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.
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 .
Our thunderquake study revealed four distinct areas where seismic waves traveled much more slowly than through the surrounding rock.
These “weak zones” could be caused by fractured or weathered rock or the presence of water or air. 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.
Titan, Saturn’s largest moon and the destination of NASA’s upcoming Dragonfly mission, is one intriguing possibility. If lightning and thunder occur there as models predict, atmospheric energy could potentially provide another way to investigate its subsurface.
