Handful of lava pebbles thrown into the air. A new study shows how strong shaking at a fault has … More rocks—briefly—defying gravity.
When Julian Lozos, a seismologist at California State University, visited the site of the magnitude 6 to 7 Ridgecrest earthquakes in July 2019, he noticed something strange. Pebble- to boulder-sized rocks clearly had been moved by the earthquakes — but there were no signs of dragging or shearing on the desert ground.
“You’ll see a hole in the ground shaped like the rock, and then the rock is next to it, but there’s no drag mark between the hole and the rock,” so Lozos.
Lozos shared his puzzling observation with Sinan Akçiz, an assistant professor at California State University. Akçiz had seen similar rock displacement after the El Mayor-Cucapah earthquake, a very strong magnitude 7.2 earthquake that occurred on April 4, 2010, south of Baja California.
To study this phenomenon, Lozos and Akçiz modified computer simulation used to model how different faults rupture during an earthquake. They suspected that the shape of the fault plays a greater role than the energy of an individual earthquake in displacing the rocks.
At the Seismological Society of America’s Annual Meeting in August 2025, Lozos will present evidence that if an earthquake ruptures along a steeply inclined fault, the ground rebounds at such speed that loose rocks can become airborne for a few moments.
Examples of rocks displaced during the 2019 Ridgecrest earthquake.
The geometry and length of the fault segments controls where the ground moves fast enough to displace the rocks.
Some earthquake ruptures achieve supersonic speed resulting in theory in even stronger ground acceleration speeds. However, if the fault is flat and long enough, accumulating friction causes the rupture to “run out of steam” and taper off.
But if a rupture reaches the end of a fault segment, it “isn’t running out of energy, it’s running out of fault, so the effect is like slamming really hard into a wall,” Lozos explains. This highly localized ground acceleration is stronger than the force of gravity, making the rocks float as the ground moves beneath them.
This curious discovery also has quite important ramifications for earthquake risk assessment. The study suggests that local seismic hazard warnings should account for much stronger ground motion in areas where faults run out.
The study, “Seismological Society of America.