Under the central part of the San Andreas fault, the deep seismic whisperings known as tremors have increased after two recent large earthquakes, and a surge in tremors appears to have foreshadowed one of them, according to a study to be published today in the journal Science.
"It reaffirms the need to be ready," said Robert Nadeau, a research seismologist at UC Berkeley who led the study. "The San Andreas fault is changing down deep and it's changing down deep in places where large earthquakes have happened in the past."
Among the findings was an unusually strong tremor episode three weeks before the magnitude-6.0 Parkfield earthquake in 2004. If these types of signals are found before other large earthquakes, they could provide a kind of early warning, said Greg Beroza, a seismologist at Stanford University who was not involved in the study.
"There have been plenty of tremor episodes that have not triggered earthquakes in other places," he said. "This one might make the strongest case."
Predicting large quakes with precision is the elusive Holy Grail of seismology. Scientists have only been able to calculate probabilities for quakes in certain areas by analyzing a timeline of ruptures and calculating the amount of stress building on a fault.
The latest study may potentially inch us closer to having an actual predictor, scientists said.
Earthquakes typically generate clear seismic waves with sharp onsets, tailing off after a minute or two, seismologists said. Tremors vibrate quietly and can continue for days. Tremors also tend to happen in a deeper, softer part of the Earth's crust, rather than in the upper part typically thought to generate earthquakes.
Seismologists used to ignore tremors because they looked like noise in the data caused by wind or cars. Until recently, scientists also had difficulty storing the enormous amounts of data required to detect tremors.
About 10 years ago, Japanese seismologists discovered deep tremors when they took a closer look at the background noise. Since then, scientists have detected tremors in the Pacific Northwest and below the San Andreas.
Nadeau's study focused on the San Andreas fault in the Parkfield region, about 170 miles northwest of Los Angeles.
Nadeau and graduate student Aurelie Guilhem combed through seismometer data from July 2001 to February 2009. That period included two strong earthquakes: the magnitude-6.5 San Simeon earthquake of 2003 and the Parkfield temblor the following year.
