On the depth extent of co-seismic rupture

We investigate the implications of deformation experiments for the coseismic down‐dip extent of rupture in quasi‐dynamic, whole‐cycle earthquake models of a fault for which the depth of the transition between seismic and aseisimic fault slip depends on strain rate. The calculations use a dislocation fault model from Tse and Rice (1986) with a vertical strike‐slip orientation, mode III rupture, and variable along‐strike length. Our reference calculation is the original rheological representation of Tse and Rice with a strain‐rate‐independent transition. The primary calculations use two different representations of a strain‐rate‐dependent transition: (1) between rate‐weakening friction and dislocation creep and (2) between rate‐weakening and rate‐strengthening frictions. For both these cases, when fault strength is high (friction between 0.5 and 0.6) and the transition is sharp, coseismic slip extends a small distance (1–2 km) below the fixed temperature (depth) that is commonly used to define the rheological transition at the plate‐motion rate. Thus, coseismic slip occurs below the depth assumed in seismic hazard models using microseismicity or a chosen fixed‐temperature contour. Though significant coseismic slip occurs below the plate‐rate transition depth, the added moment is <10%"><10% of the total. The deep extension is a region that is rheologically distinct; for example, deep coseismic slip can produce a stress increase rather than a stress drop. If friction is smaller, the deepening effect and its contribution to moment are larger. For all representations of the transition, average and surface slip increase with the along‐strike rupture length in a manner consistent with the limited data from natural observations. However, this property is not controlled by the assumed fault rheology; instead, it arises because the stiffness of the fault decreases weakly with fault length, an intrinsic and unrealistic property of the particular crustal scale fault model used.