Pauline Souloumiac

Relationship link between landward vergence in accretionary prisms and tsunami generation

The very large slip up to the subduction front of the 2011 Tohoku-Oki (Japan) earthquake has challenged our classic view of the megathrust undergoing only aseismic slip at shallow depth. Furthermore, the enhancement of tsunamis during frontal rupturing has increased concern about tsunami risks. Recent seismic reflection images from the Sumatra subduction zone show frontal landward-vergent thrusts in the accretionary prism in the area of supposed shallow ruptures and enhanced tsunamis. Using mechanical analysis, we here show that sudden and successive decreases of the effective friction along the megathrust are required to form landward-vergent frontal thrusts. These decreases are most likely caused by dynamic weakening mechanisms, such as thermal pressurization of the pore fluids related to the propagation of earthquakes to the seafloor. Therefore, landward vergence in accretionary prisms is indicative of past seafloor frontal ruptures and consequent tsunamis. The presence of landward-verging structures in the Cascadia and Sumatra accretionary prisms might indicate future frontal rupture of the shallowest portion of the megathrust, resulting in large tsunamis.

Shear Versus Tensile Failure Mechanisms Induced by Sill Intrusions: Implications for Emplacement of Conical and Saucer‐Shaped Intrusions

Sills, saucer-shaped sills, and cone sheets are fundamental magma conduits in many sedimentary basins worldwide. Models of their emplacement usually approximate the host rock properties as purely elastic and consider the plastic deformation to be negligible. However, many field observations suggest that inelastic damage and shear fracturing play a significant role during sill emplacement. Here we use a rigid plasticity approach, through limit analysis modeling, to study the conditions required for inelastic deformation of sill overburdens. Our models produce distinct shear failure structures that resemble intrusive bodies, such as cone sheets and saucer-shaped sills. This suggests that shear damage greatly controls the transition from flat sill to inclined sheets. We derive an empirical scaling law of the critical overpressure required for shear failure of the sill’s overburden. This scaling law allows to predict the critical sill diameter at which shear failure of the overburden occurs, which matches the diameters of natural saucer-shaped intrusions’ inner sills. A quantitative comparison between our shear failure model and the established sill’s tensile propagation mechanism suggests that sills initially propagate as tensile fractures, until reaching a critical diameter at which shear failure of the overburden controls the subsequent emplacement of the magma. This comparison also allows us to predict, for the first time, the conditions of emplacement of both conical intrusions, saucer-shaped intrusions, and large concordant sills. Beyond the application to sills, our study suggests that shear failure significantly controls the emplacement of igneous sheet intrusions in the Earth’s brittle crust.