FELT EFFECTS OF THE FORT WILLIAM EARTHQUAKE OF 10 DECEMBER 2005
The earthquake of the 10 December 2005 occurred at 23:21 UTC, with an epicentre approximately 8 kilometres west of Fort William. The instrumental magnitude was determined at 3.0 ML, and initial reports suggested that the earthquake had been felt throughout Lochaber. A macroseismic survey was launched on the BGS ‘Earthquakes’ web site. A total of 210 responses were received.
The questionnaires were arranged by location into squares 5x5 km, and an intensity value was assigned to each square, provided at least five responses were received from that square. Where fewer responses were received (especially the case in sparsely populated areas) the intensity was either given as “felt” or “not felt” (which is also defined as intensity 1). The large number of replies received is partly a function of the strength of shaking in the most affected area, but is also influenced by the fact that the shock occurred not far from the town of Fort William, which is quite well-populated.
The highest intensity experienced was 5 EMS (European Macroseismic Scale), which was observed over an area extending approximately 14 kilometres to the northeast and southeast of the epicentre. Unsurprisingly, Fort William returned the highest number of replies (58), followed by the settlement of Blaich (22 replies) which was the closest settlement to the epicentre. Other areas that experienced intensities of up to 5 EMS were communities close-by Spean Bridge to the east-north-east (9 replies) and Kinlochleven to the southeast (8 replies). In both these areas there was a certain amount of mingling of places where the intensity was 4 and where it was 5, and borderline cases.
Comments received described the particular effects people had experienced. These included descriptions of the noise made by the earthquake as sounding like a heavy clap of thunder, a gust of wind, or even a quarry blast. Most people who reported having felt the event described the shaking sensation as weak-moderate. A few people reported objects falling over or coming off their wall-fixing. In several reports it was stated that domestic animals were alarmed. There were no reports of damage to property.
The most distant reports were from the following places: to the southwest, the earthquake was felt in the south of Mull (75 km); to the west, the earthquake was reported as having been felt on the northern tip of Mull (62 km); to the east, the earthquake was felt at Tulloch Station; and to the northeast at the northern-most limits of Loch Lochy (27 km) The total felt area is over 7,300 sq km. The areas within each isoseismal (rounded to the nearest 100 sq km) were as follows:
| Isoseismal | Area (sq km) |
|---|---|
| 4 | 1,100 |
| 5 | 215 |
Table 1 - Isoseismal areas
Parameters for the earthquake were calculated from these macroseismic data. The magnitude was calculated to be 3.4 ML, slightly higher than the instrumental value . The macroseismic depth is around 6 km, compared to the instrumental determination of 3 km – since both values are subject to an uncertainty of a few km, the figures are consistent with one another.
Figure 1. Isoseismal lines have been drawn for intensity
5, 4 and 2-3 EMS.
Click on map for higher quality printable version.
Figure 2. Map of numbers of questionnaires received for each 5x5 km grid square. This shows the number of responses on which the strength estimates are based.
The use of Ordnance Survey topography
above is for display purposes to assist the user in locating the seismic
event described. For information regarding copyright limitations,
please refer to: BGS copyright pages
Issued 13 January 2005
