One phenomenon particular to this induced collapse is that the source is initially expansive with all diagonal moment tensor components positive. This phenomenon was not observed in spontaneous collapse studies. Figure 6 below shows the source moment tensor of a cavity collapse following an underground nuclear explosion (Stump, 1979). The results suggest negative first motions for all the diagonal moment tensor components. We compared the first motions of the ground motion data from the explosively induced collapse and a spontaneous coal mine collapse (Walter et al., 1996).
Both near-source and regional data indicate opposite polarities: positive for the induced collapse and negative for the spontaneous collapse. Figure 7 gives the regional data comparison.