Free-Surface Vanishing Traction Effects on Shallow Sources

Previous studies by Ford et al. (2009; 2009; 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 using regional distance data. However, the effect of vanishing traction at the free-surface is a complication that can bias seismic moment tensor inversions for shallow sources. In this study we address the issue of the effect of free-surface vanishing traction on recovering the seismic moment tensor, scalar seismic moment, and explosive yield. Our preliminary results indicate that the vertical dip-slip Green's functions associated with the M_xz and M_yz components are affected by the vanishing traction at the free-surface, and that this leads to biases in the recovered full moment tensor solutions. The amplitude of the Green's functions decrease systematically, but the waveforms look similar over the targeted depth range with little phase distortion. Initial synthetic testing shows a dependence on source depth and signal-to-noise ratio regarding the waveform moment tensor inversion's ability to recover the isotropic moment of a pure explosion source. However, noise and the free-surface has only minor effects on the isotropic moment recovery for a simple explosion model using a grid-search method. We will document the effects of vanishing traction on seismic moment tensor inversions considering differences in seismic velocity structure, particularly the shallow structure less than 1000 m, and investigate the development of a correctional term applied to isotropic scalar moment estimates from full moment tensor inversions, as well as an approach to damp the M_xz and M_yz terms in the inversion process to attempt to minimize the vanishing traction effect and improve the stability of the full moment tensor inversions for shallow sources.