, Green says. For the general layperson who might be interested in plate tectonics, this is also interesting, because it involves unexplained phenomena and gives insight into how the world “works.”

“Knowing how this process works can help us better understand things, such as what causes volcanoes,” Green said. “Most deep earthquakes are not particularly destructive to society, because they are so far down. Only the very largest cause significant damage.” “The connection to volcanoes is that the volcanoes of the type like we have in the Cascade range of Washington, Oregon, and Northern California also depend on production of water by similar chemical reactions.

This paper follows another Nature paper written by Green and two other UCR researchers, Haemyeong Jung and Larissa F. Dobrzhinetskaya, titled “Intermediate-Depth Earthquakes by Dehydration Embrittlement with Negative Volume Change” that appeared in the previous week’s issue of Nature.

That paper pointed 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 depth, in some parts of the earth, earthquakes do occur down to depths approaching 700 kilometers.

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 of a fluid is produced.

The dual research has Green increasingly convinced that what triggers shallow earthquakes in California and elsewhere also involves chemical reactions, and if so, this and future research will greatly enhance our ability to understand all types of earthquakes at various depths.

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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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