Non-Double-Couple Earthquake Mechanism as an Artifact of the Point-Source Approach Applied to a Finite-Extent Focus

A sizable amount of moderate and strong earthquakes exhibit a considerable percentage of non-double-couple (non-DC) components in the mechanism, as reported by agencies determining moment tensors on a routine basis from long-period seismograms. As most of them are tectonic events where a simple shear slip is anticipated along a roughly planar fault (at least in the optics of the long periods used), suspicion arises about their source origin. Leaving aside anisotropy in the source region, we assign them to be side effects of the application of the first degree moment tensor approximation to data still containing (after low-pass filtering) information about the source finiteness.