During a visit to the site of the magnitude 6 to 7 Ridgecrest earthquakes in July 2019, seismologist Julian Lozos from California State University made a mind-boggling discovery—rocks ranging from pebbles to boulders had inexplicably moved without any visible drag marks on the desert floor.

"You see a hole in the ground shaped like the rock, but the rock itself is just sitting nearby without any sign of how it got there," Lozos explained. This bizarre occurrence led Lozos to partner with fellow researcher Sinan Akçız, who had witnessed similar rock displacements after the 7.2 magnitude El Mayor-Cucapah earthquake in 2010.

To unravel this geological mystery, Lozos and Akçız tweaked a computer simulation to explore how fault lines rupture during earthquakes. They proposed that the configuration of a fault line has a more significant impact on rock displacement than the earthquake's raw energy.

At the upcoming Seismological Society of America Annual Meeting in August 2025, Lozos will unveil groundbreaking findings: when an earthquake ruptures along a sharply inclined fault, the ground can rebound so rapidly that loose rocks actually become airborne for a brief moment.

The shape and length of fault segments dictate how quickly the ground will move, potentially launching rocks into the air. Some ruptures can even achieve supersonic speeds, resulting in extremely powerful ground accelerations. Conversely, flat and extensive faults can build up friction, causing the rupture to lose momentum.

If the rupture happens to hit the end of a fault segment, it's akin to crashing into a wall, according to Lozos. This localized burst of ground acceleration can be even more forceful than gravity, allowing rocks to float momentarily as the earth shifts beneath them.

This astonishing discovery has significant implications for earthquake risk management. The researchers advocate that local seismic hazard warnings should account for the possibility of much stronger ground motions in areas where faults converge.

Their study, titled "Modeling the rupture dynamics of strong ground motion (> 1 g) in fault stepovers," has been published in Tectonophysics, shedding new light on earthquakes and their potential impacts.