Supershear earthquake

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 [ [http://news-service.stanford.edu/pr/2005/pr-agu_beroza-120705.html A century after the 1906 earthquake, geophysicists revisit 'The Big One' and come up with a new model, Press release, Stanford University] ] .

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 [ [http://www.springerlink.com/content/r376321238385220/ Broberg,K.B. 1996. How fast can a crack go?. Materials Science, 32, 80-86] ] . However, evidence of propagation at velocities between S-wave and P-wave values have been reported for several earthquakes [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, FallMeeting Suppl. 80, F648.] in agreement with theoretical and laboratory studies that support the possibility of rupture propagation in this velocity range. [Okubo, P. G. (1989). Dynamic rupture modeling with laboratory-derived constitutive relations, J. Geophys. Res. 94, 12321-12335 ] [ [http://www.sciencemag.org/cgi/content/abstract/284/5418/1337 Rosakis,A.J., Samudrala,O. & Coker,D. 1999. Cracks Faster than the Shear Wave Speed. Science, 284. no. 5418, pp. 1337 - 1340] ]

Occurrence

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 [cite book |title= The mechanics of earthquakes and faulting|last= Scholz|first= Christopher H.|authorlink= |coauthors= |year= 2002|publisher=Cambridge University Press |location= |isbn= 0521655404|pages= 471] and this may explain why supershear earthquakes have not been observed on dip-slip faults.

Examples

Directly observed

* 1999 Izmit earthquake, Magnitude "M"_w 7.6 associated with strike-slip movement on the North Anatolian Fault Zone [ [http://www.agu.org/pubs/crossref/2001/2001GL013112.shtml] 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.] ]
* 2001 Kunlun earthquake Magnitude, "M"_w 7.8 associated with strike-slip movement on the Kunlun fault [ [http://www-lgit.obs.ujf-grenoble.fr/recherche/fluides-transferts/publications/BouchonScience-2003.pdf Bouchon,M. & Vallee,M. 2003.Observation of Long Supershear Rupture During the Magnitude 8.1 Kunlunshan Earthquake, Science, 301, 824-826.] ]
* 2002 Denali earthquake, Magnitude "M"_w 7.9 associated with strike-slip movement on the Denali fault [ [http://bssa.geoscienceworld.org/cgi/content/abstract/94/6B/S256?etoc 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] ]

Inferred

* 1906 San Francisco Earthquake, Magnitude "M"_w 7.8 associated with strike-slip movement on the San Andreas Fault [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]

ee also

Slow earthquake

References

External links

[http://www.people.fas.harvard.edu/~edunham/research/supershear.html Eric Dunham's webpage on Supershear Dynamics]