Supershear Mach-waves expose the fault breakdown slip

Mikumo et al. (2003) showed that it is possible to estimate the breakdown slip (Dc) as the slip at the time of the peak slip rate for rupture propagation with subshear speeds. Cruz-Atienza et al. (2009) later attempted to extend this method to extract information about Dc as the displacement at the time of the peak particle velocity from seismic strong-motion records. However, a reasonably accurate estimate of Dc was only possible in a narrow zone adjacent to the fault (typically on the order of hundreds of meters) due to the fast decay with distance from the fault of the seismic energy related to the stress breakdown process. When the rupture propagates with supershear-speeds, on the other hand, this energy is carried much farther away from the fault by Mach waves, in particular Rayleigh Mach waves when rupture reaches the Earth's surface (Dunham and Bhat, 2008). Here, we present a new approach to estimate Dc from strong-motion records containing Mach waves. First, we show that the method by Mikumo et al. is valid for supershear rupture propagation. This method is then used to estimate Dc via an asymptotic approximation of the slip and slip-rate time histories from the Mach waves. Using spontaneous rupture simulations we demonstrate that, for a visco-elastic half-space model, Dc can be estimated within an error of 40% from Mach waves that have propagated a distance of at least 3 km from the fault. The method is applied to estimate Dc for the 2002 Mw7.9 Denali, Alaska, earthquake (∼ 1.5 m) and for the 1999 Mw7.6 Izmit, Turkey, earthquake (∼ 1.7 m).