Interaction of a Dynamic Rupture on a Fault Plane with Short Frictionless Fault Branches

Abstract

Spontaneous bilateral mode II shear ruptures were nucleated on faults in photoelastic Homalite plates loaded in uniaxial compression. Rupture velocities were measured and the interaction between the rupture front and short fault branches was observed using high-speed digital photography. Fault branches were formed by machining slits of varying lengths that intersected the fault plane over a range of angles. These branches were frictionless because they did not close under static loading prior to shear rupture nucleation. Three types of behavior were observed. First, the velocity of both rupture fronts was unaffected when the fault branches were oriented 45°to the main slip surface and the length of the branches were less than or equal to $0.75 R 0 * (where R 0 * is the slip-weakening distance in the limit of low rupture speed and an infinitely long slip-pulse). Second, rupture propagation stopped at the branch on the compressive side of the rupture tip but was unaffected by the branch on the tensile side when the branches were $1.5 R 0 * in length and remained oriented 45°to the principle slip surface. Third, branches on the tensile side of the rupture tip nucleated tensile ''wing tip'' extensions when the branches were oriented at 70°to the interface. Third, when the branches were oriented at 70°to the interface, branches on the tensile side of the rupture tip nucleated tensile ''wing-crack'' extensions. We explain these observations using a model in which the initial uniaxial load produces stress concentrations at the tips of the branches, which perturb the initial stress field on the rupture plane. These stress perturbations affect both the resolved shear stress driving the rupture and the fault-normal stress that controls the fault strength, and together they explain the observed changes in rupture speed.

Finite element simulations of dynamic shear rupture experiments and dynamic path selection along kinked and branched faults

“…[33] As in the numerical investigations of Bhat et al [2007a] and the experimental studies of Biegel et al [2007] on the effects of short branches on rupture propagation, we find that rupture activation along the branch causes alterations in the rupture velocity on the main fault. Observations of rupture velocity during the 2001 Kunlun earthquake by Robinson et al [2006] and Vallée et al [2008] demonstrated that a small change in strike along the fault corresponds to a change in rupture velocity, with rupture accelerating after encountering a small angle kink to the extensional side, and rupture decelerating after encountering a small kink angle to the compressional side.…”

“…Field observations indicate that the majority of earthquake ruptures propagate in the sub‐Rayleigh regime, but there are several cases with evidence of rupture apparently propagating in the supershear regime, such as the 1979 Imperial Valley [ Archuleta , 1984; Spudich and Cranswick , 1984], 1999 Izmit [ Bouchon et al , 2000, 2001], 2001 Kunlun [ Bouchon and Vallée , 2003; Robinson et al , 2006; Vallée et al , 2008], and 2002 Denali [ Ellsworth et al , 2004; Dunham and Archuleta , 2004; Aagaard and Heaton , 2004; Bouchon and Karabulut , 2008] earthquakes. Laboratory experiments involving mode II rupture along preweakened paths in Homalite plates have displayed definitive evidence of supershear rupture with clearly visible Mach fronts [ Rosakis et al , 1999; Rousseau and Rosakis , 2003, 2009; Xia et al , 2004, 2005; Biegel et al , 2007; Rosakis et al , 2007].…”

“…The experimental studies by Rousseau and Rosakis [2003, 2009] investigated the role of a change in fault strike and the effect of a branch junction during mode II rupture propagation along confined paths in Homalite plates. The effects of short branches extending from a main fault during dynamic rupture was studied by Biegel et al [2007] during laboratory earthquakes along a fault in Homalite plates. They found that depending on the branch angle and length, those small branches could have a large effect on rupture velocity along the main fault (see also the theoretical work by Bhat et al [2007b]).…”

An experimental study of the effect of off‐fault damage on the velocity of a slip pulse

“…The samples for our experiments were made from square photoelastic Homalite 100 plates (15.25 cm × 15.25 cm × 1 cm) cut at an angle of 25° to form a fault as in the work by Xia et al [2004] and Biegel et al [2007] (Figure 2). Each sample had a 0.1 mm diameter hole at the center of the fault into which a nickel chromium wire was inserted before the samples were placed into a loading frame.…”

Mode II intersonic crack propagation in poroelastic media

“…Still, laboratory tests show that perturbations through frictionless branches of the main crack line may slow down intersonic perturbation, and tensile wing cracks may also develop, Biegel et al (2007).…”