‘Melt welt’ mechanism of extreme weakening of gabbro at seismic slip rates

Abstract

Laboratory studies of frictional properties of rocks at slip velocities approaching the seismic range (0.1–1 m s−1), and at moderate normal stresses (1–10 MPa), have revealed a complex evolution of the dynamic shear strength, with at least two phases of weakening separated by strengthening at the onset of wholesale melting1,2,3,4. The second post-melting weakening phase is governed by viscous properties of the melt layer and is reasonably well understood5,6. The initial phase of extreme weakening, however, remains a subject of much debate. Here we show that the initial weakening of gabbro is associated with the formation of hotspots and macroscopic streaks of melt (‘melt welts’), which partially unload the rest of the slip interface. Melt welts begin to form when the average rate of frictional heating exceeds 0.1–0.4 MW m−2, while the average temperature of the shear zone is well below the solidus (250–450 °C). Similar heterogeneities in stress and temperature are likely to occur on natural fault surfaces during rapid slip, and to be important for earthquake rupture dynamics.

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Figure 1: Experimentally measured evolution of the coefficient of friction at three different ‘near-critical’ velocities.
Figure 2: Power density versus time until the onset of weakening, for experiments at four different values of normal stress.
Figure 3: Observed critical slip velocity as a function of the estimated average temperature of the slip interface at the time of weakening.
Figure 4: Initiation of melt welts at the slip interface during the onset of dynamic weakening.
Figure 5: Evolution of shear stress and temperature through the onset of weakening.

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Acknowledgements

We thank D. Lockner for comments that improved this manuscript. The SIO Marine Science Development Center provided the lathe used in our experiments. This work was supported by NSF (grant EAR-0838255).

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K.M.B. built the apparatus, K.M.B. and Y.F. designed and conducted experiments, Y.F. performed numerical modelling.

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Correspondence to Kevin M. Brown or Yuri Fialko.

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The authors declare no competing financial interests.

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Brown, K., Fialko, Y. ‘Melt welt’ mechanism of extreme weakening of gabbro at seismic slip rates. Nature 488, 638–641 (2012). https://doi.org/10.1038/nature11370

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