The two faults are located underneath Central London and Canary Wharf and are moving at a rate of 1 to 2 mm per year, the Telegraph reported. They are capable of causing a magnitude 5 or 6 earthquake.
A 5.0 magnitude earthquake would feel like standing on a platform between two trains, the Mirror noted. That's enough to cause some annoyances and some broken wall-mounted objects, but not enough to bring down buildings — probably. But the slight chance of a magnitude 6.0 tremor could seriously damage structures.
That's big news, because although Britain is crisscrossed by several small fault lines, geologists had previously believed the city was built on geologically stable ground. Now the city will have to draw up new guidelines for buildings to survive an earthquake of an intensity up to 6.5 on the Richter scale.
The strongest quake ever recorded in the region was only a magnitude 6.1 beneath the Dogger Bank in 1931, far from any inhabited land.
However, Richard Ghail, a specialist in civil and environmental engineering at Imperial College in London, said that the threat of a quake is "enough to be scary but not fundamentally a problem," noting that the faults could probably only cause such a shaker once in 1,000 years, the Mirror reported.
The Richter scale, which is used to rate the intensity of an earthquake, is logarithmic, not linear. "That just means that if you add 1 to an earthquake's magnitude, you multiply the shaking by 10. An earthquake of magnitude 5 shakes 10 times as violently as an earthquake of magnitude 4; a magnitude 6 quake shakes 10 times as hard as a magnitude 5 quake; and so on," Science magazine noted.
Neither the British Isles nor London are strangers to quakes of 4 to 6 magnitude. The UK is hit by 20 to 30 earthquakes each year, The Telegraph noted. In 2008 the northern town of Market Rasen experienced a 5.2 magnitude tremor that was felt across the island, the biggest in 25 years.
And London itself has been shaken by quakes in the past, too, with a 5.5 magnitude earthquake in April 1580 that originated underneath northern France killing two people. A previous quake rocked the city in 1382 from the same location, the Guardian reported. A quake hasn't been felt in London since the 1770s, though.
However, London is about 50 times larger than it was back in 1580, the Guardian notes. The buildings most at risk are the historic ones, not modern structures. "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," said Roger Musson of the British Geological Survey.