Eurocode 8 seismic hazard zoning maps for the UK

The earliest attempt to map seismic activity, and hence hazard, in the UK was made by O’Reilly (1884), and the (perhaps better known) map of Ballore (1896) does show a division of the British Isles into distinct zones. However, at this early date, hazard could only be expressed in terms of simple distinctions between levels of frequency of earthquake occurrence, usually subjectively assessed, and not as ground motion, which is how seismic hazard is expressed today. The first true hazard map (in the modern understanding of the word) for Great Britain was thus that of Lilwall (1976), which expressed hazard in terms of intensity with a 200-year return period, using an extreme-value technique based on the work of Milne and Davenport (1969). A subsequent study by Ove Arup (1993) used true probabilistic seismic hazard assessment (PSHA) to calculate hazard at selected points in the UK, but these points were too few to be contoured. The first contour maps of hazard on the UK territory produced using PSHA were therefore those of Musson and Winter (1996), prepared for the then Department of Trade and Industry (DTI). The UK was, of course, also covered in two major international seismic hazard mapping projects, the Global Seismic Hazard Assessment Programme (GSHAP) and SESAME (Grünthal et al 1996, Jiménez et al 2001). Both these studies used a common source model for the UK, which was derived from a simplified version of the Musson and Winter (1996) model. The results are in conformity with those of Musson and Winter (1996), although different ground motion models were used. An updated hazard map for the UK was published by Jackson (2004), but only for intensity. This is described in Musson (2004a). A specific zoning map for the UK was produced for a report on dam safety for the UK, and has since been widely circulated (Halcrow 1990). This map assesses hazard in a completely subjective way into high, medium and low classes, which are to be understood as entirely relative terms. Despite its informal nature, it proved to be a reasonable depiction of relative hazard levels when compared to later quantitative maps.