Off‐fault heterogeneities promote supershear transition of dynamic mode II cracks

Abstract

The transition from sub‐Rayleigh to supershear propagation of mode II cracks is a fundamental problem of fracture mechanics. It has extensively been studied in homogeneous uniform setups. When the applied shear load exceeds a critical value, transition occurs through the Burridge‐Andrews mechanism at a well‐defined crack length. However, velocity structures in geophysical conditions can be complex and affect the transition. Damage induced by previous earthquakes causes low‐velocity zones surrounding mature faults and inclusions with contrasting material properties can be present at seismogenic depth. We relax the assumption of homogeneous media and investigate dynamic shear fracture in heterogeneous media using two‐dimensional finite element simulations and a linear slip‐weakening law. We analyze the role of heterogeneities in the elastic media, while keeping the frictional interface properties uniform. We show that supershear transition is possible due to the sole presence of favorable off‐fault heterogeneities. Subcritical shear loads, for which propagation would remain permanently sub‐Rayleigh in an equivalent homogeneous setup, will transition to supershear as a result of reflected waves. P wave reflected as S waves, followed by further reflections, affect the amplitude of the shear stress peak in front of the propagating crack, leading to supershear transition. A wave reflection model allows to uniquely describe the effect of off‐fault inclusions on the shear stress peak. A competing mechanism of modified released potential energy affects transition and becomes predominant with decreasing distance between fault and inclusions. For inclusions at far distances, the wave reflection is the predominant mechanism.

Dynamic fields at the tip of sub-Rayleigh and supershear frictional rupture fronts

“…These stress-waves can reach the strength of the frictional interface and facilitate a transition to the supershear regime. This mechanism, known as Burridge-Andrews mechanism, has been extensively discussed in the literature [4,6,45,46,47,48].…”

Supershear bursts in the propagation of a tensile crack in linear elastic material

“…Similarly to supersonic aircraft, elastic waves radiated from supershear cracks gather to form shock wave fronts, also referred to as Mach cones, leading to particularly violent earthquakes [32][33][34][35][36]. Several studies have described how local variation in toughness or elastic properties can precisely favor the supershear transition of a mode II crack [37][38][39][40][41][42][43]. Despite the relative rarity of supershear earthquakes reported in nature, recent experiments [44] suggest that shortlived supershear events may frequently occur at small scales of crustal fault, out of the resolution of seismic inversion, yet significantly impacting the rupture dynamics.…”

A hybrid finite element‐spectral boundary integral approach: Applications to dynamic rupture modeling in unbounded domains

“…Here, we consider a slip‐weakening fault in a heterogeneous medium composed of a central stiff strip and a linear‐elastic compliant bulk. () The parameters of the problem are summarized in Table , and the model geometry is described in Figure . The initial shear stress is constant along the fault plane except in a central patch where the fault is overstressed to force abrupt nucleation of the dynamic event.…”