Plate tectonics squeeze things. But, in the upper crust in California it is not that squishy. So, the stresses cause a fault to form, or, in California, reactivate an existing fault. Some of that fault slip is absorbed by folding. In fact, if a fold axis is parallel to the strike of a fault, the fold MUST be absorbing a blind component of slip on the fault. That does not mean that ther can't be left-over slip and the fault can't come to the surface-they can and do. So, some of the slip is blind, but the fault may or may not be blind....Chris