Poster #159, Fault and Rupture Mechanics (FARM)

Modeling and observing supershear rupture: insufficient information from the presence of a daughter crack

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Poster Presentation

2020 SCEC Annual Meeting, Poster #159, SCEC Contribution #10711
Supershear rupture propagation (i.e., the propagation of an earthquake rupture at a speed higher than that of the shear wave in the local earth material) has long been theoretically predicted, and has been observed in an increasing number of recent earthquakes as seismological techniques have advanced and data availability has grown. Supershear rupture can have important implications for ground motion, and it also can indicate the physical conditions and processes taking place during earthquakes. There are many different mechanisms that can lead to the propagation of rupture at supershear speed, including the Burridge-Andrews mechanism that depends on the level of stress on the fault, mechan...isms that depend on the presence of heterogeneities like stress barriers, fault bends, and stepovers, and a mechanism that depends on wave conversions at the Earth’s free surface. Focusing on 3D dynamic models of the free-surface supershear mechanism, we discuss how different definitions of rupture velocity (and supershear rupture in particular) may lead to different classifications of ruptures as supershear or not. Much of the discrepancy hinges on whether a daughter crack ahead of the primary rupture front can be a reliable indicator of supershear rupture, and whether the apparent along-strike rupture velocity is truly indicative of large-scale supershear rupture. We discuss which definitions of supershear rupture are more consistent with the physical observables most commonly associated with supershear rupture, such as Mach cones and strong fault-parallel ground motion pulses that arrive before fault-normal pulses. The results may have implications for interpreting numerical models as well as observational evidence for supershear rupture.
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