Suggested Changes to the non-conservative low frequency criteria for the seismic design spectrum used in intraplate regions

The shape and the amplitude of the design spectrum are critical to the safe design of structures. This paper presents suggested changes to the shape constraints for the design spectrum in the 0.5 to 3 Hz range for large intraplate earthquakes. The main recommendation arises because of the impact on a large pulse loading within an intraplate earthquake with a frequency of 1 to 2 Hertz. Newmark and Hall in 1978 establish the seismic criteria for the design of nuclear power plants. This criteria still forms an integral part of the legal framework in the United States for the design of nuclear facilities. The earthquake spectrum developed for the nuclear power plant design was based on an extensive analysis of the seismicity of interplate regions and a rigorous mathematical review of the wave and pulse constraints on the limits of the design spectrum. The shape of the 0.5 to 3 Hertz region of this design spectrum was based on a number of mathematical assumptions in the analysis that were known in 1978 to be nonconservative and requiring review with the collection of additional seismic traces. Recent earthquake data from the secret Russian database held in the Lamont Doherty Observatory database, data from the New Madrid Seismic Zone, other large intraplate events and the recent 2004 Sumatra earthquake provides sufficient data to reconsider the wave and pulse constraints used by Newmark and Hall in the original mathematical review of the design spectrum. The recommendations about the spectrum shape change for major intraplate are applicable to other major structures with a low natural frequency.