Market Rasen Earthquake

Last night I was suddenly awakened by a jolt. I had just enough time to say to my better half “don’t panic – earthquake” before the surface waves hit. “How did you know?” she asked. “I felt the body waves first”* I replied. I was also able to assure her that it was a regional earthquake not a local one (our local events are much shorter and sharper) before turning the radio on to listen to the news. Five live’s up all night programme must have had the largest audience ever.The recording of the earthquake from Keele University is below:

The record is clipped in the middle as the seismometer was designed for fainter, more distant earthquakes.

The earthquake, about magnitude 5.2 according to the BGS, was centred on Market Rasen in Lincolnshire. It is the largest UK ‘quake since the Lleyn Peninsula earthquake of July 1984. The largest recorded UK earthquake , magnitude 6.1, was under the Dogger Bank, off Great Yarmouth in June 1931.

*Don’t believe this malarky about animals predicting earthquakes, they feel the smaller body waves first like I did, where has most humans are more likely to notice the later, larger surface waves.

Update:

As to the cause of this event, I’d rule out mine collapse as UK mining events rarely get above magnitude 3 and at magnitude 5.2 this is over a thousand times more powerful.

The UK is being compressed from the north-west (the mid-Atlantic Ridge) and the south-east (tail end of the Alpine orogeny). There are also vertical forces related to glacio-isostatic rebound (uplift) from the last ice age but most UK earthquakes are strike-slip with NW-SE compression.

The UK has a long and complex geological history. At England’s heart is a solid triangular Precambrian core – the Midlands Microcraton (with its upper apex somewhere near Manchester). Along the boundaries of the Midlands microcraton are a number of faults that have long and complex histories and tend to get reactivated by modern geological stresses.

To the west is the NE-SW trending Church Stretton / Pontisford Fault system (e.g. Bishop’s Castle 1990 earthquake), to the east is a NW-SE trending system (e.g. Market Rasen 2008, Dogger Bank Earthquake 1931). This triangle is also bisected by the N-S Malvern Line (e.g. Dudley Earthquake 2002)

So large earthquakes in the UK tend to be associated with lines of fundamental weaknesses in the English crust dating back to the Precambrian.

Boudica's Earthquake

Interesting article in the July BBC History Magazine about a possible Colchester, Essex earthquake at the time of Boudica in the first century AD.

Roman historians Tacitus and Dio Cassius note that just before Boudica’s revolt the statue of the goddess Victory in Colchester (then the political and religious capital of Roman Britain) rotated and fell over. ‘Otherworldly groans’ were heard and the sea ‘turned blood red’. Boudica’s followers interpreted this as divine approval for the rebellion and the Romans saw it as a bad omen.

The descriptions have been interpreted by Hull archaeologist Raphael Isserlin and BGS seismologist Roger Musson as signs of an earthquake. As well as the toppling statue, earthquakes can be accompanied by groaning sounds and the red sea colour could be the result of a turbidity current or cliff collapse triggered by the ‘quake.

Colchester was hit on April 22nd, 1884 by a magnitude 4.7 earthquake which due to its shallow depth caused widespread damage (including 1,200 buildings and 31 churches) and strange noises were also reported. There were two deaths, one of shock (presumably heart attack) and the other was a woman called Mary Saunders who viewed the earthquake as a sign that worse was to follow that she drowned herself in the River Stour.

It is interesting that it looks like we can add Boudica’s revolt to the destruction of Sodom and Gomorrah and the parting of the Red Sea to earthquake related events.

Source: David Keys, BBC History Magazine July 2007 ( Volume 8, Number 7, Page 8 )

Geology Errors From The BBC II

More geology errors from the BBC … This time from their Science and Nature – Hot Topics – Natural Disasters section on Earthquakes

Centre of the Earth
The Earth is made up of three main layers:
# The core is at the centre of the Earth

OK so far

# The mantle is a mobile semi-molten layer around the core
The mantle is a plastic solid. It is not semi-molten (except under limited circumstances)

# The outer-shell of the Earth is called the crust. Scientists call this the lithosphere – it’s the part we’re on now
The crust and lithosphere are distinctly different things. The lithosphere comprises the crust and the upper part of the mantle. Mantle and crust are compositional entities, lithosphere, aesthenosphere and mesosphere are mechanical ones.

The crust is made up of 12 individual tectonic plates.
The lithosphere is make up of numerous tectonic plates – how many major plates there are is a matter for debate, but overall there are far more than 12 plates

Below the sea, they can measure three to six miles (4km-9.6km) thick and under land this increases to 20-44 miles (32km-70.8km).

But the plates are lithosphere – about 220km thick.

… The point where the seismic activity occurs is the epicentre, where the earthquake is strongest. But it doesn’t always end there, seismic waves travel out from the epicentre, sometimes creating widespread destruction as they pass.

The focus of the earthquake is the hypocentre. The epicentre is the point on the Earth’s surface above the hypocentre.

To be continued …

Kent Earthquake

The Kent earthquake comes as no real surprise to me. This area is one of the most active areas for large earthquakes in the UK, albeit with long return times. The most significant events were on 21st May 1382 and 6th April 1580, both estimated at Local Magnitude 5.8. The latter caused much damage in the southeast of England and is one of Britain’s most fatal earthquakes with two apprentices being killed by falling masonry in a church in Newgate.

This region lies on the Artois (as in Stella) Axis, the lateral equivalent on the Variscan front that runs though South Wales and the Bristol Area – a fundamental lineament and a major zone of crustal weakness.

Note that at magnitude 4.3 today’s earthquake was about 180 times less powerful than the historic events (1 unit on the magnitude scale = ~32 x increase in power [10x amplitude]).

Note also that I’ve not mentioned the ‘Richter’ word – it only works in California with a Wood-Anderson seismograph!