Caltech seismologists use underwater cables to detect earthquakes

Seismologists at Caltech have worked with optics experts from Google to develop a new method to detect earthquakes in the ocean. Together, the team developed a method that allows the use of existing underwater telecommunications cables to detect earthquakes. They believe the new technique could lead to improved earthquake and tsunami warnings around the world.

Currently, there is a vast network of fiber-optic cables lying on the bottom of the oceans around the earth. In all, the network of fiber-optic cable spans more than a million kilometers. These cables are the backbone of international telecommunications and Internet services. Researchers have been looking for a way to use submerged cables to monitor seismic activity.

Previous efforts to use the cables to measure seismic activity focused on using "dark fibers" that are not actively being used. Caltech researchers have now devised a way to analyze light traveling through "lit" fibers in existing and functional submarine cables to detect earthquakes and ocean waves without needing additional equipment. The technique can convert the majority of submarine cables into geophysical sensors thousands of kilometers long.

Researcher Zhongwen Zhan says the team believes this is the first solution for monitoring seismicity on the ocean floor that could be feasibly implemented globally. He believes it complements the existing network of ground-based seismometers and tsunami-monitoring buoys, allowing for easier detection in many cases. These cables use lasers that send pulses of information through glass fibers bundled inside the cables to deliver data at speeds over 200,000 kilometers per second.

Operators monitor the polarization of light that travels within the fibers because laser light has an electric field that oscillates in one direction. Controlling the direction of the field allows multiple signals to travel through the same fiber simultaneously. The team focused on the Curie Cable that stretches more than 10,000 kilometers along the Pacific Ocean's eastern edge, spanning Los Angeles to Valparaiso, Chile. The team looks for sudden and rapid changes in polarization, which happens during earthquakes and windstorms that produce large waves. Those rapid changes allow researchers to identify events in the data. Polarization can be measured as often as 20 times per second, making the new method much faster than existing seismometers.