posted on Dec, 12 2004 @ 04:35 PM
Seismologists are puzzling over a series of rare tremors never before recorded in the San Andreas Fault - or in a fault of the same type, for that
matter. Parkfield, CA didn't feel the shaking deep beneath their feet, but scientists monitoring the area have recorded 110 incidents of "chatter"
in the last three years. This chatter occurs well below the average depth of an earthquake - up to 40 miles below the surface - and is usually seen
related to magma movement in volcanoes such as Mount St. Helens. Earthquakes normally occur around 10 miles below the Earth's crust. Seisimologists
have not determined any magmatic activity related to these tremors, but are hard-pressed to explain exactly what they 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, 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
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,
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The most concerning aspect of these tremors is their location. In 1857, a massive earthquake estimated at a magnitude 8 was centered at the exact
area where the current chatter is occurring. That earthquake ruptured the San Andreas for an estimated 225 miles and caused massive ground rolling.
Due to it's then sparse population, only two people died and very little damage was reported. While still a relatively rural area, growing
communities are well within the "danger zone" if a large quake were to strike. The damage and loss of life could be significant if these tremors
are preceding another tremblor like the 1857 Ft. Tejon quake or the similar 1906 San Francisco quake.
A report published on Friday noted that "...future increases in San Andreas Fault tremor activity may signal periods of increased probability for the
next large earthquake on the Cholame segment."
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[edit on 12-12-2004 by Banshee]