More than 1.2 million houses in high-hazard areas of California are particularly vulnerable to earthquakes because of the type of construction, according to the CEA. These homes are typically built before 1979, have a wood frame on a raised foundation and have a cripple wall in the crawl space under the house.
Most houses need to be seismically retrofitted in three ways: 1. The cripple walls of the house need to be braced with plywood. 2. The house needs to be bolted to the foundation. 3. The floor of the house needs to be attached to the braced cripple walls with shear transfer ties. These three steps serve to seismic retrofit and bolt a house to the foundation by converting the cripple walls into shear walls.
If any one of these three areas is not made earthquake resistant with a seismic retrofit, your un-retrofitted house can slide off of its foundation.
Above the first floor, interior finishes on the walls and partitions such as plaster, though not designed to resist earthquakes, do in fact provide a lot of earthquake resistance. Therefore, this part of the house above the crawl space does not need further bolting. The failures always occur in the crawl space, which is where the house needs to be bolted to the foundation here as well as convert the cripple walls into shear walls.