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SOUTHERN SAN ANDREAS FAULT WAITING TO EXPLODE: REPORT
By Jeremy Lovell
Reuters
June 21, 2006

http://tinyurl.com/z8emj

LONDON - The southern end of the San Andreas fault near Los Angeles, which
has been still for more than two centuries, is under immense stress and
could produce a massive earthquake at any moment, a scientist said on
Wednesday.

Yuri Fialko, of the Scripps Institution of Oceanography at La Jolla,
California, said that given average annual movement rates in other areas of
the fault, there could be enough pent-up energy in the southern end to
trigger a cataclysmic jolt of up to 10 meters (32 ft).

"The observed strain rates confirm that the southern section of the San
Andreas fault may be approaching the end of the interseismic phase of the
earthquake cycle," he wrote in the science journal Nature.

A sudden lateral movement of 7 to 10 meters would be among the largest ever
recorded.

According to the U.S. Geological Survey (USGS), the earthquake that
destroyed San Francisco in 1906 was produced by a sudden movement of the
northern end of the fault of up to 21 ft.

Fialko said there had been no recorded movement at the southern end of the
fault -- the 800-mile long geological meeting point of the Pacific and the
North American tectonic plates -- since the dawn of European settlement in
the area.

He said this lack of movement for 250 years correlated with the predicted
gaps between major earthquakes at the southern end of the fault of between
200 and 300 years.

Elsewhere on the fault, there were average slippage rates up to a couple of
centimeters a year that prevented the build-up of explosive pressure deep
underground.

When these became blocked and then suddenly broke free they produced tremors
or earthquakes of varying intensity depending on the movement that had taken
place before and the duration of the blockage.

USGS says the most recent major earthquakes in the northern and central
zones of the San Andreas fault were in 1857 and 1906.

Fialko said there were three possible explanations for the lack of observed
movement in the southern section -- creepage under the surface that had no
external manifestation, that it simply might not move as much as the rest or
a major blockage.

"Except for the first possibility above, the continued quiescence increases
the likelihood of a future event," he wrote.

Making calculations based on a wide range of land and satellite
observations, he discounted the idea of creepage and warned of impending
disaster.

"Regardless of fault geometry and mechanical properties of the ambient
crust, results presented in this study lend support to intermediate-term
forecasts of a high probability of major earthquakes on the southern SAF
system," Fialko said.

------------

QUAKE FEARS HAUNT SOUTHERN CALIFORNIA
By Patrick Barry
New Scientist
June 21, 2006

http://www.pheedo.com/click.phdo?i=f4d88311bf23ef8f2354915a93857f29

While San Franciscans justifiably worry about living near the San Andreas
fault, many forget that southern Californians too live in fear of their own
"big one". No major quake has struck the southern San Andreas fault in at
least 250 years, and scientists say that the region is now primed for a
release of the built-up tension.

A new study, by geophysicist Yuri Fialko of the University of California,
San Diego, provides the most precise measurements yet of this accumulated
strain - and it's not a pretty picture. Deep within the Earth's crust, the
west side of the San Andreas fault has moved relative to the east by as much
as 8 metres since the region's last earthquake. But closer to the surface,
the two sides of the fault are jammed against each other, building up
ever-increasing strain.

Fialko says that this 8-metre shift is on a par with the maximum movement
that the fault has ever experienced between quakes - and it has packed
enough energy in the fault to unleash a magnitude-8 earthquake if the strain
were released all at once. "If it's realised, it's going to be a major
disaster," he says.

Fialko estimated the slip rates along faults in southern California using
nine years of high-precision satellite data. Conventional seismic stations
can track ground movement very precisely but are typically spaced up to
about 10 kilometres apart, on average. Such a coarse grid of data points
makes it hard to be certain of what is going on underground. Satellites, on
the other hand, can scan the whole region with a resolution down to 20
metres. The fine detail, combined with the use of space-borne radar
interferometers and GPS satellites, allowed Fialko to measure ground motions
as slow as only a few millimetres per year. "It's only been a few years
since that became possible," says Fialko (Nature, vol 441, p 968).

"It's certainly the most precise study that I've seen," says Karen Felzer, a
geophysicist at the US Geological Survey in Pasadena, California.

These minute movements reveal the deformation of the land surface along the
faults, showing for the first time where the strain is accumulating. The
tension seems to be divided almost evenly between the parallel San Andreas
and San Jacinto faults.

Scientists were also uncertain whether slow movements in the faults at
shallow depths were partially relieving the built-up tension. But Fialko's
analysis shows that this "shallow creep" is, unfortunately, trivially small.
The southern San Andreas fault may be nearing the end of its quiet period.

------------

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