Thunderquakes make underground fiber optic telecommunications cables hum

The new research being presented today at AGU's Fall Meeting and published in AGU's Journal of Geophysical Research: Atmospheres marks the first time thunder has been heard underground by a telecommunications fiber optic array, according to the study's authors.

The new study used The Pennsylvania State University's existing fiber network for internet and phone service as a distributed sensor array to observe the progress of thunderstorms as they crossed the campus.

Traditional seismometers have recorded ground motions evoked by thunder, called thunderquakes, vibrating in the infrasound frequency range, below 20 Hertz, which is inaudible to the human ear. The fiber array, which is buried 1 meter (3 feet) underground, picked up a wider range of the frequencies heard in a peal of thunder. The bandwidth detected, from 20 to 130 Hertz, is consistent with microphone recordings of thunder and provides more information about the event, the study found.

Penn State geophysicist Tieyuan Zhu and meteorologist David Stensrud gained access to the university's telecommunication fiber optic cable in April 2019. They were listening for subtle vibrations from a variety of environmental effects, including sinkhole formation and flooding.

"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?" said Zhu. The researchers found a match when they synchronized their results with data from the U.S. National Lightning Detection Network. "We thought, yeah, this is exactly the thunderstorm data, actually recorded by our fiber array."

The passage of lightning heats the air so fast it creates a shockwave we hear as thunder. Vibrations from loud events like lightning, meteor explosions and aircraft sonic booms pass from the air to Earth's surface, shaking the ground.

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 the 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 the laser pulse takes to travel to its destination. The researchers tracked these aberrations to monitor ground motion, 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," said Zhu.

Several kilometers of continuous fiber underlay Penn State's campus, which means the array can act like a network of more than 2,000 seismometers emplaced every two meters along the cable path. With this high density of sensors, the researchers can calculate the location where the thunder originated, potentially distinguishing 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," said Zhu. "We can track the thunderstorm source movement."

The researchers said the new study demonstrates fiber optic networks under urban areas are an untapped resource for monitoring environmental hazards. They also hold potential for studying the crust and deep structures of the Earth, which cannot be measured directly.

Scientists learn about the inside of the planet by observing the way seismic waves from earthquakes are altered as they pass through it. Ground motions induced by thunderstorms, which are much more frequent than earthquakes on the east coast of North America, could help reveal the hidden shapes of Earth's interior, Zhu said.