SCIENCE science.org 10 DECEMBER 2021 • VOL 374 ISSUE 6573 1313
For several weeks in late spring, from a
small utility shack in an impromptu park-
ing lot plowed out of an old lava field,
Thrastarson and colleagues tapped the fi-
ber with an “interrogator”—a box that fired
laser pulses along the cable and recorded
the response. Every few days, a team mem-
ber drove in from Reykjavík to retrieve the
data set. From the comfort of their cozy
homes, the researchers sifted through the
data for clues to the interior fluctuations
driving the restive volcano. It was about
as easy as volcanology can get. The hard-
est part, Thrastarson says with a laugh,
was fending off tourists who thought their
shack was a bathroom.
The demands of the internet era have
draped the world in a web of fiber. Fiber
now connects neighborhoods and utilities,
traffic cameras and mass transit. For geo-
scientists, fiber had always been a means
to an end—a way to hook up a weather sta-
tion or an undersea pressure sensor. But
that has changed in the past few years.
Now, the fiber itself is the sensor, says
Jonathan Ajo-Franklin, a geophysicist at
Rice University. “We’re seeing data where
we’ve never seen data before.”
The principles of fiber optic sensing are
relatively simple. The cables are bundles
of glass fibers, each no thicker than a hu-
man hair, that carry information encoded
in light. Small, randomly oriented defects
within the fibers act like tiny mirrors, scat-
tering light. The interrogators—or boxes,
as most researchers call them—work much
like radars. They fire a laser pulse into
an unused fiber and record the pattern
of reflections coming back from defects
along the length of the cable. When an ex-
ternal pressure wave crosses a section of
the fiber—be it from an earthquake or afootfall—it stretches and squeezes the
defects. The reflections in that section
are displaced by nanometers, leading to
a phase shift in the rebounding light. By
firing thousands of pulses per second, re-
searchers can build up a picture of a pass-
ing seismic wave, at a distance of up to
100 kilometers or more along the fiber.
Unlike traditional seismometers, which
are spaced many kilometers apart, fiber of-
fers the equivalent of a seismometer every
meter or two along the cable. This density,
combined with the low cost and rugged-
ness of fiber, has prompted researchers to
lay cables on glaciers, volcanoes, perma-
frost, and earthquake fault zones—any
place the earth might crack or crunch,
grind or grate. They have also tapped into
unused “dark” fibers in existing telecom
cables to pick up vibrations from sources
as faint as pedestrians and cars. “This fiberIn 2019, researchers in the Swiss Alps brought
fiber to the foot of the Rhône Glacier (in the
valley behind the foreground ridge) and
detected motions that were traced to icequakes.