The development of optimal methods of earthquake resistant design depends on a knowledge of how the ground shakes and how real buildings oscillate during earthquakes. The San Fernando earthquake of February 9, 1971 occurred in a region well instrumented to record ground shaking and building vibration and, as a consequence, many records were obtained that illustrate the engineering features of earthquake ground motion and its effects. The magnitude 6. 5 earthquake was centered on the northern edge of the Los Angeles Metropolitan area. Although the shock was not a great earthquake in seismological terms, it produced very strong ground shaking and, therefore, was an important event for engineers. The resulting damage, estimated at over $500 million, provided valuable engineering lessons concerning the design of buildings, bridges, darns, and other facilities; and numerous special studies of the effects of the San Fernando earthquake on various structures have been published. 1-6 The shock triggered 272 accelerographs, and 241 accelerograms were recorded. These records provide valuable information about the characteristics of strong ground motions and structural responses, and make it possible to evaluate the measured performance of modern buildings during strong earthquakes. The San Fernando earthquake was of special importance to structural engineers since it provided the first sample of recorded responses of modern multistoried buildings to strong earthquake motions. The responses of more than 50 buildings were measured, most of which were located in the city of Los Angeles, and had accelerographs in the basement, at mid-height, and on the roof in accordance with building code requirements.