Earthquake rupture dynamics frozen in exhumed ancient faults

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Abstract

Most of our knowledge about co-seismic rupture propagation is derived from inversion and interpretation of strong-ground-motion seismograms1,2,3, laboratory experiments on rock4,5 and rock-analogue material6, or inferred from theoretical and numerical elastodynamic models7,8,9. However, additional information on dynamic rupture processes can be provided by direct observation of faults exhumed at the Earth's surface10. Pseudotachylytes (solidified friction-induced melts11,12) are the most certain fault-rock indicator of seismicity on ancient faults13. Here we show how the asymmetry in distribution and the orientation of pseudotachylyte-filled secondary fractures around an exhumed fault can be used to reconstruct the earthquake rupture directivity, rupture velocity and fracture energy, by comparison with the theoretical dynamic stress field computed around propagating fractures. In particular, the studied natural network of pseudotachylytes is consistent with a dominant propagation direction during repeated seismic events and subsonic rupture propagation close to the Rayleigh wave velocity.

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Figure 1: Geological sketch map of the Adamello intrusion.
Figure 2: Pseudotachylyte-bearing fault.
Figure 3: Orientation of pseudotachylyte injection veins along 28 different faults, in an area-weighted rose diagram.
Figure 4: Tensile stress field during rupture propagation in the vicinity of a fracture tip for three different rupture velocities.

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Acknowledgements

This research was supported by INGV (Italy), MIUR (Italy) and the CRdC AMRA Project. We thank M. Bouchon, M. Cocco, J. Rice, T. Tullis and J. Allen for encouragement and comments; and Z. Reches for a constructive review. Author Contributions G.D.T. and S.N. collected the field data; S.N. produced the mathematical models; G.D.T., S.N. and G.P. wrote the manuscript. G.D.T. and S.N. contributed equally to the work.

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Correspondence to Stefan Nielsen.

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Supplementary information

Supplementary Tables

Measures of the orientations of secondary fractures used for the study. Supplementary Table S1: orientation of all the 624 secondary fractures measured, organized in 28 fault segments. Supplementary Table S2: for each of the 28 fault segments, proportion of north- versus south-bound secondary fractures. Length, GPS location and altitude of the fault segment. Supplementary Table S3: number of segments that fall into a given interval of asymmetry.

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