Did the Earth Move for You as Well Darling?

by Roger Musson


Did the Earth Move for You as Well Darling?

England has a reputation as a land of seismic stability while California is considered a territory of tectonic terror. The two maps opposite show all of the earthquakes over magnitude four since 1970, and they vividly illustrate the reality that underpins this contrasting geological perception.

Feeling an earthquake in Los Angeles is not an unusual experience, in London, the last time any earthquake was felt by more than a few hundred people was in 1938. And that was not even a British earthquake, but the backwash of a Belgian one.

There is, of course, a profound geological reason why this difference exists. We have earthquakes because we live on the surface of a dynamic planet. Deep beneath our feet, vast masses of semi-molten rock slowly turn over in giant convection currents, and as they do so, the thin, brittle crust of rocks at the surface is dragged, fractured and warped. The larger discrete chunks of crust are referred to as tectonic plates, and it is where these plates collide that earthquakes are largest and most frequent: places like Alaska, Japan and Chile; places where devastating tsunamis can be caused as massive earthquakes kick up huge amounts of water by jolting apart the ocean floor.

This is not the situation in California: there is no tsunami threat from the San Andreas Fault, no matter what recent movies might show. In California, two major plates, the North American and the Pacific, are sliding past one another, with the San Andreas Fault marking the boundary. But it’s not a smooth sliding; the rocks stick until the strain on them gets larger than the friction that stops them sliding, and then they break free suddenly in an earthquake. You can try it yourself: take two pieces of rough wood, press them together, and try and slide one along the other. At first it won’t move; gradually push harder and harder, and eventually it will jump. You’ve just simulated an earthquake!

Because the San Andreas Fault is vertical, and slips sideways, and can’t displace the ocean floor, it can’t produce a tsunami. And for the same reason, all the old stories about a massive earthquake dropping western California into the sea are so much hokum.

Though the San Andreas Fault is not the only earthquake problem Los Angeles faces. If you look up a map of plate boundaries on the internet, they look like nice simple lines, but they are not. California is marked by a whole network of faults, related in one way or another to the San Andreas Fault, and all dangerous. Some of them run right under the Los Angeles metropolitan area (the San Andreas Fault does not) and it’s possible that a moderate earthquake very close could do more damage than a great one further away.

London is in quite a different situation: the nearest plate boundaries are in Iceland to the north, and Italy to the south. Does that mean no earthquakes? Not quite. Earthquakes can actually happen anywhere on the planet; those occurring away from plate boundaries are called “intraplate” earthquakes. It’s because all of the dragging of the crust sets up stresses everywhere, and even where plates are not banging together, it’s possible that some old fracture may snap under the strain.

The amount of earthquake energy released worldwide in intraplate events is a tiny fraction of what is released at the plate margins. Not only are intraplate events less common, but they are generally smaller. There is no way to get a great earthquake in an intraplate setting; energy does not build up in the same way as when one plate is determinedly trying to push past another one. So no magnitude eights for London, and certainly no tsunami.

But oddly enough, there are aspects in which Los Angeles and London share some seismological characteristics. Firstly, both sit on a geological formation known as a basin: a bowl of harder rocks containing soft alluvium or clay. This can start shaking like jelly when earthquake waves arrive, with the result that damage will be greater than in the surrounding areas. Then, just as Los Angeles is at risk from both a large fault at distance (the San Andreas) and smaller faults running right through the city (like the Newport-Inglewood Fault), historical evidence shows that the same may be true of London. You won’t find these earthquakes on the map above, because they are pre-20th century. In 1750 two small earthquakes struck London a month apart, causing much alarm and some slight damage. Both were small (probably no larger than 3 in magnitude) but because they were shallow, each delivered a good kick to a small area. The epicentre of both events was right in the heart of the City of London.

As to whether it could it happen again, with a bigger event, one can only speculate.

But London’s worst earthquake experience was back in Shakespeare’s day. On 6 April 1580, an earthquake of about 5.5 in magnitude occurred under the Dover Straits, causing damage in England, France, Belgium and the Netherlands. In London, two children were killed by falling stones (we even know their names: Thomas Gray and Mabel Everitt). And this time one can say with some certainty that such an event could happen again, because the 1580 earthquake was virtually a repeat of one in 1382 that also caused damage. However the 16th century death toll of two would probably be scaled up by the proportionate effect of having a huge population.

Ultimately, though, one must refer to the well-known saying, “earthquake don’t kill—buildings do”. And the question for both cities is: are they prepared? The buildings of Los Angeles have repeatedly been tested by earthquakes: in 1933, in 1970, and in 1994. Each time, lessons are learned and the construction of buildings is improved. For Londoners, an earthquake is probably the last thing on their mind, and it’s not the big steel and glass skyscrapers that are at risk—they wouldn’t notice even the largest British earthquake—but low-rise brick housing, a hundred or so years old and poorly maintained, is what poses the greatest hazard to life and limb.

So while Dwayne Johnson won’t need to save anyone from skyscraper-sized tsunamis this side of the San Andreas, Bob the Builder can probably spare a lot of lives.

Maps Courtesy Rogers Musson.