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.
Mode-I, Mode-II, and Mode-III cracks.
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.
- ^ A century after the 1906 earthquake, geophysicists revisit 'The Big One' and come up with a new model, Press release, Stanford University
- ^ McGuire J.J., Zhao L. & Jordan T.H. (2002). "Predominance of Unilateral Rupture for a Global Catalog of Large Earthquakes" (PDF). Bulletin of the Seismological Society of America. 92 (8): 3309–3317. doi:10.1785/0120010293.
- ^ Broberg,K.B. 1996. How fast can a crack go?. Materials Science, 32, 80-86
- ^ a b 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.
- ^ a b  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.
- ^ a b 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" (PDF). 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). Bibcode:2012GeoRL..3921307W. doi:10.1029/2012GL053622.
- ^ Yue H., Lay T. Freymuller J.; et al. (2013). "Supershear rupture of the 5 January 2013 Craig, Alaska (Mw 7.5) earthquake". Journal of Geophysical Research. 108 (11). doi:10.1002/2013JB010594.
- ^ 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
- ^ "Researchers find evidence of super-fast deep earthquake". Phys.org. July 10, 2014. Retrieved July 10, 2014.