Ground motion signatures of supershear ruptures in the Burridge-Andrews and free-surface-induced mechanisms

Abstract

We analyze the ground motions of supershear ruptures in the Burridge-Andrews (BAM) and free-surface-induced (FSI) mechanisms. BAM supershear ruptures require higher initial shear stress than FSI supershear ruptures, thus cause faster rupture speed. The supershear slip pulse initiates at the free surface or upper boundary of a fault for FSI and at the hypocenter depth in the in-plane direction for BAM in homogeneous models. Both BAM and FSI supershear ruptures can generate Mach waves and the Rayleigh wave field. The BAM and the faster FSI supershear ruptures can make stronger shear Mach waves. The shear Mach waves show a larger amplitude of negative component for a faster FSI supershear rupture. The FSI supershear ruptures make the strongest dilatational wave next to the fault while it is at a distance away from the fault for the BAM supershear ruptures. The different shapes of supershear rupture front, locations of supershear triggered and the rupture speeds contribute to these contrasts. And we find that the BAM and the faster FSI supershear ruptures can trigger more high-frequency radiation due to the sharper and stronger shear Mach waves. Finally, the comparisons between the simulated seismograms and the records of PS10 indicate that the 2002 Denali earthquake may represent a supershear one with the FSI mechanism and a slow supershear rupture speed.