Identification of supershear transition mechanisms due to material contrast at bimaterial faults

Abstract

SUMMARY Numerical modelling of dynamic rupture is conducted along faults separating similar and dissimilar materials. Supershear transition is enhanced in the direction of slip of the stiffer material (the negative direction) due to the bimaterial effect whereby a decrease in normal stress in front of the crack tip supports yielding ahead of the rupture. In the direction of slip of the more compliant material (the positive direction), an increase in normal stress ahead of the rupture tip delays or prevents the supershear transition, whereas the impact of the bimaterial effect on subshear ruptures is to promote rupture in the positive direction due to the tensile stress perturbation behind the rupture tip in this direction. We demonstrate that the material contrast and the parameter S control whether the transition from sub‐ to supershear velocity (supershear transition) is smooth or follows the Burridge–Andrews mechanism. Supershear transition along interfaces separating dissimilar materials is possible for higher values of the parameter S than supershear transition along material interfaces separating similar materials. The difference between pulse‐like and crack‐like rupture is small with regard to the supershear transition type.

Stress heterogeneities in earthquake rupture experiments with material contrasts

“…In Langer et al (2012) we show that supershear transitions can occur for larger S at a bimaterial fault than at a homogeneous fault. In the next section we explain our model for the dynamic rupture propagation.…”

“…We study numerical solutions of the 2D wave equation where the penalty method is used to enforce the contact boundary conditions (Perić and Owen, 1992;Laursen and Simo, 1993;Wriggers, 2006). The penalty method was also employed by Coker et al (2005); Povirk and Needleman (1993) and Shi et al (2008) in their implementation of elastoplasticity along homogeneous interfaces and in Olsen-Kettle et al (2008); Langer et al (2010) and Langer et al (2012) along bimaterial interfaces. For the confined and horizontally unconfined configurations our simulations allow slip at the central 150 mm of the fault.…”

“…As in Langer et al (2012) we are using the velocity-weakening friction law by Ampuero and Ben-Zion (2008). The friction coefficient along the fault is defined as…”

“…To model a dynamic slip on a fault we adapt Gmsh to contain contact joint elements (OlsenKettle et al, 2008;Langer et al, 2010Langer et al, , 2012 in a way that is similar to the traction-at-split-node method (e.g. Andrews, 1999).…”

“…An interesting result of both numerical and analytical studies, is that supershear rupture along bimaterial interfaces occurs predominantly (Harris and Day, 1997;Shi and Ben-Zion, 2006;Langer et al, 2012) or exclusively (Ranjith and Rice, 2001;Adams, 2001;Cochard and Rice, 2000) in the negative direction. Our recent numerical simulations showed that the normal stress changes in front of the rupture tip influence whether or not a supershear transition will occur (Langer et al, 2012). The supershear transition is enhanced in the negative direction, where normal stress changes ahead of the rupture unclamp the fault, making it easier for the shear wave stress peak to trigger supershear failure of the fault.…”