The velocity structure in the lithosphere is generally heterogeneous. Wave reflection, transmission, and diffraction from the boundaries of the different layers and inclusions are expected to affect rupture propagation on faults embedded in these heterogeneous structures. Here,a model of dynamic rupture on a frictional fault embedded in an elastic full space is presented, governed by plane strain elasticity, with an off-fault inclusion that has a lower rigidity than the background medium. The elastodynamics problem is solved by using the Finite Element software Pylith. The fault operates under linear slip-weakening friction law. Initiate the rupture by artificially overstressing a localized region near the left edge of the fault, considering embedded soft inclusions with 20\% to 60\% reduction in both of the shear and pressure wave speeds. The embedded inclusions are placed at different distances from the fault surface and have different dimensions. One observation is that the existence of a soft inclusion may significantly shorten the transition length to supershear propagation through the Burridge-Andrews mechanism. The higher the material contrast, the shorter the transition length to supershear propagation becomes. Another observation is that supershear rupture is generated at pretress values that are lower than what is theoretically predicted for a homogeneous medium. The implications of the results for dynamic rupture propagation in complex velocity structures as well as supershear propagation on understressed faults are discussed.
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Effect of off-fault low-velocity elastic inclusions on supershear rupture dynamics
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