Moment Tensor Analysis of Very Shallow Sources

An issue for moment tensor (MT) inversion of shallow seismic sources is that some components of the Green's functions have vanishing amplitudes at the free surface, which can result in bias in the MT solution. The effects of the free surface on the stability of the MT method become important as we continue to investigate and improve the capabilities of regional full MT inversion for source-type identification and discrimination. It is important to understand free-surface effects on discriminating shallow explosive sources for nuclear monitoring purposes. It may also be important in natural systems that have very shallow seismicity, such as volcanic and geothermal systems. We examine the effects of the free surface on the MT via synthetic testing and apply the MT-based discrimination method to three quarry blasts from the HUMMING ALBATROSS experiment. These shallow chemical explosions at similar to 10 m depth and recorded up to several kilometers distance represent rather severe source-station geometry in terms of free-surface effects. We show that the method is capable of recovering a predominantly explosive source mechanism, and the combined waveform and first-motion method enables the unique discrimination of these events. Recovering the design yield using seismic moment estimates from MT inversion remains challenging, but we can begin to put error bounds on our moment estimates using the network sensitivity solution technique (Ford et al., 2010).