The effects of variations in fluid pressure in large, thin, horizontal aquifers and reservoirs are analyzed by assuming a condition of no horizontal strain. The results indicate that the difference in induced effective horizontal and vertical stresses, coupled with the initial stresses, determines the stability to induced faulting. Conditions characterized by thrust faulting are found to be the least stable, and those characterized by normal faulting to be the most stable. The stability to induced faulting is shown to decrease with increasing values of the intermediate effective principal stress in regions characterized by normal and wrench faulting. However, this effect is not present in regions characterized by thrust faulting.