The intensity of force acting on a rock body is expressed as

I think the term you want is "stress."

Stress is expressed in terms of pressure, or force/area. So you'll see units like Pascals (Pa) or sometimes pounds per square inch (psi) and so forth.

The conventional symbol is, I think, a lower-case "sigma" ... an o with a little tail thing on the top (that I can't easily reproduce in this text box).

Fractures with very little or no displacment