Pre-earthquake changes detected in the crust

Although measurement techniques surrounding earthquakes have improved enormously over the last few decades, it has remained very difficult to measure changes in the crust that could enable earthquake prediction. Now, scientists have measured interesting changes in the speed of seismic waves that preceded two small earthquakes by 10 and 2 hours. These measurements, published in the July 10 issue of Nature, are an encouraging sign that hold promise for the field of earthquake prediction. "Detecting stress changes before an earthquake has been the holy grail in earthquake seismology for years and has motivated our research," remarked study co-author Paul Silver of the Carnegie Institution's Department of Terrestrial Magnetism. "We used a specially designed system to generate and record seismic waves before, during, and after two earthquakes on the San Andreas Fault. It's been shown in the lab that the speed of seismic waves varies with the level of stress, due to the effect on the opening and closing of cracks. So measurements of changes in wave speed should, in principle, constitute a 'stress meter' that could provide an indication of an imminent earthquake. That is, precursory changes in stress should be revealed as pre-seismic changes in wave speed. Researchers have been trying to precisely and continuously measure these velocity changes for decades, but it has been possible only recently, with improved technology, to obtain the necessary precision and reliability."

"Because we use seismic waves, this allows us to measure changes deep in the crust where earthquakes occur that may be more difficult to observe by conventional surface instruments," said lead author Fenglin Niu of Rice University.

The experiment took place in 1-kilometer-(.6-mile)-deep wells over two months at the San Andreas Fault Observatory at Depth (SAFOD) at Parkfield, California. The seismologists measured the velocity of shear waves and how it varied over time. They first "calibrated" their stress meter by measuring velocity changes due to a known source of stress, namely barometric pressure. In a previous study, Silver and colleagues found that seismic wave speed is affected by changes in the barometric pressure. They found the same thing in this study: higher barometric pressure coincided with a faster seismic wave speed, due to the closure of cracks.

The researchers found two other wave-speed anomalies. One in particular was the largest signal observed over the entire two-month period. It was unrelated to barometric pressure changes.

"We found that this anomaly occurred at the time of the largest local event, a magnitude 3 earthquake, and most importantly began 10.6 hours before the event. Such pre-seismic changes are consistent with lab experiments that exhibit precursory phenomena, namely an increase in microcrack density preceding the occurrence of an earthquake," Silver commented.

"We are very encouraged by these pre-seismic signals and are planning a series of experiments to expand on them, so that we may further understand their timing and physical basis." Niu concluded.

Source: Carnegie Institution