Clever new technique turns underwater cables into earthquake detectors
As part of a project that could greatly speed up the detection of deadly tsunamis, Caltech seismologists and Google optical experts have come up with a method that turns operating submarine communication cables into earthquake detectors without using special equipment.
Earthquakes and tsunamis are some of the most destructive events humanity faces, they are also unpredictable. With little or no warning, a shift of the Earth's crust or an undersea landslide can inflict death and destruction on a horrific scale across an entire ocean basin.
For this reason, scientists and engineers are working on more sophisticated and comprehensive detection and early warning systems. A particularly attractive option would be to find a way to turn the Earth's vast network of submarine communication cables into a giant seismic network.
Though we live in a hyper-connected world where many people think it's all wireless networks and satellite internet, 99 percent of all overseas data transmission goes by undersea cables. Except for the high polar regions, the oceans are crisscrossed with cables, and more snake around the coasts.
In contrast, land seismometers and ocean seismic networks like the Deep-ocean Assessment and Reporting of Tsunamis (DART) system, which is run by the US National Oceanic and Atmospheric Administration's National Data Buoy Center, can provide only incomplete and remote coverage of ocean seismic events like earthquakes and tsunamis.
The big problem is that the shock waves can only move at the speed of sound, which means that there is very little warning in the event of a tsunami threatening a coastline. If submarine cables could detect such events, they would be much closer to the source and the warnings would flash ashore at the speed of light.
The idea of using submarine cables isn't new, but most techniques have required the installation of special equipment, using inoperative cables, or both. The Caltech/Google method relies on the fiberoptic cables that telecom companies have been laying down since the 1980s.
As part of their normal operation, the laser pulses used to transmit multiple channels of data are polarized. As these pulses reach the other end, they are routinely monitored. If the equipment is working properly and the cable is undisturbed, the laser pulses will remain properly polarized. If there is a malfunction or the cable is disturbed, the polarization changes. This means that seismic events can be monitored on a live cable with standard equipment.
By working with the Curie Cable, which runs along the west coast of North and South America from Los Angeles to Valparaiso, the team was able to analyze the polarization of the pulses along the cable at up to 20 times per second. Most of the time, this remained normal, but if there was an earthquake or large ocean waves, these caused a large, sudden change in the polarization, which the researchers could identify the source of. Over nine months, they detected 20 moderate-to-large earthquakes along the cable, with the largest being a 7.7 magnitude event off the coast of Jamaica.
The next step will be to develop a machine learning algorithm to allow the cables to be monitored automatically, and to ignore any disturbances caused by ships or crabs moving the cable.
"This new technique can really convert the majority of submarine cables into geophysical sensors that are thousands of kilometers long to detect earthquakes and possibly tsunamis in the future," says Zhongwen Zhan, assistant professor of geophysics. "We believe this is the first solution for monitoring seismicity on the ocean floor that could feasibly be implemented around the world. It could complement the existing network of ground-based seismometers and tsunami-monitoring buoys to make the detection of submarine earthquakes and tsunamis much faster in many cases."
The research was published in Science.