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The researchers used a (source) instrument that was placed about 0.6 miles (one kilometer) down a deep well (pilot hole), contained within the center of a fault zone containing shifting rocks, at the San Andreas Fault Observatory at Depth (SAFOD).

The San Andreas Fault extends almost the entire length of the state of California as the North American and Pacific tectonic plates grind against each other.

The instrument consisted of a series of piezoelectric ceramic cylinders. The cylinders were designed to expand when voltage was applied to them—thus, the scientists were able to artificially generate small seismic-like waves.

Along side the cylinders, in another deep hole (main hole), is placed an accelerometer (receiver instrument), which measures rhythmic signals from the ceramic cylinders in the pilot hole. The scientists compare the setup to a sophisticated "stereo speaker."

The scientists state that when rocks in the interior of the Earth are compressed by the actions of an earthquake, stresses force air from induced cracks in the rocks. Seismic waves are then produced as the cracks open and close repeatedly from the stresses. These seismic waves travel faster than the air.

However, these seismic waves only travel a very tiny bit faster than the air, so very precise instruments must be used to measure them. Scientists now think they have developed such instruments needed to make these very precise detections.

American seismologist Fenglin Niu is the led researcher in the study. Niu states, “Scientists tried as early as the 1970s to measure changes in wave speed that are associated with the stress changes that precede seismic activity. For a variety of reasons, their measurements were inconclusive. Using the precision instruments built by our collaborators at Lawrence Berkeley National Laboratory, along with new signal enhancement techniques, we were able to reach the fine level of precision required." [ScienceDaily]

What are their initial conclusions? Page three explains.