A GEOMETRIC EXPLANATION FOR FAULTING-STYLE–DEPENDENT NON–DOUBLE-COUPLE COMPONENTS IN GLOBAL MOMENT TENSORS
Global moment tensor catalogs report systematic non-double-couple (NDC) components that have long been interpreted as evidence for complex earthquake sources. However, recent analyses show that most reported NDC contributions – including their polarity – can be predicted directly from the inversion methodology and wavefield sampling, without invoking non-shear rupture processes. Global catalogs show two robust patterns: (1) the polarity of NDC components is not random, but depends systematically on faulting style (thrust events predominantly positive, normal predominantly negative, strike-slip broadly negative), and (2) thrust-faulting earthquakes systematically exhibit smaller NDC amplitudes than normal or strike-slip earthquakes. I show that both effects arise from the interaction between the geometry of radiation patterns and the steep-incidence body and surface waves that dominate global inversions performed with simplified 1-D Earth models. Structural mismodeling along real 3-D paths produces coherent residuals that project differently onto the CLVD basis depending on axis plunge and mechanism orientation, explaining both polarity and magnitude patterns as predictable inversion artifacts. This explanation contrasts with previous interpretations that attributed NDC polarity to anisotropy in the source region, and the smaller NDC of thrust events to intrinsically simpler rupture processes. These results unify two previously separate observations, argue for caution in physically interpreting global NDC values, and motivate the use of 3-D Earth models, especially for regional and early warning applications where waveform fits are more sensitive to lateral heterogeneity.