It is difficult to grasp the source-type from the standard focal mechanism plot. And decompositions of the deviatoric component are non-unique, where the DC and CLVD decomposition followed here could be replaced by two DCs (Julian et al., 1998). Following the source-type analysis described in Hudson et al. (1989) we calculate and , which are given by
where , and are the deviatoric principal moments for the T, N, and P axes, respectively, and is the isotropic moment where trace. is a measure of the departure of the deviatoric component from a pure double-couple mechanism, and is for a pure double-couple and for a pure CLVD. is a measure of the volume change, where would be a full explosion and a full implosion. We calculate the source-type plot parameters for 12 earthquakes, 17 explosions and three collapses (one cavity and two mine) and produce the source-type plot (Figure 2.25). The nuclear tests occupy the region where , the earthquakes cluster near the origin (with some interesting deviations), and the collapses plot almost exactly at (1,-5/9), which is the location for a closing crack in a Poisson solid. The populations of earthquakes, explosions, and collapses separate in the source-type plot. These initial results are very encouraging and suggest a discriminant that employs the , parameters.
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