SEISMIC HAZARD MAPS FOR THE UK

Past studies of seismic hazard in the U.K. that have used modem probab ilistic methods of hazard assessment have been site-specific studies, mostly in connection with nuclear installations. There has been a need for general-purpose maps of seismic hazard to show relative variation of exposure within the U.K. and to give some guidance on absolute val ues. Such maps have now been produced, incorporating, for the first ti me, the wealth of new information on historical earthquakes in Britain that has been gathered over the last 15 years. The hazard calculation s were undertaken using a new computer code based on the USGS program SEISRISK III, but incorporating a 'logic tree' approach to model varia tion in the input parameters (e.g. focal depth) or uncertainty in the formulation of the model (e.g. attenuation parameters). An innovative approach was taken to the formulation of seismic source zones, in whic h two overlapping models were employed. The first of these uses relati vely broad source zones based loosely on an interpretation of seismici ty and tectonics, while the second uses numerous small zones that refl ect the locations of past significant earthquakes. This double approac h (using the logic tree methodology) has the merit of both considering the general trend of earthquake activity as well as focusing in on kn own danger spots. The results show that the areas of highest hazard ar e western Scotland, north-western England and Wales, where the intensi ty with 90% probability of non-exceedance in 50 years is 6 EMS.