OR/17/006 Historical and recorded seismicity

Monaghan, A A, Dochartaigh, B O, Fordyce, F, Loveless, S, Entwisle, D, Quinn, M, Smith, K, Ellen, R, Arkley, S, Kearsey, T, Campbell, S D G, Fellgett, M, and Mosca, I. 2017. UKGEOS - Glasgow geothermal Energy Research Field Site (GGERFS): initial summary of the geological platform. British Geological Survey Open Report, OR/17/006.

This section contains a broad description of the seismicity in the British Isles and a detailed description of the seismicity around the Clyde Gateway area.

The UK lies on the north-west part of the Eurasian plate and at the north-east margin of the North Atlantic Ocean. The nearest plate boundary lies approximately 1500 km to the north-west at the Mid-Atlantic Ridge, where the formation of new oceanic crust at this divergent plate boundary results in significant earthquake activity. As a result of this geographic position, the UK is characterised by low levels of earthquake activity and correspondingly low seismic hazard.

The earthquake catalogue described here is based on the BGS UK earthquake database, which contains times, locations and magnitudes for earthquakes derived from both historical archives that contain references to felt earthquakes, and from instrumental recordings of more recent earthquakes.

The primary source of data for earthquakes before 1970 is the historical catalogue of Musson (1994), along with subsequent updates (e.g. Musson 2004, 2007). It contains earthquakes of moment magnitude of 4.5 Mw and above that occurred between 1700 and 1970, and earthquakes of 5.5 Mw and above that occurred before 1700. Each event has a location and magnitude determined from the spatial variation of macroseismic intensity, a qualitative measure of the strength of shaking of an earthquake determined from the felt effects on people, objects and buildings (e.g. Musson, 1996).

The primary sources of data from 1970 to present are the annual bulletins of earthquake activity published by BGS each year (e.g. Galloway et al., 2013). These contain locations and magnitudes determined from recordings of ground motion on a network of sensors in and around the UK (e.g. Baptie, 2012). The instrumental BGS database contains all events of 3.0 Mw and above, and some smaller earthquakes are well recorded by the UK seismic network.

Figure 68 shows a map of the UK seismicity. The symbols are scaled by magnitude (Mw). It is worth noting that the location uncertainty is ±5 km for instrumental earthquakes and up to ±30 km for historical earthquakes (Musson, 1994). The terranes (i.e. basement blocks bounded by faults (Bluck et al., 1992)), are homogeneous in terms of crustal properties (e.g. distribution and style of faulting), but the seismicity within each block is heterogeneous (Musson, 2007). There are spatial variations in the level of seismic activity across the UK. Western Scotland, western England, Wales, south-western Cornwall, and the area off the coast of the south-eastern England are the areas of highest activity. The eastern coast of Scotland, north-eastern England and Northern Ireland are almost earthquake-free (Figure 68).

The largest earthquake in the catalogue, and the largest known earthquake in the British Isles, is the 5.9 Mw 7 June 1931 event in the Dogger Bank area (Neilson et al., 1984). The largest instrumentally recorded onshore earthquake in the UK is the magnitude 5.1 Mw event on 19 July 1984 near Yr Eifel in the Lleyn Peninsula.

Figure 68    Distribution of earthquakes in the UK. Historical (pre-1970) and instrumental (post-1970) seismicity is indicated by squares and circles, respectively. Black lines are the simplified major faults (after Bluck, 1992). The dashed black square indicates the Clyde Gateway area.

Regional perspective

To give a regional perspective, a wider study area is included in the rectangle between 49.9°N and 59°N latitude, and 8°W and 3°W longitude (Figure 69). This region is characterized by high levels of clustered seismicity with Mw ≤ 5 in the west coast of Scotland and low levels of seismicity with Mw ≤ 4 in the Midland Valley Terrane (Figure 69).

The largest earthquake recorded in Scotland was the 4.9 Mw 1880 Argyll event whose epicentre is around 80 km from the Clyde Gateway area. It was felt along the west coast of Scotland, east as far as Perthshire, throughout the Inner and Outer Hebrides, and in Northern Ireland (Musson 1989, 1994). The exact epicentre is uncertain. The most probable location is near Loch Awe, between Oban and Inveraray (Musson 1989, 1994). The extent of the felt area of this earthquake suggests that the focus was relatively deep (~25 km).

The seismicity in the Midland Valley Terrane is dominated by two earthquake swarms, one in Comrie and the other in the Ochil Hills (Figure 69). Earthquake swarms are defined as sequences of earthquakes clustered in time and space without a clear distinction of main shock and aftershocks. They are at a distance of ~66 km from the Clyde Gateway area. The Comrie swarms occurred in 1608, 1788–1801 and 1839–1846 and are located just north of the Highland Boundary Fault, so may be related to activation of this structure. It can be questioned whether this activity is a swarm or consist of a mainshock with many fore- and after-shocks (Musson, 2007). The largest earthquakes in the Comrie swarms occurred in 1608 (4.3 Mw), 1801 (4.3 Mw), 1839 (4.5 Mw), and 1846 (4.1 Mw). The Ochil Hills swarms may be related to the Ochil fault that bounds the southern edge of the range of the hills (Musson, 2007). The activity consists of three discrete swarms, in 1736, in 1900–1916, and in 1979–1980, where the largest earthquakes were in 1736 (2.5 Mw), in 1905 and 1908 (2.9 Mw), in 1912 (3.4 Mw), and in 1979 (3.0 Mw).

The two largest events within 20 km of the Clyde Gateway area occurred on 14 December 1910 (3.1 Mw) and 16 July 1940 (3.4 Mw) (Figure 69). The epicentre of the 14 December 1910 Glasgow earthquake was in the north-west suburbs of Glasgow, in the Hillhead-Maryhill area. Superficial damage in the epicentral area included small cracks in plaster and dislodging of a few chimney pots. The earthquake was felt within the area bounded by Lochwinnoch, Dumbarton, Strathblane and East Kilbride, and probably also in Kilcreggan, Stirling and Dunblane (Gregory, 1911; Musson, 1994). The 16 July 1940 Kilsyth earthquake was felt in most of Midland Valley and in Strathspey (Musson et al., 1984, Musson, 1994). The only report of damage was the collapse of the gable wall of a house in Carronbridge (Musson, 1994). The area of the intensity map of the Kilsyth earthquake was similar to the intensity map of the 3.1 Mw 2 February 1940 Stirling event, whose epicentre was located to the north of the Kilsyth event (Musson, 1884, 1994).

There is no recorded seismicity within the the 5 km region from the Clyde Gateway area (the star in Figure 69).

Figure 69    Historical (squares) and instrumental (circles) seismicity recorded in the study area. The symbols are scaled by magnitude and coloured by depth. The grey-shaded squares show towns and cities. The yellow star indicates the approximate Clyde Gateway area and the black square indicates the 20 km area around the area.