Science - USA (2021-12-10)

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1316 10 DECEMBER 2021 • VOL 374 ISSUE 6573 science.org SCIENCE

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fied faults that were misplaced or missing
in geological maps. “We were able to see it
all,” Zhan says.
In the future, Zhan hopes such a rapid
response won’t be necessary, because DAS
arrays exploiting unused telecom fibers
will be integrated into the permanent seis-
mic networks that continuously monitor
earthquake-prone regions and generate
early warnings. Those telecom fibers will
be especially useful in remote regions and
offshore, where traditional seismic sensors
are expensive to establish and maintain.
“DAS really shows strong potential for de-
tecting earthquakes earlier than conven-
tional sensors,” he says.
Offshore fibers could detect not just
earthquakes, but also shifting pressures
from tides and currents. Add those capa-
bilities to the standard instruments that
are usually attached to ocean cables, and
you have a new standard for ocean ob-
servation, says Charlotte Krawczyk, geo-
physicist at GFZ Potsdam. “This will be a
new kind of global monitoring,” she says.
Its reach may extend far from shore.
Last year, researchers set a new record for
DAS length with a next-generation interro-
gator that detected vibrations at a distance
of 120 kilometers on a fiber cable running
off Svalbard, the world’s northernmost in-
habited island. The team, led by Martin
Landrø at the Norwegian University of Sci-
ence and Technology, sensed earthquakes
from the Mid-Atlantic Ridge, storms, ocean
swells, ship traffic, and the lonesome calls
of blue and fin whales, raising hopes for an
unobtrusive way of monitoring the mam-
mals, Landrø says. “These are numbers
we’d like to get a better grip on.”
And in 2020, a seafloor cable deployed
from the Canary Islands picked up water-
borne sound waves from an earthquake
that shook the ocean floor. Zhan was part
of a team that used such ocean sound
waves, which travel faster in warmer wa-
ter, to measure the ocean’s temperature
change over time. If fibers start to pick up
these waves regularly, it would be “amaz-
ing,” he says—a way to identify where the
deep ocean is heating up, and whether its
capacity to absorb 90% of the heat from
global warming is diminishing.
Fiber has made inroads on ice, land, and
water. Tieyuan Zhu, a geophysicist at Penn-
sylvania State University (Penn State), Uni-
versity Park, thinks ground-based fibers
can even be used to study the air. With a
small array at Penn State, he has shown
how they can capture and locate thunder
while singling out lightning strikes, which
shake the ground in a way that’s distinct
from thunder. They can also distinguish
pelting rain from gusty winds. In June at

the nearby Shale Hills Critical Zone Ob-

servatory, he and his colleagues deployed

a fiber array to probe how storms, growing

more severe with global warming, deepen

the ground’s “weathering layer”—the tens

of meters or so of soil and rock that sit

above bedrock. Zhu says the fiber easily

heard the sounds of rain soaking into the

weathering layer from the remnants of

Hurricane Ida, which passed through the

eastern United States in September. “We

saw a beautiful signal variation.”

FIBER IS POISED to break out in science, but

its potential in the applied world may be

even greater, says Nathaniel Lindsey, a geo-

physicist at FiberSense, a startup in Cali-

fornia targeting city-sensing applications.

Already, small university arrays, sensitive

to car traffic and human footsteps, have

witnessed the stark falloff in activity be-

cause of the coronavirus pandemic, and its

resumption as lockdowns are lifted. They

can detect fires, landslides, and all sorts

of natural threats. Fibers laid into bridges

and buildings can detect when infrastruc-

ture is close to failure, and they could

replace the motion sensors and cameras

used to monitor borders and fences. Unlike

cameras, DAS “doesn’t care if it’s pouring

or there’s fog,” Biondi says. The next step

will be getting a city to allow a large, sus-

tained network of DAS arrays. In San Jose,

California, Biondi is targeting just that,

pitching the city on opening up unused fi-

bers in its telecom network for monitoring.

The downside to fiber is the staggering

amount of data it produces. In the past,

seismologists could gather data from their

isolated, widely spaced instruments and

crunch it later, on a laptop. The thousands

of distinct “sensors” along a single fiber, in

contrast, can gather hundreds of terabytes

of data in a matter of days. Long-term ar-

rays will produce petabytes, requiring pro-

cessing in real time, Fichtner says. “Then

we can’t store our data anymore,” he says.

“We will have to learn how to do science

on the fly.” Many labs are developing arti-

ficial intelligence algorithms to efficiently

sift through the data and look for patterns.

Fichtner’s lab already has a data

problem—it has done so many deployments

that it doesn’t have enough researchers to

work through the collected data. They’ve just

started to tease apart data captured from

the unnamed Icelandic volcano. One mys-

tery they have yet to unravel: low-frequency

pulses of activity every few seconds, which

could correspond to small eruptive bursts or

the collapse of bubbles in the magma.

Despite the data deluge, Fichtner’s team

isn’t stopping. The researchers just wrapped

up a monthlong deployment Greece, where

they tapped 30 kilometers of cable running

through the north of Athens, overnight add-

ing 10,000 seismometers to the 20 in the

region, hoping to capture hidden faults.

In October, they interrogated a submarine

cable that connects the islands of Santo-

rini and Ios. It runs right past Kolumbo, a

large, mysterious underwater volcano that

last erupted in 1650 but has shown worrying

signs of activity.

With the fibers watching, Fichtner says,

Kolumbo is putting on a show. The vibra-

tions just keep coming. j

A ship off Chile’s coast lays a cable that connects to California. Offshore fiber has revealed storms and swells.