July 10, 2009

San Andreas Tremors Could Indicate Increased Quake Risk

A spike in mysterious underground rumblings observed on a section of the San Andreas fault near Parkfield, California, could indicate a build-up of stress and an increased likelihood of a major earthquake, said scientists at the University of California, Berkeley.

The researchers monitored seismic activity along a heavily instrumented segment of the central San Andreas Fault from July 2001 to February 2009, and recorded more than 2,000 tremors lasting from minutes to nearly half an hour.

Seismologist Robert M. Nadeau and graduate student Aur©lie Guilhem observed an increase in underground stress at the end of a locked segment of the fault near Cholame, CA, after the 6.5-magnitude San Simeon quake in 2003 and the 6.0-magnitude Parkfield quake in 2004.   The tremors became more frequent after the quakes, and have continue to this day at a significantly higher rate than before the two quakes.

The researchers believe the increased tremor frequency may indicate a more rapid accumulation of stress along this segment of the fault -- the same portion that produced the great 7.8 magnitude Fort Tejon earthquake in 1857. 

Strong earthquakes have also occurred about every 20 to 30 years in an area northwest along the Parkfield segment of the fault.

"We've shown that earthquakes can stimulate tremors next to a locked zone, but we don't yet have evidence that this tells us anything about future quakes," said Nadeau.

"But if earthquakes trigger tremors, the pressure that stimulates tremors may also stimulate earthquakes."

While earthquakes are brief events that typically originate no deeper than 10 miles underground in California, the tremors are a continual, low-level rumbling that takes place some 10-20 miles below the surface.

Although tremors are common near volcanoes as a result of underground fluid movement,  they came as a surprise when discovered in 2002 at a subduction zone in Japan, where a piece of ocean floor is sliding under the continent.

Tremors were subsequently detected at the Cascadia subduction zone in Washington, Oregon and British Columbia, where several Pacific Ocean plates are sliding under the North American continental plate.

In 2005, Nadeau identified a mysterious "noise" detected by the Parkfield borehole seismometers as tremor activity, and has studied them ever since.

The Parkfield area of the San Andreas is a strike/slip fault, where the Pacific plate is moving horizontally against the North American plate, making it different than the Japanese and Cascadia tremor sites.

"The Parkfield tremors are smaller versions of the Cascadia and Japanese tremors," Nadeau explained.

"Most last between three and 21 minutes, while some Cascadia tremors go on for days."

Nearly all known instances of the tremors originated 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.  For this reason, seismologists have speculated that increases in their activity may indicate the buildup of stress just prior to an earthquake. 

Nadeau said the new study strengthens that theory.

He and Guilhem identified the location of some 2,200 tremors recorded between 2001 and 2009 by borehole seismometers implanted along the San Andreas Fault as part of UC Berkeley's High-Resolution Seismic Network.  During that time, two nearby earthquakes occurred: one in San Simeon on Dec. 22, 2003, and one in Parkfield on Sept. 28, 2004.

Tremor activity was low beneath the Parkfield and Cholame segments of the San Andreas Fault prior to the San Simeon quake.  However, it doubled in frequency after the quake, and was six times more frequent after the Parkfield quake.

Most of the activity occurred along a 16-mile segment of the San Andreas Fault south of Parkfield, around the town of Cholame, while  less than 10 percent occurred at an equal distance above Parkfield, near Monarch Peak. Cholame is at the northern end of a long-locked and hazardous segment of the San Andreas Fault, while Monarch Peak is not.

Nevertheless, Monarch Peak is in a complex part of the San Andreas Fault, having ruptured southward some 220 miles in 1857 in the magnitude 7.8 Fort Tejon quake, Nadeau said.

Today, the tremor activity remains about twice as high as before the San Simeon quake.  Periodic peaks of activity have emerged that began repeating roughly every 50 days, but 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.

The continued tremors are worrisome, he said, because of the history of major quakes along this segment of the fault. 

A series of pre-tremors were detected a few days before the Parkfield quake, something that makes Nadeau hopeful of observing similar tremors ahead of future quakes.

However, the underlying source of tremors is still somewhat of a mystery, he said.

Some scientists believe fluids moving underground may be to blame, just as movement of underground magma, water and gas can cause volcanic tremors.

However, Nadeau favors an alternative theory that non-volcanic tremors are generated in a deep region of hot soft rock, that, except for a few hard rocks embedded like peanut brittle, typically flow without generating earthquakes.  The fracturing of the brittle rocks may be generating groups of many small quakes that combine into a faint rumble.

"If tremors are composed of a lot of little earthquakes, each should have a primary and secondary wave just like large quakes," but they would overlap and produce a rumble, explained Guilhem.

But tremors due to shear (tearing) stress rather than compressional (opening and closing) stress are more consistent with deformation in the fault zone than with underground fluid movement, Nadeau said.

The mapping of the underground rumblings shows the tremors are not restricted to the plane of the fault, suggesting that faults spread out as they reach deeper into the Earth's crust.

Whatever their cause, 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," said Nadeau.

Guilhem said seismologists around the world are looking for tremors along other fault systems, although such shakings can be hard to detect due to noise from oceans and civilization.

Brief tremor activity has been observed along a few faults, triggered by large, distant quakes, and these may be areas to focus on. For instance, tremors were triggered along Northern California's Calaveras Fault by Alaska's Denali quake in 2002, Nadeau said.

The research, which was supported by the U.S. Geological Survey and the National Science Foundation, is reported in the July 10 issue of Science.


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