Effect of Shallow Mechanics and Fault complexity on Earthquake Surface Rupture

We performed dynamic rupture simulations to investigate the impact of fault geometry and shallow subsurface constitutive properties on surface rupture. Our study is based on the observations and models collected after the 2019, Mw 4.9, reverse fault Le Teil, France earthquake. We used a 3D half-space Boundary Integral Equation Method (BIEM) code to simulate spontaneous rupture propagation through the La Rouvière fault and a co-seismically broken secondary structure. To estimate the frictional variations during the earthquake for each position on the fault, we computed the stress change history assuming a kinematic rupture model. Then, the constitutive relation was estimated by combining the shear stress variation and slip for each time step. From this initial step, we obtained a spontaneous rupture model of Le Teil Earthquake. Based on this model, we tested the shallow surface properties and the inclusion of a secondary fault. We first conducted a parameter study to investigate the impact of shallow fault frictional behavior on surface displacement. We tested slip-strengthening and slip-weakening shallow fault behaviors, with different stress change and critical slip displacement values. Our results show that the surface displacements are controlled essentially by the amount of shallow stress drop or by the rate of slip-strengthening. At the same time, we found that the amount of deep slip is following the amount of surface rupture. In a second study, we explored different assumptions about secondary fault geometry. We compared the results from 4 models having different possible geometries to the surface rupture profiles obtained from satellite images. Our results show that the interaction between the principal and secondary faults modifies slip velocity and shear stress distribution, leading to different surface deformation patterns and displacement amounts. The comparison with data shows that secondary fault is more likely to be vertical and connected to the principal La Rouvière fault at 1 km depth.