When it comes to detecting seismic activity, locations far from main fault lines are often inadequately sampled, making it difficult to research localized and minor natural phenomena. This undersampling is especially troublesome when examining the effects of oil and gas extraction, wastewater injection, and carbon dioxide sequestration on ground movement, all of which occur in places remote from seismically active areas.
Researchers repurposed nearly 8 kilometers of abandoned telecom fiber optic cable into a seismic array that detected scores of aftershocks missed by permanent seismic stations just days after a magnitude 5.1 earthquake in Tangshan, China, in 2020.
According to a study published in Seismological Research Letters, the quick deployment of distributed acoustic sensing (DAS) technology increased the total number of aftershocks identified during the seismic event and supplied data for ground motion estimation.
According to Xiangfang Zeng of the Chinese Academy of Sciences and colleagues, the findings provide a real-world demonstration of how “dark fiber” can be used to create an ultra-dense seismic array for post-earthquake monitoring in urban areas, reducing the deployment time and cost associated with more traditional monitoring.
The majority of aftershocks happened to the south of the mainshock’s epicenter. However, relocated aftershocks were dispersed along a fault beneath the city, which may delineate the mainshock’s ruptured fault.
Xiangfang Zeng
DAS arrays are “extremely useful in achieving rapid damage estimates with additional information from increased seismic monitoring capabilities,” according to Zeng, who also stated that the technology’s dense observations are vital for high-resolution seismic hazard mapping.
The microscopic internal faults in a long optical fiber act as thousands of seismic sensors in distributed acoustic sensing. An interrogator, which is located at one end of the fiber, sends laser pulses down the cable, which are reflected off the faults in the fiber and returned back to the instrument. Researchers can learn more about the resultant seismic waves by examining changes in the size, frequency, and phase of the reflected pulses after an earthquake disrupts the fiber.
Tangshan was the epicenter of one of the deadliest earthquakes in recorded history in 1976. The magnitude 7.8 earthquake destroyed more than 85 percent of buildings and killed at least 240,000 people.
Prior to the Tangshan incident in their study, which occurred on July 12, 2020, Zeng and his colleagues had been experimenting with DAS monitoring of earthquakes for years. “The idea of combining DAS and black cable to monitor aftershocks came to me in the morning when I learned about a highly felt earthquake near the Tangshan earthquake in 1976,” Zeng added. “It took roughly two days to obtain clearance from a local telecom provider to use the black fiber. In the early hours of July 15, a field crew led by Dr. Bao from my group flew to Tangshan and installed the interrogator.”
From the 15th to the 23rd of July, the researchers used the DAS system to detect seismicity in the area. The array identified 32 earthquakes that were not in the catalog of the local permanent array. During the observations, the strongest aftershock identified was a magnitude 1.9 earthquake. Zeng and colleagues discovered that several earthquakes caused considerable ground motion.
“The majority of aftershocks happened to the south of the mainshock’s epicenter,” Zeng said. “However, relocated aftershocks were dispersed along a fault beneath the city, which may delineate the mainshock’s ruptured fault.”
“The researchers hope that their findings will inspire the development of more permanent DAS systems in the region. The Tangshan region’s seismicity is highly active,” Zeng stated, citing a magnitude 4.3 earthquake that occurred on April 16, 2021. “A permanent DAS network, in addition to the existing seismometer network, will give additional information about seismogenic faults and seismicity, as well as ground motion site effects from better resolved shallow crustal structure in this region.”
The research team “borrowed” from previous groups that have developed distributed acoustic sensing (DAS) systems in this new experiment. Laser pulses are used in DAS to detect minute vibrations along an optical fiber/cable. Researchers place interrogators along the optical fiber/cable. Short infrared laser pulses are sent out and detected by these interrogator units. Tiny strains on the optical fibers caused by seismic activity cause some of the laser light to be reflected and then rebounded back to the sensor. The scientists can measure changes in light scattering over time by transmitting fast pulses. They can locate the location of the action by understanding the speed of light.
