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Earthquake Table 10-8. Age of Housing Units in Planning Area Time Period Number of Current Planning Area Housing Units Built in Period % of Total Housing Units Significance of Time Frame Pre-1933 15,588 8.0% Before 1933, there were no explicit earthquake requirements in building codes. State law did not require local governments to have building officials or issue building permits. 1933-1940 10,025 5.2% In 1940, the first strong motion recording was made. 1941-1960 87,547 45.1% In 1960, the Structural Engineers Association of California published guidelines on recommended earthquake provisions. 1961-1975 40,454 20.8% In 1975, significant improvements were made to lateral force requirements. 1976-1994 24,970 12.9% In 1994, the Uniform Building Code was amended to include provisions for seismic safety. 1994 – present 15,468 8.0% Seismic code is currently enforced. Total 194,052 100% Note: Number and percent estimates are approximation as housing unit age information does not correspond directly with the time periods indicated. In addition, there are significant margins of error associated with the Census estimates. Source: 2018 American Community Survey, San Mateo County, California Soft-Story Buildings A soft-story building is a multi-story building with one or more floors that are “soft” because of structural design. If a building has a floor that is 70-percent less stiff than the floor above it, it is considered a soft-story building. This soft story creates a major weak point in an earthquake. Since soft stories are typically associated with retail spaces and parking garages, they are often on the lower stories of a building. When they collapse, they can take the whole building down with them, causing serious structural damage that may render the structure unusable. These floors can be especially dangerous in earthquakes because they cannot cope with the lateral forces caused by the swaying of the building during a quake. As a result, the soft story may fail, causing what is known as a soft-story collapse. Soft-story collapse is one of the leading causes of earthquake damage to private residences. Exposure rates and vulnerability analysis associated with soft-story construction in the planning area are not currently known. ABAG and other agencies in the Bay Area have programs generating this type of data, but it is not known when such data will be available for San Mateo County. This type of data will need to be generated to support future risk assessments of the earthquake hazard. Unreinforced Masonry Buildings Unreinforced masonry buildings are constructed from materials such as adobe, brick, hollow clay tiles, or other masonry materials and do not contain an internal reinforcing structure, such as rebar in concrete or steel bracing for brick. Unreinforced masonry buildings pose a significant danger during an earthquake because the mortar holding masonry together is typically not strong enough to withstand significant earthquakes. Additionally, the brittle composition of these houses can break apart and fall away or buckle, potentially causing a complete collapse of the building. In San Mateo County, unreinforced masonry buildings are generally brick buildings that were constructed before modern earthquake building codes and designs were enacted. The State of California enacted a law in 1986 that required all local governments in Seismic Zone 4 (nearest to active earthquake faults) to inventory unreinforced 10-21