Effects of three-dimensional Earth structure on CMT earthquake parameters

We investigate errors in centroid earthquake parameters due to unmodeled structural heterogeneity. We generate a simulated dataset consisting of synthetic seismograms for 50 earthquakes and 150 stations distributed globally. To generate the synthetic seismograms we use a spectral-element wave-propagation package (SPECFEM3D_GLOBE) that accounts for the Earth’s three-dimensional structure. An established centroid–moment-tensor inversion algorithm from the Global CMT project is used to invert the synthetic dataset, with and without added noise, for earthquake source parameters. This algorithm uses a one-dimensional earth structure, together with approximate corrections for three-dimensional structure, to model the seismograms. We interpret the differences between the estimated source parameters and the parameters used to compute the synthetic dataset as errors due to unmodeled structural heterogeneity and the presence of noise. We expect that the errors obtained in this study are representative of the errors in the Global CMT catalogue. We find that the errors in scalar moment, moment-tensor elements and location are small on average. The depth and centroid time are, however, biased by a small amount. We find that the error in depth can be reduced significantly by applying corrections for the difference in the velocity structure at the source and receiver locations from the Earth’s average structure in the CMT inversions. This modification has a minimal effect on the errors in centroid time. We do not find large errors in scalar moments, even where the crustal thickness at the source is very different from the Earth’s average crustal thickness.