When lightning strikes, it superheats the air round it and creates a shock wave that we hear as thunder. However thunder doesn’t simply journey by way of the ambiance to our ears.
When that power reaches the bottom, a few of it’s transformed into seismic waves that ripple by way of the soil and rock beneath. The ensuing vibrations are referred to as thunderquakes.
Till now, scientists haven’t understood these indicators properly sufficient to make sensible use of them. Our staff’s analysis exhibits that thunderquakes can be utilized to map the construction beneath the Earth’s subsurface – resembling groundwater motion, environmental contamination, sinkholes, or for evaluating constructing websites and foundations – identical to X-rays picture contained in the human physique.
We found this by listening to thunder with one thing present in communities world wide however uncommon as a seismic measuring instrument: fibre-optic cable.
Turning fiber optics into vibration sensors
Utilizing a way known as distributed acoustic sensing, we turned an peculiar fiber-optic cable, the sort used for web or telephone service, right into a string of hundreds of vibration sensors.
A laser-pulsing laptop, known as an interrogator, sends gentle by way of the cable and measures tiny adjustments attributable to vibrations alongside its size. In our experiment, a part of the Penn State FORESEE mission, we used a cable that’s over 2 miles (greater than 4 kilometers) lengthy. That gave us greater than 2,100 sensors, spaced only some toes aside, all listening to the bottom without delay.
Over two years, we used an previous telecom fiber to report and determine 458 clear, high-quality thunderquakes.
The big quantity of element captured by this course of has allowed us to see one thing that had been tough to look at earlier than: particular sorts of seismic waves produced when sound from the ambiance interacts with the bottom.
An important of those seismic waves are what we name air-coupled Rayleigh waves. They’re measurable on the floor however can be utilized to review the subsurface far beneath, even down 300 toes (round 100 meters).
How a thunderquake ‘X-rays’ the bottom beneath
The trick to this system is named seismic dispersion: Waves of various frequencies journey at completely different depths. By measuring how the thunderquake’s wave speeds modified with frequency, we might reconstruct the seismic wave pace at completely different depths, which permits us to interpret the properties of the bottom beneath the fibre.
The end result was the equal of an X-ray of the subsurface extending roughly 300 toes underground, with out drilling a single gap.
Imaging the subsurface sometimes requires specialised gear, resembling truck-mounted vibration sources or arrays of sensors that must be put in for surveys. These approaches could be costly and tough to deploy over massive areas.
Thunderstorms provide a naturally occurring supply of seismic power that’s already unfold throughout the panorama. And fiber-optic cables are buried beneath cities and cities world wide.
As a substitute of bringing a seismic supply to the bottom, we are able to hearken to the storms passing overhead. That would finally make it attainable to watch the shallow subsurface constantly, utilizing infrastructure that’s already within the floor and seismic power that’s already coming from the sky.
What we discovered at our check web site
In State Faculty, Pennsylvania, the place we performed the exams, the geology is generally limestone and dolomite, rocks that may slowly dissolve as groundwater strikes by way of them. Over time, this course of creates fractures, caves and sinkholes.
Our thunderquake research revealed 4 distinct areas the place seismic waves travelled rather more slowly than by way of the encircling rock. These “weak zones” may very well be attributable to fractured or weathered rock or the presence of water or air. Two of them coincide with areas the place satellite tv for pc radar exhibits that the bottom is actively subsiding. The depths of all 4 are in step with fractures and voids documented at close by websites.
These outcomes matter as a result of karst landscapes just like the one beneath State Faculty are widespread. They cowl roughly 20% of the world’s continental land space and have an effect on practically 1 / 4 of the worldwide inhabitants. Sinkholes, groundwater contamination and different hazards related to these landscapes can threaten buildings, infrastructure and public security. Realizing the place these hazards are permits for higher development and planning.
Our outcomes show a elementary course of: Sound waves within the ambiance could be transformed into helpful seismic waves within the stable Earth. However thunder is just one instance.
Not only for thunder, and never simply on Earth
Sonic booms, volcanic eruptions and meteor airbursts can produce related atmospheric shock waves, doubtlessly creating different pure seismic sources.
The thought might even lengthen past Earth. Earth-like tectonic earthquakes are usually not anticipated to be widespread on another planets and moons, however atmospheric disturbances should be current.
Titan, Saturn’s largest moon and the vacation spot of NASA’s upcoming Dragonfly mission, is one intriguing chance. If lightning and thunder happen there as fashions predict, atmospheric power might doubtlessly present one other technique to examine its subsurface.
Again on Earth, the ambiance is continually interacting with the bottom beneath our toes. Our work means that this interplay may very well be a supply of details about the hidden world beneath us.
Nolan Roth, The Ohio State College and Tieyuan Zhu, Penn State. Republished from The Dialog through PTI.
Printed – August 24, 2026 02:17 pm IST
