Supershear earthquake

In seismology, a supershear earthquake happens when the movement along a fault happens faster than the speed of seismic S waves. This unusual speed can create effects similar to a sonic boom, which is the loud noise you hear when something travels faster than the speed of sound. Supershear earthquakes are rare but very important for scientists who study earthquakes. They help us understand more about how energy moves during these powerful events. These earthquakes give clues about the strength of rocks and how faults behave under extreme stress.

Rupture propagation velocity

During earthquakes, movement along a fault starts at a point called the focus and spreads outward. Usually, this movement happens in one direction, as seen in big earthquakes like the 2008 Sichuan and 2004 Indian Ocean earthquakes. Scientists once thought the fastest this movement could go was about 92% of the speed of seismic shear waves, known as Rayleigh waves. But later studies showed that sometimes the movement can be faster, even reaching speeds close to compressional waves (P waves). Research suggests that supershear rupture—where movement exceeds shear wave speed—is common in large strike-slip earthquakes.

Occurrence

Some big earthquakes happen along strike-slip faults. In these earthquakes, the rupture moves sideways, like a crack opening up and shifting horizontally. This is different from other types of earthquakes where the rupture moves up and down. Scientists have found that these sideways-moving ruptures can sometimes move faster than the speed of certain earthquake waves, which is very unusual. This special fast movement has not been seen in the up-and-down moving types of earthquakes.

Initiation of supershear rupture

Supershear rupture starts when a new, smaller rupture begins in an area of very high stress. This high stress allows the smaller rupture to move faster than usual, creating a supershear effect. Experiments using special materials have shown that this process matches what is expected from the Burridge-Andrews mechanism.

Main article: shear stress

Further information: photoelastic

Geological effects

When a supershear earthquake happens, the ground near the fault can change in a special way. The rocks can break into tiny pieces, creating what scientists call "pulverized rocks." This happens because the earthquake moves so fast, it makes lots of tiny cracks in the rocks. These pulverized rocks have been found far from big faults like the San Andreas Fault. Tests in labs show that very fast movement is needed to make rocks break this way.

Examples

Some big earthquakes have been found to break apart the Earth’s surface faster than usual seismic waves move. This is called a supershear earthquake.

Examples include the 1999 Izmit earthquake in Turkey, the 2002 Denali earthquake in Alaska, and the 2023 Turkey–Syria earthquakes. These events show how powerful earthquakes can be when they move faster than typical seismic waves.