Using thunderquakes to X-ray Earth – a new study shows urban seismology in action

When lightning strikes, it heats the surrounding air to temperatures hotter than the surface of the sun, creating a sudden expansion that generates a shock wave. This shock wave is what we hear as thunder. However, the energy from thunder doesn’t only travel through the air to reach our ears. When this energy reaches the ground, some of it is converted into seismic waves—vibrations that travel through the soil and rock beneath us. These vibrations are called thunderquakes. Until recently, scientists did not fully understand these signals or know how to use them for practical purposes. Thunderquakes are a natural phenomenon where atmospheric energy is transferred into the Earth’s subsurface, providing a way to study underground structures without invasive methods.

Scientists turned ordinary fiber-optic cables—typically used for internet or phone service—into thousands of vibration sensors using a technique called distributed acoustic sensing. A device called an interrogator, which sends laser pulses through the cable, measures tiny changes in the light caused by vibrations along the cable’s length. In an experiment conducted as part of the Penn State FORESEE project, researchers used a fiber-optic cable over 2 miles (4 kilometers) long, which provided more than 2,100 sensors spaced just a few feet apart. Over two years, this setup recorded 458 high-quality thunderquakes. This method allowed scientists to capture unprecedented detail about seismic waves generated by thunder, revealing patterns that were previously difficult to observe.

Thunderquakes can act like X-rays for the Earth’s subsurface by producing specific types of seismic waves called air-coupled Rayleigh waves. These waves travel along the surface but can also provide information about the ground up to 300 feet (100 meters) below. The key to this technique is seismic dispersion, where waves of different frequencies travel at different depths. By analyzing how the speed of thunderquake waves changes with frequency, scientists can reconstruct the seismic wave speed at various depths, creating a detailed image of the subsurface. This process is similar to how X-rays create images of the human body, but it uses natural atmospheric energy instead of artificial sources.

Traditional methods for imaging the subsurface often require specialized equipment, such as truck-mounted vibration sources or arrays of sensors, which can be expensive and difficult to deploy over large areas. Thunderstorms, however, provide a naturally occurring and widespread source of seismic energy. Additionally, fiber-optic cables are already buried beneath cities and towns worldwide, making them readily available for seismic monitoring. This approach eliminates the need to bring a seismic source to the ground, as scientists can instead listen to storms passing overhead. The technique could enable continuous monitoring of the shallow subsurface using existing infrastructure and natural energy sources.

In State College, Pennsylvania, where the study was conducted, the geology consists mostly of limestone and dolomite—rocks that can dissolve over time due to groundwater movement, creating fractures, caves, and sinkholes. The thunderquake study identified four distinct areas where seismic waves traveled much more slowly than through the surrounding rock, indicating weak zones that could be caused by fractured rock, weathering, water, or air. Two of these zones align with areas where satellite radar shows the ground is actively subsiding. The depths of these weak zones match documented fractures and voids in nearby sites. These findings are significant because karst landscapes, like the one beneath State College, cover about 20% of the world’s continental land area and affect nearly a quarter of the global population.