Pennsylvania, When lightning strikes, it superheats the air around it and creates a shock wave that we hear as thunder. But thunder doesn’t just travel through the atmosphere to our ears.
Our thunderquake study revealed four distinct areas where seismic waves traveled much more slowly than through the surrounding rock. Until now, scientists haven’t understood these signals well enough to make practical use of them. Our team’s research shows that thunderquakes can be used to map the structure beneath the Earth’s subsurface – such as groundwater movement, environmental contamination, sinkholes, or for evaluating building sites and foundations – just like X-rays image inside the human body. We discovered this by listening to thunder with something found in communities around the world but unusual as a seismic measuring instrument: fiber-optic cable. A laser-pulsing computer, called an interrogator, sends light through the cable and measures tiny changes caused by vibrations along its length. The most important of these seismic waves are what we call air-coupled Rayleigh waves. Two of them coincide with areas where satellite radar shows that the ground is actively subsiding. Sinkholes, groundwater contamination and other hazards associated with these landscapes can threaten buildings, infrastructure and public safety. Knowing where these hazards are allows for better construction and planning. Our results demonstrate a fundamental process: Sound waves in the atmosphere can be converted into useful seismic waves in the solid Earth. But thunder is only one example.
In our experiment, part of the Penn State FORESEE project, we used a cable that is over 2 miles long. That gave us more than 2,100 sensors, spaced only a few feet apart, all listening to the ground at once. They are measurable at the surface but can be used to study the subsurface far below, even down 300 feet . They cover roughly 20% of the world’s continental land area and affect nearly a quarter of the global population.
Because karst landscapes like the one beneath State College are widespread, these results matter.
These “weak zones” could be caused by fractured or weathered rock or the presence of water or air.
