Mar 29, 2009 9:57 pm US/Pacific
300 Aftershocks Jolt Salton Sea Area
SALTON SEA, Calif. (CBS) ―
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A magnitude 4.8 quake struck Tuesday at 4:55 a.m. in the Imperial County.
CBS
More than 300 aftershocks, some of them approaching the moderate range, have pulsed through the Salton Sea area since last Tuesday's 4.8 magnitude quake, as the region is on the move.
Seismologists monitoring "creep meters" and other high-tech, sensitive measuring devices say the fractured pattern of faults under the Salton Sea are moving all at once. Instead of one major strike-slip fault crack, as in most of California, the San Andreas Fault in the Coachella Valley is a web of
"ladder pattern" faults.
U.S. Geological Survey scientist Susan Hough told the Desert Sun newspaper that the ladder faults are "an energenic cluster" that has thrown
off 300 aftershocks, including 10 with a magnitude above 3.0, since Tuesday.
The largest aftershock was a 4.0 shaker at 3:25 a.m. Thursday.
Scientists from the University of Colorado at Boulder say the western half of the Coachella Valley has moved to the northwest .002 of an inch
following the Tuesday quake. Geologist Roger Bilham said his devices are accurate to one thousandth of an inch.
"There was a very small offset at two locations on Durmid Hill last week," Bilham told the Desert Sun, referring to a hill at the northern end of
the Salton Sea.
Such simultaneous ladder fault movements has long been predicted as a possible precursor to a major quake along the southernmost reach of the San Andreas, which has not had a major earth movement since the year 1680.
Scientists calculate that massive, destructive-force fault movements have occurred in the Coachella Valley an average of every 220 years, making a
major movement along the San Andreas Fault way overdue.
A large quake at the Salton Sea is capable of causing significant damages in Los Angeles and San Diego, seismologists warn.
(© 2010 CBS Broadcasting Inc. All Rights Reserved. This material may not be published, broadcast, rewritten, or redistributed. Wire services contributed to this report.)
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