California has gained notoriety for many things, one of which is earthquakes. Because southern California has so many earthquakes and a large population, strict zoning laws and building codes are in place.
Where homes can be built
The Alquist-Priolo Earthquake Fault Zoning Act prevents buildings for human occupancy from being constructed upon active faults. Statutes require that cities and counties use these zones as part of their construction permitting process. Additionally, sellers of existing residences must disclose to potential buyers if the property is located in a designated fault zone.
The Seismic Hazards Mapping Act requires the state to prepare maps of the zones in California most susceptible to landslide and liquefaction hazards during earthquakes. Landslide Zones identify where the stability of hill slopes must be evaluated, and countermeasures undertaken in building design and construction. Liquefaction Zones identify where the stability of foundation soils must be investigated, and countermeasures undertaken in building design and construction. Statutes require that cities and counties use these zones as part of their construction permitting process. Additionally, sellers must disclose to buyers whether the property is in either of these zones, after the map for that area has been issued officially.
How homes must be built
New construction is tightly governed by building codes that will create a continuous load path from the roof to the foundation. Older homes must be reinforced to prevent earthquake-induced injury and damage. Following are the most common problems with older homes in California:
Weak cripple walls. Wooden floors and stud walls are sometimes built on top of an exterior foundation to support a house and create a crawlspace. These are called cripple walls and they carry the weight of the house. During an earthquake, these walls can collapse if they are not braced to resist horizontal movement. If the cripple wall fails, the house may shift or fall.
Unbraced pier-and-post foundations. The outside of the house is supported by wood posts resting on unconnected concrete piers. Siding is often nailed to the outside of the posts, making them not easily visible. During an earthquake these posts can fail, if they are not braced against swaying. If the posts fail, the house may shift or fall.
Unreinforced masonry foundations. Brick, concrete block, or stone foundations often cannot resist earthquake shaking. They may break apart, or be too weak to hold anchor bolts. Homes may shift off such foundations during earthquakes, damaging the walls, floors, utility lines, and home contents.
Unreinforced masonry walls. Houses built of unreinforced masonry such as bricks, hollow clay tiles, stone, concrete blocks, or adobe are very likely to be damaged during earthquakes. The mortar holding the masonry together is generally not strong enough to resist earthquake forces. Walls may fall away or buckle, resulting in damage.
Rooms over garages. The large opening of a garage door and the weight of a second-story room built over the garage can result in the walls being too weak to withstand earthquake shaking. When the narrow sections of the wall on each side of the opening are not reinforced or braced, the weakness is worse.
Unreinforced masonry chimneys. Many chimneys are built of unreinforced brick or stone. During an earthquake these can collapse or break and fall on the roof. When the chimney fails, the falling stones and bricks can cause injuries, damage the home, and damage nearby cars. Tall slender chimneys are the most vulnerable.
For more information on building in earthquake zones, see our related article "Standing Firm in the Face of Disaster." More Best Practices® for planning, designing and constructing your homes to resist damage from earthquakes and other natural forces can be found in BuildIQ University’s online training course, Building for Performance.