What is the Seismic Hazard in the UK
Seismic hazard calculations in regions of low seismicity, such as the UK, are generally based on probabilistic methodology. Probabilistic seismic hazard assessment (PSHA) uses a combination of interpreted geological and seismological data to calculate the probability that a certain level of ground motion will be exceeded, or not exceeded, in a given period of time.
This methodology can be divided into three principal components as follows:
- A set of seismic source zones which define the geographical variation of earthquake activity based on the distribution of observed seismic activity together with geological and tectonic factors. These represent areas where there is an equal chance that a given earthquake will occur at any point in the zone.
- An understanding of earthquake recurrence with respect to earthquake magnitude, as described in the previous section.
- A relationship that defines what ground motion might be expected at Location A due to an earthquake of known magnitude at Location B.
Results
Musson and Winter (1996) produced general purpose maps of seismic hazard in the UK using the PSHA methodology. This study used the SEISRISK III computer program and incorporated a "logic tree" approach to model variation in input parameters.
As might be expected, the areas of highest hazard are those where earthquakes have been most common in the past, particularly those places with repeated earthquake activity. Areas where hazard is higher than average include the west of Scotland and both north and south Wales. The places with lowest seismic hazard are Northern Ireland and northeast Scotland.
The overall values of hazard are not particularly high, since the predicted intensity for the higher zones is only 6 EMS. In other words, even in areas of relatively high exposure to earthquakes in the UK, there is only a 10% chance of experiencing shaking equivalent to intensity 6 in a 50 year period. If we assume that less than 5% of buildings of normal construction will be damaged when the intensity of shaking is 6, the probability of damage for a single house in 50 years is therefore less than 0.5%.
