Researchers Use Large, Unused Fiber-Optic Network as an Earthquake Sensor

In conventional seismology, scientists rely on sensors that are highly expensive to build and install underground to study the movement of Earth in the moments before, at the time of, and following an earthquake.

Moreover, due to the labor and expense involved, just a handful of seismic sensors have been installed around remote areas of California, rendering it difficult to perceive the impacts of future earthquakes and even the small earthquakes that occur on unmapped faults.

Currently, scientists at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab) have found out a technique for solving these problems by converting parts of a 13,000-mile-long testbed of “dark fiber,” unused fiber-optic cable, owned by the DOE Energy Sciences Network (ESnet), into a highly sensitive seismic activity sensor with the ability to potentially enhance the performance of earthquake early warning systems that are being developed at present in the western United States. The study describing the research, which was the first one to use a large regional network as an earthquake sensor, was reported in Nature’s Scientific Reports this week.

Shaking Up Seismology with Dark Fiber

Jonathan Ajo-Franklin, a staff scientist in Berkeley Lab’s Earth and Environmental Sciences Area who headed the study, stated that there are roughly 10 million kilometers of fiber-optic cable across the globe, and nearly 10% of that comprises of dark fiber, remnants of the dot-com boom that led telecom companies to rush and install vast networks of underground cable to fulfill the requirements of a growing new industry. However, with improvement in the technology for data transmission, a lesser number of cables were required, leaving behind a legacy of unlit dark fiber, waiting to be put to use.

The Ajo-Franklin team has been working on an experiment of this type for many years. In a study performed in 2017, a fiber-optic cable was installed by the team in a shallow trench in Richmond, California, and it was shown that it was feasible to use a new sensing technology known as distributed acoustic sensing (DAS) for imaging of the shallow subsurface. The DAS technology involves measuring seismic wavefields by shooting short laser pulses over the length of the fiber. As part of a follow-up study, the team and a group of collaborators showed for the first time that it is possible to use fiber-optic cables as sensors for the detection of earthquakes.

In the current study, although the same DAS technique is used, rather than deploying its own fiber-optic cable, the team performed its experiments on a 20-mile segment of the 13,000-mile-long ESnet Dark Fiber Testbed extending from West Sacramento to Woodland, California.

To further verify our results from the 2017 study, we knew we would need to run the DAS tests on an actual dark fiber network.

Jonathan Ajo-Franklin, Staff Scientist, Earth and Environmental Sciences Area, Berkeley Lab.

Ajo-Franklin also heads Berkeley Lab’s Geophysics Department.

“When Jonathan approached me about using our Dark Fiber Testbed, I didn’t even know it was possible” to use a network as a sensor, stated Inder Monga, Executive Director of ESnet and director of the Scientific Networking Division at Berkeley Lab. “No one had done this work before. But the possibilities were tremendous, so I said, ‘Sure, let’s do this!”

Chris Tracy from ESnet worked in close collaboration with the researchers to decipher the logistics of implementation. CenturyLink, a telecommunications company, offered fiber installation information.

Due to the regional coverage of the ESnet Testbed, the scientists could monitor environmental noise and seismic activity with finer detail compared to earlier studies.

The coverage of the ESnet Dark Fiber Testbed provided us with subsurface images at a higher resolution and larger scale than would have been possible with a traditional sensor network. Conventional seismic networks often employ only a few dozen sensors spaced apart by several kilometers to cover an area this large, but with the ESnet Testbed and DAS, we have 10,000 sensors in a line with a two-meter spacing. This means that with just one fiber-optic cable you can gather very detailed information about soil structure over several months.

Verónica Rodríguez Tribaldos, Study Co-Author, Earth and Environmental Sciences Area, Berkeley Lab.

Rodríguez Tribaldos is also a postdoctoral researcher in Ajo-Franklin’s lab.

After using DAS for seven months to record data through the ESnet Dark Fiber Testbed, the scientists demonstrated the manifold advantages of using a commercial fiber.

Just by listening for 40 minutes, this technology has the potential to do about 10 different things at once. We were able to pick up very low frequency waves from distant earthquakes as well as the higher frequencies generated by nearby vehicles.

Jonathan Ajo-Franklin, Staff Scientist, Earth and Environmental Sciences Area, Berkeley Lab.

Using the technology, the researchers were able to distinguish between a car or moving train and an earthquake, as well as to detect both distant and local earthquakes, from Berkeley to Gilroy to Chiapas, Mexico. It is also possible to use this technology to provide information on aquifers, characterize soil quality, and be integrated into geotechnical studies, he added.

According to Rodríguez Tribaldos, since the technology offers a detailed picture of the subsurface, it looks promising to be used in time-lapse studies of soil properties. For instance, in the field of environmental monitoring, this tool can be used to detect the melting of permafrost, long-term groundwater changes, or the hydrological variations involved in landslide hazards.

The findings of this study also indicate that scientists may no longer have to choose between affordability and data quality. “Cell phone sensors are inexpensive and tell us when a large earthquake happens nearby, but they will not be able to record the fine vibrations of the planet,” stated co-author Nate Lindsey, a UC Berkeley graduate student who headed the field work and earthquake analysis for the 2017 study. “In this study, we showed that inexpensive fiber-optics pick up those small ground motions with surprising quality.”

Since 300 terabytes of raw data have been collected for the study, the researchers now face the challenge of finding ways to efficiently manage and process the “fire hose” of seismic information. Ajo-Franklin believes that someday, he could develop a seismology data portal that couples ESnet as a sensor and data transfer mechanism, with Berkeley Lab’s supercomputing facility, NERSC (National Energy Research Scientific Computing Center) managing the analysis and long-term storage of data.

Monga added that although the Dark Fiber Testbed will be lit for the next generation of ESnet, dubbed “ESnet 6,” very soon, there might be sections that can be used for seismology. “Although it was completely unexpected that ESnet—a transatlantic network dedicated for research—could be used as a seismic sensor, it fits perfectly within our mission,” he stated. “At ESnet, we want to enable scientific discovery unconstrained by geography.”

The study was funded by Laboratory Directed Research and Development Funding with the earlier study supported by the Strategic Environmental Research and Defense Program (SERDP), U.S. Department of Defense.