Abstract
Large earthquakes on mid-ocean ridge transform faults are commonly preceded by foreshocks1,2,3 and changes in the seismic properties of the fault zone3. These seismic precursors could be linked to fluid-related processes2,3. Hydrothermal fluids within young, hot crust near the intersection of oceanic transform faults become significantly more compressible with decreasing pressure, with potential impacts on fault behaviour. Here we use a theoretical model to show that oceanic transform faults can switch from dilatant and progressive deformation to rupture in response to fluid-related processes. We assume that the fault core material behaves according to a dilatant and strain-softening or contractant and strain-hardening constitutive law (Cam-clay-type4), depending on the effective stress state. According to our model, after the initial purely rigid-elastic phase, dilatancy is found to occur within the fault core, causing pore pressure to decrease, hence fluid compressibility to increase. The plastic regime starts with a stable phase, with effective stresses gradually increasing over a timescale of years in response to tectonic loading. The fault then evolves into a metastable phase, lasting a few days, as the pore pressure decreases, inducing large variations or even a discontinuous jump in fluid compressibility, depending on fluid properties. This in turn triggers fault-slip instability that creates foreshock swarms. In the final phase, the fault fails in the mainshock rupture. Our results imply that seismic precursors are caused by the decrease in fluid pressure which results in an increase in fluid compressibility, in response to fault core dilatancy just before rupture.
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Change history
24 December 2015
The version of this Letter originally published described hydrothermal fluids circulating within oceanic fracture zones near ridgetransform intersections as super-critical fluids. However, Daniel Broseta, Matthew Steele-MacInnis and Thomas Driesner pointed out that, even if this term is of common use in the scientific literature, this description is incorrect and potentially misleading for multicomponent fluids such as seawater. In addition, some of the values of compressibility cited in the original Letter were incorrect. Correct values of isothermal compressibility of seawater are one order of magnitude higher compared with those used in the original manuscript and exhibit a sharp discontinuous increase near dew-point conditions. As a consequence, the effect of variations in fluid compressibility on seismicity is enhanced. This strengthens our confidence in the Piau-Maury-Fitzenz model. Figures 2 and 3 and some of the calculations invoking the real fluid properties at the relevant conditions have been corrected in all online versions of the Letter. Daniel Broseta, Matthew Steele-MacInnis and Thomas Driesner have been added to the author list in recognition of their contributions to these amendments.
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Acknowledgements
This work is a tribute to late Jean Francheteau, who passed away on 21 July, 2010. Jean supported the ideas developed in this paper since the very beginning. The work was initiated in 2005, when L.G. was a Cecil and Ida Green Scholar at IGPP/SIO, University of California, San Diego, at the invitation of J. Orcutt and M. Zumberge, who are both greatly acknowledged here. Discussions with Y. Fialko, Y. Hamiel and J. Sclater were very useful. This paper is NOAA/PMEL contribution number 4109.
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L.G. initiated the work based on previous work by R.D., who provided the initial dataset from the Blanco FZ and greatly contributed to the ideas developed in the manuscript. L.G. and P.H. discussed different models, before employing the Piau–Maury–Fitzenz model, produced by J-M.P., V.M. and D.F. D.B., M.S.-M. and T.D. pointed out the thermomechanical effects of phase changes for multi-component fluids. J-M.P. wrote Supplementary Appendix 2. L.G. wrote Supplementary Appendix 3, with contributions from Q.C. and V.M. All authors discussed results and contributed to the manuscript.
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Géli, L., Piau, JM., Dziak, R. et al. Seismic precursors linked to highly compressible fluids at oceanic transform faults. Nature Geosci 7, 757–761 (2014). https://doi.org/10.1038/ngeo2244
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DOI: https://doi.org/10.1038/ngeo2244
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