Hi Mark. The Coulomb Hypothesis would probably be better defined if it were called by the name in the form it’s being used, which is Coulomb Failure Function.

As I understand it “B Value” plays a big part in this. The lower the “B Value” the greater the probability of a major quake.

The Coulomb criterion for failure (e.g., Jaeger and Cook, 1979; Scholz, 1990) leads naturally to a quantity or function, called (by various authors) the Coulomb Failure Stress or Coulomb Failure Function (CFF), which provides a measure of the proximity of a fault to failure.

There are several ways in which to apply the CFF to earthquake problems, and, to my mind, it is still not a settled question as which of these ways is most appropriate in different cases, or even if the CFF is the appropriate measure of failure to use. Mother nature will have the last word in settling this issue, of course.

Approaches to Using Coulomb Failure Function (CFF)

In an isotropic medium, planes of all orientations would be equally susceptible to failure, and the optimal failure planes are determined by the orientation of the principal axes of the stress tensor. In the most general case, in order to calculate the change in CFF (Delta_CFF) at a location in the medium caused by an earthquake, it is necessary to know the deviatoric stress tensor before the earthquake and after the earthquake. It is possible to calculate the changes in static stress caused by the earthquake from a model of slip in the earthquake, but this is not enough. One must also know the "regional" state of stress that existed before the earthquake so that one can add the stress changes and determine the final stress field. The pre-existing regional field determines the initial CFF at a location, and the final stress field determines the optimal failure planes, slip direction on them, and the final value of CFF. Note that the CFF values calculated before and after the earthquake may be for differently oriented planes and different directions of slip on those planes. This approach was taken by Oppenheimer and others (1988) to study aftershocks to the 1988 Morgan Hill, California earthquake. It is implemented in program stroop (stress on optimal planes).

The southern section of the Rodgers Creek fault is a good example of this. There is movement south and north of this section, but very little movement within that section. However the one big problem is that the stress value for that section of the fault is not known. The same thing applies to the San Gregario fault.

Beyond this my understanding it is sketchy at best. Take Care…Don in creepy town.