JPL QUAKE RESEARCH
- From: datakoll <datakoll@xxxxxxxxx>
- Date: Thu, 18 Jun 2009 16:13:10 -0700 (PDT)
June 17, 2009
Scientists Search for a Pulse in Skies Above Earthquake Country
NASA Gives California's San Andreas, Other Faults a 3-D Close-up
Story Highlights
New NASA 3-D airborne radar to study California’s earthquake faults.
Radar sees below the surface to measure buildup and release of strain
along faults.
Data can be used to guide rescue and damage assessment efforts after a
quake.
LA basin, San Francisco Bay among areas to be studied.
When a swarm of hundreds of small to moderate earthquakes erupted
beneath California's Salton Sea in March, sending spasms rumbling
across the desert floor, it set off more than just seismometers. It
also raised the eyebrows of quite a few concerned scientists. The
reason: lurking underground, just a few kilometers to the northeast,
lays a sleeping giant: the 160-kilometer-(100-mile) long southern
segment of the notorious 1,300-kilometer- (800-mile) long San Andreas
fault. Scientists were concerned that the recent earthquake swarm at
the Salton Sea's Bombay Beach could perhaps be the straw that broke
the camel's back, triggering "the big one," a huge earthquake that
could devastate Southern California.
The southern end of the San Andreas has remained silent, at least for
now. But the earthquake swarm and more recent, widely felt earthquakes
in the Los Angeles area have stirred renewed interest in earthquake
research. A multi-year project currently under way at NASA’s Jet
Propulsion Laboratory, Pasadena, Calif., is seeking to improve our
understanding of these mysterious and sometimes deadly natural hazards
by using a groundbreaking, JPL-developed airborne radar to study
earthquake processes along the San Andreas and other California
faults.
The 'Mother of California Faults'
Formed 15 to 20 million years ago, the San Andreas has defined
California's seismic history and dramatically altered its landscape.
It serves as the boundary between the two massive tectonic plates upon
which the Golden State rides: the Pacific and North American plates.
Grinding horizontally past each other in a roughly north-south
direction at up to 3.5 centimeters (1.4 inches) a year, the fault is a
battle zone of pulverized rock, extending to depths of at least 16
kilometers (10 miles). In some places, the plates "creep" quietly past
each other, producing small to moderate earthquakes, in a process
known as aseismic creep.
But other parts of the fault get "stuck." They lock in place for
sometimes hundreds of years before eventually releasing their pent-up
frustrations in epic lunges, such as those responsible for the large
magnitude 7.9 earthquakes that struck a then sparsely populated
Southern California near Fort Tejon in 1857, and San Francisco in
1906.
From San Luis Obispo south to the Cajon Pass near San Bernardino, theSan Andreas forms a largely unbroken line that is often clearly
visible from the ground and air. South of Cajon Pass, however, the
fault zone becomes more complex. Here, several different faults share
the "burden" of moving the tectonic plates, including the San Andreas
and the parallel and intersecting San Jacinto and southern San Andreas
faults, among others. North of San Luis Obispo, the fault zone
similarly splits into nearly parallel faults, with the Hayward and
Calaveras faults sharing the plate motion with the San Andreas in the
San Francisco Bay area.
Paleoseismological studies dating back 1,500 years have shown that
large earthquakes occur on the southern San Andreas about every 250 to
300 years, on average. Yet the extreme southern segment of the fault
hasn't budged for about 320 years. It is apparently overdue, primed
for another large event.
Last year, the United States Geological Survey estimated that such a
large earthquake, originating near the Salton Sea and rupturing the
ground northward to near Lake Hughes in Los Angeles County, could
devastate an eight-county region, killing up to 1,800, injuring
50,000, displacing a quarter million people, significantly damaging
300,000 buildings and causing an estimated $213 billion in damage.
Searching for Clues From Above and Below
Like doctors assessing the health of a patient, scientists use a broad
array of tools to "listen" to the San Andreas and other faults,
looking for clues about their past, present and future behavior. They
dig trenches across faults, and place instruments, such as
seismographs, creep meters and stress meters, into the ground to try
to detect any changes that might be occurring above or below Earth's
surface.
Increasingly, they also rely on space-based technologies, such as
those being developed at JPL. Space-based instruments can image minute
Earth movements to within a few centimeters (fractions of an inch),
measuring the slow buildup of deformation along faults and mapping
ground deformation after earthquakes occur. Among these tools are the
Global Positioning System and interferometric synthetic aperture
radar, or InSAR.
Until recently, the only InSAR data available for the San Andreas and
other California faults have come from European Space Agency, Canadian
and Japanese radar satellites. But those satellites aren’t dedicated
to or optimized for studying earthquakes, and the availability of
their data is limited.
A New 3-D Radar Tool
Now, JPL scientists have added a new airborne radar tool to their
arsenal. Called the Uninhabited Aerial Vehicle Synthetic Aperture
Radar, or UAVSAR, this L-band wavelength radar flies aboard a modified
NASA Gulfstream III aircraft from NASA's Dryden Flight Research
Center, Edwards, Calif. The compact, reconfigurable radar, housed in a
pod under the aircraft's fuselage, uses pulses of microwave energy to
detect and measure very subtle deformations in Earth's surface, such
as those caused by earthquakes, volcanoes, landslides and glacier
movements.
UAVSAR works like this: flying at a nominal altitude of 13,800 meters
(45,000 feet), the radar collects data over a selected region. It then
flies over the same region again, minutes to months later, using the
aircraft's advanced navigation system to precisely fly over the same
path to an accuracy of within 4.6 meters (15 feet). By comparing these
camera-like images, called interferograms, over time, scientists can
measure the slow surface deformations involved with the buildup and
release of strain along earthquake faults.
(UAVSAR is currently wrapping up a two-month expedition in Greenland
and Iceland to study the flow of glaciers and ice streams. See
http://www.jpl.nasa.gov/news/news.cfm?release=2009-075 and
http://www.jpl.nasa.gov/news/features.cfm?feature=2156 ).
'Mowing the Lawn'
Last November, JPL scientists began conducting a series of UAVSAR
flights over regions of Northern and Southern California that are
actively deforming and are marked by frequent earthquakes. About every
six months for the next several years, the scientists will precisely
repeat the same flight paths to produce interferograms. From these
data, 3-D maps will be created for regions of interest, including the
mighty San Andreas and other California faults, extending from the
Mexican border to Santa Rosa in the northern San Francisco Bay. Last
month, the scientists completed their first full map of the San
Andreas. Some regions, such as Parkfield on the central San Andreas,
and the Hayward fault, have already had more than one flyover.
"We'll be 'mowing the lawn,' so to speak, mapping the San Andreas and
adjacent faults, segment by segment, and then periodically repeating
the same radar observations," said Andrea Donnellan, one of three JPL
principal investigators on the UAVSAR fault mapping project, and
program area lead for Natural Disasters in NASA Headquarters' Science
Mission Directorate, Washington.
"By comparing these repeat-pass radar observations, we hope to measure
any crustal deformations that may occur between observations, allowing
us to 'see' the amount of strain building up in the San Andreas and
adjoining faults,” Donnellan said. “This will give us a much clearer
picture of which faults are active and at what rates they're moving,
both before earthquakes and after them."
Donnellan said the UAVSAR fault mapping data will substantially
improve our knowledge of regional earthquake hazards in California.
"The 3-D UAVSAR data will allow scientists to bring entire faults into
focus, allowing them to see the faults not just at their surfaces, but
also at depth," she said. "When integrated into computer models, the
data should give scientists a much clearer picture of California's
complex fault systems, such as those in the Los Angeles basin and in
the area around the Salton Sea."
The scientists will estimate the total displacement occurring in each
region. As more observations are collected, they expect to be able to
determine how strain is partitioned between individual faults. They’ll
also be able to measure ground signals caused by human activities,
such as pumping water into or out of the ground or drilling for oil.
The UAVSAR flights will serve as a baseline for pre-earthquake
activity. Should earthquakes occur during the course of this project,
the team will measure the deformation at the time of the earthquakes
to determine the distribution of slip on the faults, and then monitor
longer-term motions after the earthquakes to learn more about fault
zone properties.
“Airborne UAVSAR mapping can allow a rapid response after an
earthquake to determine what fault was the source and which parts of
the fault slipped during the earthquake,” said Eric Fielding, another
JPL principal investigator on the UAVSAR project. “Information about
the earthquake source can be used to estimate what areas were most
affected by the earthquake shaking to guide rescue and damage
assessment response.”
The UAVSAR data will also be used to test the earthquake forecasting
methodology developed by UC Davis scientist John Rundle under NASA's
QuakeSim project (see http://www.jpl.nasa.gov/news/news.cfm?release=2003/-074
). The experiment identifies regions that have a high probability for
earthquakes in the near future.
Mapping Faults from the Salton Sea to Santa Rosa
Donnellan's research will focus on Southern California between the
Salton Sea and the Pacific coast, along with the Los Angeles basin,
the seismically active Transverse Ranges (the east-west-oriented
mountain ranges located between San Diego and Santa Barbara), and the
San Francisco Bay area up through Santa Rosa.
Meanwhile, JPL colleagues Paul Lundgren and Zhen Liu will focus on the
central San Andreas fault between the Bay Area and Los Angeles. This
area is a transition zone between the creeping part of the fault north
of the Parkfield segment, which has experienced fairly regular
moderate earthquakes of around magnitude 6, and the Carrizo Plain
segment, which ruptured in the 1857 Fort Tejon earthquake. They will
also integrate UAVSAR with GPS and satellite InSAR data to form more
complete models of how the fault slips over time.
JPL's Eric Fielding will focus on the Hayward fault along the east
side of San Francisco Bay, identified as having the highest risk of a
damaging earthquake in the Bay Area. The Hayward fault creeps in some
parts, but also ruptured in a magnitude 6.8 to 7.0 earthquake in 1868
that caused extensive damage due to its location in the heart of the
Bay Area. Fielding will analyze this creep to determine how much of
the fault's overall motion is being released gradually, without large
earthquakes, and estimate how much of the fault has accumulated stress
since the 1868 quake that could rupture again. From these data,
Fielding's team hopes to develop models of how stress and strain is
evolving on the fault system and infer properties of the fault zone.
“Previous studies of the Hayward fault using satellite InSAR were
limited by fixed satellite orbits and shorter radar wavelengths that
only provided useful measurements in the urbanized areas of the San
Francisco Bay," said Fielding. "UAVSAR will give us a complete picture
of the 3-D deformation and map much finer details than are possible
from space.”
Initial science results from the UAVSAR fault mapping project will be
available some time after the second round of mapping flights are
completed. In the meantime, the science team is busy constructing
computer models to compare with the actual UAVSAR data once they
become available.
What's Next?
Donnellan said UAVSAR is also serving as a flying testbed to evaluate
the tools and technologies for future space-based radars, such as
those planned for a NASA mission currently in formulation called the
Deformation, Ecosystem Structure and Dynamics of Ice, or DESDynI. That
mission, which will study hazards such as earthquakes, volcanoes and
landslides, as well as global environmental change, will use both a
light detection and ranging sensor, or lidar, and an L-band radar that
is very similar to UAVSAR's but with a much wider ground swath.
DESDynI will be capable of providing repeat-pass interferometric data
every eight days.
Once DESDynI is in orbit, UAVSAR will be used to calibrate its data
and will complement its measurements by filling in gaps in its
coverage.
"The Earth science community is anxiously awaiting the launch of
DESDynI in a few years," Donnellan said. "In the meantime, UAVSAR data
will give us a head start on better understanding California's complex
fault systems. Its data will also help state and local governments
mitigate losses from future earthquakes, including the inevitable 'big
one' we all know is in our future."
To learn more about UAVSAR, visit: http://uavsar.jpl.nasa.gov .
To learn more about other ongoing JPL earthquake research programs,
visit: http://quakesim.jpl.nasa.gov/ .
-end-
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