Researchers at Penn State have developed a method for using thunder-triggered "thunderquakes" to image the structure of the Earth's upper crust, offering a middle ground between waiting for natural earthquakes and deliberately setting off explosives. Some of the acoustic energy from thunder penetrates the ground, but the resulting seismic signals are notoriously complex, having deterred scientists from analysing them until now. The new model helps disentangle that complexity, potentially opening up a cheap and passive way to map subsurface terrain wherever thunderstorms occur.

The complexity stems from how lightning produces thunder: superheated plasma bubbles strung along the lightning's irregular path each generate their own acoustic shock wave, which expand, interfere with one another and lose power as they travel. Once these waves reach the ground, they interact unpredictably with soil, rock and infrastructure, splitting into surface-hugging Rayleigh waves and others that penetrate deeper. To model this, the team adapted the SPECFEM3D Cartesian software, making compromises such as treating the atmosphere as a uniform 3.6km layer and stretching the top 20 metres of ground to 200 metres to match the software's processing frequency, before testing the approach using fibre-optic cabling to reconstruct the terrain beneath the Penn State campus.