Berkeley, CA - Mysterious underground tremors on the San Andreas fault could mean a big quake is on the way, according to University of California seismologists.
A team led by Robert M Nadeau found that after the 6.5-magnitude San Simeon quake in 2003 and the 6.0-magnitude Parkfield quake in 2004, underground stress increased at the end of a locked segment of the San Andreas Fault near Cholame, California, at the same time as tremors became more frequent. The tremors have continued to this day at a rate significantly higher than before the two quakes.
The UC Berkeley researchers conclude that stress may be accumulating more rapidly than in the past along this segment of the San Andreas Fault - putting it at risk of breaking just as it did in 1857 to produce the great 7.8 magnitude Fort Tejon earthquake.
Because in nearly all known instances tremors originate from the edge of a locked zone - a segment of a fault that hasn't moved in years and is at high risk of a major earthquake - seismologists reckon that increases in their activity may forewarn of stress build-up just before an earthquake. "If earthquakes trigger tremors, the pressure that stimulates tremors may also stimulate earthquakes," warned Nadeau.
For the new study, Nadeau pinpointed the location of nearly 2,200 tremors recorded between 2001 and 2009 by borehole seismometers along the San Andreas Fault. During this period, two nearby earthquakes occurred: one in San Simeon, in December 2003, and one in Parkfield on the San Andreas Fault in September 2004.
Before the San Simeon quake, tremor activity was low, but it doubled in frequency afterwards and was six times more frequent after the Parkfield quake. Most of the activity occurred along a 25km segment of the San Andreas Fault south of Parkfield, around the town of Cholame. Fewer than 10 percent of the tremors occurred at an equal distance above Parkfield, near Monarch Peak. While Cholame is at the northern end of a long-locked and hazardous segment of the San Andreas Fault, Monarch Peak is not.
Tremor activity remains about twice as high today as before the San Simeon quake, while periodic peaks of activity have emerged that are now repeating about every 100-110 days. "What's surprising is that the activity has not gone down to its old level," Nadeau said. Tremors "are not relieving a lot of stress or making the fault less hazardous, they just indicate a changes in stress next to locked faults," he added.
The research is reported in Science.