Fiber-optic cables can sense aftershocks

Scientists transformed telecom cables beneath Tangshan, China, into a rapidly deployed aftershock monitoring network following the 2020 magnitude-5.1 earthquake.
 

By Mariah C. Hoskins, Ph.D., science writer (@DrMCHoskins)
 

Citation: Hoskins, M. C., 2021, Fiber-optic cables can sense aftershocks, Temblor, http://doi.org/10.32858/temblor.170
 

Scientists have recently started exploring whether internet fiber-optic cables can be used as earthquake monitors. In a study presented at the Seismological Society of America’s annual meeting last week, a team of researchers turned a fiber-optic cable into a rapidly deployed, high-density seismic network to monitor aftershocks following a major earthquake.

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

 

Fiber-optic seismology

“Optical fibers are designed to be ideal transmitters of light,” explains Ariel Lellouch, a seismologist at Stanford University, who was not involved in the new study. Information is converted into light and transmitted along the cable. Imperfections in optical fibers cause some light to be reflected back to its source in a consistent pattern. When an earthquake strikes nearby, the cable is stretched or compressed, changing the pattern of the reflected light. By sending encoded laser pulses on unused fibers in the cable — so called “dark fibers” — researchers can calculate the amount of stretching or compressing at different points along the cable and determine whether an earthquake has occurred. This method is called “Distributed Acoustic Sensing” (DAS) and has recently gained traction in the seismology community as a potential supplement to traditional seismometer networks.
 

Rapid aftershock deployment

Aftershocks following a large earthquake provide critical information to better understand earthquake hazard in the area. “The aftershock occurrence will decrease really fast … For example, you maybe get thousands of aftershocks the first day after the mainshock, but you maybe only get hundreds the second day and get tens the third day,” says Zeng. So the time it takes to deploy a network is critical. Traditional seismometers could take days to weeks to deploy after a major earthquake, whereas telecom cables are already in place.

Following the Tangshan earthquake last year, within half an hour of tapping into the telecom cable, the researchers turned the entire cable into a seismic array, equivalent to 3,800 sensors with 6.5-foot (2-meter) spacing. The aftershock distribution and high-resolution shaking maps calculated from this dense array are consistent with a previously known fault beneath Tangshan.
 

 

Potential for improving earthquake monitoring

The high-resolution capabilities of DAS, along with the telecom infrastructure already in place in urban areas — where it is difficult to install seismometers — makes DAS a promising addition to earthquake monitoring tools, says Verónica Rodríguez Tribaldos, a seismologist researching DAS at Lawrence Berkeley National Lab, who was not involved in the study. While DAS shows promising potential for improving earthquake detection, there remain challenges. Low signal quality plagues DAS. This can be offset by having a large number of signals, but that presents its own challenge. DAS generates a huge amount of data, says Zeng. “We need new tools, for example, the machine learning to handle it.”

Data from DAS arrays are different from data collected by traditional seismometers, so at this point the data is used separately. “I hope that we can combine the permanent seismic network and the DAS array,” says Zeng. Research to accomplish that is underway, according to Rodríguez Tribaldos. Lellouch and Rodríguez Tribaldos both say that DAS could compliment traditional networks and has the potential to expand monitoring in the ocean, where installing and maintaining traditional seismometers is particularly expensive, but where fiber-optic cables are already installed.

Monitoring applications of DAS go beyond earthquakes, says Lellouch. DAS is already routine in some applications of subsurface imaging, including hydrocarbon exploration, and is currently used in some places for CO₂ sequestration monitoring. Rodríguez Tribaldos suggests that some applications could go hand-in hand with earthquake monitoring using DAS. “With one cable and one dataset, you can do many different things.”

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

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.