FELT EFFECTS OF THE HARTLAND POINT, DEVON AND CORNWALL EARTHQUAKE OF 31 MAY 2001

By RMW Musson, PHO Henni & J Bukits

The magnitude 3.6 ML earthquake of the 31 May 2001 occurred at 23:42 UTC, with an epicentre approximately 10 kilometres west of Hartland Point, Devon.

A questionnaire survey was launched using the usual BGS methodology as described in Musson and Henni (1999). Questionnaires were placed in the following newspapers giving coverage over the felt area:

Somerset County Gazette, Western Morning News, Herald Express, Weekender, Evening Herald, Plymouth Extra, Express and Echo, Exeter Leader, North Devon Journal, Cornish Guardian, South Wales Evening Star, Carmarthen Journal, Western Mail, Wales on Sunday, Wellington Weekly News, Taunton Times.

Additionally, an electronic questionnaire was made available from the BGS ‘Earthquakes’ web site. About 120 emailed responses were received, out of a total of 518 responses, including 48 negative responses. These were sorted by area to give 119 different places for the assignment of intensity.

The distribution of effects of this earthquake was rather strange, and not what would be expected from the instrumentally derived parameters. The earthquake was very perceptible over a tract of country running roughly from Bodmin Moor to the western edge of Dartmoor. Even allowing for population distribution, the earthquake was felt much more strongly in SE Cornwall and SW Devon than close to the epicentre. It was also felt rather more strongly than would be expected for an earthquake of 3.6 ML magnitude. News reports immediately after the earthquake spoke of damage to property (albeit very minor, being confined to cracks in exterior rendering) and considerable alarm at Gunnislake, about 60 km away from the instrumental epicentre.

Reports of damage were few; most consisted of reports that perhaps pre-existing cracks had become enlarged, which cannot be taken as indicating real damage. Besides Gunnislake, cracks were also reported at Camelford, St Columb Major, St Tudy and Tintagel; loose plaster fell out of cracks in Ladock, a window was cracked at St Clether; and cement pointing fell from a roof at St Stephen.

Because the earthquake happened at night, most people were asleep in upstairs rooms, and this may have contributed to the perceptibility of the shock. The intensity was generally 4 EMS (European Macroseismic Scale) in SE Cornwall and SW Devon and only at a few places has intensity 5 been assigned (somewhat tentatively) on account of observations of the fall of books, plates, glasses and similar items, and the general concordance of other observations (degree of alarm, hanging objects swinging etc). These places were Callington, Camelford, and Tintagel.

Figure 1 (large file) shows the isoseismal map for the event

Intensity 4 EMS was also observed in the area between Hartland Point and Barnstaple, but between this area and the area of most observations the intensity seems to have been lower - at Bude it was only 3 EMS. As a result, in the isoseismal map the main isoseismal 4 EMS has been drawn between Newquay and Ivybridge, with the area around Barnstaple shown as an outlier, probably, but not conclusively separate.

In eastern Devon and western Cornwall the shaking was not much observed, with intensities of 2 or 3 EMS. In Somerset, it was weakly perceived in Taunton only. A few low-intensity observations were received from South Wales, the furthest being Abergavenny and Cenarth. There is no indication that the shock was felt in the cities of Cardiff and Swansea.

References

Musson, R.M.W. and Henni, P.H.O., 1999. From questionnaires to intensities – Assessing free-form macroseismic data in the UK, Phys. Chem. Earth (A), vol 24 no 6, pp 511-515.