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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 velocity. This causes a form of sonic boom to occur.
 Rupture propagation velocity
During seismic events along a fault surface the displacement initiates at the focus and then propagates outwards. Typically 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 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.
 Directly observed
- 1999 Izmit earthquake, magnitude Mw 7.6 associated with strike-slip movement on the North Anatolian Fault Zone
- 1999 Düzce earthquake, magnitude 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.
- 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
 See also
- A century after the 1906 earthquake, geophysicists revisit 'The Big One' and come up with a new model, Press release, Stanford University
- Broberg,K.B. 1996. How fast can a crack go?. Materials Science, 32, 80-86
- Archuleta,R.J. 1984. A faulting model for the 1979 Imperial Valley earthquake, J. Geophys. Res., 89, 4559–4585.
- Ellsworth,W.L. & Celebi,M. 1999. Near Field Displacement Time Histories of the M 7.4 Kocaeli (Izimit), Turkey, Earthquake of August 17, 1999, Am. Geophys. Union, Fall Meeting Suppl. 80, F648.
- Okubo, P. G. (1989). Dynamic rupture modeling with laboratory-derived constitutive relations, J. Geophys. Res. 94, 12321-12335
- Rosakis,A.J., Samudrala,O. & Coker,D. 1999. Cracks Faster than the Shear Wave Speed. Science, 284. no. 5418, pp. 1337 - 1340
- Scholz, Christopher H. (2002). The mechanics of earthquakes and faulting. Cambridge University Press. p. 471. ISBN 0-521-65540-4.
-  Bouchon, M., M.-P. Bouin, H. Karabulut, M. N. Toksöz, M. Dietrich, and A. J. Rosakis (2001), How Fast is Rupture During an Earthquake ? New Insights from the 1999 Turkey Earthquakes, Geophys. Res. Lett., 28(14), 2723–2726.]
- Bouchon,M. & Vallee,M. 2003.Observation of Long Supershear Rupture During the Magnitude 8.1 Kunlunshan Earthquake, Science, 301, 824-826.
- Walker, K.T.; Shearer P.M. (2009). "Illuminating the near-sonic rupture velocities of the intracontinental Kokoxili Mw 7.8 and Denali fault Mw 7.9 strike-slip earthquakes with global P wave back projection imaging". Journal of Geophysical Research 114 (B02304). Bibcode:2009JGRB..11402304W. doi:10.1029/2008JB005738. Retrieved 1 May 2011.
- Dunham,E.M. & Archuleta,R.J. 2004.Evidence for a Supershear Transient during the 2002 Denali Fault Earthquake, Bulletin of the Seismological Society of America, 92, S256-S268
- Wang, D.; Mori J. (2012). "The 2010 Qinghai, China, Earthquake: A Moderate Earthquake with Supershear Rupture". Bulletin of the Seismological Society of America (Seismological Society of America) 102 (1): 301–308. Bibcode:2012BuSSA.102..301W. doi:10.1785/0120110034. Retrieved 24 April 2012.
- Wang D., Mori J. Uchide T. (2012). "Supershear rupture on multiple faults for the Mw 8.6 Off Northern Sumatra, Indonesia earthquake of April 11, 2012". Geophysical Research Letters 39 (21). doi:10.1029/2012GL053622.
- Song,S. Beroza,G.C. & Segall,P. 2005. Evidence for supershear rupture during the 1906 San Francisco earthquake. Eos.Trans.AGU, 86(52), Fall Meet.Suppl., Abstract S12A-05