cally needed tools for mitigating the loss of life and property from future large earthquakes.
Although little can be done to alter the seismic hazard—which is geologically fixed by proximity to potential earthquake sources, local rock or soil type, and exposure to secondary ground failures—it is possible to limit the exposure to earthquake destruction by land-use policies. This approach is feasible when the hazard is localized, as in fault rupture. Unless specially designed, any structures built across an active fault will be forced to follow the ground deformation caused by fault rupture. The slip across faults in major earthquakes can be as large as 10 meters, and it is usually not practical or possible to design structures to withstand this type of displacement without severe damage. Therefore, restricting land use in active fault zones is the primary strategy for mitigating fault rupture as a seismic hazard (Box 3.2; Figure 3.24). These policies are less effective where the hazard is distributed across a broad area and all sites face significant seismic hazards; here the primary mechanisms for reducing risk are good engineering design and construction standards and effective emergency management.
U.S. building codes have been successful largely in achieving a high degree of life safety during earthquakes. From 1983 to 2001, in the western United States, 129 people died in eight severe earthquakes (82), while more than 160,000 people worldwide were killed in earthquakes (83). Some of this success is attributable to revisions in building codes prompted by vigorous postearthquake investigations of structural failure and improved understanding of ground motion and dynamic structural behavior (Section 2.7). One notable example is the change in codes after the near collapse of Olive View Hospital during the 1971 San Fernando earthquake. To allow for more open space on the first floor, many buildings of that era, including the hospital, were designed with a major discontinuity in the structural system between the first and successive stories, with the upper stories much stiffer and stronger. This condition was usually referred to as the “soft first story,” and the codes were modified to require that buildings have no significant discontinuities in stiffness and strength from the top floor to the foundation.
The San Fernando earthquake also exposed a deficiency in bridge design that resulted in improvements to the design standards for these structures. This earthquake also spawned a major seismic retrofit pro-