Rock Fracture
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Supershear

Supershear refers to very fast, spontaneously generated ruptures. Conceptually, mode II fractures, such as strike slip faults, propagate within two steady-state velocity regimes: either below the Rayleigh wave speed, called sub-Rayleigh, or between the S- and P-wave speeds, called supershear (Burridge, 1973; Andrews, 1976; Rosakis et al., 1999; Xia et al., 2004). Seismological observations suggest that supershear rupture propagations have occurred, at least locally, in several crustal scale strike-slip events, like the 1979 Imperial Valley earthquake (Archuleta, 1984), the 1999 Izmit and Duzce earthquakes (Bouchon et al., 2001), and the 2001 Kunlunshan earthquake (Bouchon and Vallee, 2003).

Occurrence of melt, also referred to as psuedotachylyte, within and around faults (Figure 1) has been known for a long time (see for example, Swanson, 1988; Di Toro and Pennacchioni, 2004). These observations have recently been interpreted as a product of high velocity ruptures based on laboratory experiments (Griffith et al., 2009; Ngo et al., 2012) and mechanical models of plastic strain around high velocity ruptures (Dunham et al. 2011). Please see 'Mechanisms and Mechanics of Splay Joints.'

Psuedotachylyte melt along and diagonal to a fault, the Adamello tonalities (southern Italian Alps). The picture by D.D. Pollard. See Di Toro and Pennacchioni (2004) for the original description.Figure 1. Psuedotachylyte melt along and diagonal to a fault, the Adamello tonalities (southern Italian Alps). The picture by D.D. Pollard. See Di Toro and Pennacchioni (2004) for the original description.
Reference:

Andrews, D.J., 1976. Rupture velocity of plane strain shear cracks. Journal of Geophysical Research 81: 5679–5687.

Archuleta, R.J., 1984. A faulting model for the 1979 Imperial Valley earthquake. Journal of Geophysical Research 89: 4559–4585.

Bouchon, M., Bouin, M.P., Karabulut, H., Toksoz, M.N., Dietrich, M., Rosakis, A.J., 2001. How fast is rupture during an earthquake? New insights from the 1999 Turkey earthquakes. Geophys. Res. Lett. 28: 2723–2726.

Bouchon, M., Vallee, M., 2003. Observation of long supershear rupture during the magnitude 8.1 Kunlunshan earthquake. Science 301: 824–826.

Burridge, R., 1973. Admissible speeds for plane-strain shear cracks with friction but lacking cohesion. Geophys. J. R. Astr. Soc. 35: 439–455.

Di Toro, G., Pennacchioni, G., 2004. Superheated friction-induced melts in zoned pseudotachylytes within the Adamello tonalities (southern Italian Alps). Journal of Structural Geology 26: 1783–1801, doi:10.1016/j.jsg.2004.03.001.

Dunham, E.M., Belanger, D., Cong, L., Kozdon, J.E., 2011b. Earthquake ruptures with strongly rate-weakening friction and off-fault plasticity, Part 2: Nonplanar faults. Bulletin of Seismological Society of America 101: 2308–2322, doi: 10.1785/0120100076.

Griffith, W.A., Di Toro, G., Pennacchioni, G., Pollard, D.D., Nielsen, S., . 2009. Journal of Geophysical Research 114: B02402, doi:10.1029/2008JB005879.

Ngo, D., Huang, Y., Rosakis, A.J., Griffith, A.A., Pollard, D.D., 2012. Off-fault tensile cracks: A link between geological fault observations, lab experiments and dynamic rupture models. Journal of Geophysical Research 117: B01307, doi:10.1029/2011JB008577.

Rosakis, A.J., Samudrala, O., Coker, D., 1999. Cracks faster than the shear wave speed. Science 284: 1337–1340.

Swanson, M.T., 1988. Pseudotachylyte-bearing strike-slip duplex structures in the Fort Foster Brittle Zone, S. Maine. Journal of Structural Geology 10: 813–828, doi:10.1016/0191-8141(88)90097-1.

Xia, Rosakis, A.J., Kanamori, H., 2004. Laboratory earthquakes: the sub-Rayleigh-to-supershear transition. Science 303: 1859–1861.



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