Researchers have turned 20 kilometers of undersea fiber-optic cable into the equivalent of 10,000 seismic stations along the ocean floor. A 3.5 magnitude quake and seismic scattering from underwater fault zones were recorded. The technique, which had previously been tested with fiber-optic cables on land, could provide much-needed data on quakes that occur under the sea, where few seismic stations exist, leaving 70% of Earth's surface without earthquake detectors.
The cable stretches 52 kilometers offshore to the first seismic station ever placed on the floor of the Pacific Ocean, put there 17 years ago by the Monterey Bay Aquarium Research Institute. A permanent cable to the Monterey Accelerated Research System (MARS) node was layed in 2009, 20 kilometers of which were used in this test while offline for yearly maintenance.
This study on the frontier of seismology is the first time anyone has used offshore fiber-optic cables to look at these types of oceanographic signals or for imaging fault structures. The ultimate goal is to use the dense fiber-optic networks around the world — probably more than 10 million kilometers in all, on both land and under the sea — as sensitive measures of Earth's movement, allowing earthquake monitoring in regions that don't have expensive ground stations.
The technique 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. The systems are sensitive to changes of nanometers to hundreds of picometers for every meter of length — a one-part-in-a-billion change.
During the underwater test, the researchers 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, CA, 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.
To make use of the world's lit fiber-optic cables, the researchers need to show that they can ping laser pulses through one channel without interfering with other channels in the fiber that carries independent data packets.