> Photonic seismology
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> The technique the researchers use is Distributed Acoustic Sensing, which employs a photonic device that sends short pulses of laser light down the cable and detects the backscattering created by strain in the cable that is caused by stretching. With interferometry, they can measure the backscatter every 2 meters (6 feet), effectively turning a 20-kilometer cable into 10,000 individual motion sensors.
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> During the underwater test, they were able to measure a broad range of frequencies of seismic waves from a magnitude 3.4 earthquake that occurred 45 kilometers inland near Gilroy, California, and map multiple known and previously unmapped submarine fault zones, part of the San Gregorio Fault system. They also were able to detect steady-state ocean waves — so-called ocean microseisms — as well as storm waves, all of which matched buoy and land seismic measurements.
https://news.berkeley.edu/2019/11/28/underwater-telecom-cables-make-superb-seismic-network/
In: Engineering
Telecom cables are big bundles of fiber optic cables. Laser light travels down the fibers to transmit data. If you stress the cable it changes how the light travels and a small but measurable amount will bounce back to the sender, exactly how much depends on the forces acting on the cable.
The researchers were clever in the shape and timing of the laser signal sent down the fiber and this allowed them to gather a lot of data about the ocean around the cable over very long distances.
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