“The Day We Trembled... ”
THE MELTON MOWBRAY EARTHQUAKE OF 28 OCTOBER 2001
by Paul Henni
The epicentre of the earthquake of 28 October 2001 was about 4km east of Long Clawson. The magnitude was 4.1 ML* (see notes at bottom of article) with a depth of 11 km, and reports suggested that the earthquake was felt throughout Lincolnshire, Leicestershire, Warwickshire, Yorkshire, Shropshire and Nottinghamshire.
What is an earthquake?
An earthquake is the sudden release of strain energy in the Earth's crust resulting in waves of shaking that radiate outwards. When stresses in the crust exceed the strength of the rock, it breaks along lines of weakness. The point where an earthquake starts is termed the focus or hypocentre and may be many kilometres deep within the earth. The point at the surface directly above the focus is called the epicentre.
Earthquakes may occur anywhere, but they are unevenly distributed over the Earth with the majority occurring at the boundaries of the major crustal plates. They also occur, less frequently, within the plates and far from the plate boundaries, as in eastern USA. Australia and the United Kingdom.
One of the largest earthquakes ever was in Chile on 22 May 1960 with magnitude of 9.5 Mw. Other large earthquakes include Lisbon in 1755 (magnitude 8.7 Ms); Assam in 1897 (8.7 Ms); and Alaska in 1964 (9.2 Mw).
How often do earthquakes occur in Britain?
Between 200 and 300 earthquakes are detected and located in the UK by the British Geological Survey annually. The risk from these earthquakes is not insignificant and must be considered when engineering for sensitive installations. Earthquakes of magnitude 5.0 ML2 and higher occur in UK on average every 8 years; magnitude 4.0 to 4.9 every 2 years; there are on average 3 a year of magnitude 3.0 to 3.93; 26 a year magnitude 2.0 to 2.9; and 140 a year magnitude 1.0 to 1.9.
The biggest earthquake Britain has ever had was the North Sea earthquake of 7 June 1931, with a magnitude of 6.1ML and with an epicentre offshore in the Dogger Bank area, 120 km NE of Great Yarmouth. Damage was reported from 71 different places in Britain, with the strongest effects at Filey, where the top of a church spire was rotated. Bridlington, Beverley and Hull were also affected with most of the damage affecting chimneys and plaster. A factory roof fell in at Staines (Surrey) and cliffs collapsed at Flamborough Head and Mundesley, Norfolk.
The Lleyn earthquake (North Wales) in July 1984, with a magnitude of 5.4 ML was the largest onshore earthquake this century in the UK: damage consisted of widespread cracks in plaster and falls of some chimneys and weak plaster. The April 1990 Bishop's Castle earthquake in the Welsh Borders, with a magnitude of 5.1 ML, was the second largest onshore earthquake in recent years: it was felt over an area of 140,000 sq. km.
The Colchester earthquake of 1884 (4.6 ML) was the most damaging earthquake in the UK for several centuries. It was described by Peter Haining in “The Great English Earthquake” (Robert Hale, 1976). There was considerable damage to churches, including the top of a spire falling, falling masonry from roofs, falling turrets and parapets. Damage to residential properties included shattering of brick walls, and chimney falls, often through roofs. Even London was affected: in the House of Commons, Peter Haining reports, books were shaken from shelves, and the Government was shaken; both literally and metaphorically.
Personal injury resulting from earthquakes in the UK is rare. However, eleven people are known to have died as the result of British earthquakes. Six were killed by falling stones, two fell from upper floors, two died of shock, and one, apparently, committed suicide.
The “Keyworth” earthquake of 2001
Following the earthquake, BGS launched a survey: questionnaires were placed in newspapers covering the felt area, and an electronic questionnaire was made available. Around 2000 e-mailed responses were received, and an estimated 3000 paper questionnaires.
The total number of places from which replies were received was 285 (after amalgamating replies from very close settlements less than 2 km apart).
The large number of replies received is partly a function of the strength of shaking in the most affected area, but is also influenced by the fact that the shock occurred in the heart of the English Midlands, a well-populated area.
The highest intensity experienced was over an area around and south of Newark-on-Trent and east of Loughborough. This area was distinguished by an increased number of reports of objects thrown down, a greater level of alarm, and a greater tendency for the shaking to be described as strong.
Objects thrown down included a large model boat falling off a shelf, books thrown from shelves, candlesticks falling from a windowsill, a picture thrown off a wall, small ornaments and children's toys being knocked over, and so on.
In a number of cases various types of alarm were set off, and in one case a traveller on the Loughborough train reported passengers screaming as the train momentarily lurched on the tracks, evidently reacting to quite a strong shaking. In some places (e.g. Belvoir, Leicestershire) the shock was perceptible out of doors. There were a few reports of people running out in fright, and many reports of people going outside to investigate. Animals (pets, cage birds, horses) were alarmed in many cases. A very few reports mentioned people falling off their chair or over-balancing. A very common report was the creaking of house joists. There were a number of reports of minor damage, not all of which can be authenticated. (It is common that after an earthquake householders believe their pre-existing cracks have widened - but unless measurements before and after have been made, this could be put down to imagination much of the time!) The following reports more or less sum up all the damage:
Leicester North - “... cracked pane of glass in back door...”
Market Harborough - “... a couple of roof tiles were dislodged...”
Rotherby - “... plaster from the decaying ceiling fell around me...”
Coventry South East - “... cement pointing fell out of roof - not there day before ...”
Whitwick - “... cracks widened around window frames ...”
Carlton Scoop - “... slight cracks in wall plaster surrounding door and window frames...”
Granby - “...wall plaster dislodged from joint between wall and ceiling...”
Keyworth - “... long stepped cracks have appeared in the mortar in a wall ... rendering bowed away from bricks on a neighbor's wall... a little mortar and brickwork loosened under a window ... a piece of cement about 6” long fell off the gable end of the house.”
Long Eaton - “... a chimney stack on some old terraced houses nearby collapsed...”
West Bridgford - “... crack in big window...”
The most distant reports were from the following places: in the west, the earthquake was felt near Chester; in the east, at Great Ryburgh, 5 km south east of Fakenham, Norfolk; in the north, the limit of observation was marked by Knaresborough; and in the south, the shock was felt as far as Bedford -with a single very distant observation from Salisbury.
* Several different scales for measuring earthquake intensity exist. The Richter local magnitude (ML) is used for local' earthquakes up to 600 km away, and is the magnitude scale used by BGS when locating UK earthquakes. For comparison purposes, a magnitude 5 ML earthquake is equivalent to the explosion of 1,000 tons of TNT whereas a magnitude 6 ML earthquake is the energy equivalent of 30,000 tons of TNT or a 30 kilotonne nuclear explosion. Surface wave magnitude (Ms) is used for observations near the earthquake epicentre where the surface wave is larger than the body wave; moment magnitude (Mw) is considered the best scale to use for larger earthquakes as the Ms saturates at about magnitude 8.
Paul Henni suggests that if you have access to the Internet you can learn more about earthquakes from BGS website at http://www.gsrg.nmh.ac.uk/hazard/faq1.htm.
he interim report on the Melton Mowbray earthquake was published at http://www.gsrg.nmh.ac.uk/.