Shaking near the centers of such major earthquakes likely exceeds the levels recorded in magnitude 7 shocks. This new information has already led to proposed increases in building-code strength requirements for structures built near potential earthquake sources. Until 1957, scientists had never accurately measured how different ground types affect the local intensity of earthquake shaking, although they had long noticed that structures built on soft soil were more severely damaged in earthquakes than those built on firm soil or bedrock. The records of ground motion obtained during the 1989 Loma Prieta earthquake and its aftershocks vividly confirmed that soft clay soil shakes more violently than firmer sandy soil, which in turn shakes much more than hard rock. The collapse of the part of the Cypress structure that stood on soft soil was clearly due to the stronger shaking of that ground type.
Building codes are the public's first line of defense against earthquakes.
The codes specify the levels of earthquake forces that structures must be designed to withstand. These specifications are based on current information from strong-motion instruments. As ground motions of greater intensity have been recorded, the minimum earthquake requirements specified in building codes have been raised. In addition, provisions for different soil conditions have been added to the codes as scientists have documented the significant influence of soil type on shaking intensity. In recent earthquakes, buildings built to modern codes have generally sustained relatively little damage.