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16 CHAPTER 2
Figure 2.9 Ambiguity in the focal mechanism solution of a normal fault. Legend as for Fig. 2.8.
focal sphere projection is obtained. Similar results are It is apparent that the different types of faulting can
obtained when the faulting is normal (Fig. 2.9). In be identified in a focal mechanism solution by the dis-
theory the fault plane can be distinguished by making tinctive pattern of compressional and dilational regions
use of Anderson’s simple theory of faulting (Section on the resulting focal sphere. Indeed, it is also possible
2.10.2) which predicts that normal faults have dips of to differentiate earthquakes that have originated by a
more than 45° and thrusts less than 45°. Thus, f 1 is combination of fault types, such as dip-slip accompanied
the fault plane in Fig. 2.8 and f 2 the fault plane in by some strike-slip movement. The precision with which
Fig. 2.9. the directions of the nodal planes can be determined is
