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Thursday, September 16, 2010

London is Past due for Earthquake

London is overdue for a major earthquake, warns seismologist
The Guardian

London is overdue for a major earthquake, warns seismologist

After earthquakes in 1382 and 1580, London can expect a similar quake at some time in the coming decades that would cause considerable damage and loss of life

View of the London skyline from the top floor of the National Portrait Gallery.Photograph: Paul Owen
A magnitude 5.5 earthquake in London would probably spare modern office buildings, but older buildings would be at risk. Photograph: Paul Owen/Guardian

London is overdue for an earthquake that could cause billions of pounds worth of damage, a leading seismologist warned today.

Britain is not close enough to any tectonic plate boundaries for a large earthquake like the catastrophic magnitude 7 quake that hit Haiti earlier this year. But small or moderate quakes can still cause damage.

In April 1580, a magnitude 5.5 earthquake caused extensive damage in the south-east of England and north-east of France. In London, two people were killed. The epicentre of the quake was in the Dover Straits 86 miles from London, originating 10-20 miles underground.

"This earthquake can certainly happen again because even the quake in 1580 was a repeat of a previous one that occurred in 1382, with almost the same epicentre, size and results," said Roger Musson of the British Geological Survey.

Speaking at the British Science Festival in Birmingham, Musson said that precise predictions of the next major earthquake to hit London were difficult. "All we can do is say that something that has happened twice can and probably will happen three times, but as to whether it happens tomorrow or in two years' time or in 20 or 50 years' time, that is something we would love to know but we don't."

He added: "What we can be sure of is that, in the years since 1580, the exposure of society to earthquakes has increased enormously. The same earthquake happening tomorrow will impact on far more people than was the case in the 16th century. The size of London in terms of population is about 50 times more today than it was in 1580."

Britain as a whole typically gets a magnitude 3.5 earthquake once a year, a magnitude 2.5 quake 10 times a year, and around a hundred magnitude 1.5 quakes a year. Every 10 years the country will feel a magnitude 4.5 quake and, on average, a magnitude 5.5 every 100 years.

The British Isles sit in the middle of a tectonic plate that is being squashed from two sides: north-east along a boundary with north Africa and eastwards along a plate boundary in the Atlantic Ocean.

Because of old geological movements, there are lots of small cracks that have developed in our tectonic plate during old phases of mountain-building. "Think of it as a dinner plate that has been broken several times and glued back together again and you're squeezing it," said Musson. "If one bit is not glued terribly well, then it can give a little."

The cost of an earthquake in the south-east of England would be huge. An earthquake in 1999 that hit the city of Newcastle in New South Wales, Australia, caused about £4bn of damage at today's prices, said Musson, despite being smaller than the 1580 English quake.

He said that a magnitude 5.5 earthquake would likely spare modern office buildings, but older Victorian buildings would be at risk. "What's tended to get damaged most was buildings of the Victorian period that are in bad repair. You'll remember there was a small earthquake in Folkestone in 2007. What was damaged most was old chimneys – they came down. Newer houses were not damaged at all ... It may not sound very dramatic compared to buildings collapsing but if people are walking in the street and a chimney falls on you, that's bad news."



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