Mexico, San Francisco, Los Angeles and Kobe : What next ?

Some analogies in the distribution of damage in the 1985 Mexico, 1989 Loma Prieta, and 1995 Kobe earthquakes may be attributable to similarities in the history of reclamation of bayshore or lakeshore environments by emplacing artificial fill on soft mud. In all three cases, a transitional environment has generated similar soil types and analogous forms of human settlement. These similarities may translate into hazardous situations because of amplification of seismic waves in wedge-shaped low-velocity layers; nonlinearity of seismic wave propagation in soft water-saturated soils; transitions from solid-like to liquid-like behavior, including liquefaction and the emergence of prograde surface waves; and other unforeseen conditions arising from surface geology. Severe stability problems may arise in tall, top-heavy structures and in structures with horizontal spans of the order of the wavelength of surface waves. Effective strategies of hazard reduction include a recognition of the many unanticipated ways in which earthquake hazard may become an emergent property of complex nature-society systems.