9 Free-Surface Vanishing Traction Effects on Shallow Sources

Waveform inversion to determine the seismic moment tensor is a standard approach in determining the source mechanism of natural and man-made seismicity. The moment tensor is a general representation that includes the double-couple (DC) and non-double-couple sources such as explosions, opening and closing cracks, and volume compensated linear vector dipoles (CLVD). Previous studies (Ford et al., 2009; Ford et al., 2009; Ford et al., 2010) have shown that regional full waveform moment tensor inversion can discriminate isotropic events due to nuclear explosions from other seismic sources such as earthquakes and mine collapses. The method is robust and capable for source-type discrimination applications at regional distances. There can be complications, however, and as part of our continuing efforts to investigate and improve the capabilities of regional full waveform moment tensor inversion for source-type identification purposes, we address the issue of the effect of free-surface vanishing traction on recovering the seismic moment tensor, scalar seismic moment, and explosive yield.