Facts about the Folkestone Earthquake

Folkestone Earthquake 28 April 2007

Alice Walker and Roger Musson, British Geological Survey, Edinburgh

The earthquake which shook Folkestone and the rest of Kent at 08:18 on 28 April, was by no means unusual for the UK as a whole, although the intensity of shaking and level of damage have rarely been exceeded in the few hundred years for which we have records. At a magnitude of 4.2 ML, we experience this size of earthquake, or greater, every 5 years or so somewhere on land or in the UK offshore areas.

The damage from the Folkestone earthquake was locally heavy to a degree seldom seen in the UK. In parts of Folkestone many houses suffered damage to chimneys and walls, particularly collapse of brickwork from the upper parts of chimneys, which in some cases caused secondary damage to roofs on the way down. In some cases chimney stacks were twisted, and in a few cases buildings suffered structural cracks to the extent that they may need to be demolished. Severity of earthquake shaking, in terms of its effects, is classified according to an intensity scale (The European Macroseismic Scale - EMS) which runs from 1 (not felt) to 12 (total destruction). The strongest effects in Folkestone can be considered to be intensity 7 EMS. There is no direct correlation between an earthquake's magnitude ("Richter Scale"), which is a measure of the energy released, and the intensity of shaking, which depends also on the distance to the epicentre, the depth at which the fault moved, and the surface geology.

The epicentral location of the Folkestone earthquake was determined using data from the BGS UK seismic monitoring network supplemented with readings from European neighbours. The analysis places the epicentre more or less directly adjacent to the town, and the focus (where the fault actually moved) was at a depth of 2 km. Following the mainshock a small number of aftershocks, which were not felt, were also recorded by the local seismograph station.

The felt area of the earthquake extends throughout Kent and as far as Bognor, 130km to the west, and Norwich, 175km to the north, with a few unconfirmed reports at greater distances. It was felt weakly in Calais and Boulogne. The map shows contours for intensity 3, 4, 5 and 6 (EMS), with the highest intensity being too localised to be contoured.

Historically the people of Kent have felt earthquakes centred in, or near, the Dover Straits in 1950, 1776, 1580 and 1382; the two earliest ones had magnitudes estimated at just under 6, and caused damage as far as London, where in 1580 two people were killed by falling masonry. The last earthquake felt by residents in Folkestone was actually a Dutch earthquake (epicentre near Maastricht) in 1992. Comparisons can be made between the Folkestone event and the Colchester earthquake of 1884, magnitude 4.6 ML. At Colchester, there was also much damage in the epicentral region, and it is likely that similarities between Folkestone and Colchester will strengthen as more analysis is conducted; the shallow depth of both earthquakes in a ‘soft' geological environment, resulting in a concentration (and possibly amplification) of the ground-shaking in their epicentral areas.

The geological explanation of earthquakes such as the Folkestone event is uncertain. Detailed study of the data from the Folkestone earthquake will, hopefully, shed more light on this difficult problem. Most British earthquakes are understood to occur as a result of reactivation of ancient lines of geological weakness in response to regional pressure from the widening of the Atlantic. Additional monitoring stations are being installed by the British Geological Survey (BGS) to record any further activity in the area.