The ubiquitous squeak of sneakers on a basketball court may be caused by more than just friction, a new study suggests.

Researchers have found that the sharp chirp of rubber on a hard floor happens when tiny areas of slipping between the shoe’s sole and the floor move at supersonic speeds — and, in some experiments, the process involved miniature, lightning-like sparks. What’s more, the findings could lead to an improved understanding of earthquakes and aid in the design of grippy surfaces.

The new study, published Feb. 25 in the journal Nature, shows that soft rubber does not slide the way many people imagine. Instead of the whole sole sticking and then slipping at once, motion bunches into fast, wrinkle-like fronts called “opening slip pulses” that detach and reattach the rubber across the contact zone. Those repeating pulses generate the vibrations that our ears hear as squeaks.

But soft materials like rubber behave differently when they slide across rigid surfaces.

To understand the physics of this process, researchers at the Harvard John A. Paulson School of Engineering and Applied Sciences (SEAS) teamed up with experts from the University of Nottingham in the U.K. and the French National Center for Scientific Research. They used high-speed optical imaging and synchronized audio to watch soft rubber move quickly along smooth glass.

But what they saw was not smooth sliding. Instead, motion bunched up into opening slip pulses, sweeping across the rubber in starts and stops.