21 April 2022–Unused telecommunications fiber could form the backbone of earthquake early warning systems that complement existing warning systems and fill in critical gaps such as offshore detection, Robert Mellors said at the Seismological Society of America’s Annual Meeting.
Distributed acoustic sensing, or DAS, uses the tiny internal flaws in a long optical fiber as thousands of seismic sensors. An instrument called an interrogator at one end of the fiber sends laser pulses down the cable that are reflected off the fiber flaws and bounced back to the instrument. When the fiber is disturbed in any way, researchers can examine changes in the timing of the reflected pulses to learn more about the resulting seismic waves.
Mellors, a research scientist at the University of California, San Diego, discussed two recent tests of DAS as an addition to the earthquake early warning toolkit. Mellors and his colleagues looked at a strong ground motion dataset recorded 80 meters from a buried chemical explosion, and data collected about 12 kilometers from the epicenter of a magnitude 5.3 earthquake in Calipatria, California.
Their investigations suggest DAS could be “a cheaper way to get more seismic stations that are closer together for better results” in earthquake early warning, he said. “But there is still a fair amount of work that needs to be done before it can be one of the data sources that would go into an existing system.”
One of the first big hurdles, Mellors said, is to “make absolutely sure that we can distinguish between a magnitude 6 and a magnitude 3 quickly.”
The low cost of using telecom fiber in challenging environments such as urban areas or offshore, where it would be difficult to deploy traditional stations, are part of what make DAS earthquake early warning appealing, Mellors said. But more work needs to be done to figure out how to collect DAS data and convert it to a form that would support existing warning methods.
For instance, the high spatial density coverage of DAS, with its thousands of seismic sensors, could provide information for earthquake early warning methods that rely on estimates of the location of large ground motions. Offshore DAS deployments could provide data on earthquake magnitude and epicenter locations for source-based warning, Mellors noted.
Another challenge is with the equipment itself, he said. DAS interrogators are often designed for other uses, and may not be configured to capture the high-amplitude signals needed for warning systems.
“Another challenge there is that each one of these DAS interrogators is producing several megabytes per second of data,” Mellors added. “You’re not going to be able to process that in an early warning system quickly, so you need to figure out some way of reducing the data but keeping the same information.”
In addition to using DAS data collected from earthquakes and explosions, as Mellors and colleagues did, researchers should place synthetic models of DAS data into existing earthquake early warning systems to see how they perform, he suggested.