The UK is not well known for its seismic activity, and through the course of history earthquakes have only caused 11 deaths in 400 years, however, it is feared that although seismic activity has not increased, vulnerability has. Today, urban centres are now more numerous and larger than ever before. For example, an earthquake of a magnitude that did little damage 400 years ago would be far more damaging today, and more people would be at risk due to higher population densities. It has therefore become increasingly important to monitor UK earthquake events in order to learn more about them and predict them.
After an earthquake has been felt extensively in the UK, a questionnaire survey is usually carried out to find out what intensities were felt in what settlements. These intensities can then be plotted on a macroseismic map and isoseismic lines showing areas of equal seismic shock may be drawn. From this, the epicentre of the earthquake can be seen, and its approximate magnitude can be calculated. .
For this exercise, data was used from a fictitious earthquake centred in the Bishop's Castle area of Shropshire. Fictitious questionnaire data was used to calculate what intensities were felt in different settlements, and this information was plotted on a map of the area. From this, it was easy to see where to draw isoseismic lines. Isoseismals were not drawn for EMS values of 2 or less. .
After constructing isoseismic lines, the area within the intensity value 3 (marked in red - see map) was calculated. This figure was then put into an equation to calculate the magnitude (ML) using the following formula:.
Area (A3) = 20,857km2.
ML = 1.03log - 0.19.
= 1.03log x 20,857 - 0.19.
= 4.13ML.
As this ML value was calculated using a macroseismic map, the value is said to be "macroseismically derived". In areas without sufficient seismographs, this is the only was of calculating the magnitude of an earthquake.
