The waves behind the weird phenomenon are called shear waves, which normally are relatively slow seismic waves that move over the surface in a manner similar to ocean waves. These are the waves that are felt as rolling and shaking motions after the initial shock of normal earthquakes. The initial shock is another, much faster, kind of wave that behaves more like pressure waves that make sound in the air.
In a supershear earthquake, however, the shear waves are created very quickly when a long fault, like the San Andreas, breaks loose faster than the speed shear waves normally travel. When this happens the shear waves mach cone is created that can reach the same speed as the pressure waves, explained the paper’s lead author, François Passelègue of the Geology Laboratory at École Normale Supérieure in Paris, France.