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A supershear earthquake is an earthquake in which the propagation of the rupture along the fault surface occurs at speeds in excess of the seismic shear wave (S-wave) velocity. This causes an effect analogous to a sonic boom.
Rupture propagation velocity
During seismic events along a fault surface the displacement initiates at the focus and then propagates outwards. Typically for large earthquakes the focus lies towards one end of the slip surface and much of the propagation is unidirectional (e.g. the 2008 Sichuan and 2004 Indian Ocean earthquakes). Theoretical studies have in the past suggested that the upper bound for propagation velocity is that of Rayleigh waves, approximately 0.92 of the shear wave velocity. However, evidence of propagation at velocities between S-wave and compressional wave (P-wave) values have been reported for several earthquakes in agreement with theoretical and laboratory studies that support the possibility of rupture propagation in this velocity range.
Evidence of rupture propagation at velocities greater than S-wave velocities expected for the surrounding crust have been observed for several large earthquakes associated with strike-slip faults. During strike-slip, the main component of rupture propagation will be horizontal, in the direction of displacement, as a Mode II (in-plane) shear crack. This contrasts with a dip-slip rupture where the main direction of rupture propagation will be perpendicular to the displacement, like a Mode III (anti-plane) shear crack. Theoretical studies have shown that Mode III cracks are limited to the shear wave velocity but that Mode II cracks can propagate between the S and P-wave velocities  and this may explain why supershear earthquakes have not been observed on dip-slip faults.
- 1999 Izmit earthquake, magnitude Mw 7.6 associated with strike-slip movement on the North Anatolian Fault Zone
- 1999 Düzce earthquake, magnitude Mw 7.2 associated with strike-slip movement on the North Anatolian Fault Zone
- 2001 Kunlun earthquake magnitude, Mw 7.8 associated with strike-slip movement on the Kunlun fault
- 2002 Denali earthquake, magnitude Mw 7.9 associated with strike-slip movement on the Denali Fault
- 2010 Yushu earthquake, magnitude Mw 6.9 associated with strike-slip movement on the Yushu Fault
- 2012 Indian Ocean earthquakes, magnitude Mw 8.6 associated with strike-slip on several fault segments - the first supershear event recognised in oceanic lithosphere.
- 2013 Craig earthquakes, magnitude Mw 7.6 associated with strike-slip on the Queen Charlotte Fault - the first supershear event recognised on plate boundaries.
- 1906 San Francisco earthquake, magnitude Mw 7.8 associated with strike-slip movement on the San Andreas Fault
- 1979 Imperial Valley earthquake, magnitude Mw 6.4 associated with slip on the Imperial Fault
- 2013 Okhotsk Sea earthquake magnitude Mw 6.7 aftershock was an extremely deep (640 kilometers (400 miles)) supershear as well as unusually fast at "eight kilometers per second (five miles per second), nearly 50 percent faster than the shear wave velocity at that depth."
- A century after the 1906 earthquake, geophysicists revisit 'The Big One' and come up with a new model, Press release, Stanford University
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