Nature, 436, 1009-1012, doi:10.1038, 2005
G. Di Toro, S. Nielsen and G. Pennacchioni
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.