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Subject: Fw: Space Survey Yields New Info On California's Landscape, Quakes
From: "Larry Cochrane" cochrane@..............
Date: Fri, 11 Jan 2002 00:28:03 -0800

Saw this in the sci.geo.geology newsgroup. -Larry

----- Original Message -----
From: "Ron Baalke" 
Newsgroups: sci.geo.geology
Sent: Thursday, January 10, 2002 9:57 PM
Subject: Space Survey Yields New Info On California's Landscape, Quakes

> PASADENA, CALIFORNIA 91109. TELEPHONE (818) 354-5011
> Contact: Alan Buis (818) 354-0474        January 10, 2002
>      A space-based survey by a research team from NASA's Jet
> Propulsion Laboratory, Pasadena, Calif., and Rice University,
> Houston, Texas, offers new insights into the history of
> central California's varied topography and the region's
> earthquake hazards.
>      Using several years of data from precise space-based
> surveying methods such as the Global Positioning System,
> researchers Dr. Donald Argus of JPL and Dr. Richard Gordon of
> Rice University found a strong correlation between the degree
> to which the Pacific tectonic plate and its adjacent Sierran
> microplate push against one another (converge) or pull apart
> from one another (diverge) and the height, extent and age of
> California's coastal mountains.  Their results were published
> recently in the Geological Society of America Bulletin and
> were featured as a recent "Editor's Choice" in Science.
>      "This precise positioning data is allowing us to better
> understand why central California's coastal mountains are
> where they are and where they're growing," Argus said.
>      Much of coastal California rides on the Pacific plate,
> while the Sierran plate serves as a buffer zone of sorts for
> the North American plate, which carries the rest of the
> continental United States.
>      North of the 'big bend' in the San Andreas fault, the
> relative motion of the Pacific and Sierran plates in central
> California nearly parallels the San Andreas and related
> faults.  In most places, the plates are converging at rates up
> to 3.3 millimeters (.13 inches) per year, horizontally
> shortening Earth's crust across the fault and raising
> California's coastal mountains.
>      "We found the greater the rate of convergence, the larger
> the size and extent of the mountains," said Argus.
>      The affected mountains include the Temblor and Diablo
> Ranges, those on the west flank of the Sacramento-San Joaquin
> Valley, others near the San Andreas fault system and those
> strictly near the coast.   These ranges block drainage of the
> watershed comprising the Sierra Nevada and great central
> valley of California into the Pacific Ocean.
>      In contrast, he and Gordon found that just north of San
> Francisco, the Pacific and Sierran plates are slowly pulling
> apart at a rate of 2.6 millimeters (.1 inches) per year,
> opening a hole manifested as a topographic low in San Pablo
> Bay.  Here, rivers originating in the Sierra Nevada mountains
> drain through the coastal mountains on their way to passage
> under the Golden Gate Bridge and out into the Pacific.
>      Argus and Gordon's study also addresses overall
> earthquake hazards in the region.  They calculated the lateral
> rate of motion between the Pacific and Sierran plates at
> approximately 39 millimeters (about 1.5 inches) per year.
> This rate differs significantly from a previous estimate of 34
> millimeters (about 1.3 inches) per year obtained by measuring
> and dating creek displacements across the San Andreas fault.
> The scientists attributed this difference to inelastic
> deformation, slip along other faults or both.  These
> observations limit the total amount of strain that may be
> released in earthquakes along the fault system, Argus said.
>      The researchers also found a general relationship between
> the degree of convergence and the degree of stable sliding
> along the San Andreas and other northwest-striking strike-slip
> faults in central California.  Where convergence rates are low
> or negative, sliding tends to be stable, manifesting itself as
> steady "creep" or small to moderate earthquakes; where
> convergence rates are high, the faults tend to be unstable,
> resulting in great earthquakes such as the 1906 San Francisco
> quake.  In most cases, the stable fault sections move parallel
> to the direction of relative plate motion.
>      Argus and Gordon found prominent exceptions to this rule,
> however, that make their hypothesis at best a partial
> explanation for the observed distribution of locked and
> nonlocked fault sections.  They speculate that other unknown
> factors are at work in these areas.
>      Based upon present rates of fault convergence and
> neglecting the effects of erosion, the two calculated the age
> of California's coastal ranges to be at least 3 to 6 million
> years, with the Diablo Range estimated at approximately 10
> million years old.  Most previous age estimates range from 1
> to 3 million years.
>      This research was funded as part of NASA's Earth Science
> Enterprise, a long-term research effort dedicated to
> understanding how human-induced and natural changes affect our
> global environment.
>      JPL is a division of the California Institute of
> Technology in Pasadena.
>                    #####


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