In order to calculate changes in Coulomb failure stress, one approach is to specify a pre-existing regional stress field onto which the stress changes caused by an earthquake can be superposed. Then Delta_CFF, the change in the Coulomb Failure Function, can be calculated by comparing the final combined stress field with the pre-existing field.
Other approaches to calculating Delta_CFF are discussed elsewhere.
If a regional stress field is desired, the dialog that appears will look like this:
Add regional stresses? (y/n): [n] y
Poissons ratio: [0.25]
Regional options:
h = general (with 2 axes horizontal)
p = general (with 2 axes horizontal, asks for phi)
s = regional Shear stress
u = regional uniaxial tensional or compressive stress
t = regional Tectonic (e or c) stress
l = Lithostatic stress (no hor disp)
i = Lithostatic stress (isotropic)
c = Continue: exit from regional options.
Give regional option (g/h/p/s/e/f/d/t/l/i/c): [c] e
Azimuth of tension or compression: [0.] 0
Magnitude of stress (bar, + is tension): [0.] -100
Give regional option (g/h/p/s/e/f/d/t/l/i/c): [c]
Sorry that these codes are so cryptic. The actual translation of the input quantities into a stress tensor in the global 1,2,3 coordinate system can be found in subroutine regional.f. Tectonic (t) and lithostatic (l,i) stresses are discussed in Jaeger and Cook (Fundamentals of Rock Mechanics, 1979)
A convenient way to see the regional in the global 1,2,3 coordinate system is to run program reg2trac, which prints the regional and allows the resultiong tractions on any plane to be calculated.