High above Earth where seismic waves never
reach, satellites may be able to detect earthquakes - before they
For many people, earthquakes
are synonymous with unpredictability. They strike suddenly on otherwise
normal days, and despite all the achievements of seismology, scientists
still can't provide warning of an impending quake in the way that
weathermen warn of approaching storms.
seem to strike out of the blue, the furious energy that a quake
releases builds up for months and years beforehand in the form of
stresses within the Earth's crust. At the moment, forecasters have
no direct way of seeing these stresses or detecting when they reach
critically high levels.
That may be changing,
however. Satellite technologies being developed at NASA and elsewhere
might be able to spot the signs of an impending quake days or weeks
before it strikes, giving the public and emergency planners time
"There are several satellite-based methods that show promise as
precursors to earthquake activity," says Jacob Yates, a researcher
at the Goddard Space Flight Center. "One method is Interferometric-Synthetic
Aperture Radar (InSAR). Basically, InSAR is when two radar images
of a given tectonic area are combined in a process called data fusion,
and any changes in ground motion at the surface may be detected."
image showing the shift in the ground height due to the
1999 Hector Mine earthquake. The radar data were acquired
by the European Space Agency ERS-2 satellite on September
15 and October 20, 1999.
This technique is sensitive
enough to detect slow ground motions as tiny as 1 mm per year. That
kind of sensitivity, combined with the landscape-wide view that
satellites can offer, lets scientists see the tiny motions and contortions
of land around a fault line in more detail than ever before. By
watching these motions, they can figure out where points of high
strain are building up.
A group of NASA and
university scientists led by Carol Raymond of JPL recently studied
the feasibility of forecasting earthquakes from space. Their report,
which was released in April 2003, outlines a 20-year plan to deploy
a network of satellites - the Global Earthquake Satellite System
(GESS) - using InSAR to monitor fault zones around the world.
With some practice,
says Raymond, scientists eventually should be able to use the InSAR
data to infer when stresses in the Earth's crust have reached a
dangerous level, issuing a monthly "hazard assessment" for a given
fault. Forecasters might report that the likelihood of having a
major quake on, say, the San Andreas fault during the coming month
is 2%, or 10%, or 50%.
Current methods are
less certain. For example, the US Geological Survey recently released
an updated assessment of the earthquake risk in the San Francisco
Bay Area based on the seismic history of the area, its geology,
and computer models. The study
reported a 62% chance of a major quake (magnitude 6.7 or greater)
hitting the area sometime within the next 30 years - not
exactly something to plan your day around.
InSAR is one way to
forecast quakes, but perhaps not the only one. While InSAR satellites
merely improve the data available to orthodox seismology, there
are other techniques that break with orthodoxy.
Credit: MODIS onboard NASA's Terra satellite.more
image of the region surrounding Gujarat, India, on January
21, 2001. Yellow-orange areas trace thermal anomalies
that appeared days before the Jan. 26th quake. The boxed
star denotes the quake's epicenter.
One of these ideas
is to look for surges in infrared (IR) radiation. Friedemann Freund,
adjunct professor of physics at San Jose State University and a
scientist at the Ames Research Center, explains: "In the 1980s and
90s, Russian and Chinese scientists noticed some strange thermal
anomalies associated with earthquakes in Asia - for example, the
1998 Zhangbei earthquake near the Great Wall of China. This earthquake
occurred when ground temperatures in the region were around -20o
C. Just before the quake, thermal sensors detected temperature variations
as large as 6o to 9o, according to Chinese
with IR cameras could be used to detect these hot spots from space.
In fact, when Freund and colleague Dimitar Ouzounov of the Goddard
Space Flight Center (GSFC) examined infrared data collected by NASA's
Terra satellite, they discovered a warming of the ground in western
India just before the powerful January 26, 2001, quake in Gujarat.
"The thermal anomaly was as large as +4 C°," says Freund.
What causes rocks under
pressure to emit infrared radiation? No one is certain. The frequency
spectrum of the emissions shows that internal heat from friction - e.g.,
rocks rubbing together - is not responsible for the radiation.
In one laboratory experiment,
Freund and colleagues placed red granite blocks under a 1,500 ton
press - mimicking in some ways what happens miles below Earth's surface.
A sensitive camera developed at JPL and GSFC monitored the rock
and detected infrared emissions. Furthermore, a voltage built up
on the rock's surface. This leads Freund to believe the cause might
Ordinary rocks are
insulators. Rocks placed under great stress, however, sometimes
act like semiconductors. Freund believes that, before a quake, pairs
of positive charges called 'defect electrons' or 'positive holes'
split up and migrate to the surface of stressed rocks. There they
recombine with each other and, in the process, release infrared
radiation. This explanation has some support from experiments, but
it's still a young theory that hasn't gained widespread acceptance
among scientists, he notes.
red granite is subjected to extreme crushing pressures
in the laboratory, as in this experiment conducted by
Freund and colleagues, its surface emits infrared radiation.
in rock might explain another curious observation: Scientists doing
research with magnetometers just before major earthquakes have serendipitously
recorded tiny, slow fluctuations in Earth's magnetic field. One
example happened during the Loma-Prieta earthquake that devastated
San Francisco in 1989. Almost 2 weeks before the quake, readings
of low-frequency magnetic signals (0.01-0.02 Hz) jumped up to 20
times above normal levels, and then spiked even higher the day of
The cause of these
signals is unknown. In addition to Freund's idea, theories include
the movement of deep, ion-conducting groundwater into cracks opened
up by the crushing of rocks, electromagnetic energy released by
electrons that are sheered from crystalline rocks such as granite,
and a piezo-magnetic effect triggered by pressure applied to certain
kinds of rocks.
A company called QuakeFinder
is hoping that these faint magnetic signals (typically less than
1 nanotesla) can be detected by a satellite in low-Earth orbit.
Ground-based sensors can detect these fluctuations as well, but
polar-orbiting satellites have the advantage of covering most of
the Earth's surface each day.
On June 30, 2003, Quakefinder launched QuakeSat.
Measuring only 4 in. x 4 in. x 12 in., the satellite will operate
for a year to see whether it can sift out magnetic signals generated
by tectonic activity. The first six months of the mission will be
spent calibrating the satellite and gathering baseline data. After
that ground controllers will be looking in earnest for quakes.
Both the infrared and
magnetic methods of quake detection are controversial. For now InSAR
seems to be a safer bet for earthquake forecasting. All three, however,
offer a tantalizing possibility: Someday the local weather report
will forecast not only of the storms above us, but also the ones
brewing beneath our feet.
Here to read a special Earthquake Fact File