Keywords: dynamic, geomaterials, soil-structure interaction, uncertainty, seismic, intensity.

Nowadays seismic events are, in the main, recorded by accurate instruments, with the use of intensity recording generally considered to be of less importance and to have a larger degree of associated uncertainty. However, in areas of moderate seismic activity, intensities still play an important role in extending the seismic catalogues to include historic events. Improving the understanding of intensities is therefore beneficial.

Previous investigators have considered the relationship between intensity and ground acceleration and velocity by applying purely statistical approaches using seismic ground motion recordings as the basis. A new approach is described in this paper; it applies engineering analysis to establish a relationship between intensity levels and the response frequency of key intensity indicators and thereby introduces an additional constraint to the data.

Whilst the key intensity indicators were originally chosen because they are easy to observe and report against, the same does not apply to understanding their engineering characteristics. Significant in this respect are; the perception of humans to seismic motion, the behaviour of domestic properties sited on various soil types with the associated quantification of uncertainties inherent in their dynamic properties, and the rocking and sliding of furniture and other domestic items. However, significant work has taken place in these areas of research over the last couple of decades and that has enabled a detailed evaluation of those intensity indicators. The developments to utilise that research and, for the first time apply it systematically to the issues of seismic intensities, are reported in this paper along with the findings.

The following significant conclusions can be drawn from the findings in this paper. Through the relationship that is established between the intensity level and the frequency, it is clear that intensity is related to the spectral acceleration and not the peak ground acceleration. Two factors have been determined to govern the response of structures and systems under seismic input. Firstly, the maximum intensity indicator frequency has been determined to be less than 6 Hz. Secondly, under increasing ground acceleration a typical structure will begin to respond in a nonlinear manner with a consequential reduction in its fundamental response frequency. This means that the fundamental response of a typical structure will always be within the velocity and displacement region of the response spectra, this explains the often quoted argument that intensity is more closely related to velocity than acceleration. This paper provides an explanation for that argument and demonstrates that intensity is nonetheless a function of spectral acceleration.

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