Predicting earthquakes is the Holy Grail of seismology, but despite intensive research, not a single warning sign has proved reliable. Now seismologists have stumbled upon a signal that has preceded two small quakes on the San Andreas fault by hours and could possibly provide a warning. "We are encouraged," says seismologist Paul Silver of the Carnegie Institution of Washington's Department of Terrestrial Magnetism in Washington, D.C., a co-author of a paper appearing tomorrow in Nature. "It's a very intriguing signal."
Seismologist Fenglin Niu of Rice University in Houston, Texas, Silver, and their colleagues were not looking for precursory signals when they lowered instruments down 1 kilometer in two holes drilled just off the San Andreas fault. The holes, near the central California town of Parkfield, are part of a fault-monitoring project known as the San Andreas Fault Observatory at Depth (SAFOD) (Science, 12 October 2007, p. 183), and the team was looking for signs that the slowly varying weight of the atmosphere as weather systems passed overhead was alternately squeezing and releasing pressure on deep rock. The resulting changes in stress on the rock should, according to theory and lab experiments, squeeze shut or force open tiny cracks in the rock. That in turn would speed up or slow down the artificial seismic waves the group was passing from one hole to the other.
The group found this atmospheric pressure effect, implying that they were indeed measuring stress changes a kilometer down. But the researchers also found a signal of bigger shifts in stress: large declines in the speed of seismic waves that began hours before two small earthquakes that shook the region in December 2005. The waves continued to slow during the quakes, magnitude 3.0 and 1.0. Their velocities returned toward normal after a couple of days.
Earthquakes release large amounts of stress, but in these two cases stress seemed to start changing before the quakes. If such changes were triggered by the coming quakes, they could provide a warning.
Given decades of disappointment, all concerned are being cautious. Are the preceding stress changes and the quakes related? "Maybe," says seismologist and co-investigator William Ellsworth of the U.S. Geological Survey in Menlo Park, California. "Is it worth pursuing? Absolutely." Much remains to be done, he says, including understanding why a third nearby quake of magnitude 2.6 failed to register on the borehole system. Niu's seismic stress meter will be reinstalled in the SAFOD holes in September to extend the 2-month-long record.
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