Most of what we know about the Earth’s interior comes from observing seismic waves. These travel at somewhat different speeds depending on the details of the rock they’re moving through—whether it’s solid or semi-molten, how much water is present, whether it’s fractured or solid material, and so on. Get enough data from enough seismic events, and you can start piecing together a picture of what’s present at different depths below the surface.
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In many cases, we can get this data from naturally occurring events like earthquakes. In others, we intentionally create waves using things like explosives, providing the opportunity to do imaging in specific areas without needing to wait for an earthquake. Now, a team of scientists at Penn State suggests there’s a potential option that sits between waiting for an earthquake and triggering your own seismic event: thunderstorms.
Some of the energy carried by thunder enters the Earth’s upper crust, triggering what are termed “thunderquakes.” But, for various physical reasons, the seismic signals are extremely complex, making it difficult to extract clear signals from them. The Penn State team says it has finally constructed a model that can help make sense of this complexity and used it to reconstruct the terrain under the local campus.
Why are thunderquakes so hideously complex? It starts with the phenomenon that creates thunder in the first place. Lightning creates thunder by forming superheated bubbles of plasma along its path, creating a structure that has been compared to a string of beads. Each of those beads has the potential to generate an acoustic shock wave, leading to a chain of expanding shock waves that trace the lightning’s path through the area, which is anything but a straight line. These waves also have the pot
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