Download American Scientist

A reprint from
American Scientist
the magazine of Sigma Xi, The Scientific Research Society
This reprint is provided for personal and noncommercial use. For any other use, please send a request to Permissions,
American Scientist, P.O. Box 13975, Research Triangle Park, NC, 27709, U.S.A., or by electronic mail to [email protected].
©Sigma Xi, The Scientific Research Society and other rightsholders
Sightings
See How the Earth Moves
Interferometric Synthetic Aperture Radar (InSAR) measurements, collected by satellites circling Earth, can
provide big insights into major earthquakes. Richard Walters and John Elliott demonstrated this after a 6.3-magnitude quake struck central Italy in April, killing close to 300 people and severely damaging the medieval town
of L’Aquila. By comparing measurements taken before and after the earthquake, the geophysicists pinpointed the
responsible fault, measured changes aboveground and calculated likely shifts underground. Walters and
Elliott are researchers at the University of Oxford and the United Kingdom’s National Centre for Earth
Observation. In an e-mail exchange, Walters explained their studies to American Scientist associate
editor Catherine Clabby.
American Scientist: How do you capture the sort
of detail you achieved for the L’Aquila earthquake?
Richard Walters: To construct an interferogram, we
look at differences between two radar phase images. Changes in phase come from ground displacements between the image acquisitions, changes in
satellite position, topographic effects and changes
in the atmosphere, among other things. We correct for changes in satellite position with precisely
calculated orbits and for topographic effects with a
high-resolution digital elevation model, leaving an
interferogram that shows ground displacements.
It is difficult to correct for the atmospheric signal,
but we propagate atmospheric errors through our
model to look at how this factor could affect our
understanding of an earthquake.
A. S. What information does this provide regard-
ing an earthquake?
R. W. InSAR’s main strength is its high spatial
resolution. Compared with seismology, InSAR
can identify an earthquake’s precise location. For
instance, there are many faults in the L’Aquila region, and it wasn’t immediately apparent which
had ruptured. InSAR located the earthquake on
the Paganica fault, which was less well known
than other nearby faults. The data showed one
side of the fault moved up a maximum of 8 centimeters while the other moved down a maximum
of 25 centimeters. Using our model, we also estimated a maximum slip of 90 centimeters about 7
kilometers below the surface.
A. S. What is the usefulness of this data?
R. W. It is often assumed that active faults are
associated with large topographic features such
as high mountain ranges. However, the InSAR
data showed that this fault was associated with
smaller topographic features. This has implications for Italy and similar regions. In addition,
by modeling, we can try to understand the nature of an earthquake and its source fault. And
408
American Scientist, Volume 97
we can calculate how an event may have
moved other nearby faults closer to failure. This
is important for assessing seismic hazards. Several faults near L’Aquila that were moved closer
to failure are near the historic towns of Amatrice
and Campotosto and near a large reservoir with
a hydroelectric dam.
A. S. How did you obtain the data you needed
for these studies?
R. W. We used the Environmental Satellite (ENVISAT), part of the European Space Agency (ESA),
to study the L’Aquila earthquake. To produce images as quickly as possible after an earthquake,
we need fast access to the data and precisely
calculated satellite orbits. ESA’s response was
extremely rapid. They ensured that all new data
was acquired by ENVISAT each time it passed
over the area. A data catalog, constantly updated,
was made freely available. Orbit information was
available within several days. ESA collaborates
with the Japanese Space Agency and it also uploaded data from the Japanese Advanced Land
Observing Satellite.
A. S. This technology can be used to observe
changes in volcanoes and glacial ice too. How
else is it useful?
R. W. InSAR can be used to monitor non-tectonic
ground movement, such as subsidence in mining areas; measure long-term slip rates on faults;
and help us explore the mechanical behavior of
the shallow earth in the weeks and years after
an earthquake. InSAR also has been used to produce 90-meter resolution topographic maps for
all continental areas between 60 latitude degrees
north and south. The hope is that InSAR will be
used to routinely monitor ground motions all
over Earth.
A. S. Where are our blind spots now?
R. W. We have coverage only up to around 80
degrees latitude north and south due to the slight
© 2009 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].
/
m
m
m
-"RVJMB
1BHBOJDBGBVMU
m
MJOFPGTJHIUEJTQMBDFNFOUJODFOUJNFUFST
The image above shows an interferogram, a map of ground movements produced with measurements made remotely by Interferometric Synthetic Aperture Radar (InSAR), plotted atop digital topography. United Kingdom
geophysicists Richard Walters and John Elliott used measurements acquired by a European Space Agency satellite
on February 1 and April 12 to capture ground movements from the April 6 earthquake in central Italy. The color
bands (red through blue) represent contours of ground motion toward or away from the satellite, within its line of
sight. Moving towards the center of each lobe, each contour represents an additional 2.8 centimeters of ground motion. The image shows that during the earthquake, the region to the northeast of the Paganica fault moved toward
the satellite by about 8 centimeters, whereas the region to the southwest moved away by about 25 centimeters. Each
pixel in the interferogram represents an area of about 80 meters squared. Walters’ and Elliott’s research is funded
by the United Kingdom’s Natural Environmental Research Council. The ESA data is copyrighted.
tilt of the satellite orbital plane relative to Earth’s
spin axis. In addition, SAR radar sensors are only
one set of several sensors mounted on ENVISAT
and require a lot of energy to operate. The satellite
can’t acquire data continuously, especially while
traversing the dark side of the Earth. And InSAR
only captures continental ground motions. There
are lots of interesting earthquakes in the oceans
that we cannot measure with this technique.
A. S. What improvements are needed? Are they
coming?
R. W. Most early SAR satellites used C-band
radar with 5.6-centimeter wavelength. This
wavelength is easily scattered by vegetation.
Several newer satellites use L-band radar with
a longer wavelength, about 20 centimeters, enabling views of areas, such as tropical forests,
that previously were inaccessible. Also, the European Space Agency plans to launch Sentinel1, a constellation of two new satellites, in 2011
and 2013. The satellites will have a combined
revisit time of six days, a huge improvement
over ENVISAT’s 35-day repeat time. Scientists
from multiple backgrounds are also working on
reducing atmospheric errors in interferograms.
Space agencies will improve the stability of satellite orbits. That will help reduce measurement
errors and allow us to study smaller ground
motions with more confidence.
In Sightings, American Scientist publishes examples of innovative scientific imaging from diverse research fields.
www.americanscientist.org
© 2009 Sigma Xi, The Scientific Research Society. Reproduction
with permission only. Contact [email protected].
2009 September–October
409