Evaluation of site seismicity is the first step in a sequence of steps involved in developing seismic design criteria for a proposed construction site in a seismically active area. The time of occurrence, the location, and the magnitude of future earthquakes and the attenuation of ground motions with distance from the source represent major uncertainties in an assessment of seismic hazard. The development and application of a straightforward and flexible procedure for quantitative evaluation of seismic hazard associated with combined random and nonrandom earthquake occurrence processes are described. The proposed procedure uses the basic methodology originally suggested by Cornell1 and provides a logical framework for extending the classical seismic risk analysis technique to a class of prediction in evaluating site seismicity.
Quantitative description of the hazard is a fundamental requirement for the design of structures which are subject to environmental loading. The major environmental threat to an offshore platform generally results from waves and currents generated by storms. In seismically active areas of the world, however, the occurrence of earthquake generated ground motions may either contribute to or dominate the environmental loading threat. A rational design process involves a balance between environmental and operational load and platform resistance to achieve an optimum risk or acceptable probability of failure. An underlying notion here is that the design objectives should include both safety and economy.
For platforms sited in seismically active regions, a necessary first step is to define the seismicity of the proposed site. As used here, the term "seismicity" is intended to imply a quantitative description of the seismic threat at a site of interest due to the future occurrence of earthquakes on potential sources which surround the site. In order to be useful input to the selection of earthquake design criteria, the evaluation of site seismicity must lead to a quantative description of both the severity and the likelihood of occurrence of possible future ground motions at the proposed platform site. The time of occurrence, the location, and the magnitude of future earthquakes and the attenuation of ground motions with distance from the source represent major uncertainties in an assessment of seismic hazard.
Although by no means, universally accepted, it is generally agreed that earthquakes of engineering significance are the result of the sudden release of strain energy accumulated in the earth's crust which develops seismic wave emissions from the fault slippage surfaces involved in the event. The characteristics of the resulting ground motions at a given site are influenced by a number of factors. In developing a model for evaluating site seismicity, three distinct physical phenomena can be identified. These three important aspects of the problem are:
Source characteristics-the time of occurrence, location, and magnitude of future earthquake events.
Source-to-site transmission and attenuation characteristics-the distance and azimuth of site from the source and the geologic conditions along the travel path.
Local site modulation effects-the influences of local surface and subsurface topography and soil conditions in modifying the incoming seismic waves.
