strain is where something is deforming (moving). Stress is a force over an area. There is stress everywhere; stress levels in the rocks in the eastern US are probably higher than in California, because the rocks are stronger. There more or less is no such thing as seismic stress, or at least it does not mean anything unless defined. The stress we are interested in is the force resolved onto a fault surface. The stress perpendicular to a fault holds it together, the stress parallel to the fault tries to make it slip. Water pressure counteracts the perpendicular stress (called "normal stress") and makes it take less shear stress to make it slip. Earthquakes can perturb (sp?) the stress in the area. Perhaps that is what you mean by seismic stress.

It's been a long time since I actually took a structural geology class, but I think the above is essentially correct; someone may correct me if it is not.

Chris