RMW Musson (BGS Scotland)
I used to be woken abruptly at 1:30am by the ringing of my cellphone, which was sitting on the shelf above the bed room radiator. The cellphone ringing in the course of the evening normally means just one factor: unhealthy information. I took the cellphone into the corridor in order to not disturb my spouse greater than want be. A colleague of mine was on the road:
There’s been a big earthquake in Lincolnshire. It’s about 5.”
Visions of injury raced via my thoughts. Magnitude 5 just isn’t massive in world phrases, however is in regards to the higher finish of what’s skilled in Britain. And measurement just isn’t every little thing: location is crucial. We’ve got been lucky within the UK lately, as the most important British earthquakes have principally occurred effectively away from giant cities. The 2 earlier 5s (in 1984 and 1990) have been within the excessive north-west of Wales and the Shropshire countryside respectively. Nonetheless, an earthquake in Lincolnshire in 1185, most likely about 5 in magnitude, introduced down a big a part of Lincoln cathedral and, if folklore is to be believed, so destroyed some close by villages that they have been by no means rebuilt. Was this a repeat of the 1185 earthquake?
OK, I’m on my method.”
The streets of Edinburgh have been utterly abandoned as I drove as much as the BGS workplace on the southern fringe of town. Three of my colleagues have been already there, mentioning knowledge from the seismic monitoring community remotely over radio and pc hyperlinks. By triangulating the instances at which the shockwaves had reached every instrument, they’d already mounted the epicentre at some extent roughly midway between Lincoln and Grimsby – the closest place of any measurement being the small city of Market Rasen.
The magnitude, that measure of size that journalists and the public call the ‘Richter Scale’ (a term never used by seismologists if they can avoid it), was currently set at 5.3 ML (ML stands for ‘local magnitude’, the correct term for Richter’s original scale; there are others in widespread use also). Magnitude is calculated from the wave amplitude, corrected for distance, at each monitoring station. The more stations, the better the estimate and, for this earthquake, the final value was eventually corrected to 5.2 ML once all the data had come in.
Already the phones were beginning to ring. Journalists from as far away as Australia were calling to get the details. By now, we had a full team in the office, some continuing to refine the interpretation of the data, others preparing the press release, the web pages and the alert information for urgent dispatch to government offices and managers of critical facilities. And everyone was taking a hand in answering the phones and giving out information to the media.
One colleague grabbed a camera and headed for Waverly railway station to take the first train south and start surveying damage in the epicentral area as soon as possible (damage is generally cleared up quickly, for obvious reasons – seismologists have to get in fast if they want to see what happened on the ground). The field crew started preparing equipment to take down by Land Rover to install extra stations for recording aftershocks.
By 6am, the first TV camera crew had arrived at the office. Schedules of interviews were being drawn up – some of the TV interviews to be filmed on location in the BGS office, others in the BBC and ITN studios in the centre of town. And practically every radio station wanted at least a few minutes’ talk over the phone.
By now, it was becoming apparent that the scale of the damage wrought by the earthquake was less – far, far less – than I had feared when first taking that phone call over five hours earlier. Quite evidently, Lincoln Cathedral was still standing. There were no reports of evacuated homes as there had been for the more modest (4.2 ML) Folkestone earthquake the previous year. Nor were there reports of streets full of fallen chimneys. There was one report of a serious injury – a man suffering a broken pelvis, as parts of a chimney collapsed and fell though his bedroom ceiling – but this was in Wombwell in South Yorkshire, far from the epicentre and not even in the same county.
This was turning out to be the pattern of the earthquake: isolated damage, a chimney down here and there, over a very wide area (even over 100km away) but no concentration of damage near the epicentre. Indeed, my colleague with the camera had picked up a hire car at Doncaster and joined up with a small team who came up from the BGS headquarters at Keyworth in Nottinghamshire. Together, as they drove around the North Lincolnshire countryside, they found very little to photograph. At Market Rasen itself, there was practically no damage whatsoever, other than some stones fallen from the roof of the parish church. And where fallen chimneys were in evidence, they were mostly old and decayed, and had evidently been fairly insecure before the earthquake.
Such a pattern can mean only one thing: the focus was deep. Whereas in seismology the epicentre denotes the point on the Earth’s surface beneath which the rupturing of the fault that produced the earthquake initiated, the three-dimensional point where the earthquake began is known as the hypocentre, or focus. When an earthquake occurs, a geological fault (usually a pre-existing one) gives way under the strain resulting from the shifting of the planet’s tectonic plates. The fault tears as the rocks on either side suddenly shift in opposite directions. That tearing starts at the hypocentre and then spreads out over the fault surface until the rocks stick again and movement ceases. The part of the fault that breaks in this way is known as the rupture plane; the larger this is, and the bigger the displacement of rocks across it, the bigger the earthquake.
Because the fault breaks suddenly and violently, it releases a lot of pent-up energy in the form of seismic waves, which become shaking when they reach the surface of the Earth. These waves radiate out from the rupture plane, especially from where the displacement (or slip) was largest. However, in the case of an earthquake like the Market Rasen one, the entire rupture most likely only extended for several hundreds of metres, with a slip to be measured in centimetres. Given that the fault break occurred over 20km down, in the lower part of the crust, even if you were standing directly at the epicentre, the shockwaves would have travelled through 20km of rock before they reached you, with some of the energy lost on the way. Also, it is typical of such deeper earthquakes that, as the seismic waves get refracted by strata and structure on their way upwards, the pattern of effects at the surface tends to be more complex than a simple decrease in strength of shaking with distance.
This is why there was such a contrast with the Folkestone earthquake of 2007. That earthquake, about 30 times less powerful in terms of actual energy release, occurred at a depth of around 5km. It therefore had a much greater kick in the area around the epicentre, but was felt over a much smaller area; outside of Kent, it was barely perceptible.
In fact, a feature of the Market Rasen earthquake was the very large area over which it was perceptible to at least some degree. Because it occurred at a few minutes to one in the morning, at which time most people are asleep, it was mostly only people still up and sitting quietly, or lying awake, who would have been able to notice the weakest shaking at long distances from the epicentre. Over the course of the day, more and more distant reports began to filter in. It had been felt in Yeovil, Somerset. Someone in Aberdeen had felt it. It was rumoured to have been felt on the Dutch coast (this turned out to be true – it was felt in Belgium also). And in the evening, after I had finally got home again, a seismologist friend in Dublin phoned to say it had definitely been felt in Ireland as well.
The pattern of effects in full was revealed by the results of a questionnaire survey. The study of earthquake effects is known as macroseismology. The strength of earthquake shaking at any place can be measured by instruments in terms of ground acceleration, velocity or displacement, but if one has no instruments, an intensity scale can be used, similar to the Beaufort Wind Scale. Intensity scales date back to the nineteenth century and were the seismologist’s main tool in the days before the development of seismometers for recording earthquakes. The most modern scale, which is the one used in Britain, is the European Macroseismic Scale (EMS or EMS‑98). This is a twelve-degree scale ranging from 1 (imperceptible) to 12 (total destruction). The first six degrees can be summarised in a simple fashion as follows:
- The earthquake was not felt at all.
- The shaking was very feeble, felt only by isolated individuals in favourable circumstances.
- The shaking was weak and felt by a few people.
- The shaking was felt by many people indoors and was strong enough to cause windows and doors to rattle and rafters to creak.
- The shaking was strong enough to cause alarm and shift or knock down some small objects. Some slight damage might occur, such as fine cracks in plaster.
- The shaking was strong enough to cause people to run out of houses, to knock books off shelves and cause minor damage to buildings: cracks in plaster, falls of parts of chimneys.
Practice in seismology used to be that questionnaires would be sent out to the public and the results collated, before a seismologist would assess each locality against the scale. This was an extremely laborious process. After the 19 July 1984 Lleyn Peninsula earthquake, the 12,000 or so questionnaires collected were never fully analysed just because of the immensity of the task. Today, matters are easier because of the Internet. A questionnaire was placed on the BGS seismology web page to collect the data, which were then converted from postcodes to latitudes and longitudes, and the intensity values assessed for each place using an automatic algorithm. As a result, it was possible to produce a first contour map of earthquake intensity (called an isoseismal map) the day after the earthquake – a task that would previously have taken many weeks.
In addition to the BGS questionnaire survey, similar data sets were collected by the USGS and the European-Mediterranean Seismological Centre (EMSC) as part of permanent data collecting programmes. The results of all three surveys were combined to produce a massive data set of over 31,000 responses from around 2,700 places. It is from this that the isoseismal map shown here was derived. Although intensity 6 EMS was assessed for a number of locations, these were widely scattered over an area in which the intensity was predominantly 5 EMS – thus there is no isoseismal for intensity 6 EMS.
However, for those who felt the earthquake, the first question was: “Will there be any more?”. It is a well-known feature of earthquakes that they are frequently followed by ‘settling-down’ events known as aftershocks. While these are generally a good deal smaller than the first or main shock, they can be disproportionately damaging, as they finish off chimneys or walls that were already weakened by the main shock. While no one can reliably predict earthquakes in general, there are some useful rules of thumb regarding aftershocks. Firstly, they tend to decline rapidly in strength and frequency with time after the main shock. Secondly, the largest aftershock is typically one magnitude less than the main shock. Thirdly, the largest aftershock usually occurs within 48 hours. Fourthly, in Britain, it seems to be the case that the largest earthquakes produce either long sequences of aftershocks or virtually none at all. Which type would this one be?
The nearest equivalent in modern times was probably the East Midlands earthquake of 11 February 1957, a 5.3 ML event that was followed by a 4.2 ML aftershock the next day. We would soon know if the Market Rasen earthquake was going to behave in a similar way. It didn’t. The main shock (at 00h 56m on 27 February) was followed by the first aftershock at 02h 46m – a mere 1.8 ML in magnitude and practically imperceptible. At 09h 03m, a second occurred, also 1.8 ML. Then, at 16h 54m the same day, a 2.2 ML was recorded… And then nothing.
The additional stations installed by the BGS field team around the epicentre, at Belchford, Broxholme and Metheringham, would have picked up even tiny events. It seems clear that fault movement stopped almost immediately.
In the days following the earthquake, the instrumental data collected by BGS was augmented with recordings from overseas to start modelling the event in more detail. Using a process known as moment tensor inversion, one can resolve an image of the fault causing an earthquake, with a limitation that the technique always produces two complementary solutions, either of which could be correct. The type of fault that moves in an earthquake is related to the tectonic processes at work. A common question received from the public was: “Why should we have an earthquake in England, when it isn’t on the edge of a tectonic plate?”. The answer is that the stresses placed on any tectonic plate don’t stop at the margins. The entire plate is under stress and, even in the centre of it, if local stress concentrations rise above the strength of some ancient fault, minor slippage will occur. In the context of Western Europe, the driving force is the opening of the Atlantic Ocean, which exerts compressive stress on the rocks of the British Isles from a north-westerly direction.
Given that minor mid-plate adjustments, in the form of intraplate earthquakes, typically involve purely or predominantly horizontal displacements, it follows that the sort of fault most likely to be reactivated in such conditions is one that trends either north-south or east-west. The principle is exactly the same as in sailing: a yacht moves fastest if the sails are angled at 45o to the direction of the following wind. This is what has been observed in most significant British earthquakes of the past 30 years, and proved to be true in the Market Rasen event also. The causative fault was strike-slip (it moved horizontally) either north-south or east-west, with a small compressive element. Had there been a long aftershock sequence, visualising the pattern of hypocentres would have given a clear indication as to which solution was correct, as one would be able to see the distribution line up along one of the possible planes. Looking at a geological map is not necessarily helpful; the structures visible at the surface are not necessarily a good guide to what exists 20km down. Therefore, precisely what fault caused the earthquake remains unknown.
So, what can we make of the 2008 Market Rasen earthquake, the largest British event since 1984? Obviously, it represents an escape in some ways. Thanks to the depth, the damage caused was nowhere severe. Even so, it is estimated that insured losses will run into several tens of millions of pounds, simply because of the sheer size of the area over which damage occurred. Even if damage only occurred to a tiny fraction of houses, apply that over a large enough area (including the conurbations of South Yorkshire and Manchester) and you get a lot of claims.
Lincoln cathedral, however, did not suffer as in 1185 (or even in 1931 when very minor movement was detected in some buttresses following a North Sea earthquake), and a visual inspection made by the cathedral works department immediately after the earthquake revealed no damage or movement at all. Considering that many British cathedrals are in a rather fragile condition, and given that earthquake damage to British cathedrals has occurred in the past, it remains to be seen how long we can continue without another incident. In fact, any future British earthquake causing significant damage is likely to change attitudes regarding the seismicity of this country. Up to now, British earthquakes are a peril usually far from people’s minds.
Perhaps it is not altogether just good fortune. Looking at the record of past British earthquakes, there is definitely a correlation between magnitude and depth. The larger events generally do seem to be consistently deeper, and the contrast between the Folkestone and Market Rasen earthquakes bears this out.
Nevertheless, there have been earthquakes in the past that clearly were highly destructive, like the 1185 event and, one day, we may be less fortunate.
In the sequel to this article, I will write about British earthquakes more generally.
Acknowledgements
Seismology is a team effort. In writing the above words, I am of course reflecting the work of all my colleagues. The monitoring and study of earthquakes in the UK is supported by a Customer Group led by the Dept of Communities and Local Government. This article is published with the permission of the Executive Director of the British Geological Survey (NERC).
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