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
and
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.
![]() |
Berkeley Seismological Laboratory
215 McCone Hall, UC Berkeley, Berkeley, CA 94720-4760
Questions or comments? Send e-mail: www@seismo.berkeley.edu
© 2007, The Regents of the University of California