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%.