Page 27 - Global Tectonics

14 CHAPTER 2

Figure 2.6 Lambert equal area net.

fi rst motions, respectively, left the focal region. A focal
mechanism solution is thus obtained by plotting all the
observational data on the projection of the focal sphere

and then fitting a pair of orthogonal planes which best
divide the area of the projection into zones of compres-
sional and dilational first motions. The more stations

recording the earthquake, the more closely defi ned will
be the nodal planes.

2.1.7 Ambiguity in focal
mechanism solutions

It is apparent from Fig. 2.7 that the same distribution of
compressional and dilational quadrants would be
obtained if either nodal plane represented the actual
fault plane. Thus, the same pattern of fi rst motions
Figure 2.7 Ambiguity in the focal mechanism solution would be obtained for sinistral motion along a north–

of a strike-slip fault. Regions of compressional first south plane as for dextral motion along an east–west
motions are shaded. plane.