One of the most innovative developments in modern seismology is the use of existing telecommunication infrastructure as a large-scale seismic sensing network. Telecommunication systems often contain unused optical fibers, commonly known as "dark fibers", that are not carrying internet traffic. These spare fibers may extend for tens to hundreds of kilometers beneath roads, railways, pipelines, or across the seafloor. Instead of installing thousands of conventional seismometers, researchers can transform these unused optical fibers into dense arrays of virtual seismic sensors using a technique known as "Distributed Acoustic Sensing (DAS)".
The principle behind DAS is both elegant and remarkably sensitive. At one end of the optical fiber, a device called a "laser interrogator" repeatedly injects extremely short, coherent laser pulses into the fiber. Although optical fibers are manufactured to be highly transparent, they contain microscopic variations in density and composition. These naturally occurring imperfections scatter a tiny fraction of the incident light back toward the interrogator through a phenomenon known as "Rayleigh scattering". Under normal conditions, this backscattered light is simply part of the fiber's optical background. However, when the fiber experiences mechanical deformation, the scattered light carries information about that disturbance.
During an earthquake, seismic waves—including primary (P) waves, secondary (S) waves, and surface waves—propagate through the ground and produce extremely small strains, typically on the order of microstrain or even nanostrain. As these waves pass beneath a buried or submarine fiber-optic cable, they cause the fiber to stretch and compress by minute amounts. These tiny deformations alter both the physical length of the fiber and its refractive index through the "photoelastic effect". Together, these changes modify the phase of the Rayleigh backscattered light. By comparing the phase of successive laser pulses using interferometric techniques, the interrogator can detect these changes with extraordinary precision. Because the wavelength of the laser light is only about 1550 nm, even displacements far smaller than the diameter of an atom can produce measurable phase shifts.
The location of each disturbance is determined from the travel time of the backscattered light. Since the speed of light inside the optical fiber is known, the interrogator can accurately calculate where along the cable the strain occurred. This measurement process is repeated thousands of times every second, allowing continuous monitoring of dynamic ground motion in real time.
A key feature of DAS is that the optical fiber itself serves as the sensing element. No electronic sensors, batteries, or geophones are installed along the cable. Instead, the interrogator effectively divides the fiber into thousands of closely spaced sensing intervals, often only a few meters long. Each interval behaves like an individual seismic sensor, collectively forming a dense "virtual seismic array". Unlike conventional seismic networks, where measurements are available only at discrete station locations, DAS provides continuous spatial measurements along the entire length of the fiber. Moreover, only the interrogator requires electrical power, whereas traditional seismic arrays require every individual station to be powered and maintained.
This capability is particularly valuable for submarine seismology. Thousands of kilometers of fiber-optic telecommunication cables already traverse the ocean floor, yet large regions of the oceans remain poorly instrumented because deploying and maintaining seafloor seismometers is technically challenging and prohibitively expensive. Many submarine communication cables contain unused fibers that can be repurposed for DAS measurements, offering an opportunity to dramatically improve seismic monitoring in areas where conventional instrumentation is sparse or absent. This approach has already demonstrated the ability to detect earthquakes, ocean-generated microseisms, volcanic activity, and even passing ocean waves using existing telecommunications infrastructure.
Distributed Acoustic Sensing belongs to the broader family of "Distributed Fiber Optic Sensing (DFOS)" technologies, which exploit the interaction between light and the optical fiber to measure physical quantities continuously along its length. Different DFOS techniques rely on different scattering mechanisms depending on the parameter being measured. "Distributed Temperature Sensing (DTS)" typically uses Raman or Brillouin scattering to measure temperature, while "Distributed Static Strain Sensing (DSS)" commonly relies on Brillouin scattering to monitor long-term deformation. DAS, in contrast, utilizes coherent Rayleigh backscattering to measure rapidly varying dynamic strain, making it particularly well suited for seismic monitoring and vibration sensing. The technology was initially developed by the oil and gas industry for geophysical monitoring before being adapted for regional and global seismic observation.
Despite its considerable advantages, dark-fiber seismology also has important limitations. Unlike conventional seismometers, DAS measures "strain" along the optical fiber rather than the three-dimensional ground displacement or acceleration. Consequently, the recorded signals require different interpretation methods and cannot always be compared directly with conventional seismic records. The measurements are also highly directional, with maximum sensitivity to deformation occurring along the axis of the fiber, making cable orientation an important factor in data quality. Signal strength further depends on how well the cable is mechanically coupled to the surrounding ground; fibers directly buried in soil or attached firmly to the seafloor generally produce stronger signals than fibers loosely installed inside protective ducts. In addition, DAS systems generate enormous volumes of data. A 50–100 km fiber monitored at kilohertz sampling rates can produce several terabytes of data every day, demanding substantial storage capacity and computational resources for processing. Finally, optical attenuation limits the maximum distance that can be monitored from a single interrogator, requiring multiple interrogation units for very long fiber links.
Despite these challenges, dark-fiber seismology represents a major shift in earthquake monitoring. By transforming existing telecommunication infrastructure into continuous, high-density sensing networks, DAS provides an economical and scalable means of observing seismic activity over unprecedented spatial scales. As interrogation technology, signal processing algorithms, and computational capabilities continue to improve, dark fibers are expected to play an increasingly important role in both terrestrial and submarine seismic monitoring, complementing rather than replacing conventional seismometer networks.
- Marra et al. (2018) Ultrastable laser interferometry for earthquake detection with terrestrial and submarine cables. Science. https://doi.org/10.1126/science.aat4458.
- Zhan Z. (2019) Distributed Acoustic Sensing Turns Fiber Optic Cables into Sensitive Seismic Antennas, Seismological Research Letters, DOI: 10.1785/0220190112.
- Williams et al. (2019) Distributed sensing of microseisms and teleseisms with submarine dark fibers. Nature Communications. https://doi.org/10.1038/s41467-019-13262-7
- Zhu et al. (2023) Seismic arrival-time picking on distributed acoustic sensing data using semi-supervised learning. Nature Communications. https://doi.org/10.1038/s41467-023-43355-3
- Li et al. (2023) The break of earthquake asperities imaged by distributed acoustic sensing. Nature. https://doi.org/10.1038/s41586-023-06227-w