1990 Bishop's Castle earthquake

Earthquake
title = Bishop's Castle earthquake
date = 02 April 1990




magnitude = 5.1 M_L
depth = 14.1 km
location = coord|52.43|N|3.03|W
countries affected = ENG WLS
intensity = 6 EMS - Slightly damaging
casualties = 0

The 1990 Bishop's Castle earthquake was an earthquake that hit near the town of Bishop's Castle, Shropshire, England on 2nd April 1990.

Location, date and time

On April 2nd, 1990, a powerful earth tremor was felt across much of England and Wales at 13:46 34.2s UTC. Early news reports in the immediate aftermath speculatively attributed the epicentre to places as far apart as Nottingham and a valley in the east of Wales and then settled on Wrexham [cite web | title=New Scientist | work= Contemporary account of limitations to the original seismological reporting, which rates the earthquake at 5.4 on the Richter scale |url= http://www.newscientist.com/article/mg12617120.400-how-britains-seismometers-got-it-wrong-.html | accessdate=2008-02-02] , before geologists finally concluded that it had in fact been in the vicinity of the small town of Clun near the town of Bishop's Castle, Shropshire. [cite web | title=BGS Bishop's Castle page | work=British Geological Survey web-page on the Bishop's Castle earthquake |url=http://www.quakes.bgs.ac.uk/macroseismics/bishopscastle_macro.htm| accessdate=2008-02-02] .

Cause

Bishops Castle lies atop an ancient geological fault line - the Pontesford Linley fault. A sudden movement in the fault sent shock waves through the rock [cite web | title=Shropshire County Council| work= Landscape of contrasts|url=http://www.shropshire.gov.uk/discover.nsf/open/81F8010DA4FE7DCC80256D6B005664A7| accessdate=2008-02-02] . The local rocks are predominantly Jurassic Middle Lias [cite web | title=Shropshire Rocks| work= |url=http://www.shropshirerocks.org/shropshiregeologicaltrail| accessdate=2008-02-02] .

Magnitude

The magnitude of the earthquake was originally measured as being between 5.0 and 5.4 on the Richter scale [cite web | title=New Scientist | work= Contemporary account of limitations to the original seismological reporting, which rates the earthquake at 5.4 on the Richter scale |url= http://www.newscientist.com/article/mg12617120.400-how-britains-seismometers-got-it-wrong-.html | accessdate=2008-02-02] [cite web | title=New Scientist | work= Another contemporary account from the same issue of New Scientist, 12 April 1990, but rating the magnitude at only 4.9|url= http://environment.newscientist.com/article/mg12617120.100-a-case-of-the-shakes-.html | accessdate=2008-02-02] [cite web | title=EERI | work=Contemporary newsletter of the Earthquake Engineering Research Unit, April 1990 |url= http://www.eeri.org/lfe/pdf/uk_bishops_castle_1990_newsletter.pdf] . It has since been rated below the midpoint of that range, at 5.1. [cite web | title=BGS Bishop's Castle page | work=Recent British Geological Survey page on the Bishop's Castle earthquake |url=http://www.earthquakes.bgs.ac.uk/macroseismics/bishopscastle_macro.htm] . Nevertheless, it remained the greatest earthquake to have struck anywhere in the UK since 1984 at least until February 27, 2008, when an earthquake at Market Rasen was provisionally rated at 5.2 by the British Geological Survey, which provisional rating, though exactly matching the midpoint of early ratings for the 1990 Bishop's Castle Earthquake, is 0.1 higher than the final rating accorded by the BGS to the latter.
The largest earthquake on land in the UK measured 5.4, and had an epicentre on the Lleyn Peninsula, north Wales, in July 1984 [cite web | title=Daily Telegraph| work= |url=http://www.telegraph.co.uk/news/main.jhtml?xml=/news/2007/04/28/nkent228.xml| accessdate=2008-02-02] .

Impact

The earthquake was felt by people as far away as the east of the Republic of Ireland to the west, the city of Newcastle Upon Tyne to the north-east, the county of Kent to the south-east, and the county of Cornwall to the south-west.

In Shrewsbury, the county town of Shropshire, which lies some miles to the north of Bishop's Castle, there was damage to masonry, with a number of chimney stacks being broken off from roofs and collapsing partially or completely into gardens and streets. Some others were knocked askew. Several of the worst affected buildings, including shops, were evacuated. Police cordons were put up around houses at risk of chimney-collapse until they had been made safe, with at least fifty properties in the town reported as requiring emergency attention within the twenty-four hours immediately following the event, while others requiring less urgent treatment were tended to on subsequent days [cite web | title=EERI | work=Contemporary newsletter of the Earthquake Engineering Research Institute, April 1990 |url= http://www.eeri.org/lfe/pdf/uk_bishops_castle_1990_newsletter.pdf] .

There was also damage to ornamental features such as crosses and gargoyles built into the masonry of some of Shrewsbury's medieval churches, and to Clun Castle.

Electrical power was lost from areas served by some substations situated approximately thirty kilometres (seventeen miles) from the epicentre after the earthquake caused transformers at the substations to trip offline [cite web | title=EERI | work=Contemporary newsletter of the Earthquake Engineering Research Institute, April 1990 |url= http://www.eeri.org/lfe/pdf/uk_bishops_castle_1990_newsletter.pdf] .

Residents of the worst affected areas, including parts of Shrewsbury, reported lateral shaking and swaying to the walls of their houses at the height of the tremor, which was preceded and then accompanied by a rumbling noise that gained strength over a period of 15 to 30 seconds before reaching and sustaining peak intensity during the most severe shaking. Finally, the movement and accompanying sound tailed off much more rapidly than it had first built up, stopping altogether within just a few seconds from the peak activity [cite web | title=Google Books | work= papers presented at the International Conference on Earthquake, Blast and Impact held at the University of Manchester Institute of Science and Technology, UK, 18-20 September 1991, graph
url=http://books.google.com/books?id=dxtbhw-UE7AC&pg=PA5&vq=%22response+spectra%22&dq=bishops+castle+earthquake&sig=g4tLvTEl3y3kt7jQJY9xAdynsYk] , [cite web | title=Google Books | work= papers presented at the International Conference on Earthquake, Blast and Impact held at the University of Manchester Institute of Science and Technology, UK, 18-20 September 1991, discussion
url=http://books.google.com/books?id=dxtbhw-UE7AC&pg=RA1-PA1&lpg=RA1-PA1&dq=bishops+castle+earthquake&source=web&ots=Rro-7BBcoA&sig=6JiaXpHkwbrX7JS3x-pnkbQ5VNw | accessdate=2008-02-02] .

Damage to buildings was also reported in Wrexham, and some minor damage as far north as Liverpool and Manchester. [cite web | title=USGS | work= Significant earthquakes of the world 1990 - see first April entry reporting geographical extent of masonry damage
url= http://earthquake.usgs.gov/eqcenter/eqarchives/significant/sig_1990.php | accessdate=2008-02-02]

No serious injuries were reported.

An engineering consultant, using the 1990 Bishop's Castle earthquake specifications as a model, has estimated that a slightly weaker earthquake measuring 4.8 on the Richter scale which struck at Dudley in September 2002 would have caused structural damage of an order costing less than 5% of the cost of the complete reconstruction of an entire property to repair, to 1% of buildings situated in towns in the vicinity of the epicentre [cite web | title=Will Re Civil Engineers| work= Catastrophe Report |url=http://www.willisre.com/html/reports/catastrophe/DUDLEY.pdf| accessdate=2008-02-02] . While some indication of the typical percentage of homes damaged to a similarly minor extent by a slightly stronger earthquake measuring 5.1 on the Richter scale may be extrapolated from this estimation, a representative of the British Geological Survey has stated that 'numerous' but fewer than 20% of properties in Shrewsbury suffered the partial or total loss of their chimneys or damage of equivalent gravity "(see below)". Reliable, exact figures of the percentage of buildings damaged close to the epicentre of the 1990 Bishop's Castle earthquake are not readily available, but the aforesaid sources taken together appear clearly to indicate that between 1% and 20% of properties in towns near the epicentre suffered damage of European Macroseismic Scale Grade 7 severity.

The Welsh Borders suffer frequent earthquakes by British standards. Over the last century, there have been sizeable shocks near Shrewsbury in 1932, near Ludlow in 1926 and near Hereford in 1896 and 1924 [cite web | title=New Scientist Magazine | work= Ancient faults and modern earthquakes
url=http://www.newscientist.com/article/mg12717244.000-ancient-faults-and-modern-earthquakes-britains-recentearthquake-happened-on-a-fault-that-was-active-hundreds-of-millions-of-yearsago--faults-of-similarly-ancient-lineage-could-be-the-sites-of-futuretremors-.html | accessdate=2008-02-02] .

Impact Assessment Controversy

As of February 3rd 2008, the tremor was rated by the British Geological Survey at only 6 on the European Macroseismic Scale (EMS 98), a subjective rating of the impact of earthquakes at ground level. This rating was disputed by those who lived in the worst affected regions and witnessed with their own eyes the many partially and totally collapsed chimneys, which according to the EMS scale as described at the British Geological Survey's own website [cite web | title=BGS EMS Synopsis | work=Synopsis of the European Macroseismic Scale as described by the British Geological Society |url=http://www.quakes.bgs.ac.uk/macroseismics/ems_synopsis.htm] could be seen as conferring to the Bishop's Castle Earthquake a rating of at least 7, the same rating granted to the lower-magnitude Folkestone earthquake of 2007 according to the website of the British Geological Survey [cite web | title=BGS Folkestone page | work=EMS Rating of the 2007 Kent Earthquake according to the British Geological Society website |url=http://www.quakes.bgs.ac.uk/macroseismics/folkestone_macro.htm] .

However, on February 5th 2008 a representative of the British Geological Survey, Roger Musson [cite web | title=BGS Seismology Enquiries | work=BGS Seismology Contact Information and Staff List Page |url=http://www.earthquakes.bgs.ac.uk/aboutus/contacts/contacts.htm] , clarified in a formal written response to enquiries that 'the critical distinction between intensity 7 and 6 is that the higher degree entails that fall of parts of chimneys, or equivalent damage to ordinary masonry structures, occurs to about 20% of buildings. While it is certainly the case that in Shrewsbury the 1990 earthquake caused damage to numerous properties, the number of houses damaged as a proportion of the total building stock was not that high. Hence the intensity assessment of 6'.

He added 'After extensive discussion with the engineering community, the maximum intensity at Folkestone has also been downgraded to 6, on the basis that the houses damaged were in particularly poor condition and thus failed to qualify as "ordinary"'.

References