Searching for Real Meaning

Unfortunately, as magnitudes for these horizontal stresses are often not available due to the difficulty of the measurement, a valuable check on the observed relationships to come have not been made. That is not to say such data is completely unavailable: (4) propose a method to estimate this quantity from earthquake focal mechanisms, and (2) explain its measurement from inducing borehole hydraulic fractures. The suggestion then for this next level of investigation would be to compare the contour plots of stress magnitude with those generated from this methodology.

To address the question posed earlier of whether the combination of various stress regimes is a legitimate logical step, I have prepared the below figure illustrating a confluence of stress directions that intersect in a region. These ideas are in Figure 4. Careful study of caption and cartoon should clarify the concept. By thus considering the Sh azimuth on the normal faulting regime, I believe it is plausible to map using all faulting regime data as input.

As to the meaning of the concentrations of force shown in the maps, there are three interpretations that seem plausible. The first, and least exciting, is that these patterns are merely a function of the uneven distribution of data. This is suggested distinctly by the thrust regime data from the Pacific plate map, and is the reason why all maps are produced with the data points used in their calculation are included on the one of the outputs. I beleive investigation of this interpretation could be made by making smaller area maps utilizing small subsets of the data.

The physical explanations that are left I refer to as the Force or Focus question and are the final two options I have considered. Basal drag on mountain roots in continental crust by mantle convection could be a source of intraplate stress fields. The concentration of stress azimuth intersections shown by the maps could indicate the location of crustal deformation due to such local accelaration. Alternatively, the focus idea would be related to a ridge-push argument for intraplate stress fields. If an arcuate force source applies force evenly along its length, then it is easy to imagine a focusing of force as if a parabolic reflector of EM energy. In this scenario, the locations highlighted by the stress maps could indicate sites of potential secondary geologic phenomena such as volcanism, compression, or even tight interlocked stability.

 

regime
Figure 4 By rotating each of the simple block model faulting styles around an axis somewhere to the left of the block diagrams, it is possible to produce toroids of fault planes that will have conical expression in three dimensions. With a small step in logic, one can then arrive at the above earth model. This idea is scale independent and does not mandate the presence of the faults.

("Pink" fault planes have turned light gray through the file conversion process)