Posted on 07/21/2004 7:04:04 PM PDT by NormsRevenge
LOS ANGELES (AP) - A new study concludes that a long stretch of the mighty San Andreas Fault close to highly populated Southern California has usually only had very large earthquakes. The researchers' finding challenge an idea that frequent small temblors slowly relieve accumulating strain on faults and therefore reduce the likelihood of big quakes.
Digging trenches at a site on the San Andreas 120 miles northwest of Los Angeles, geologists identified six events and found evidence that about 95 percent of slippage there has occurred during big quakes, with magnitudes ranging from 7.5 to 8, according to the study appearing in the current issue of the journal Geology.
The study's authors included professor Kerry Sieh of the California Institute of Technology, his former student and postdoctoral fellow Jing Liu, Charles Rubin of Central Washington University in Ellensburg, Wash., and Yann Klinger of the Institut de Physique du Globe de Paris, France.
The San Andreas Fault system, a huge, visible gash running through western California, is more than 800 miles long and marks the boundary of the Pacific Plate, which is on the west side and is moving northwestward relative to the North American Plate on the east side.
Scientists have long known the average period between big earthquakes on the section of the San Andreas nearest Los Angeles is 130 years. The most recent large earthquake happened 147 years ago, a magnitude-7.9 temblor that struck in 1857.
The researchers dug trenches parallel and perpendicular to a section of the fault lying between the southern San Joaquin Valley city of Bakersfield and the coast.
The trenches allowed them to study how movement of the San Andreas offset gullies that crossed the fault and were subsequently buried by sediment over the centuries.
The researchers found six offsets, the youngest related to the 1857 quake. Older gullies were progressively offset further by the fault.
Of the six, the offsets of five ranged from 5.72 yards to 8.8 yards. The other was about 1 1/2 yards.
According to scientists, offsets of several yards are typical of earthquakes that are very large and rupture over very long distances. The 1857 quake had a rupture length of 225 miles, extending from Parkfield in Central California to the Cajon Pass east of Los Angeles.
Rubin noted in an interview that the smallest offset was probably not the result of a small quake, but represented the tail end of a rupture that began on a distant section of the fault.
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On the Net:
San Andreas Fault guide: http://www.scec.org/wallacecreek/
Interestingly the "border" between the Pacific Plate and the North American Plate actually extends all the way to the Wasatch Range in Utah....all of Nevada is being stretched and sliding NW, too.
It's all Bush's fault. :-]
I still think they ought to pump a lubricant in there, get everybody to a safe spot with adequate provisions, and set it off with a nuke. The damage may be bad, but the loss of life and property would be minimized.
That would be the Hurricane Fault. The tops of the mesas in Southwestern Utah are volcanic, many only 10,000 years old. Think of the erosion in the intervening period! We live on a very active continent. There may be a damn good reason civilization didn't develop here.
Sounds like the last time I had sex.
Bragging, eh? It is one way of getting yer rocks off. Hey, maybe we can get Bubba to volunteer somewhere in the Santa Monica Mountin's!
Ahem
Calico: A 200,000 Year Old Site In The Americas?
(Central Mojave Desert Of California)
One of the best books I've ever read is "Guns, Germs, and Steel".....
Explains why advanced civilizations developed in some parts of the world and not others.
Main reasons had to do with domesticable plants and animals.
I suppose it's some consolation that a volcanic eruption is overdue in the Mammoth Mountain - Mono Lake region. When the Long Valley Caldera last erupted the layer of ash extended into eastern Kansas. What a way to go!
Actually it's God tryiny to correct his mistakes.
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