California mountains are several million years older

CONTACT: B.J. Almond
PHONE:
(713) 348-6770
EMAIL: balmond@rice.edu

CALIFORNIA MOUNTAINS ARE SEVERAL
MILLION YEARS OLDER
Geologist at Rice University also finds lower
potential for earthquakes

A new analysis of the
topography of California indicates that the coastal ranges might be more than
three million years older than previously estimated and that one part of the
plate movement that could cause earthquakes is occurring more slowly.

Using several years of
data from satellites, lasers and radio telescopes, a research team from Rice
University in Houston and NASA’s Jet Propulsion Laboratory (JPL) in Pasadena,
Calif., studied changes outside California’s deforming zone, the areas where
plates or segments of the earth’s crust and mantle slide past one another while
also pushing together or pulling apart.

“We used more accurate
data than obtained within the deforming zone itself,” said Richard Gordon, the
W.M. Keck Professor of Geology and Geophysics in the Department of Earth Science
at Rice, who collaborated with Donald Argus at the JPL. “The measurements within
the deforming zone don’t capture all the motion of the plates, some of which
occurs offshore and under water.”

The researchers used the
measurements outside the deforming zone to infer the changes that have taken
place where the Pacific plate and the adjacent Sierran plate meet. Much of the
California coast rests on the Pacific plate, and most of the continental United
States rides on the North American plate. The smaller Sierran plate serves as a
buffer between the two.

Along the San Andreas
fault system, Gordon and Argus estimate that the Pacific plate slides
horizontally past the Sierran plate at about 39 millimeters (1.5 inches) per
year, which is significantly more than previous estimates of 34 millimeters (1.3
inches). But in addition to this horizontal sliding along the fault, the plates
near the fault are also pushing against each other at about 3.3 millimeters (.13
inch) per year; this converging of plates raises California’s coastal mountains,
but the new estimated rate is lower than previous estimates.

“If the rate is lower,
it takes more time to build the mountains to their present size,” Gordon said.

North of San Francisco, the researchers found that the Pacific and Sierran
plates are slowly pulling apart at a rate of 2.6 millimeters (.1 inch) per
year.

Based on the new rates
of fault convergence, Gordon estimated California’s coastal mountains to be at
least three million to six million years old – quite a leap from the previous
estimates of one million to three million years.

“That’s quite a bit
older than what I had been taught as a student,” said Gordon, who used to hike
in the foothills of these coastal ranges as a child in San Jose. “People have
been assuming that the convergence rate was much higher than what we
found.”

The researchers also
studied the relationship between the degree of convergence and the degree of
stable sliding along the San Andreas fault and nearby fault lines. Low
convergence rates were associated with stable sliding that is expressed by small
to moderate earthquakes; high convergence rates have the opposite effect,
resulting in great earthquakes.

“Prior workers thought
the rate of convergence was 15 millimeters a year, but we calculated much
smaller numbers – approximately three millimeters a year – so the potential
number of earthquakes and the severity of those quakes is much smaller,” Gordon
said.

The research was funded
as part of NASA’s Earth Science Enterprise, a long-term research effort focusing
on the effects of human-induced and natural changes on the global environment.