Moment tensor inversions and source models

The use of a moment tensor formulation in source mechanism inversions is becoming more widespread. However, this method does not yield results that correspond to a double-couple source, which is still the preferred source model. This is because the double couple is characterized by extra constraints on the general symmetric moment tensor. A method is presented of imposing these non-linear constraints while retaining the linear nature of the inversion. An application of this result shows (1) that the double-couple model fits the body-wave data as well as more general models if one explicitly considers the fewer number of free parameters in the double couple and (2) that the principal axes of the moment tensor of the unconstrained model do not necessarily correspond to the P and T axes of the double couple.