Science - USA (2021-12-10)

dered down the slope, the fiber detected

internal structures in the flows, including

“roll waves” produced by instabilities in

their dense flowing core. “You see waves

that had only been predicted theoretically,”

Fichtner says. “You see them immediately.”

Buoyed by these results, the team shifted

its sights to one of Iceland’s largest and

most dangerous volcanoes, Grímsvötn,

which is buried beneath an ice cap but

monitored only by a lone seismic station.

It last erupted for 4 days in 2011, blowing

a hole in the ice cap and lofting ash into

the stratosphere that grounded hundreds

of transatlantic flights. As global warming

accelerates melting of the ice cap, it could

uncork Grímsvötn; the declining weight

could allow pressurized magma to more

easily fracture its rock jailhouse and escape.

The researchers wanted to know whether

Grímsvötn was growing more restless.

That led Fichtner, Thrastarson, and

others to make the 2-day trek to the cal-

dera in April. Using a sled weighed down

by an oil barrel and towed by a snowcat,

they plowed a half-meter trench in the ice

and snow, burying a 12-kilometer-long ca-

ble that circumscribed the caldera before

bending into the center. They left the in-

strument box running for several months,

beaming its data back. Fichtner was aston-

ished at the upheaval they detected: some

1800 small earthquakes within 10 kilo-

meters of the caldera, more than 10 times

the number seen by the seismometer.

They also picked up a distinct hum,

smoother than any known kind of volca-

nic tremor. Their best guess is that it is a

resonance, generated as tremors ring the

300-meter-thick ice cap like a bell. If so,

changes in the hum could signal changes

in ice thickness, or provide warning of in-

creased volcanic activity, Fichtner says.

Fichtner isn’t alone in his focus on fro-

zen worlds. In 2016, Ajo-Franklin led a

team to Fairbanks, Alaska, to see whether

fibers could monitor permafrost, the sub-

surface layer of frozen soil, chock full of

preserved organic matter, that is threat-

ened by climate change across much of

the Arctic. They laid a 4-kilometer-long

fiber cable in a crisscrossing array at a

military research site. More than 100 small

borehole heaters warmed the surround-

ing soil, and an instrument that wobbles

like a badly loaded washing machine set

off vibrations at the surface. The resulting

data—hundreds of terabytes, flown back in

hard drives in students’ luggage—showed

the fibers could indeed detect thawing:

Meltwater significantly slowed the speed

of the seismic waves. The study, to be pub-

lished this year, raises the possibility of

using dark fiber and ambient earthquake

waves to track permafrost thaw—which

could inform projections of how much car-

bon these thawing soils will release with

continued Arctic warming.

TWO YEARS AGO, a 7.1 magnitude earthquake

struck Ridgecrest, California, 180 kilometers

north of Los Angeles—the most powerful

quake to hit California in 20 years. Zhongwen

Zhan, a seismologist at the California Insti-

tute of Technology, was elated. He knew

Ridgecrest would ring for days with after-

shocks; by deploying a fiber array to detect

and analyze them, he could learn about

other earthquake hazards. The local tele-

com helped his team hook up four boxes

on unused fibers, including one stretching

8 kilometers under the town. In just a few

days, the team had installed the equivalent

of thousands of seismic sensors. And then

they watched while the ground continued

to shake.

On a traditional seismic hazard map,

Ridgecrest was a single pixel, the whole

area lumped together with the same risk.

The aftershocks caught by the fiber ar-

ray revealed the drastic variations within

that pixel, with one side of the town shak-

ing three times more strongly than the

other. Using a different set of vibrations—

mini–seismic waves from traffic—to image

the shallowest parts of subsurface, Zhan

and colleagues found that the shaky side

of town sat on far looser sediments, a risk

unknown to residents. The array also identi-

PHOTO: CREDIT GOES HERE AS SHOWN; CREDIT GOES HERE AS SHOWN

Utility

station

Earthquake

Fiber optic cable

Individual

glass fiber

Trac noise

Seismic waves

shift a defect

Laser pulse

Backscattered pulse

Defect

Scattered

light

Good vibrations

Fiber optic cables don’t just carry data from sensors; now, they are the sensors. By offering the equivalent of a

seismometer every meter or so along their length, fibers portend a cheaper way to study motions on Earth.

Earthquakes

Fibers not only locate distant quakes,

but also reveal the composition

and structure of surrounding rock

“illuminated” by seismic waves.

Volcanoes

In regions that are barely monitored

by traditional seismometers, fibers

can be used to detect the small tremors

that often foreshadow eruptions.

Cryosphere

Fibers have revealed hidden motions in

glaciers and avalanches. They could

also be used to detect the timing and

extent of thawing permafrost.

Oceans

Offshore cables have detected storms,

ship traffic, and acoustic waves that

probe temperatures in the deep

ocean—a way to assess its capacity to

buffer climate change.

City monitoring

Shifts in traffic and footfalls have been

correlated to pandemic lockdowns.

The data could also be used to track

commercial activity.

Dark fiber

Fiber optic cables are bundles

of glass fibers, each no thicker

than a human hair. Telecoms often bury them

with extra capacity. This unused “dark” fiber

can be tapped cheaply and safely, without

interrupting existing data streams.

In the firing line

When seismic waves cross a fiber, they

displace defects in the fiber by nanometers—

creating a shift in the backscattered pulse of

laser light.

SCIENCE science.org 10 DECEMBER 2021 • VOL 374 ISSUE 6573 1315

GRAPHIC: C. BICKEL/

SCIENCE

A world of noise

Existing or newly deployed fibers have much

to listen to.