Washington, July 10 : Scientists have measured how changes in stress in rocks affect changes in the speed of seismic waves at depths where earthquakes begin, which could lead to the development of a "stress meter" for better understanding how fault-zone stress is related to earthquakes.
The team of scientists that carried out the measurements was from Rice University, the Carnegie Institution of Washington, and the US Department of Energy's Lawrence Berkeley National Laboratory (Berkeley Lab).
"The goal of our project was to develop a method for measuring stress changes, especially at depths where earthquakes originate," said Fenglin Niu of Rice University's Department of Earth Science. "We call it a seismic stress meter," he added.
The research team used the twin boreholes ("wells") of the National Science Foundation's San Andreas Fault Observatory at Depth (SAFOD) near Parkfield, CA, to send signals from a source one kilometer deep in the pilot hole to a receiver at the same depth in the main hole.
"The source is a stack of donut-shaped piezoelectric ceramic cylinders that expand when voltage is applied," explained Tom Daley of Berkeley Lab's Earth Sciences Division.
"The source is suspended in the water that fills the hole, and when it expands it exerts pressure on the water, which exerts pressure on the rock; the seismic wave travels through the rock to the detector, which is in contact with the sides of the main bore hole and measures movement with accelerometers," he added.
The instruments were sensitive enough to detect changes in rock stress a kilometer deep, caused only by changes in the barometric pressure of the atmosphere - a mere change in the weight of the air on the surface.
During the first month of data collection, the team found a consistent relationship between barometric pressure and minute changes in the travel time of seismic waves between the source and the detector.
Higher barometric pressure (corresponding to greater stress on the rock) meant less travel time; the seismic waves moved faster because tiny cracks in the rock closed up under pressure.
During the second month of data collection, the quality of the data actually improved, but the researchers detected two anomalous departures from the established relation of barometric pressure to travel time.
These excursions corresponded to two earthquakes in the Parkfield region, an area so well instrumented that earthquake magnitude and location, including depth, can be determined with great precision.
One earthquake measured magnitude 3, the largest local event during the observation period; the other earthquake measured magnitude 1, but occurred closer to the experiment.
According to Daley, "What we've seen are interesting stress changes associated with earthquakes. It encourages us to continue this kind of observation."