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Science 7 March 2003:
Vol. 299. no. 5612, pp. 1557 - 1559
DOI: 10.1126/science.1080650
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Reports

A Supershear Transition Mechanism for Cracks

Eric M. Dunham,1* Pascal Favreau,2 J. M. Carlson1

Seismic data indicate that fault ruptures follow complicated paths with variable velocity because of inhomogeneities in initial stress or fracture energy. We report a phenomenon unique to three-dimensional cracks: Locally stronger fault sections, rather than slowing ruptures, drive them forward at velocities exceeding the shear wave speed. This supershear mechanism differentiates barrier and asperity models of fault heterogeneity, which previously have been regarded as indistinguishable. High strength barriers concentrate energy, producing potentially destructive pulses of strong ground motion.

1 Department of Physics, University of California, Santa Barbara, CA 93106, USA.
2 Institut de Physique du Globe de Paris, 75252 Paris, France.
*   To whom correspondence should be addressed. E-mail: edunham{at}physics.ucsb.edu

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