February 27, 2008

earthquake in England

UK residents were woken to the largest earthquake in the UK for nearly 25 years. The quake, measuring 5.2 on the Richter scale, had its epicentre near Market Rasen in Lincolnshire.

Prof Bill McGuire, Director of UCL’s Benfield Hazard Research Centre, said:

“Nowhere on the planet is entirely quake-free, and while the UK does not experience major earthquakes like that which struck Sumatra in 2004, small quakes occur every few years. These rarely do more than dislodge a few chimneys or crack the odd wall. A quake on the scale of last night’s can be expected at least once a decade or so, and is nothing to write home about. The largest earthquake ever recorded in the UK occurred beneath the North Sea’s Dogger Bank in 1931. This was 10 times stronger than the Market Rasen event, and even generated a small tsunami.”

Prof Bob Holdsworth, Head of Earth Sciences, University of Durham, said:

“Most potentially damaging earthquakes like this occur near to plate boundaries such as the San Andreas Fault, but the UK lies in the interior of a plate, so this is quite unusual. The causes and controls of UK seismicity are still poorly understood, but it is possible that it reflects the reactivation of an old fault zone that has lain dormant for tens or hundreds of millions of years. The UK crust is riddled with such old faults which form an important part of our geological heritage. Perhaps this one is just reminding us that it is still there.”

Professor Roger Searle, Department of Earth Sciences, University of Durham, said:

“Although most earthquakes occur along tectonic plate boundaries such as the San Andreas Fault, Britain does experience a moderate level of seismic activity. This reflects minor adjustments to the stresses that occur in all plates. A ‘quake as large as the Market Rason quake is quite rare in Britain: It is almost 25 years since the last one of this size, which was in Wales. Enormous earthquakes like the 2004 Banda Aceh Boxing Day earthquake are about a million times more energetic than this one.”

Edmund Booth, Consulting Engineer, said:

“Some pointers, from the point of view of a structural engineer specialising in earthquake resistant design:
1),Last night’s earthquake appears to be entirely in line with previous expectations of the sort of earthquake that can occur in the United Kingdom. Seismologists advise that earthquakes of magnitude up to 6 are possible almost anywhere in the world.
2),The amount of damage from last summer’s floods probably far exceeded the total damage from earthquakes in the UK over the last century.
3),Earthquakes occur without any warning and are frightening to experience, even at levels far below those needed to cause damage. This is especially true in a country like the UK where the level of seismicity is low to very low. So a startling UK earthquake is not necessarily one of much engineering significance.
4),The new European engineering standard for the design of structures in seismic regions, Eurocode 8, is currently under discussion at the British Standards Institute, to decide the extent of its applicability within the UK. http://www.seced.org.uk/view_news.php?display=9&PHPSESSID=c8a40559d792dd59010e1287dfe8351c gives some details, including downloads for two relevant documents. The first is by the British Geological Survey, Edinburgh, which discusses the range and intensity of likely seismic activity in the UK. The second gives recommendations on the circumstances when it may be advisable to consider earthquake effects when designing UK structures. Both documents were written last year (i.e. before last night’s earthquake.)
5),Earthquakes pose special problems for structural engineers. Unlike other natural effects such as flooding or high winds, they occur without warning and shake not only the basic structure of facilities like buildings, power stations and bridges, but also their contents, including things such as vital safety systems within industrial facilities. Moreover, hundreds of years of data are needed to get a reliable assessment of the most intense shaking that can occur at any particular location, particularly in an area like the UK; the worst that has occurred during the last hundred years at any location may be well below the worst shaking that is possible. This is why earthquakes cause particular difficulties in the design of ‘high consequence of failure facilities’ such as petrochemical plant or nuclear power stations, and it is accepted that such facilities should be designed against earthquakes in the UK. It is generally agreed, however, that more ordinary facilities which are well designed for wind and gravity loads don’t need any special anti-earthquake measures.”

Dr. Bruce D. Malamud, Reader in Natural and Environmental Hazards, Kings College London, said:

“The Market Rasen earthquake has been measured by the British Geological Survey (BGS) with a magnitude 5.2 (ML) on the Richter Scale.

“The magnitude scales used today all stem from that proposed by Charles Richter of the California Institute of Technology, in 1935. His Richter scale takes into account how much the ground moves (as measured by a seismograph) at a distance of 100 km from where the earthquake originated. Not all seismographs sit at 100 km from the earthquake’s centre, so corrections are made for the distance. ML stands for the “local magnitude”, and is just one of many scales based on the original Richter scale.

“Since 1 January 2008, there have worldwide been more than three earthquakes per day with magnitudes greater than 5.0. So, an event of magnitude 5.2 (the Market Rasen earthquake) is not uncommon around the world. But, this is the first time we have had this size event in the mainland UK since 1984.

“Earthquakes (which can be located at depth–such as this one, recorded by the BGS at 5 km below the surface of the ground) generate energy, sending waves through the earth. These waves can be up and down, side to side, and ‘compressional’ [imagine taking a slinkey, stretching it out, and hitting it on one end–a wave will travel ‘through’ the slinkey].

“A seismograph is a sensitive monitor of how much these waves are moving the ground at the location that the seismograph is located. These monitors of earthquake activity were first believed to have been used by the Chinese, many thousands of years, and have become increasingly sophisticated every since. If you want to try your own ‘home made’ seismometer, using a plastic bag filled with water, a vehicle (car, bus, tube), and pen.

For many years, scientists have invested a large amount of time on whether earthquakes can be predicted (their location, size, and time). But, although many theories have been proposed, ranging from animals changing behaviour just before an earthquake, changing chemistry of water in water wells, and the change in ‘temperature’ of an area (as viewed by remote sensing) before an earthquake, no one theory has been found to consistently work in terms of prediction of an earthquake hours to days before its occurrence.”