From its long history of stress measurement in sedimentary strata Sigra has definitively shown that its tectonic strain model describes the state of stress in 70% of cases, the other cases being complex due to active faulting, unconformities or extreme folding.
The tectonic strain is the lateral strain to which a sequence of strata are subject.
It is derived by measuring the stress at various depths in the sequence and then subtracting from these the lateral stress that would be caused by the self weight of the rock mass above in a zero lateral strain environment. This is a function of rock mass and Poisson’s ratio.
The remaining horizontal stress is that caused by tectonic effects. The values of these varies depending on the stiffness of the rock in question. It is generally found that the tectonic strain is even or varies monotonically through the sequence.
This concept is shown in Figure 1
Figure 1. A conceptual model showing uniformly varying major and minor tectonic strains causing widely varying major and minor horizontal stresses depending on rock stiffness
The shallower values of tectonic strain frequently reflect local structures such as faulting and folding and may not reflect tectonic plate effects.