Inelastic off-fault response and three-dimensional dynamicsof earthquake rupture on a strike-slip fault

摘要

Large dynamic stress off the fault incurs an inelastic response and energy loss, whichcontributes to the fracture energy, limiting the rupture and slip velocity. Using anexplicit finite element method, we model three-dimensional dynamic ruptures on a verticalstrike-slip fault in a homogeneous half-space. The material is subjected to a pressure-dependent Drucker-Prager yield criterion. Initial stresses in the medium increase linearlywith depth. Our simulations show that the inelastic response is confined narrowly tothe fault at depth. There the inelastic strain is induced by large dynamic stresses associatedwith the rupture front that overcome the effect of the high confining pressure. The inelasticzone increases in size as it nears the surface. For material with low cohesion (-5 MPa)the inelastic zone broadens dramatically near the surface, forming a "flowerlike" structure.The near-surface inelastic strain occurs in both the extensional and the compressionalregimes of the fault, induced by seismic waves ahead of the rupture front under a lowconfining pressure. When cohesion is large (~10 MPa), the inelastic strain is significantlyreduced near the surface and confined mostly to depth. Cohesion, however, affects theinelastic zone at depth less significantly. The induced shear microcracks show diverseorientations near the surface, owing to the low confining pressure, but exhibit mostlyhorizontal slip at depth. The inferred rupture-induced anisotropy at depth has the fast wavedirection along the direction of the maximum compressive stress.