Seismic sources at discontinuities: unambiguous moment and potency components

It is well known that for a seismic source on or near a material discontinuity, the moment tensor is ambiguous in the sense that its value depends on the exact location of the source. In this short paper, we demonstrate that this ambiguity can be resolved by considering components of the source moment and potency tensors separately. For a general source and any location ‘at’ an interface, certain components of its moment tensor and certain components of its potency tensor can be determined unambiguously. The other components depend on the exact location of the source. To investigate these results it is convenient to allow for an asymmetric moment tensor, where the antisymmetric part is physically equivalent to a torque source. These results are illustrated using ray theory for a source at a strong material contrast.