Shear stress on a vertical strike-slip fault in a layered medium due to tectonic loading: San Andreas Fault system

Depth distribution of shear stress at a transform boundary induced by tectonic loading is analyzed. The lithospheric plate is assumed to be made up of two elastic layers and is loaded by the far-field shear stress. The upper part of the plate boundary is locked. The slipping region that underlies the locked part is modeled by a mode III crack containing a breakdown zone. Owing to the existence of a breakdown zone, the stress possesses a pronounced peak at the tip of the slipping region, and the loading stress takes high values both in front of the slipping region and within the breakdown zone. The stress in the upper part of Earth's crust is little influenced by the form of the slipping region and is mainly affected by elastic properties of the medium. The comparison of the present results with those based on observational data, in particular, the data pertaining to the 1979 Imperial Valley earthquake, allow the formulation of two general suggestions related to the faulting process. First, the depth difference between the point of initiation and the point of deepest penetration of rupture associated with major earthquakes correlates with the size of the breakdown zone. Second, the process of faulting in the top part of Earth's crust is primarily controlled by medium rigidity, which explains why faulting and seismicity are suppressed in sediments.