Mysterious tremors deep beneath the San Andreas Fault

[Guest guest]

<http://www.sfgate.com/cgi-bin/object/article?f=/c/a/2004/12/10/MNGJRA9UQA1.DTL & \

o=0 & type=printable>

 

 

 

Mysterious tremors deep beneath the San Andreas Fault near the quake-

prone town of Parkfield are shaking the earth's brittle crust, far

below the region where earthquakes normally strike -- and scientists

say they can't understand what's happening or what the motions mean.

 

Seismic researchers are monitoring the strange vibrations closely. But

whether the faint underground tremors -- termed " chatter " by some

seismologists -- portend an increased likelihood of a major quake in

the area is an unsolved puzzle.

 

Robert Nadeau, a geophysicist at the UC Berkeley Seismological

Laboratory, has charted more than 110 of the faint vibrations since

they were first detected by the lab's High Resolution Seismic Network

in Parkfield three years ago. What concerns Nadeau and his colleagues

is that the epicenter of the great 1857 Fort Tejon earthquake, whose

magnitude has been estimated at 7.8 to 8, was located almost exactly

where the deep tremors are now occurring -- beneath the San Luis

Obispo County village of Cholame, some 17 miles south of Parkfield.

 

The episodes of chatter last from four to 20 minutes and are being

recorded from as deep as 40 miles beneath the surface -- up to four

times the depth of normal earthquakes, which originate in what

scientists call the " seismogenic zone. " That zone reaches no deeper

than 9 or 10 miles below the Earth's surface.

 

What's most striking is that deep tremors like the Cholame series have

never been recorded before on a strike-slip fault such as the San

Andreas, Nadeau said.

 

" We see this kind of tremor activity inside volcanoes like Mount St.

Helens, " Nadeau said, " but that's due to the movement of rising magma,

and in the tremors we've recorded there's no evidence of volcanism and

no seismic waves typical of ordinary earthquakes. "

 

Nadeau and David Dolenc, a graduate student in his lab, are publishing

the first report on the mysterious sequence of deep tremors today in

Science Express, the online edition of the journal Science. They

conclude that " future increases in San Andreas Fault tremor activity

may signal periods of increased probability for the next large

earthquake on the Cholame segment. "

 

The Fort Tejon event rocked the ground violently and ruptured the

fault for 225 miles, from northwest of Parkfield to San Bernardino. It

was at least as large as the 1906 San Francisco quake. But because the

Cholame region was virtually unpopulated at the time, it killed only

two people and destroyed only the Tejon Army post, midway along the

affected section of the fault.

 

The area is still sparsely populated; Cholame itself boasts only 2,125

inhabitants. But Paso Robles, with a population of more than 25,000,

is only 25 miles west of the village -- and it was badly damaged by a

magnitude 6.5 quake only a year ago.

 

Scientists have estimated that the Cholame segment of the fault has

ruptured in a large quake roughly every 140 years. It is now 148 years

since the Fort Tejon event, so the possibility of another one may be

steadily increasing, they say.

 

Similar deep tremors have been detected recently along the coast of

the Pacific Northwest, known as the Cascadia Subduction Zone, as well

as in Japan -- and there, too, scientists are struggling to understand

what their import is. In those areas, giant slabs of the earth's crust

are dipping downward and sliding ponderously beneath other great

crustal slabs, and scientists believe that fluids -- most likely

seabed water saturating the slabs -- are causing the tremors,

according to Herbert Dragert of Canada's Geological Survey in British

Columbia and Kazushige Obara of Japan's National Research Institute

for Earth Science and Disaster Prevention.

 

In an interview, Dragert said the tremors appear to add stress to a

major thrust fault in the Puget Sound region, and that scientists in

Canada and Washington are trying to determine whether the tremors

might " play a significant role in triggering great earthquakes. "

 

In California, the most mystifying feature of the unexplained tremors

is that they are occurring right on the deepest part of the San

Andreas -- a fault that does not involve subduction or volcanic

activity. Instead, two sides of the earth's crust are sliding

horizontally past each other in a motion seismologists call

" right-lateral strike slip. " In an earthquake, that slip can be an

abrupt jolt, and in big quakes, a violent one.

 

The tremors are occurring at such great depth, Nadeau said, that they

must be at the very bottom of the brittle crust -- where the earth's

hot, viscous upper mantle begins -- which has been under stress for

millions of years.

 

It's possible that the mantle there resembles something like Silly

Putty, Nadeau said, with great chunks of embedded rock grinding

against each other to generate the tremor signals. That's purely a

speculation, Nadeau conceded, but so far it's the only one around.

 

" No one really knows what the tremors mean, " said David Schwartz, a

geophysicist at the U.S. Geological Survey in Menlo Park. " As to what

they imply for the possibility of some future quake, we can't tell,

and right now we can only wait and see. "

 

A long-awaited magnitude 6 quake struck Parkfield in September at a

depth of about 5 miles. That quake was seen as the latest in a series

of quakes that have hit around Parkfield on an average of every 22

years between 1857 and 1966.

 

The Parkfield section of the San Andreas, in southern Monterey County,

is the most intensively instrumented seismic danger region in the

United States. A borehole 2 miles deep, carrying an array of

instruments and called the San Andreas Fault Observatory at Depth, is

to be completed next summer.

 

Whether its instruments solve the mystery of the tremors and determine

whether they portend a future Cholame earthquake remains to be seen.

 

E-mail David Perlman at

<dperlmandperlman.

 

Page A - 1

URL:

http://sfgate.com/cgi-bin/article.cgi?file=/c/a/2004/12/10/MNGJRA9UQA1.DTL