Monday, 5 February 2018

London and Earthquakes

Londoners leave the city in advance of a third earthquake in 1750, but it never happened.

Obviously, London is not a city one normally associates with earthquakes and seismic damage, but there is such a connection, and it is quite surprising.

There are tales of the effects of earthquakes upon London and its inhabitants in at least a dozen cases before the 20th century. Other accounts may be hidden and it is quite probable that other earthquakes were felt but not written about in surviving literature. A few events have occurred with epicentres that were likely to have been under London, but the majority come from the seismogenic areas of the United Kingdom: the Midlands and Lincolnshire, north Wales, Kent and the English Channel into France and the Netherlands.

In this brief and unscientific account, intensities are given in the roughly coincident part of the main, relatively modern, scales (MM, MSK, etc.) and, because of the low values, magnitudes are quoted in the by now redundant and somewhat inaccurate Richter scale, ML, or local magnitude.

An earthquake is known to have rocked London in December 1164, but no details are forthcoming. Another occurred on 13th or 20th February 1247 with possible seiching of the Thames. Still others occurred on 14th December 1269, 11th September 1275 and 4th January 1299. A more substantial seismic event took place on Wednesday, 21st May 1382 at about 2 p.m. It caused enough seiching on the Thames to capsize boats, and it significantly damaged old St Paul's Cathedral. The earthquake occurred while a Synod of the Church was being held in Blackfriars in order to discuss dissenters. The Archbishop of Canterbury, William Courtnay, was prescient and level headed enough to attribute it to natural causes, something that his successors in the church rarely did. Overall, the 1382 eatthquake caused effects in London to about intensity VI. A further earthquake struck London on 23rd April 1449, and one occurred in Croydon on 25th May 1551.

One of the most widely discussed historical earthquakes was that which occurred on Wednesday, 6th April 1580, at 6 p.m.. It had an estimated magnitude of 5.7-5.8, and an inferred hypocentral depth of 20-30 km. Its estimated recurrence interval was about 200 years. The epicentre was either in the English Channel or in France south of Calais. London fell at the western end of a band of intensity VII effects that extended across the Channel to Lille and beyond. It appears that this event caused seiching in the Thames, if not a minor tsunami, and it definitely resulted in localised flooding. Several chimneys collapsed in London, a pinnacle fell from Westminster Abbey, and damage was particularly significant in Shorditch. Two young people, Thomas Gray and Mabel Everite, were killed by falling stones at Christchurch, Newgate. The boy died instantly and the girl succumbed several days later. The 1580 earthquake may have come from a source akin to that which caused a magnitude 4.3 event (with a hypocentral depth of 5.3 km) under Folkstone in 2007.

A more northerly source of seismicity lies in the North Sea, and it delivered tremors to London on 24th December 1601. This brings to mind the largest recorded earthquake in the British Isles, the 1931 Dogger Bank tremors (magnitude 6.1), which was also felt in London.

Relatively large earthquakes are also generated in north Wales and the Irish Sea area. Those of 7th October 1690 (magnitude circa 5.2) and 9th November 1852 (magnitude estimated at 5.3), had epicentres at Caernarfon and were felt in London.

A smaller event occurred at noon on Thursday 6th February 1750, with a magnitude of about 2.6. It was followed a month later, on 8th March, by a magnitude 3.1 event, which was experienced at 5:30 a.m. The shaking for this was violent in London, and the epicentres for the two events were estimated to have been near Leadenhall Street and near Lambeth, respectively. A rumour was propagated that earthquakes would occur monthly, which led to a mass exodus from London (and gridlock on the roads) on 8th April 1750. Needless to say, there were no tremors. The Church of England attributed the 1750 earthquakes to God's displeasure at the publication of Memoirs of a Woman of Pleasure ("Fanny Hill", by John Cleland, 1748-9).

A peculiarly destructive earthquake occurred at 09:18 on Monday, 22nd April 1884, with an epicentre at Wivenhoe, Essex. It lasted 20 seconds, and had an estimated magnitude of 4.6 and a hypocentral depth of about 70 km. In the Colchester area this event destroyed one mediaeval church and severely damaged at least four others. It damaged 1,250 other buildings in the area. One building, which was in a precarious state, was said to have collapsed in east London.

Other, more recent earthquakes that were felt in London include the 22nd September 2002 event at Dudley, West Midlands (magnitude 4.7), the 2007 Folkstone event mentioned above, and the earthquake of Wednesday, 27th February 2008 at Market Rasen, Lincolnshire (magnitude 5.2).

The leading expert on UK earthquakes and British seismicity is Dr Roger Musson of the British Geological Survey. He has warned that London is overdue for a damaging seismic event. Such is our critical infrastructure, that next time the effects are likely to be more serious and more complex than they were in 1580 and 1750.

Select Bibliography

Davison C. 1924. A History of British Earthquakes. Cambridge University Press, Cambridge: 332-335.

Guardian 2010. London is overdue for a major earthquake, warns seismologist. The Guardian, 16 September 2010.

Musson, R.M.W. 2004. A critical history of British earthquakes. Annals of Geophysics 47(2/3): 597-609.

Musson, R.M.W. and P.W. Winter 1996. Seismic hazard maps for the U.K. Natural Hazards 14(2-3): 141-154.

Neilson, G., R.M.W. Musson and P.W. Burton 1984. The  “London” earthquake of 1580, April 6. Engineering Geology 20: 113-141.

Scott, R.F. 1977. The Essex earthquake of 1884. Earthquake Engineering and Structural Dynamics 5: 145-155.