State Of The Practice Of Seismic Hazard Evaluation

The main use of seismic hazard analyses is to develop rock outcrop or stiff soil ground motions for use in design. The quantitative descriptions of the ground motions can be in terms of simple scalar values (e.g. peak acceleration, peak velocity, peak displacement, response spectral values, or Arias intensity) or it can be in terms of time histories of acceleration, velocity, and displacement. Typically, the design ground motions are then used in geotechnical and/or structural engineering analyses. This paper does not address site-specific site response analyses. Site response is addressed in the accompanying theme lecture paper by Dobry. The topic of this paper is the state-of-the-practice of seismic hazard analysis. The actual practice of seismic hazard analysis varies tremendously from poor to very good. The large variability in practice is not simply a reflection of project budgets; a large variation in practice exists for similar projects. Although the basic methodologies used in seismic hazard analysis are well established, as represented by short courses and seminars on seismic hazard analysis as well as numerous seismic hazard reports, these basic methodologies are generally not well understood. Usually, the state-of-the-practice is defined as what the "average" practitioner would do. This applies to both those conducting the seismic hazard evaluation and those applying the results in an engineering analysis. In general, the high end of the practice is very good, but the results are often not well understood by the engineers who need to apply them. To address with this situation, we first discuss the some of the shortcomings of the current practice, and then discuss the state-of-the-practice at the high end. As a start, we describe many common misunderstandings regarding the development and interpretation of design ground motions that contribute to poor practice.