Space Survey Yields New Info on California’s Landscape, Quakes

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.