As you might imagine, installing seismic sensors on the ocean floor isn't an easy task. Recently, however, scientists were able to detect seabed seismic activity using something that was already down there – a fiber optic telecommunications cable.
The project involved researchers from the University of California-Berkeley, the Lawrence Berkeley National Laboratory, the Monterey Bay Aquarium Research Institute, and Texas-based Rice University. Over a four-day period, they utilized approximately 20 km (12 miles) of a 52-km (32-mile) cable that had been laid in 2009 along the bottom of California's Monterey Bay. That cable connects an oceanographic-data-gathering "science node" to a lab at the Aquarium.
A photonic device was used to first send short pulses of laser light down the cable, and then detect the backscattering of that light, which was created by strain in the cable that was caused by stretching. It was possible to measure that backscatter once every 2 meters (6.6 ft), essentially turning the 20 kilometers of cable into 10,000 separate sensors. The science node at the end of the cable wasn't involved in the process, which is known as Distributed Acoustic Sensing.
The scientists were thus able to detect a magnitude 3.4 earthquake that occurred 45 km (28 miles) inland near the city of Gilroy, California, plus they accurately detected both "steady-state" and storm waves on the surface. Additionally, the technology allowed them to map previously unmapped submarine fault zones within the San Gregorio Fault system.
It is now hoped that their findings could be applied to the already-installed worldwide network of undersea telecommunications cables, greatly boosting the amount of seismic data that can be gathered – perhaps even providing earlier warnings of earthquakes.
"We have huge knowledge gaps about processes on the ocean floor and the structure of the oceanic crust because it is challenging to put instruments like seismometers at the bottom of the sea,” says UC Berkeley's Prof. Michael Manga. "This research shows the promise of using existing fiber optic cables as arrays of sensors to image in new ways."
A paper on the research – which was led by UC Berkeley grad student Nate Lindsey and Rice's Prof. Jonathan Ajo-Franklin – was recently published in the journal Science.
Sources: UC Berkeley, American Association for the Advancement of Science
