|
Update as of 12/27/08:
Great Indian Ocean quake in 2004 set off tremors in the San Andreas
Fault!
This is new information In addition to the original article below!
Tremors from the Great Sumatran quake of 12/26/04 were set off near
Parkfield, California, nearly 9,000 miles away, according to Abhijit Ghosh from the University of Washington. His findings
will be presented at the upcoming meeting of the American Geophysical Union in San Francisco.
Scientists used data from bore holes which are part of the High-Resolution
Seismic Netowrk operated by the University of California, Berkely, and information from the Northern California Seismic Network
(operated by the USGS). Non-volcanic tremors were recorded precisely at the time the seismic waves were passing
through Parkfield, and were recorded on numerous instruments as far as 125 miles apart. "It's fairly obvious.
There's no question...." according to Ghosh.
Readers, please note that these are additional events over and above those
reported originally below.
Selected portions of an article recently received have been posted
below:
Deep Tremors
Detected Along San Andreas Fault
Stefan Lovgren in Los Angeles
for National Geographic News
December
9, 2004
Seismologists have detected non-volcanic tremors deep along the San Andreas Fault in central California,
near the epicenter of a 7.8-magnitude earthquake that struck Fort Tejon in 1857.
Such tremors had previously
only been recorded in subduction zones, such as Japan and the Pacific Northwest, where one tectonic plate dives beneath
another. The San Andreas fault, on the other hand, forms what is known as a transform plate boundary zone, an area where
two plates slide past each other.
Scientists identified 110 tremor events lasting four minutes or more in
the Parkfield area of central California during a three-year search period that ended with the 6.5-magnitude San Simeon
earthquake on December 22, 2003.
These tremors may signal an increased likelihood of earthquakes on the
San Andreas fault. Scientists have long expected California's next big earthquake—of magnitude 8 or higher—to
occur on the San Andreas fault.
"Because these tremors occur directly below the epicentral region of the
1857 quake, and because this part of the fault is locked and could rupture again soon, it is possible that increases
in tremor activity may signal times of increased likelihood for a large event in the area," said Robert Nadeau of the Berkeley
Seismological Laboratory in California.
Nadeau is the lead author of the study, which is described in tomorrow's
issue of the journal Science.
CHAOTIC SHAKING
The San Andreas Fault, the boundary between the
North American and Pacific tectonic plates, runs 800 miles (1,300 kilometers) along the California coast and extends
10 miles (16 kilometers) down into the Earth. It is like a master fault in an intricate network of smaller faults
that branch from the San Andreas and join it.
The scientists detected the tremors at a depth of 12.5 to 25 miles
(20 to 40 kilometers) below the San Andreas fault near Cholame in central California, some 15 miles (25 kilometers)
southeast of the town of Parkfield.
"The tremors are shaking of the ground that differ from earthquakes in that
they last for up to 20 minutes, compared to earthquakes which last for less than 30 seconds," Nadeau said. "Unlike
earthquakes, the tremor shaking is chaotic."
The discovery marks the first time such deep, non-volcanic tremors
have been reported on the San Andreas fault, suggesting that the deformation causing earthquakes may have deeper
origins than previously thought.
"Analyzing these tremors may help us to better understand deeper processes
associated with earthquakes, therefore improving our knowledge of them, including our forecasting capabilities,"
said Andres Chavarria, a seismologist at Duke University in Durham, North Carolina.
SEISMIC SIGNALS
California's
two largest earthquakes—Fort Tejon in 1857 and San Francisco in 1906—occurred on the San Andreas fault,
and scientists expect the next "big one" to occur on this fault.
"It certainly is the big dog," Nadeau said.
In recent decades, seismologists have taken particular interest in the Parkfield segment of the San Andreas
Fault because of the moderate-size earthquakes (around magnitude 6) that occur there at fairly regular intervals.
Seismologists
have installed a dense network of instruments in the Parkfield area to learn more about the earthquakes. Researchers
have placed seismometers in bore holes 100 to 200 yards (90 to 180 meters) deep, away from noise sources that typically
mask seismic signals from small earthquakes.
"The high sensitivity and low noise level of seismic signals
recorded by this network were also extremely helpful for the tremor analysis we performed," Nadeau said.
On
September 28, nine months after the scientists ended their analysis, another magnitude 6 earthquake struck the
area close to the tremor region.
"Many kinds of instruments were able to record [this event], providing an invaluable
source of data for earthquake studies," Chavarria, the Duke University seismologist, said.
PREDICTING
QUAKES
The discovery in 2002 of non-volcanic tremors in Japan represented a major advance in seismology. Fluids
from diving ocean bottom rocks were believed to be critical for generating the tremors in the subduction zones
of Japan and the North American Pacific Northwest.
However, the San Andreas Fault does not subduct. As a result,
no source of fluids from the rock of a descending tectonic plate is available.
Researchers hope that tremors
will prove to be a useful tool for forecasting larger earthquakes.
"We still have only a rudimentary understanding
of the process that creates the tremor and its significance for earthquakes," said William Ellsworth, chief scientist
for the Earthquake Hazards Team at the U.S. Geological Survey in Menlo Park, California.
"This discovery
opens an important new window into the workings of the San Andreas Fault that will undoubtedly become a major research
focus in the coming years," he added.
|
