A new study shows that underground fiber optic cables can pick up the rumble of thunder underground, potentially providing scientists with a new way of imaging the inside of Earth. Researchers used Pennsylvania State University’s existing fiber network for internet and phone service as a distributed acoustic sensing (DAS) array to observe the progress of thunderstorms as the storms crossed the campus. According to the researchers, the study is the first to demonstrate the use of preexisting underground telecommunication fiber optics to characterize thunder-induced seismic waves in the near surface. Penn State geophysicist Tieyuan Zhu and meteorologist David Stensrud gained access to the university’s telecommunication fiber optic cable in April 2019. During a half-hour interval, they identified 18 thunder-induced seismic events in the DAS array data. The high-fidelity DAS data showed that the thunder-induced seismics were broadband, with their peak frequency ranging from 20 to 130 Hz. Traditional seismometers have recorded ground motions evoked by thunder vibrating in the infrasound frequency range, below 20 Hz. “Once we set up, we found a lot of very strong events in our fiber optic data, so I was very curious. What’s the cause of these signals?” Zhu said. The researchers used the arrival times of the 18 events to estimate the phase velocity of the near surface, the back azimuth, and the location of thunder-seismic sources. They were able to track the direction of the storm based on the intensity of the thunderquake events and synchronize their results with data from the U.S. National Lightning Detection Network. The DAS data further enabled the researchers to simulate thunder-seismic wave propagation and full waveform synthetics and locate the thunder-seismic source by time-reversal migration. Several kilometers of continuous fiber underlie the Penn State campus — the equivalent of a network of more than 2000 seismometers placed every 2 m along the same path. Such a high density of sensors enabled the researchers to calculate where the thunder originated and potentially distinguish between cloud-to-ground and cloud-to-cloud lightning. “Compared to the seismometers, the fiber optic array can provide fabulous spatial, and also temporal, resolution,” Zhu said. “We can track the thunderstorm source movement.” Fiber optic cables carry telecommunications information in bursts of laser light conducted by strands of transparent glass about as thick as a human hair. Vibrations in Earth such as those created by thunderstorms, earthquakes, or hurricanes stretch or compress the glass fibers, causing a slight change in light intensity and the time a laser pulse takes to travel to its destination. Using the DAS array, the researchers tracked these aberrations to monitor ground motion by converting the laser pulses back to acoustic signals. “The laser is very sensitive. If there is a subtle underground perturbation, the laser can detect that change,” Zhu said. The researchers believe the new study shows that the fiber optic networks underlying urban areas are an untapped resource for monitoring environmental events. Underground fiber optics also hold potential for studying the crust and deep structures of Earth, which cannot be measured directly. The research was published in the Journal of Geophysical Research: Atmospheres (www.doi.org/10.1029/2019JD031453). Thunder rumbles underground in this recording of thunder-induced seismic events picked up by the fiber optic telecommunications cable under Penn State’s campus on April 15, 2019. The top graph shows stretching and compression strain in the fiber optic cable caused by ground motion from the vibrations of the thunder. The colored lower graph visualizes the frequency in hertz of the vibrations. Courtesy of Tieyuan Zhu and David Stenrud/AGU.