Are UK earthquakes a cause for real future concern?
After a 4.4-magnitude earthquake in South Wales, Professor Iain Stewart discusses the cause and impact of these seismic events
How common are earthquakes in the UK?
Small earthquakes – anything less than a magnitude 3 – are a fairly common annual occurrence. The geology of much of the UK is pretty old – hundreds of millions of years across much of the west of mainland Britain – and it is riddled with ancient fault lines that were once very active but are now virtually extinct. The trouble is that unlike volcanoes, old fault lines never go completely extinct and often have very small crustal shifts on them, which triggers small earthquakes. But while we have this backdrop of so-called ‘tectonic’ quakes, many of the small earthquakes that are registered in the UK are caused by human activity, such as the collapse of disused mine workings.
What causes them – are we on a fault line?
It is often tricky to identify a specific fault line with a particular earthquake – there are just too many faults down there. That’s certainly the case in South Wales, which was once the leading northern edge of an ancient mountain belt. From the seismic waves recorded, seismologists at the British Geological Survey have determined that the fault involved in this earthquake involved a small horizontal shunt, either on a near-vertical fault line trending NW-SE or one trending NE-SW.
What influences the strength of an earthquake?
The size of the fault that moves determines the energy it releases, but then the depth is important too. The shallower the quake, the less its shaking will be dampened by the rocks the seismic waves pass through. In areas of very hard bedrock, the waves travel more effectively and the shaking is stronger, whereas if the rock is broken up then the shaking is softened. By and large, you’re more likely to feel the earth move if you’re living on top of hard, rigid geology below.
Was this a particularly strong earthquake in UK terms?
We would expect to see an event of this size (M 4.4) every four years or so, and in fact a slightly smaller (M 4.2) seismic shock occurred beneath the Bristol Channel in 2014. One of the UK’s biggest quakes of the 20th century happened near Swansea in 1906 – a magnitude 5.2 quake that caused minor structural damage between Swansea and Cardiff. If you go back a few more centuries there are other quakes with magnitudes 5 or above, so this region definitely has ‘form’.
An illustration of the Swansea earthquake of 1906 (By Geological Society of London via Wikimedia Commons)
Why don’t we experience the catastrophic earthquakes seen elsewhere in the world?
The crustal stresses that build up beneath the UK are the long-distant effects of tectonic forces applied at two far-flung plate boundaries – Iceland to the NW and the Mediterranean to the SE. So we’re being squeezed ever so gently in a slow tectonic vice – nothing dramatic and calamitous, just the odd reverberation unpicking old weak fault lines.
Is there a risk that we might see them at some point in the future?
There is no reason to expect any increase in the UK’s seismicity over coming decades. But we do know that in past times the UK, and the London area in particular, have been affected by damaging earthquakes that go up to magnitudes of 6 and above, causing substantial damage and even deaths. A repeat of those earthquakes is entirely possible, and it is questionable the extent to which the modern glass and steel structures of The City are designed for significant earthquake vibrations, particularly since the soft muddy ground of the Thames flood plain is likely to amplify seismic shaking. But apart from a potential risk in the south-east corner of the country, I’m not sure there’s much to be concerned about with UK earthquakes.
Is there anything that we can do to prepare for them, and are more efforts needed to make people aware of the science behind earthquakes in the UK?
Although there is no real threat from earthquakes in the UK, the modern reality is that many of us climb on a plane and jet off to countries that do have very severe earthquake threats. The likes of Italy, Turkey, Greece and much of the Mediterranean are earthquake-prone, and then there are obvious places like California and Japan. So there is an argument that even those living in the seismically quiet corner of Europe should know some basic earthquake safety. Equally, those that are responsible for our safety abroad – holiday tour operators and hotel management – ought to make visitors aware of the threat in the same way that people are made aware of fire safety. So, teaching earthquake safety in Geography lessons at school would certainly make sense, even if we’re not likely to need it here.
Is more research needed to understand why they happen, and where they might occur?
The UK has some of the best seismologists in the world, but for most of them the concern is not here in the UK but abroad in those regions where seismic calamities are a regular affliction. In that context, we are always trying to improve the science and to use modern technology, especially satellite monitoring, to reduce the risk of earthquake disasters. So there is much work still to be done on the global seismic threat, but thankfully little to concern us here in the UK.
The aftermath of the 2015 Nepal earthquake (By Rajan Journalist via Wikimedia Commons)
What research is taking place at the University to develop a greater understanding of earthquakes generally?
We have specialists working across a range of natural hazard threats – earthquakes, volcanoes, tsunamis, landslides and floods. From coastal engineering through to geography and geology, researchers are working on how best to reduce the disaster potential of these naturally occurring calamities. In some cases, the issue is understanding better the physical mechanisms by which these events occur.
We have a research project led by Dr Luca Menegon who is studying ancient fault lines in northern Norway (a region not known as earthquake country) as an analogue for the deep crustal fault lines that are responsible for major earthquakes deep beneath the Himalayas. In other cases, the problem is how to more effectively communicate what we already know about these threats to those communities at risk.
My main interest has been concerned with the seismic threat to Istanbul, a city of 13 million people that lies along a lethal seismic fault line. Seismologists are anticipating a destructive earthquake here in the coming years to decades, so the challenge is how earthquake scientists can get their message across. We’ve just published a paper that discusses the ethical responsibility that geoscientists have in communicating to those living in at-risk neighbourhoods of the city and the difficulties that are faced in situation where a geological threat becomes a complex political issue.
Sometimes, the hardest challenge is not understanding the complex science of the earthquake but how to motivate people and authorities to become better prepared.
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