Fault activity and palaeoseismicity during Quaternary time in Scotland
Field study at seven Scottish sites has resulted in the following evidence for late- and post-glacial earthquakes and fault movements. a) Glen Roy, western Highlands: Earthquake-induced deformation structures are observed in 10,000 year-old lake deposits, and can be related to a surface fault rupture and several landslides. The deformation structures have been mapped over an area of 100 sq. km and display most intense deformation in a central area, with decreasing degrees of deformation in peripheral zones. This zonation is interpreted in terms of varying intensities of ground-shaking during a major earthquake. The field data indicate a magnitude 6.25 event. b) Kinloch Hourn, western Highlands: A prominent, 14-km long fault displays evidence for recurrent movement. A magnitude 5.5-6.0 event occurred between 3500 and 2400 years ago, and unquantified movement has occurred since then. c Firth of Lorn (west coast): Levelling survey, at two sites, indicates several vertical displacements of up to 3m, of a 10,000 year old raised shoreline. d) Lismore (west coast): Lateral fault displacements of c. 0.5m have disrupted present rock and soil morphology and indicate movement in the last few thousand years. e Tayside, eastern Scotland: Two sites display soft-sediment deformation structures in late-glacial sands and silts. The structures are interpreted as the result of (unquantified) earthquake ground-shaking. This field evidence is collectively evaluated in terms of crustal stress, earthquake recurrence and present-day earthquake hazard. Earthquakes as large as magnitude 7 are thought to have occurred but were probably triggered by glacial rebound stesses. Earthquakes upto magnitude 6 have certainly occurred, some as recently as 3000 years ago, and are likely to recur. Present-day surface fault displacements of up to 0.1m are considered likely on fractures favourably orientated with respect to the present-day stress field.