Page 28 - Global Tectonics

THE INTERIOR OF THE EARTH 15

Figure 2.8 Ambiguity in the focal mechanism solution of a thrust fault. Shaded areas represent regions of

compressional first motions (C), unshaded areas represent regions of dilational first motions (D), f refers to a fault plane,
ap to an auxiliary plane. Changing the nature of the nodal planes as in (a) and (c) does not alter the pattern of first

motions shown in (b), the projection of the lower hemisphere of the focal sphere.
In Fig. 2.8a an earthquake has occurred as a result which the two nodal planes are also shown. Because
of faulting along a westerly dipping thrust plane f 1 . f 1 Fig. 2.8a is a vertical section, the first motions indicated

and its associated auxiliary plane ap 1 divide the region plot along an east–west azimuth. Arrivals at stations at
around the focus into quadrants which experience other azimuths would occupy other locations within
either compression or dilation as a result of the fault the projection space. Consider now Fig. 2.8c, in which
movement. The directions in which compressional fi rst plane ap 1 becomes the fault plane f 2 and f 1 the auxiliary
motions C 1 and C 2 and dilational fi rst motions D 1 and plane ap 2 . By considering the movement along the
D 2 leave the focus are shown, and C 2 and D 2 are plotted thrust plane it is obvious that the same regions around
on the projection of the focal sphere in Fig. 2.8b, on the fault are compressed or dilated, so that an identical