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March 24, 2004
The Seismic Signature of Rock Fractures

Berkeley Lab researchers are learning how to read the seismic signature of shear stress on rock fractures, a development that could help scientists monitor the stability of fractures in boreholes and underground oil and gas reservoirs. And further in the future, their work could help seismologists gauge the accumulation of stress along active fault zones, one of the precursors to earthquakes.

P waves (P stands for primary or pressure or push-pull) move through the earth like a Slinky, while S waves (S stands for secondary or shear or shake) move up and down or side to side. When crossing a stressed fracture, waves can change from P to S and vice versa.

"Monitoring shear stress is important because rocks generally fail under such stress," says Aoife Toomey of the Earth Sciences Division, who is studying the phenomenon with Seiji Nakagawa. "But up until recently there has not been a way to differentiate between the effects of normal stress and shear stress on a fracture. We're working to make that distinction more clear."

In a rock fracture, whether it's the San Andreas Fault or a tiny fissure emanating from a borehole, shear stress acts parallel to the fracture, grinding the two sides against each other. The more shear stress applied to the fracture, the more the fracture opens up or dilates, which increases its instability and permeability. And if the stress continues to intensify, the more likely it is to fail.

Fortunately, determining whether a fracture is subject to shear stress recently became a little easier, thanks to laboratory experiments that reveal how seismic waves change when they cross a stressed fracture: P waves, which move like a Slinky, convert to S waves, which move in an S-shaped pattern, and vice versa, even for normally incident waves. When detected on the surface and within boreholes, these anomalous wave conversions yield insights into the magnitude and direction of the shear stress, and provide a window into monitoring the stability of fractures.

To gain an even better understanding how these wave conversions yield clues about stressed fractures, Toomey and Nakagawa are using numerical models to simulate mode-converted waves. Their work is enabling them, for the first time, to develop a quantitative relationship between converted waves and a fracture's geometry, seismic frequency, and in-situ shear stress.

So far, their simulations have revealed that fracture dilation—one of the indications of likely failure—is accompanied by a significant increase in the amplitudes of converted waves. When the waves spike, the fracture is dilated and shear stress is high.

Someday, monitoring shear stress along the San Andreas Fault, shown here as it runs through Central California's Carrizo Plain, could help scientists predict earthquakes.

"Possible applications of this phenomenon include ensuring the stability of fractured reservoirs during fluid production and injection, or mapping the distribution of permeability in a reservoir," says Toomey.

In addition, because lab experiments have shown that dilation is a precursor to the failure of a fracture, their research could give scientists a tantalizing clue about the stability of a fault. For example, if the amplitude of a fault's converted waves increases, perhaps it is ready to rupture. However, much more research is needed before such earthquake prediction becomes a reality.

"We need to know if converted waves are produced at the lower frequencies used in the field," Toomey says. "But even if our work is not feasible for earthquake prediction, it could someday be used in civil engineering to analyze the stability of manmade structures such as bridges."

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