When lightning strikes, it superheats the surrounding air, creating a shock wave that manifests as thunder. However, thunder’s journey doesn’t end in the atmosphere.
When this energy hits the ground, some of it transforms into seismic waves that travel through the earth, resulting in vibrations known as “thunderquakes.”
Previously, these signals were not well understood by scientists and were not utilized effectively. Our research indicates that thunderquakes can be harnessed to map the Earth’s subsurface structure, such as tracing groundwater movement, identifying environmental contamination, and evaluating sinkholes and building sites, much like how X-rays reveal the human body’s internal structures.
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We made this discovery by using a common yet unconventional seismic measuring tool: fiber-optic cable, found globally in communities.
Turning fiber optics into vibration sensors
We employed a technique called distributed acoustic sensing to convert a standard fiber-optic cable, typically used for internet or phone services, into a series of thousands of vibration sensors.
A laser-pulsing computer, known as an interrogator, sends light through the cable and detects minute changes caused by vibrations along its length. In our study, part of the Penn State FORESEE project, we utilized a cable extending over 2 miles (more than 4 kilometers). This setup provided us with more than 2,100 sensors, spaced just a few feet apart, all simultaneously monitoring the ground.
Over two years, we recorded and identified 458 distinct, high-quality thunderquakes using an old telecom fiber.
The detailed information gathered through this method enabled us to observe specific seismic waves that occur when atmospheric sound interacts with the ground.
Among these seismic waves, the air-coupled Rayleigh waves are particularly significant. Although they can be detected at the surface, they offer insights into the subsurface, reaching depths of up to 300 feet (around 100 meters).
How a thunderquake ‘X-rays’ the ground below
This method relies on a concept known as seismic dispersion, where waves of different frequencies penetrate to various depths. By examining changes in wave speed with frequency, we reconstructed seismic wave speeds at different depths, allowing us to interpret the properties of the ground beneath the fiber.
The outcome resembles an X-ray of the subsurface, reaching about 300 feet underground, achieved without any drilling.
Typically, subsurface imaging requires specialized equipment, like truck-mounted vibration sources or sensor arrays set up for surveys. These methods can be costly and challenging to deploy over extensive areas.
Thunderstorms inherently provide widespread seismic energy across landscapes. Additionally, fiber-optic cables are already installed beneath cities and towns globally.
Instead of introducing a seismic source to the ground, we can listen to passing storms. This approach might eventually allow for continuous monitoring of the shallow subsurface using existing infrastructure and naturally occurring seismic energy from the sky.
What we found at our test site
In State College, Pennsylvania, where our tests took place, the geology consists mainly of limestone and dolomite, which can gradually dissolve as groundwater flows through them, creating fractures, caves, and sinkholes.
Our thunderquake research identified four distinct zones where seismic waves traveled significantly slower than in the surrounding rock. These “weak zones” may result from fractured or weathered rock or the presence of water or air. Two of these zones align with areas where satellite radar indicates active ground subsidence. The depth of all four zones is consistent with fractures and voids found at nearby sites.
These findings are crucial because karst landscapes, like the one under State College, are common. They cover around 20 percent of the world’s continental land area and impact nearly a quarter of the global population. Hazards such as sinkholes, groundwater contamination, and others associated with these landscapes pose risks to buildings, infrastructure, and public safety. Identifying these hazards enables better planning and construction.
Not just for thunder, and not just on Earth
Our findings illustrate a fundamental process: Atmospheric sound waves can be transformed into useful seismic waves within the solid Earth. Thunder is merely one example.
Sonic booms, volcanic eruptions, and meteor airbursts can create similar atmospheric shock waves, potentially serving as other natural seismic sources.
This concept might even apply beyond Earth. While Earth-like tectonic earthquakes might not be common on some planets and moons, atmospheric disturbances could still be present.
Titan, Saturn’s largest moon, and the target of NASA’s upcoming Dragonfly mission, presents an intriguing possibility. If models predicting lightning and thunder there are accurate, atmospheric energy could offer another method to explore its subsurface.
On Earth, the atmosphere continually interacts with the ground. Our research suggests this interaction could provide valuable insights into the hidden world beneath us.
