Microscale fracture opening in host–inclusion systems: evaluating a potential alternative mechanism for shallow earthquakes

Due to weak rheology and high confining pressure, the proper mechanism for nucleation of Earthquakes in the lower crust remained an open question for the past 70 years. The main mechanisms proposed so far require either the presence of hydrous minerals or pre-existing ductile shear zones. However, many studies on pseudotachylytes from Norway and Australia provided evidence for paleo Earthquakes where neither the prerequisites were satisfied (Dunkel et al., 2020; Hawemann et al., 2018). In this work, we explore the hypothesis for a new mechanism for earthquake nucleation triggered by the mechanical instability of host inclusion pairs. Upon changes in external pressure and temperature conditions, a host-inclusion system can develop considerably large deviatoric strains which in turn create a significant non-hydrostatic stress that could be responsible for nucleation of fractures and their subsequent propagation in the host. Cracks developed from two or more inclusions can interact and coalesce to form a macro crack, that can ultimately result in a fault. In order to assess this possibility, we performed numerical simulations on host-inclusion system using the extended finite element method present in the code ABAQUS. To mimic the material properties in the lower crust, a Traction Separation Law (TSL) was introduced to enable modelling the quasi-brittle behaviour for the material by introducing a linear softening between two crack planes. Our results demonstrated that (1) we could successfully trigger fracture nucleation; (2) fractures nucleated between two inclusions can coalesce to form a macro crack which seems to support our initial hypothesis for fracture nucleation without large external deviatoric stresses.