- PALO ALTO, Calif (UPI) --
To geophysicists' dismay, a method widely used to assess long-term earthquake
hazards in the United States, Japan, New Zealand and other rattle-prone
areas has failed to hold up at what was considered the ideal testing ground.
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- A long-anticipated temblor forecast by the technique
for the tiny central California town of Parkfield -- one of the most seismically
active and closely monitored sites in the world -- never materialized.
The town's sustained stillness, when it was supposed to toss and turn within
a predictable period of time, has shaken scientists' belief in the validity
of what has become a standard tool for predicting quakes in the most active
regions.
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- Government agencies in a number of Pacific Rim countries
routinely use the strategy for long-range hazard assessments, such as the
widely publicized 1999 U.S. Geological Survey report projecting a 70 percent
probability a large quake will strike the San Francisco Bay area by 2030.
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- "The message I would send to my geophysical colleagues
about this model is, 'Use with caution,'" said Eric Segall, professor
of geophysics at Stanford University and lead author of the study, which
will be published in the Sept. 19 issue of the British journal Nature.
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- "(The new results) should disturb many of us who
have relied upon the concept of time-predictability in our forecasts,"
Ross Stein of the USGS in Menlo Park, Calif., who analyzed the findings,
told United Press International. "Alternative approaches to earthquake
forecasting must now be explored."
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- The time-predictable recurrence model assumes a temblor
will strike on a segment of a fault -- a jolt-producing rupture in Earth's
crust -- when the stress released from a previous shaker has built back
up to the pre-quake level. Although that premise remains intact, the means
of timing the recurrence does not.
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- According to computations based on the model, the moderate,
magnitude-6 earthquake that rocked Parkfield -- a picturesque rural town
halfway between San Francisco and Los Angeles -- in 1966 should have been
followed by another in 1987, said Segall and Stanford geophysics graduate
student Jessica Murray. The fact that it has not casts serious doubt over
the model developed in 1980 by Japanese geophysicists building on the premise
that earthquakes in fault zones are caused by the constant buildup and
release of strain in Earth's crust.
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- "We think it's rather damning that it doesn't work
here," Segall said in an interview with UPI. "If it's going to
work anywhere, you'd think it would work here."
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- Parkfield provides the perfect perspective, perched along
a 15.5-mile (25-kilometer) stretch of the notorious 30-million-year-old
San Andreas fault, which suddenly slipped on one side and swelled on the
other by up to 21 feet (6.4 meters) on April 18, 1906, setting off the
great San Francisco earthquake.
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- More than 20 years of intense monitoring by instruments
on or near the fault surface have produced heaps of geological clues to
the physical processes underlying recurring earthquakes, including a wide
array of measurements spanning an entire quake cycle, since the last moderate
one in 1966.
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- The time lapse between shakers at Parkfield is relatively
short, an average of 22 years, compared to a century or two that typically
separates temblors in earthquake country elsewhere. Moderate-size temblors
of about magnitude 6 have occurred on the Parkfield section of the San
Andreas fault at fairly regular intervals -- in 1857, 1881, 1901, 1922,
1934 and 1966, prompting the National Earthquake Prediction Evaluation
Council in 1994 to declare Parkfield "the best identified locale to
trap an earthquake."
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- As an added bonus, Parkfield's tectonic setting is study-perfect
in its simplicity. Nearly all the earthquake action centers on the San
Andreas rupture, in contrast to such hotbeds of quake activity as the San
Francisco Bay area where rumblings emanate from a network of faults, including
the San Andreas, Calaveras and Hayward.
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- The San Andreas defines an 800-mile (1,300-km) section
of the boundary between the Pacific and North American plates, two of a
number of gigantic shifting slabs that break up Earth's surface.
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- "With a plate boundary like the San Andreas, you
have the North American plate on one side and the Pacific plate on the
other," Segall said. "The two plates are moving at a very steady
rate with respect to one another, so strain is being put into the system
at an essentially constant rate."
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- When an earthquake occurs on the fault, a certain amount
of accumulated strain is released, Murray explained.
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- "Following the quake, strain builds up again because
of the continuous grinding of the tectonic plates," she noted. "According
to the time-predictable model, if you know the size of the most recent
earthquake and the rate of strain accumulation afterwards, you should be
able to forecast the time that the next event will happen simply by dividing
the strain released by the strain-accumulation rate."
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- Applying the model to the Parkfield data, the scientists
came up with a forecast of when the next earthquake would strike.
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- "According to the model, a magnitude-6 earthquake
should have taken place between 1973 and 1987 -- but it didn't," Murray
said. "In fact, 15 years have gone by. Our results show, with 95 percent
confidence, that it should definitely have happened before now, and it
hasn't, so that shows that the model doesn't work -- at least in this location."
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- The investigators doubt it would work any better elsewhere,
including in the densely populated metropolitan areas of Northern and Southern
California.
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- "We used the model at Parkfield where things are
fairly simple," Murray observed, "but when you come to the Bay
Area or Los Angeles, there are a lot more fault interactions, so it's probably
even less likely to work in those places."
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- To determine for certain, the method should be tested
at other sites, including in Japan, Segall said.
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- "We're in a tough situation, because agencies like
the USGS -- which have the responsibility for issuing forecasts so that
city planners and builders can use the best available knowledge -- have
to do the best they can with what information they have," he said.
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- "These findings illustrate that earthquake generation,
like many natural processes, is not cut-and-dried -- it cannot be summed
up in a simple rule," Murray told UPI. "However, these findings
benefit the public in as much as they will be an impetus for further research
and improvements in earthquake probability estimates."
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- The great Parkfield earthquake experiment will continue
to delve into the mechanisms operating before, during and after an earthquake.
Scientists hope to "capture" the elusive earthquake, whenever
it strikes, on the array of instruments on or near the fault surface and,
in the latest expansion of the project, deep underground. The tools set
1.8 to 2.4 miles (3 km to 4 km) beneath the surface will directly reveal,
for the first time, the physical and chemical processes controlling earthquake
generation within a seismically active fault.
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- These and other technological advances could turn long-range
forecasting into a science, Murray said.
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- "As scientists we must continually question our
assumptions and methods; test and revise our approaches, and develop new
concepts and techniques that capture our evolving understanding of the
earthquake machine," Stein told UPI. "We have an obligation to
provide society with useful tools to assess and monitor for the hazards
of earthquakes and to freely acknowledge what we know and what we don't
know."
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- In the meantime, people living in earthquake-prone regions
should plan for the inevitable.
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- "I always tell people to prepare," Segall said.
"We know big earthquakes have happened in the past, we know they will
happen again. We just don't know when."
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