Crisscrossing the seafloor is an extensive web of optical-fibre telecommunication cables used for internet, television, and telephony. But it’s the capacity of these cables for sensing Earth’s tectonic plates that has researchers interested.
In a study described in the journal Science, 20 kilometres of unused cable off the coast of California was turned into the equivalent of 10,000 seismic sensing stations.
The researchers explored the tectonic plates deep below Earth’s surface that meet at fault lines where rocks are actively folding and shifting. These fault lines are responsible for triggering destructive earthquakes and tsunamis.
Mapping Earth’s fault zones is challenging, and many remain unknown. As a consequence, the potential for offshore natural disasters is not fully understood.
To resolve this, a team led by Nate Lindsey, from the University of California Berkeley, investigated the use of fibre-optic sensing that can measure physical qualities such as temperature, pressure, strain, voltages and acceleration.
They experimented using Distributed Acoustic Sensing (DAS), a type of fibre-optic sensing that uses pulses of laser light to continuously detect slight movements along the seafloor.
“There is a huge need for seafloor seismology,” says Lindsey. “Any instrumentation you get out into the ocean, even if it is only for the first 50 kilometres from shore, will be very useful.”
During their four-day experiment in Monterey Bay, the researchers recorded a 3.5 magnitude quake from underwater fault zones.
The study also quantified offshore seismic hazards such as fault lines of the San Gregorio Fault system, underwater volcanoes, steady-state ocean waves, and storm waves.
Study co-author Jonathan Ajo-Franklin says the ultimate goal is to use all of the active fibre-optic networks around the world. He estimates there’s more than 10 million kilometres of cables, allowing earthquake monitoring in regions previously inaccessible.
Before they can fulfill this goal, researchers need to show they can collect DAS measurements without interfering with the data that online fibre-optic cables are carrying.
They’ve now turned to conducting experiments with online fibres, while also planning fibre-optic monitoring of seismic events in a geothermal area in the Brawley Seismic Zone, south of Southern California’s landlocked Salton Sea.