The seismic speed limit
In the Earth's crust, an earthquake rupture has a theoretical speed limit: the velocity of seismic shear waves, or S-waves. These waves, which shake the ground back and forth, typically travel through solid crustal rock at 3 to 4 kilometers per second. For most earthquakes, the fracture that rips along a fault line propagates more slowly than the S-waves it generates.
A supershear earthquake shatters this limit. The rupture front accelerates beyond the S-wave velocity, an effect like a jet breaking the sound barrier. This phenomenon is most common in large (magnitude 6.7 or greater) strike-slip earthquakes, where two tectonic plates grind sideways against each other. Recent analyses suggest that about 14% of such events since the year 2000 have been supershear quakes, a much higher frequency than previously assumed.
The specific coordinates for this location, 40.748° N, 29.939° E, pinpoint a region near İzmit, Turkey. The 1999 magnitude 7.6 İzmit earthquake was one of the first events where supershear rupture was directly observed. While parts of the fault broke at a conventional speed of about 3 km/s, a central segment nearly 50 kilometers long ruptured at a supershear velocity of approximately 4.8 km/s.
Seismic sonic booms
When a rupture goes supershear, it creates a "Mach cone" of coalesced shear waves. This cone is a focused, narrow front of intense seismic energy that trails the rupture, similar to the sonic boom from a supersonic aircraft. The focused energy within this Mach cone can cause more violent and potentially more destructive ground shaking than a standard earthquake of the same magnitude.
Several notable earthquakes have been identified as supershear events. The 2001 magnitude 7.8 Kunlun earthquake in Tibet was the first instrumentally confirmed case, with a rupture that reached speeds between 4.2 and 4.8 km/s and possibly as high as 6 km/s in some segments. The 2002 magnitude 7.9 Denali Fault earthquake in Alaska also went supershear, with an average rupture velocity of 3.2 km/s.
More recently, the 2018 magnitude 7.5 Palu earthquake in Indonesia showed a sustained supershear velocity of 4.1 km/s almost from its initiation. This was unusual, as many supershear quakes are thought to begin at slower, subshear speeds before accelerating. The conditions that allow for supershear rupture are still being studied, but they appear to involve long, straight fault lines fault lines. Laboratory experiments using rock analogs and high-speed cameras help scientists understand the physics of how these ruptures transition to supersonic speeds.