Optical fiber cables can sense aftershocks

Scientists have switched communication cables under Tangshan, China, into a rapidly expanding aftershocks monitoring network following the 5.1-magnitude earthquake in 2020.

Written by Maria C. Hoskins, Ph.D., science writer (DrMCHoskins)

Quotation: Hoskins, MC, 2021, Optical Fiber Cables Can Sensing aftershocks, Tumblr, http://doi.org/10.32858/temblor.170

Scientists have recently begun to explore whether fiber optic cables for the Internet can be used as seismographs. In a study presented at the American Seismological Society’s annual meeting last week, a team of researchers converted a fiber-optic cable into a fast-spreading high-intensity seismic network to monitor aftershocks in the aftermath of a major earthquake.

On July 12, 2020, Tangshan, China, and neighboring cities were hit by a 5.1-magnitude earthquake, the strongest earthquake in the region in fourteen years. Xiangfang Zeng, a seismologist with the Innovation Academy of Precision Measurement Sciences and Technology, CAS, Wuhan, China, and colleagues worked with a local telecom company to use a 4.7-mile (7.6 km) fiber-optic internet cable to monitor aftershocks. . Over the course of seven days, the team detected twice as many aftershocks as the permanent seismic network.

Optical fiber strings are similar to the optical fiber strings for the Internet. Fiber-optic cables have recently been used in seismic studies. Credit: Compare Fiber, Unsplash

Seismology using optical fibers

“Optical fibers are designed to act as ideal transmitters of light,” explains Ariel Lelouch, a seismologist at Stanford University who was not involved in the new study. The information is converted into light and transmitted over the cable. Imperfections in optical fibers cause some light to be reflected back to its source in a fixed pattern. When an earthquake strikes soon, the cable is stretched or compressed, changing the pattern of the reflected light. By sending encoded laser pulses onto unused fibers in the cable – so-called “dark fibers” – researchers can calculate the amount of stretch or pressure at various points along the cable and determine whether or not an earthquake has occurred. This method is called “distributed acoustic sensing” (DAS) and has recently gained traction in the seismology community as a potential complement to conventional seismometric networks.

Rapid spread of aftershocks

Aftershocks following a large earthquake provide important information to better understand earthquake hazards in the area. “The aftershocks will decrease very quickly … for example, you might get thousands of aftershocks in the first day after the main shock, but you might only get hundreds on the second day and get tens on the third day,” Zeng says. So the time taken to deploy the network is critical. Conventional seismometers can take days to weeks to deploy after a major earthquake, when communications cables are already in place.

After last year’s Tangshan earthquake, within half an hour of tapping the communications cable, researchers converted the entire cable into a seismic array, equivalent to 3,800 sensors 6.5 feet (2 meters) apart. The distribution of aftershocks and high-resolution vibration maps computed from this dense cluster correspond to a previously known error below Tangshan.

Researchers exploit a communications cable signal. Credit: Xiangfang Zeng

Potential for improved seismic monitoring

The high-resolution capabilities of the DAS, combined with the communications infrastructure already in place in urban areas – where seismometers are difficult to install – make DAS a promising addition to earthquake monitoring tools, says Veronica Rodriguez Tripaldus, a seismologist researching DAS at Lawrence Berkeley. The national laboratory, which did not participate in the study. While DAS shows promising potential for improving seismic detection, challenges remain. Low signal quality plagues DAS. This can be compensated for by having a large number of signals, but this is a challenge of its own. Zeng says the DAS system generates an enormous amount of data. “We need new tools, for example, machine learning, to deal with it.”

The data from the DAS arrays differs from the data collected by conventional seismometers, so the data is used separately at this point. “I hope we can combine the permanent seismic network with the DAS group,” says Zeng. According to Rodriguez Tripaldus, research is underway to do this. Lellouch and Rodríguez Tribaldos both say that DAS can complement traditional networks and have the potential to extend monitoring in the periphery, where the installation and maintenance of conventional seismometers is particularly expensive, but where fiber-optic cables have already been installed.

Lelouch says monitoring DAS applications goes beyond earthquakes. DAS is already routine in some subsurface imaging applications, including hydrocarbon exploration, and is currently used in some places to monitor carbon dioxide sequestration. Rodríguez Tribaldos suggests that some applications could go hand in hand with earthquake monitoring with DAS. “With one cable and one set of data, you can do a lot of different things.”

Zeng, Lellouch, and Rodríguez Tribaldos all say that despite the challenges, fiber-optic seismology has a promising future for seismology. Zeng says he hopes people can support cooperation between seismologists and telecom companies. “We also hope we can get more data and we can [can] Getting a better understanding of earthquakes, which may help reduce earthquake risks. ”

References

Zeng, X., Bao, F., Lin, R., Wang, S., LV, H., Song, Z., 2021, A Fast Aftershock Monitoring Network with DAS and Internet Fiber-Optic Cable in Tangshan, China, SSA Annual Meeting.

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