SEISMIC MOTION DUE TO KINEMATIC DISLOCATION ON FAULTS WITH FINITE THICKNESS

Abstract

In the absence of recordings of strong ground motions, it is useful to simulate these motions in order to assess their probable impact on engineering structures. One of the reliable, convenient and commonly used simulation techniques are those based on the physics of faulting. In one such popular technique, the ‘fault slip’ is modelled as a discontinuity in the displacement field (also referred to as kinematic dislocation) across an internal surface embedded in an elastic medium. Further, the thickness of this (internal) surface is often assumed to be zero. Although this model of the source may hold for synthesis of far-field motion, its applicability at near-fault locations requires an investigation. This is because the actual interface between the rock masses comprising the fault is of finite width and this is likely to influence the motions closest to the fault (and thus, the motions that are potentially most damaging for engineering structures). Accordingly, this study aims to offer insights into the influence of a fault with finite (but small) thickness on the synthetic ground motions. First, an overview of Betti’s reciprocity theorem is presented, followed by its application to synthesis of seismic motion due to dislocation on a fault with infinitesimal (zero) thickness. It is shown that in the case of a shear-type dislocation, the faulting process under the infinitesimal fault-thickness model can be exactly emulated by applying double-couples at the locations of slip. A short introduction on how the representation theorem may be extended for the case of a finite fault-thickness model is finally presented.