Fig. 1

Geometry and notation of an elastic body \(V\) bounded by a traction-free external surface \(S_{{{\text{ex}}}}\) and an internal closed surface \(S_{{{\text{in}}}}\). The unit normal vectors \({\mathbf{n}}\) are taken to be outward to the surfaces. \({\mathbf{n}}^{ + }\) and \({\mathbf{n}}^{ - }\) indicate the unit normal vectors to the front side \(\Sigma^{ + }\) and back side \(\Sigma^{ - }\) of a fault \(\Sigma\), respectively