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287 Stress fields

practical importance of these techniques is clear as well as their local usefulness, when
appropriatedly calibrated. However, it needs to be remembered that the use of such
empirical techniques in areas where they have not yet been calibrated has appreciable
uncertainty. The data presented in Figure 9.7b show this clearly as there is considerable
variance in the magnitude of least principal stress in the deep water Gulf of Mexico for
agiven value of the vertical effective stress.

Compressional stress states in sedimentary basins

Thestressmagnitudedataandstressestimationtechniquesdiscussedintheprevioustwo
sections of this chapter focused on normal faulting enviroments where both S hmin and
S Hmax are less than the vertical stress, S v .In this section, I present stress magnitudes at
depth from more compressional environments, in order of increasing stress magnitudes
at depth.

Normal/strike-slip

These areas are those where S hmin is less than the vertical stress but S Hmax ≈ S v .If
the state of stress is in frictional equilibrium (as predicted by Coulomb faulting theory
using µ = 0.6), equation (4.45) will accurately describe the state of stress. In other
words, S hmin will be significantly below S v ,at the value predicted by equation (4.45)
and S Hmax ≈ S v . One such area is shown in Figure 9.10a, for a field in southeast Asia.
The way to think about this type of transitional stress state is that S max ≈ S v ≡ S 1 and
S hmin ≡ S 3 .In this case, both equations (4.45) and (4.46) could be satisfied and both
normal and strike-slip faults could be critically stressed, assuming, of course, that they
have the appropriate orientation to the principal stress axes (as discussed in Chapter 4).
This type of stress state is found in many parts of the world, including much of western
Europe (Zoback 1992).

Strike-slip faulting

As discussed previously in Chapters 1 and 4, these stress states are those in which the
vertical stress is the intermediate stress. If the state of stress is in frictional equilibrium,
the difference between the horizontal stresses are described by equation (4.46), and
S Hmax will be appreciably greater than S hmin .Anexample is shown in Figure 9.10b,
from an area of the Timor Sea (Castillo, Bishop et al. 2000). Note that in this area,
the values of both S hmin and S Hmax are elevated with respect to the area represented
by the data in Figure 9.10a. At depths between 1.6 and 4.2 km, the magnitude of the
maximum horizontal stress is exactly what would be predicted from equation (4.46)