Scientists have found a new use for thunder that has nothing to do with its rumble in the sky. Researchers at Penn State used 458 thunder-generated seismic events to create a geological image of the rock beneath a Pennsylvania campus without drilling any new holes.
Their findings, published in the journal Science Advances, show that an overhead storm can act as a useful tool for identifying subsurface geology.
How Thunder Can Be Used to Probe the Deep
Geophysicists have traditionally relied on strong outside forces to investigate geology hidden beneath the surface. An earthquake is one example, as are vibrations from large vehicles or from controlled active sources placed at the surface.
What the field had not been able to use effectively in the past is the vibrations caused by thunder, since a distant lightning strike sets off a complicated series of mixed shock waves that rarely travel the same path.
The issue is one of physics. The channel of ionized air created by a strike takes the form of a non-uniform line, causing numerous pockets of energy to form as a single flash dies down.
Each of these pockets takes the form of a shock wave that dissipates into the air as sound, with the collective noise being recognizable as thunder.
When lightning bolts discharge, the tortuosity and nonuniform plasma temperatures along the superheated channel generate multiple semi-discrete shock bubbles along the bolt akin to a string of pearls.
As one of these waves touches down on the surface, part of it couples into the ground and produces a seismic wave. The speed of these waves is partially determined by the properties of the earth, with different wave speeds providing information about subsurface materials.
A careful analysis of waves at a given point reveals important information about the layer of earth at that depth, with longer wavelengths providing information about deeper geology.
Subsurface layering creates a uniquely dispersive wavefield, providing a physical basis for thunder as a source for seismic tomography.
The challenge with using thunder for imaging lies in the fact that the energy it produces is scattered in all directions, interrupted by buildings, hills, and variations in bedrock.
This challenge was overcome by the researchers, who used distributed acoustic sensing and advanced processing to isolate useful signals from this complexity. The result was a new way to identify underground formations with useful resolution.
Real-world infrastructure, topography, and bedrock heterogeneity can severely complicate analysis of thunderquakes, which has caused them to go largely untapped as practical sources.
How a Simple Phone Line Became a Sensor
The team at Penn State made their discovery with the help of existing telecommunications lines. An approximately 2.5-mile, 4-kilometer cable, buried just a few feet underground beneath the University Park campus, served as the foundation for the research led by Professor Tieyuan Zhu, an associate professor of geosciences at Penn State.
The researchers developed a new way to use distributed acoustic sensing technology, or DAS, to better understand how atmospheric acoustic waves transfer energy into the ground.
Zhu and his team used fiber-optic strain sensing to read the vibrations that made their way along the length of the cable.
The array, part of the FORESEE network, which stands for Fiber-Optic foR Environmental SEnsEing, recorded continuous seismic data across 2,137 channels at 2-meter channel spacing from 2019 to 2021, with a 10-meter gauge length and sampling at 250 or 500 Hz.
Using a laser beam shot down the fiber, the team recorded how the phase of backscattered light shifts due to tiny strains caused by seismic waves moving through the ground.
Nolan Roth, a doctoral candidate at Penn State who has since moved to Ohio State University as a postdoctoral researcher, manually reviewed more than 2 years of data.
Roth and his colleagues were able to identify 458 high-quality thunderquake events and match their timing to reports made by the National Lightning Detection Network, which provided information on the location and peak current of the lightning strikes.
“With DAS, we are recording hundreds of samples every second and every few meters along the cable,” Roth said while describing the thoroughness of the work. This allowed the team to examine the data set for patterns that could be used to identify geological features previously unseen.
What the Underground Map Revealed About the Campus
A single thunderquake did not have enough resolution for the researchers to work with, so the team stacked together multiple readings using cross-correlation virtual-source interferometry.
Using the 2,137 sensor segments, they created an averaged model of the vibrations where randomness canceled out, leaving behind patterns caused by repeated readings.
The processing generated about 141,000 virtual-source combinations from the 458 thunderquakes, which were ultimately reduced to 309 virtual shot gathers for inversion.
Taken together, these results formed a cross-section of the campus that reached approximately 100 meters, about 330 feet, underground. Embedded within this profile were four unusual zones where the speed of the vibration was slower than surrounding areas.
The most notable features are four low-velocity weak zones, WZ-1 to WZ-4 which are interpreted as areas containing weaker materials such as sediments, weathered rock, fractures or fluids.
The four pockets had average widths of about 40 to 100 meters, and depths of about 28 to 50 meters, in the Nittany Dolomite and Stonehenge Limestone underlying the campus.
The researchers interpreted the dimensions and locations of the weak zones as consistent with fracture complexes, which are concentrated zones of fracturing that can provide pathways for groundwater and contribute to weathering of surrounding rock.
The research team cross-referenced their findings with older drill samples taken from the campus, as well as two sets of engineering surveys. The results were broadly consistent with known rock layers and nearby geophysical measurements.
Independent engineering Multichannel Analysis of Surface Wave surveys and stratigraphy from the deepest available borehole log were broadly consistent with nearby models.
Additional readings by satellite using time-lapse Interferometric Synthetic Aperture Radar observations from 2017 to 2025 revealed that areas around WZ-1 and WZ-2 showed ongoing line-of-sight subsidence, further supporting the findings.
One of the zones sits about 0.6 kilometers southeast of the university’s football stadium, near an area where voids in the rock had previously been documented, while another is about 1.6 kilometers from the Mount Nittany Medical Center where subsurface voids were known.
The other two pockets showed no signs of active subsidence which may reflect increased ground stability due to urban development, a lesser degree of fracturing or masking of the signal by infrastructure.
The limits of the research were defined by the tools used, as extremely shallow sediments with shear-wave phase velocities less than the air-wave velocity cannot be resolved, and simulations were limited to a dominant frequency of about 4 Hz without introducing significant numerical dispersion.
Why This Discovery Goes Far Beyond One College
Limestone is a sedimentary rock found across the central United States, with the ability to dissolve underground, forming unique landforms known as karst. Karst terrains exist across much of the Central and Eastern United States and about 20 percent of U.S. land surface is classified as karst.
Karst is important because it contains highly productive but vulnerable groundwater systems and is characterized by features such as caves, springs, sinkholes, fractures, and underground conduits.
Thunderstorms provide a valuable alternative to investigating local geology. The central and eastern United States experience frequent thunderstorms and comparatively few earthquakes in many areas, making passive seismic surveys based on earthquakes more difficult.
The Penn State researchers therefore see thunderquakes as a potential source of seismic energy in regions where traditional seismic sources are limited.
“What we’ve done,” said Zhu, “is demonstrate that we can actually use those lightning signals to study the geology beneath the surface. In fact, the same principle can be used to great effect in a number of situations.”
Geologists are already considering future applications, since seismic imaging can be used to evaluate geohazards such as sinkholes and landslides, assess groundwater and mining resources, and study volcanoes and magma pockets.
The passive nature of the study also means it could potentially be used in places far removed from development, including the Arctic and densely populated urban centers where large-scale excavation is difficult.
What has yet to be seen is whether the four weak zones found beneath the Penn State campus could be used as an early indicator for potential sinkholes in the region.
The four areas were not directly drilled into as part of this study, and the researchers did not establish that they are future sinkhole sites.
Roth and his colleagues have suggested that further research could allow the findings to go beyond one campus, with additional applications in the field of geoscience still being explored.
Zhu has also noted that the findings could serve a purpose beyond simply studying the Earth. The results could prove useful for scientists looking at tectonic movements on other planets, since quakes on other worlds are not well understood, so having a different source for seismic imaging might be necessary.
Lightning and thunder are not unique to Earth, and thunder-induced seismicity could potentially provide alternative sources for subsurface exploration on other worlds, although this remains a future possibility rather than a demonstrated application.
Following the scans, Zhu described the process as a means of allowing people to see underground with the help of a storm in the sky.
Sources
Thunderquakes: A new way to image the Earth’s subsurface — Penn State University
Roth, N. et al., Science Advances (DOI: 10.1126/sciadv.aeg8096)
Thunderstorms offer scientists a new way to see underground — Earth.com
Thunderquakes Could Help Scientists See Beneath Earth’s Surface — Discover Magazine
Scientists Planted Telecom Cables to Find ‘Thunderquakes.’ Here’s What Happened — Gizmodo
Scientists use thunderquakes to X-ray Earth beneath cities — The Weather Network