Analysis of Differences in Seismic Moment Tensors between Global Catalogs

Catalogs of moment tensors form the foundation for a wide variety of studies in seismology. Despite their importance, assessing the uncertainties in the moment tensors and the quantities derived from them is difficult. To gain insight,  we compare 5000 moment tensors in catalogs of the USGS and the Global CMT Project for the period from September 2015 to December 2020. The GCMT Project generally reports larger scalar moments than the USGS, with the difference between the reported moments decreasing with magnitude. The effect of the different definitions of the scalar moment between catalogs, reflecting treatment of the non-double-couple component, is consistent with that expected. However, this effect is small and has a sign opposite to the differences in reported scalar moment. Hence the differences are intrinsic to the moment tensors in the two catalogs. The differences in the deviation from a double-couple source and in source geometry derived from the moment tensors also decrease with magnitude. The deviations from a double-couple source inferred from the two catalogs are moderately correlated, with the correlation stronger for larger deviations. However, we do not observe the expected correlation between the deviation from a double-couple source and the resulting differences in scalar moment due to the different definitions. There is essentially no correlation between the differences in source geometry, scalar moment, or fraction of the non-double-couple component, suggesting that the differences reflect aspects of the inversion rather than the source process. Despite the differences in moment tensors, the reported location and depth of the centroids are consistent between catalogs.