Thunder + fiber-optic cabling used for seismic imaging

Things don’t get less complex once the Earth gets involved. The acoustic shock waves may strike soft soil, hard rock, various forms of human infrastructure, and so on, each of which will affect how energy gets transmitted. Some of the energy gets converted into what are called Rayleigh waves, where the energy is transmitted as a wave that moves along the Earth’s surface. The rest go deeper, forming waves that may move through some combination of loose material or the underlying bedrock. To extract information about the Earth’s structure, you have to understand what the seismic waves from a thunderclap would normally look like. Which, to an extent, requires modeling all of the above processes. Since each thunderquake is going to be unique due to the different locations and conditions, this model is going to be, at best, an approximation. The fear that any approximation wouldn’t be good enough to generate usable data probably kept people from trying to analyze thunderquakes sooner. To get their approximation, the team started with a software package called SPECFEM3D Cartesian, which is dedicated to 3D reconstructions of seismic waves. Already, that choice necessitates a few compromises. For example, the software treats the atmosphere as a 3.6 km-thick homogeneous layer, even though the atmosphere near a thunderstorm is anything but. The model also updates events at a frequency that’s slower than the waves moving through the Earth-air interface. So, to compensate for that, the researchers simply stretched the top 20 meters of Earth out to cover 200 meters. These and other factors mean that there were plenty of reasons to think that the model wouldn’t be sufficient to handle real-world data. So, the people who developed it tested it against the real world, using thunderstorms that passed by their campus. Two years of data netted them 458 well-resolved thunderquakes, each of which was confirmed using records from the US’s National Lightning Detection Network (something I had not realized existed). These quakes were characterized by multiple signals arriving from different altitudes, as you’d expect from a chain of beads reaching from clouds to the Earth’s surface. Once the signals arrived at the Earth’s surface, things happened quickly: “The impingement of each bubble onto the ground or environment generates a high-energy impulsive wavelet followed by a decaying wave train dominated by surface-wave content lasting one to two seconds.” From there, the signal spread out and started to interact with the features of the Earth under the campus. Using this data, the team identified four “weak zones,” where seismic signals slow down as they interact with less rigid materials. These can include sediments, fractured rock, or areas with high water content. The Penn State campus happens to sit on a karst formation, where water has slowly altered limestone bedrock, potentially creating a variety of weak spots. In these cases, the team was able to confirm that these four sites actually have something unusual going on there. This was done using a mixture of radar that measured surface deformation, engineering surveys, boreholes made at the sites, and independent seismic data. All of which gives the researchers confidence that, despite all the approximations it required, their model is performing reconstructions that are sufficiently accurate to obtain informative seismic data. And the thunderquakes have a number of advantages, including their relative frequency in many areas of the globe, and the fact that they’re best for reconstruction of the areas closest to the surface, which is where almost all of our infrastructure is located. So, this definitely appears to be a case where we have a model that’s wrong, but also useful. Science Advances, 2026. DOI: 10.1126/sciadv.aeg8096 (About DOIs).