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Rock pulverization at high strain rate near the San Andreas fault

Abstract

In the damage zone around faults, strain is usually localized along fractures, whereas the blocks enclosed by the fractures remain relatively undamaged1,2. Some rocks near the San Andreas fault, however, are pervasively pulverized at distances of up to 400 m from the fault’s core3; intense fragmentation at such distances is rarely observed along other fault zones. Moreover, these rocks preserve their original grain shapes, indicating that they experienced low total strain3. Here we use laboratory experiments to show that the intense fragmentation of intact rocks sampled near the San Andreas fault requires high rates of strain (>150 s−1). Our calculations suggest that the combination of the low amount of strain experienced by the pulverized rocks and the high rates of strain indicated by our experiments could be explained by a supershear rupture—a rupture that propagated along the fault at a velocity equal to or greater than that of seismic shear waves.

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Figure 1: States of the samples after the experiments.
Figure 2: Experimental results.
Figure 3: Time-lapse snapshots of a sample being pulverized.
Figure 4: Dilatational strain rate versus dilatational strain induced by a subshear rupture 100 m from the fault core.

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Acknowledgements

We thank R. Barre, C. Rousseau, J.-B. Toni, R. Leau and R. Guiguet for their technical help and F. Hild, K. Safa, M. Bouchon and P. Favreau for discussions on this paper; V. D’Hour made preliminary work on Tarn granite. We thank E. Brodsky, F. Renard, J.-P. Gratier, Y. Ben-Zion, S. Boutareaud, P. Molnar and R. Hellmann for reading previous drafts of this manuscript. This work was financially supported by the 3F INSU/CNRS program and by the PPF ‘Systèmes Complexes’ of the University Joseph Fourier.

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M.L.D. proposed and participated in the experiments, processed the data and made the computations. M.L.D. also made the grain size distribution measurements and modal analysis. G.G. developed the SHPB apparatus at the École Polytechnique and was in charge of the experiments.

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Correspondence to Mai-Linh Doan.

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Doan, ML., Gary, G. Rock pulverization at high strain rate near the San Andreas fault. Nature Geosci 2, 709–712 (2009). https://doi.org/10.1038/ngeo640

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