A cripple wall is a wood stud foundation wall that takes the place of a concrete stemwall (the part of the foundation between the “footing” in the ground and where the framing of the house starts).  If your home has a wood cripple wall, it is most important to have it strengthened by installing bracing or plywood structural panels nailed to the cripple studs to prevent collapse (kind of like the “domino” effect) in a seismic event.