While California won't meet this serious fate, a University of South Carolina geologist has found that unusual geologic deformation has caused a major earth boundary under the San Joaquin Valley, called the Moho, to disappear.

Tom Owens, collaborating with researchers from the University of Arizona, the University of Colorado and the California Institute of Technology, used seismic studies to examine the layers of earth beneath the southern Sierra Nevada mountains in California. Their research, featured recently in Nature, found that the Moho—a boundary
between the Earth's rocky crust and the underlying mantle—is missing.

The finding is the first report of such a disappearance in California, and it could lead to a better understanding of the occurrence and distribution of earthquakes around the Sierra Nevada, the periphery of which has experienced large earthquakes in the past.

Owens said the researchers used data of earthquake waves to study the deep structure of the Earth, much like doctors use X-rays or CAT scans to examine the body. Their research determined that the Moho has vanished because of a geologic occurrence called a "drip," caused when the upper portion of the Earth's mantle flows deeper into the mantle, pulling part of the overlying crust down with it.

The researchers believe that this process started between 3 and 10 million years ago and that the downward flow is now producing an inverted mountain of crust protruding into the mantle to a depth of five to six times the height of the highest peaks in the Sierra Nevada. At the surface, in the wake of this subterranean flow, volcanic eruptions have been triggered, and the courses of rivers are being diverted.

"There is good evidence from our earthquake data that material has become detached from the overlying crust," Owens said. "The drip of high-density materials will pull down the crust and cause some uplift, producing erosion and possibly changing the course of rivers."

Owens likens the process to what happens when a lava lamp is turned off. Some of the waxy material sticks at the top of the fluid in the lamp—even though it is more dense than the fluid below it. But, if the lamp is disturbed (by shaking, for example), then a piece of the waxy material will fall to the bottom. The dense mantle material sits in a similarly unstable manner at the base of the crust until a geologic event disturbs it and produces conditions that allow it to begin sinking deeper into the earth.

The research could shed light on how the shift in material is triggered and how it, in turn, influences the equilibrium of the earth in the surrounding region.

"Interestingly, it has been suggested that there could be a curious correlation between formation of the mantle drip and distribution of earthquakes in the region," he said, strongly cautioning that the study cannot be used to predict earthquake activity.

The researchers have applied for a federal grant that would enable them to continue their studies northward along the Sierra Nevada to complete the picture of this unique feature in eastern California.

Owens is one of two USC geology professors to be featured this month in Nature. Geology researcher Robert Thunell is co-author of a study on global climate change.

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