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1314 10 DECEMBER 2021 • VOL 374 ISSUE 6573 science.org SCIENCE
is really everywhere, and that is a game
changer,” says Biondo Biondi, a geophysicist
at Stanford University. “We can have seis-
mic sensors everywhere on the cheap.”
For the past few years, many fiber
measurements were mere proofs of prin-
ciple. But the field is now maturing, says
Andreas Fichtner, a seismologist at ETH
Zürich and Thrastarson’s adviser. Fiber is
revealing previously unknown earthquake
faults, the hidden mechanics of glaciers
and avalanches, and volcanic gurglings
that could aid in predicting eruptions. “It’s
getting beyond the hype, to where people
start doing science with it,” Fichtner says.
LIKE MANY BREAKTHROUGH scientific tech-
niques, fiber sensing has its origins in
U.S. military research. Beginning in the
1980s, the Navy towed fiber optic cables
behind ships to sense the sounds of enemy
submarines—one reason the technique is
still sometimes called distributed acoustic
sensing (DAS). By the late 2000s, the oil
and gas industry was lining its pipelines
and boreholes with fiber. Technicians used
backscattered laser light to look for sharp
temperature changes—a sign of a ruptured
well or pipe—or to detect artificial seismic
waves from air guns on the surface, in or-
der to probe the structure of the surround-
ing rock. Today, the fracking industry uses
borehole fibers to monitor rock fracturing
and the microearthquakes caused by the
high-pressure injection of water.
By the mid-2010s, academic scientists
were adopting the technique. A pioneer-
ing effort came in 2015, when a team of
scientists at the German Research Centre
for Geosciences (GFZ Potsdam) took ad-
vantage of unused fibers on a 15-kilometer
cable connecting two geothermal power
plants in Iceland. Philippe Jousset and his
colleagues were able to not only detect dis-
tant earthquakes, but also locate the rup-
ture sources by measuring differences in
the arrival times of earthquake waves on
either side of small bends in the cable.
In California, meanwhile, Eileen Martin,
then a student with Biondi, was running
a 2.5-kilometer loop of fiber in utility tun-
nels under Stanford. In that hushed milieu,
the array picked up not only earthquakes,
but also the vibrations of traffic, footfalls—
even waves on the ocean. It only went
offline once, Martin adds, when someone
jostled the fiber in the computer room.
The nascent field’s biggest splash came
when Ajo-Franklin and colleagues tapped
a fiber the Monterey Bay Aquarium Re-
search Institute runs off the California
coast to undersea instruments. They de-
ployed their interrogator box in 2018,
during a 4-day maintenance shutdown of
the undersea instruments, and detected
a small earthquake that struck California
at the time. More interesting was what
the arrival times of the earthquake waves
revealed in the surrounding rock: a previ-
ously unknown fault zone under the cable,
just 10 kilometers off the coast. The finding
showed that fiber could detect unknown
earthquake threats. For fiber, says Martin,
now at the Colorado School of Mines, “thatwas the single most convincing case any-
body has made.”
Fichtner had specialized in using data
collected by others to image the planet’s
deep interior. But in DAS, he saw a po-
tential paradigm shift in the making. The
promise of the technique turned him into
a field seismologist. He decided to target
remote, frozen environments where tradi-
tional seismometers are especially expen-
sive and difficult to deploy.
Those efforts started close to home with
the Rhône Glacier in the Swiss Alps, where
15 square kilometers of ice is retreating in
the face of global warming. In 2019, near
the front of the glacier, Fichtner’s group
laid a kilometer-long cable in the shape of
triangle, covered it with snow, and left it
in place for 1 week. Despite the short de-
ployment, the fiber captured a set of fast
motions that were traced to occasional
icequakes—previously seen only on ice
sheets, not smaller glaciers. The fiber tri-
angle traced the source of the quakes to
the same slip patch—suggesting some parts
of the glacier bed behave almost like earth-
quake fault zones. “This changes our under-
standing of how glaciers move significantly,”
Fichtner says.
In January, the researchers studied a far
faster flow of snow and ice: avalanches.
They attached their box to a cable run-
ning up a snow-draped mountain in south-
western Switzerland at the Sionne Valley
avalanche test site, where researchers use
helicopter-dropped explosives to trigger ar-
tificial avalanches. As the avalanches thun- PHOTOS: (LEFT TO RIGHT KRISTÍ N JÓ NSDÓ TTIR; HILDUR JONSDOTTIRAt a new, unnamed volcano near Reykjavík, Iceland (left), fiber has revealed noise that may correspond to eruptive bursts. Along the rim of Iceland’s ice-capped Grímsvötn
volcano (right), researchers buried fiber with a weighted sled—and heard a mysterious hum that could be magma-fueled tremors ringing the ice cap like a bell.