OR/18/029 Seismic activity
| Baptie, B. 2018. Earthquake seismology 2017/2018. British Geological Survey Internal Report, OR/18/029. |
The details of all earthquakes, felt explosions and sonic booms detected by the BGS seismic network have been published in monthly bulletins and compiled in the BGS Annual Bulletins.
There were 218 local earthquakes located by the monitoring network during 2017–2018, with 26 having magnitudes of 2.0 ML or greater, and nine having magnitudes of 3.0 ML or greater. Fourteen events with a magnitude of 2.0 ML or greater were reported felt, together with a further 11 smaller ones, bringing the total to 25 felt earthquakes in 2017–2018.
The largest earthquake was a magnitude 4.7 ML event on 30 June 2017 in the central North Sea. The epicentre was 215 km southeast of Lerwick, Shetland and 310 km northeast of Aberdeen. It was felt in Shetland, Orkney, Wick, Thurso and in Fraserburgh with a maximum intensity of 3 EMS.
The South Wales earthquake of 17 February 2018 (4.6 ML) was the largest earthquake on mainland Britain in almost 10 years, since a magnitude 5.2 ML earthquake near Market Rasen on 27 February 2008. The epicentre was approximately 18 km north-northeast of Swansea but it was felt across all of Wales and much of England, with a maximum intensity of 5 EMS.
On 4 August 2017 at 14:43 UTC, an earthquake of magnitude 4.0 ML occurred in the locality of Moidart on the west coast of mainland Scotland. The epicentre was approximately 22 km south of Mallaig, 50 km west of Fort William and 145 km northwest of Glasgow. The earthquake was the largest event in the region since a magnitude 4.0 earthquake near Arran on 4 March 1999 that was felt widely across southwest Scotland.
A magnitude 3.4 ML earthquake occurred approximately 5 km south of Cockermouth, Cumbria, on 28 February. We received over 110 reports of the earthquake being felt, most of them from people living close to the epicentre and in the nearby towns of Whitehaven and Workington. The intensity of the shaking was generally weak or moderate. It was the largest earthquake in Cumbria since a magnitude 3.5 ML earthquake on 21 December 2010 near Coniston.
The Moidart earthquake of 4 August 2017
The Moidart earthquake of 4 August 2017 (4.0 ML) was the largest earthquake in Scotland for 18 years. The earthquake was felt widely across the west of Scotland. Only five other earthquakes of this size or greater have been observed in the period of instrumental recording from 1970 to present.
On 4 August 2017 at 14:43 UTC, an earthquake of magnitude 4.0 ML occurred in Moidart on the west coast of Scotland. The epicentre was approximately 22 km south of Mallaig, 50 km west of Fort William and 145 km northwest of Glasgow. The earthquake was the largest event in the region since a magnitude 4.0 earthquake near Arran on 4 March 1999 that was felt across southwest of Scotland.
The earthquake was followed by at least four aftershocks, the largest of which had a magnitude of 3.4 ML and which occurred two minutes after the mainshock. The two largest aftershocks were also felt.
Analysis of the BGS earthquake catalogue shows that there have been only five other earthquakes in this region with magnitudes of 4 ML or above in the period of instrumental monitoring from 1970 to present. The largest of these was a magnitude 4.4 ML earthquake near Kintail in 1974. This was one of a sequence of over 20 earthquakes that occurred over several months in 1974/1975. Two other earthquakes in this sequence also had magnitudes of above 4.0 ML. A magnitude 4.1 ML earthquake near Oban in 1986 was 24 km south-southeast of the Moidart earthquake.
Historical observations of earthquake activity in Scotland date back to the 16th century (Musson, 1996[1]). These show that despite many accounts of earthquakes felt by people, damaging earthquakes are relatively rare. Scotland's largest recorded earthquake, a magnitude 5.2 ML event in Argyll in 1880, was 75 km to the southeast of the Moidart earthquake. Only two other earthquakes with a magnitude of 5.0 ML or greater have been observed in the last 400 years.
Some 350 members of the public from 121 different postcodes completed our online questionnaire, allowing EMS intensities to be calculated in different locations. A minimum number of five reports from a given 5 km by 5 km square are required to estimate the intensity. Where there are fewer than 5 different observations, we assign an intensity value of ‘Felt’. A maximum intensity of 5 EMS was observed in the villages of Acharacle and Roshven, 10 km from the epicentre. An intensity of 5 EMS was also observed at Tobermory and Mallaig, approximately 20 km from the epicentre. Intensities of 4 EMS were observed at Lochailort (10 km), Strontian (12 km) and Arisaig (12 km). There were too few observations to determine a value for the intensity from most of the other locations close to the epicentre, perhaps as a result of the low population density. Intensities of 4 EMS were observed in Oban and Fort William at distances of 45 km to the southeast and 48 km to the east, respectively. The earthquake was felt at distances of up to 150 km from the epicentre, including Inverness and Invergarry to the northeast and Glasgow to the southeast. Five reports from central Glasgow suggest an intensity of 4 EMS, however, this does not seem consistent with other observations.
Over half of the reports state that people considered the shaking to be moderate in strength, while around one third thought that it was weak. Many people reported hearing a moderate to loud bang or rumble. There were 31 reports of objects falling over but no reports of damage.
The South Wales earthquake of 17 February 2018
The South Wales earthquake of 17 February 2018 (4.6 ML) was the largest earthquake on mainland Britain in almost 10 years, since a magnitude 5.2 ML earthquake near Market Rasen on 27 February 2008. The earthquake occurred in a part of South Wales that has experienced bursts of earthquakes with magnitudes of 5 ML or above at regular intervals in the last few hundred years, however, there has been relatively little seismicity in the region in the last few decades.
On 17 February 2018 at 14:31 UTC, a magnitude 4.6 ML earthquake occurred in South Wales. The epicentre was approximately 18 km north-northeast of Swansea and 55 km northwest of Cardiff, but it was felt across all of Wales and much of England. It was the largest earthquake on mainland Britain in almost 10 years, since the magnitude 5.2 ML Market Rasen earthquake on 27 February 2008 (Ottemöller and Sargeant, 2010[2]).
The earthquake occurred in a part of South Wales that has been struck by a number of other significant earthquakes in the last few hundred years, although there has been relatively little seismicity in the last few decades. A magnitude 5.2 ML earthquake in 1906 was one of the most damaging British earthquakes of the 20th Century, with damage to chimneys and walls reported across South Wales (Davison, 1907[3]). Earthquakes with magnitudes of 5.2 and 5.1 occurred near Swansea in 1727 and 1775, respectively. The epicentre of the earthquake on 17 February 2018 is close to the epicentre of the 1775 event.
More recently, a magnitude 4.1 ML earthquake in the Bristol Channel on 20 February 2014, approximately 50 km to the southwest was also felt widely in South Wales. Three earthquakes with magnitudes of 4.1, 3.9 and 3.0 ML occurred in 1974, near Newport, approximately 50 km east. A cluster of instrumentally recorded seismicity approximately 40 km to the east, near Bargoed, may be associated with mining activity in the South Wales coalfields.
7811 members of the public completed our online macroseismic questionnaire. Data were grouped by postcode into 5 km by 5 km squares and an EMS (European Macroseismic Scale) intensity was calculated in each. We received data for 1363 different squares. An intensity of 5 EMS was reported widely throughout South Wales. An intensity of 5 EMS was also observed in North Devon (approximately 80 km); Bristol (100 km); Stroud, Gloucester and Cheltenham (approximately 120 km). Intensities of 4 EMS were observed at Swindon (145 km), Birmingham (155 km) and Liverpool (190 km). Reports are clearly biased towards areas of higher population density, with relatively few reports from Pembrokeshire or North Wales.
The earthquake was felt as far away as Blackpool, 240 km north-northeast of the epicentre; in the East Midlands, 200 km northeast; Oxford, 180 km east; Southampton, 200 km southeast; and as far east as Slough and Windsor, 225 km from the epicentre. The earthquake was also felt in much of Devon and Cornwall.
Over half of the reports (4543) stated that people considered the shaking to be moderate in strength, while 2833 reports stated that it was weak. The shaking was described as severe by 317 people. There were just over 200 reports of superficial damage, but on closer examination, many of these refer to existing cracks in plaster.
Coloured squares in (b) show the number of observations used to determine each intensity value. Yellow star denotes the epicentre.
References
- ↑ Musson, R M W. 1996. Determination of parameters for historical British earthquakes. Annali di Geofisica, 39, 1041–1048.
- ↑ Ottemöller, L, and Sargeant, S. 2010. Ground-Motion Difference between Two Moderate-Size Intraplate Earthquakes in the United Kingdom. Bulletin of the Seismological Society of America, 100, 4, 1823–1829.
- ↑ Davison, C. 1907. The Swansea Earthquake of June 27th, 1906. Quarterly Journal of the Geological Society, 63, doi.org/10.1144/GSL.JGS.1907.063.01-04.25.