For houses with short (less than full-story height) wood-framed walls between the foundation and the wood-framed first floor—in the crawl space under the house—a retrofit is needed to strengthen this area and make it less vulnerable to shake damage.
Both stem wall and crawl space wall (cripple wall) houses may require bolting. Foundation anchor-bolts are installed to these walls.
If a house has crawl space walls, in addition to bolting, plywood bracing may also be needed to further protect against earthquake damage. The walls are braced by nailing plywood sheathing to the inside of the walls that surround the crawl space.