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THE DAY THE EARTH DID NOT STAND STILL.
THE UK, AN EARTH QUAKE ZONE?
In Hertford, Herefordshire and Hampshire hurricanes hardly ever happen however earthquakes are another matter. In the past five years there have been two earthquakes in the UK that were approaching the severity on the Richter scale that can be life threatening.
In 2002 the unlikely site of the last earthquake of note was Dudley in the West Midlands, almost as far inland as it is possible to be in the UK. It was initial thought to have a magnitude of 4.8 on the Richter scale but was later upgraded to 5. It was the most severe quake to date and raised questions of residual ground stability where mining was once commonplace, the area surrounding Dudley is “Honeycombed” with disused mines that have either been capped or abandoned long ago.
On Saturday 28th April another earthquake awoke the sleepy county of Kent in south- east England. It was estimated to have a magnitude of 4.3 on the Richter scale by the British geological society but the USGS puts its magnitude at 4.7. Its epicentre was thought to be under the English Channel some 7.5-8 miles off the coast of Kent near Folkestone. Strangely the County of Kent and North Worcestershire around Dudley have a commonality of the past, which has to a certain degree left both counties similarly unstable below ground level.
The area around Dudley was mined extensively in the eighteenth, nineteenth and the first half of the twentieth centuries, the prize to be gained was coal, iron, lead, limestone gravel and clay. Dudley was the crucible that wrought the beginnings of the industrial revolution. The played out workings, gravel pits and exhausted limestone excavations left scars below the earth in the rock strata, which amounted to huge cathedrals of caverns.
Kent on the other hand has something in common with Dudley and two features that are unique to Kent. In the seventeenth, eighteenth, nineteenth and twentieth centuries Kent fuelled the cauldron of a growing London with a massive amount of coal. Most was mined from Kent itself but some mines were known to extend under the sea. The water supply in Kent holds another of the features that could affect the rock strata too.
Whilst some of Kent’s water comes from “Surface reservoirs” a great deal of it is sourced from sumps deep underground in the limestone and chalk caverns, the problem is that most of the surface reservoirs are less than 50% full and many of the underground sumps are virtually empty. The reason that the water stocks are so low stems from two causes and the first is rainfall. In the past two years the rainfall has been diminishing in Kent in fact in eighteen of the past twenty-four months the rainfall has been well below average and the water table is dropping rapidly as a result. The second reason is wastage Kent has a high number of leaks that have not so far been rectified because it is cheaper not to fix leaking pipes whilst a captive consumer base will continue to pay for it. The consumer in Kent is also part of the wastage problem. They have a usage rate that is far in excess of many other areas; the average consumption in Kent is 160-litres/per person per day higher than most comparable areas. It born of a complacency that assumes that water will always flow from a tap but the day is fast approaching when it will not. With many of the water collection sumps at such a low level the absorption of energy from movements within the subterranean rock strata has diminished. The other unique feature of Kent is the Channel Tunnel, its route under the English Channel brings it ashore very close to Folkestone but it is not thought to be a contributory factor.
Earthquakes are usually caused by the release of energy at the edges of the Earths tectonic plates as one plate sub-ducts the plate it abuts. The problem is that there are no Tectonic plate abutments in the English Channel so the recent tremor felt in Folkestone was not as a result of subduction. There are however fault lines in the channel, fissures in the rock strata that do move and release energy as the movement normalises.
Folkestone is no stranger to tremors, it has happened before. In the past eight hundred years there have been four earth tremors, which have affected Folkestone some felt as far away as London.
The first recorded quake was in 1382 and though details were sketchy it was thought to have a magnitude approaching 6 on the Richter scale. The second Quake of note came in 1580 and was almost certainly in excess of 6 on the Richter scale, this quake was a strong one and was said to be responsible for two deaths in south London.
In 1776 an earthquake struck with a magnitude thought to be between 4 and 5 and the most recent prior to this incident came in 1950 with a similar force to this incident. It may seem as though several modern developments such as the Channel Tunnel construction, or mining, or excavations have contributed to the current tremors but since the land bridge between the British Isles and what is now mainland Europe was flooded by the rising water levels that followed the end of the last ice age England has been moving away from what is now France at a rate of approximately 4 centimetres a year. The movement however is not due to tectonic plate drift but erosion; the chalk cliffs of Kent are slowly crumbling into the sea and widening the Channel. The cliffs are what we call chalk and think of as a very soft rock but they were formed by the compaction of billions of tiny sea creatures over millions of years before the last ice age. The integrity of the cliffs is therefore brittle, they collapse into the sea frequently as fissures form, and water erosion undermines their security. When earth tremors happen the weaker structures of the chalk cliffs crack more easily and accelerate the collapse. On Saturday 28th April the intensity of the tremors 8 miles out to sea forced a split in the cliffs not near Folkestone but 150 miles away in Hampshire a reminder that a shock wave radiates outward from an epicentre and is rarely localised to the immediate area.
The tremors however left a scar upon Folkestone that will not be forgotten for a long time, it is estimated that the cost of this short interlude will run to £20 million but there were no fatalities and the most serious injury was a head/ neck injury to a woman in her thirties.
The UK is not in an “Earthquake zone” it does not sit at the edge of a tectonic plate, on the contrary it sits well toward the middle of a plate so “Severe earthquakes” do not happen but from time to time small tremors are felt and are often reactive to events elsewhere that have either happened or are about to happen. In the past 100 years there have been 14-recorded earthquakes in the UK [to be recorded an earthquake must have an intensity grater than 3.5]. Most are between 4 and 5.5 and some bear close comparison to earthquakes elsewhere, for instance the Swansea earthquake of 1906 came just 70 days after the San Francisco earthquake of 1906 and the Dogger Bank earthquake of 1931 that measured 6.1 on the Richter scale came after the catastrophic earthquake in Napier, New Zealand. The earthquake in Kent of Saturday 28th April follows several earthquakes in the south pacific in January 2007 and one on Sumatra in March. Earthquakes can accompany, precede or follow a volcanic eruption too, though it is more than 1200 miles from Kent mount Etna in Italy has been active since March, it has been reasonably benign since its last eruption in 1992 but in the past five years there have been several minor eruptions. On May 1st 2007 the volcano erupted again opening a new vent to the southwest spewing out white hot lava. Etna rests upon the edge of the African tectonic plate, which has been moving northward at a rate of one inch per year. The African tectonic plate has been sub-ducting the Eurasian tectonic plate for many years and as there have been few seismic events since the last eruption of Etna it could be assumed that a limit of plasticity has been reached and the safety valve on this occasion may well be Mount Etna, only time will tell.
Reactive Earthquake Theory is not an accepted science and is discounted by many as speculation but in the same way that Newton’s Cradle demonstrates the conservation of energy Reactive Earthquake theory dissipates energy stored by the Earth’s Tectonic plates. The problem with the theory is that it is imprecise; the variations that can affect timescale, intensity, magnitude and vectoring are globally diverse. It is a science that for the moment can only be applied retrospectively, one day that will change and earthquakes will be predictable.
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