Friends of the Ochils Newsletter 16: Summer 2000
To a fault: Earthquakes in the Ochils
WHEN ASKED to write on this topic, I wasn't quite sure what I would be able to find out. At school we were told that Comrie in Perthshire and Menstrie were the "earthquake capitals" of Scotland, a tradition that ties in with their being located on major faults, or dislocations. Comrie lies on the Highland Border Fault Zone, Menstrie on the Ochils Fault.
The most obvious place to find out more was the British Geological Survey in Edinburgh: any time there is a disastrous earthquake, the media descend on here. Hence, for what follows, I must express my gratitude to the BGS and their library staff.
My first surprise was to find that, at least in recent years, my now home town of Clackmannan is very seismically active. These tremors are all of very low strength, shallow (ie around 1km deep) and located near Forestmill. They are put down to colliery activities, and the regularity with which they occur suggests shot-firing, which in turn gives some idea of the sensitivity of the BGS's monitoring equipment.
So to the objective of this article. The teachers who told us of Menstrie's claim to fame were quite correct. Between 1900 and 1916 there were scores of tiny events, known as "swarm activities". Most are now believed to have been due to quarry blasting, but some were the real thing.
27 September 1905
This, the first of the larger events, was located some 4km down and estimated at 3.1ML (a variant of the Richter Scale). It caused considerable alarm in the Alva/Alloa area but there was no reported damage apart from some tiles loosened in Alloa and a chair knocked over in Greenloaning. A very loud explosion-like noise accompanied the shock.
20 October 1908
Another "larger" event, also located at a depth of 4km and estimated at 3.1ML. At Tillicoultry, "whole shelves of dishes were broken". There seems to have been no damage to buildings. It was felt in Falkirk, and - as with the 1905 event - was surprisingly felt in West Linton.
3 May 1912
The most intense of the swarm: located near Dunblane at 4km and estimated at 3.7ML. Plaster was dislodged at Dunblane, slates loosened and walls and ceilings cracked at Glendevon, while many ran out of doors in Alva. But people did seem to be growing accustomed to the shocks. This one was felt in Comrie, Glasgow and Musselburgh.
The earliest known Ochils event occurred on 30 April 1736 but is poorly documented. It was felt in Stirling and the Hillfoots villages where "it rent some houses" and caused many people to run out in alarm. It was obviously severe - perhaps around 2.7ML.
8 August 1872
An isolated event with its epicentre (the point on the surface above where the movement took place) between Dunblane and Braco. It caused windows and glassware to be broken, tools to fall off a smithy wall and slates to tumble from the roof at Ardoch House.
In 1941 an earthquake occurred with its epicentre near Stirling, but was probably unrelated to the Ochils events. It was however felt over much of central Scotland. Objects were thrown from shelves although no serious damage was reported.
19 February 1979
Strength 3.2ML, depth 6km. Felt in villages north and south of the hills and as far away as Perth, Callander and Stirling. It could have been responsible for damage to installations at Glen Devon dam - one of three events thought to have damaged dams. This formed part of a short-lived recurrence of activity.
Some recent isolated smaller events:
31 October 1991 Felt at Bridge of Allan: 1.1ML at 3.9km
26 January 1992 Dunblane: 1.3ML at 5.6km
4 August 1992 Located below Dollar golf course: 0.4ML at 23.1km
7 September 1992 Near Rumbling Bridge: 1.3ML at 4.6km
29 September 1992 Sheriffmuir: 0.5ML at 0.3km
9 October 1992 Braco: 0.4ML at 4.8km
25 October 1992 Stirling (felt at Bridge of Allan): 1.5ML at 2.4km
Since 1970, the BGS has operated an extensive network of highly sensitive seismic recording apparatus, including some in the Ochils. These have to be remote from roads, industrial sites and even trees (the latter move about in the wind). The less disturbance, the better for the recording of wholly seismic activity.
A strength of 4.0ML is regarded as the threshold of minimum magnitude for seismic hazard assessment. The Richter Scale is a well known method of measuring the magnitude of earthquakes. Each step involves a 30-fold increase in the energy released: hence a 5 event releases 30 times the energy of a 4 event. A 5 earthquake releases the equivalent of 1000 tons of TNT.
Some observations on the geology of the Ochils will appear in the autumn newsletter.