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Seafloor Seismology Illuminated Using “Dark” Fiber-Optic Cables

fiber optic on bay floor map
Map of MARS cable in Monterey Bay with the pink illuminated portion used for sensing. | Lindsey et al., Science (2019)

A new fault system on the seafloor was discovered off California's coast by
temporarily transforming a pre-existing underwater fiber optic cable into
an array of nearly 10,000 seismic sensors, according to a new
study published in the Nov. 29 issue of Science.

Distributed Acoustic Sensing — DAS for short — is a new method of
fiber-optic sensing that uses pulses of laser light to detect slight
movements along an optical fiber cable. Through the study, the authors
showcase the potential of leveraging the extensive web of unused subsea
optical fiber telecommunications cables — also known as "dark fiber" —
already spanning the ocean's floor to monitor and record oceanographic and
seismic processes in unprecedented detail as they occur in the most remote
and difficult to study environments on Earth.

University of California Berkeley researcher Nate Lindsey, the lead author
of the study, and his colleagues temporarily repurposed an undersea
fiber-optic cable to collect DAS measurements across the continental shelf
of California's Monterey Bay. The 32-mile long cable, part of the Monterey
Accelerated Research System (MARS), usually carries data to and from an
undersea network of oceanographic instruments and the observatory on shore.

The researchers used DAS to convert the MARS observatory's decade-old
communication cable into a dense array of thousands of seismic sensors —
spaced only meters apart and spanning nearly 12.5 miles seafloor.

"This is extraordinarily dense when compared with a current seismological
experiment, which will be composed of arrays of single seismometer stations
and spaced anywhere from 100 meters to 100 kilometers apart for various
applications," said Lindsey.

Over the four-day duration of the study, the authors were able to map a
previously unknown fault system and observe several dynamic tidal and
storm-driven processes in the water column above. By chance, the
researchers detected a weak local earthquake during the study. The
temblor's seismic energy revealed several seafloor faults as it traveled
along the length of DAS cable. While some matched the locations of known
faults, others had not yet been identified.

"That was exceptionally lucky. Not only to record a local magnitude 3.4
inside the four-day window, but also to have crossed seafloor faults and
see them lit up as a result of the earthquake energy," said Lindsey.

Deep below the surface, tectonic forces fracture and fold the planet's
crust. When rocks at the surface succumb to the strain of the unrelenting
geological processes beneath, they break and move, forming faults along the
fractures. Like geological scars, the surface of Earth is striated with
fault lines. The largest and most lively — where rocks are actively
snapping and shifting — are responsible for triggering destructive seismic
events. Valuable geothermal energy and mineral deposits like oil and gas
are often found along these seams, too.

However, charting Earth's fault zones is challenging and many remain
unknown, particularly those that lie miles deep on the bottom of the ocean.
As a result, what we know about deep-sea geophysical processes remains
incomplete and the potential for offshore seismic hazards — the
earthquakes, volcanic eruptions, landslides and tsunamis that threaten
coastal populations — is not fully understood.

Fault properties, such as their length, orientation and rate of movement,
can inform scientists about fault activity including their likelihood of
causing ground-shaking earthquakes. But these data are rare and incomplete
for most ocean faults, said Lindsey, even for faults that are known.

"Even offshore California, where the U.S. Geological Survey has conducted
some of the most detailed seismic mapping anywhere on the planet, there are
still many unmapped faults and a need to characterize these fault zones,"
said Lindsey.

In 2011, a fault underlying the Japan Trench released centuries of built-up
tectonic stress and triggered the Tohoku earthquake, which originated below
the North Pacific Ocean. The earthquake devastated northeastern Japan and
led to tsunamis that damaged coastal communities throughout the Pacific.
The magnitude 9.1 event was unexpected. Despite the quake-prone nation's
extensive seismic hazard maps, it was not known that the fault would — or
could — produce such a powerful earthquake.

Like Japan, the faults offshore on the West Coast of North America carry
similar seismic risks. In California, major cities like San Francisco and
Los Angeles are built alongside dense networks of active faults and are
threatened by the highest levels of earthquake risk in the nation.

"To put some real numbers on the situation for context, there are currently
252 seismometers [worldwide] in the [National Science Foundation's]
ocean-bottom seismometer pool," said Lindsey. However, the instruments are
temporary due to their limited battery life and storage space, and the
number of continuously operating seafloor seismometers along the Pacific
coast is fewer than a dozen.

While these and other remote sensing systems are helping to fill the gaps
in our understanding of offshore faults and seismic hazards, coverage
remains spotty and instruments are difficult and often expensive to deploy.

"There are many different opportunities using DAS, including offshore
seismic hazard analysis, studying seafloor fault properties, recording
small earthquakes offshore, offshore earthquake early warning, studying and
monitoring coastal erosion, and understanding how ocean waves convert and
dissipate energy at the coasts," said Lindsey.

According to Lindsey, applying the technique to other high hazard regions
is a priority for the next stages of the research, as well as recording DAS
data alongside other types of sensors and in different marine environments
and depths.

[Credit for associated photo: Lindsey et al., Science (2019)]