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Seismogenesis and state of stress in the UK

By Brian Baptie

Abstract

In this paper I present a compilation of focal mechanisms for earthquakes with magnitudes greater than 3.0 ML in the British Isles that can be used to help constrain our understanding of seismicity and it's driving forces in the British Isles. The fault plane solutions consist of both previously published mechanisms for significant British earthquakes, and new solutions calculated from regional and local data for more recent and smaller earth- quakes that were previously unpublished. Focal mechanisms for earthquakes in the UK are dominantly strike-slip with northwest-southeast compression and northeast-southwest tension,or reverse, with northwest-southeast com- pression. In many cases there is also an oblique component to the slip. P and T axes from individual solutions are relatively well constrained in azimuth, though less so in dip, with P-axes orientation for most events clustering between north and north-west, indicating sub-horizontal compression. However, some spatial variation in P- and T-axes orientation is also apparent, with near north/northeast compression and east-west extension in north- west Scotland, changing to northwest-southeast compression in England and Wales.\ud \ud I estimate a best-fitting stress tensor, under the assumption of uniform stress using two different inversion methods for both the entire focal mechanism data set and two different subsets of the data. The results from the two different datasets suggest that there is a significant difference in the stress state between northwest Scotland and England and Wales. Calculated σ1 directions for England and Wales are northwest-southeast, consistent both with existing stress data and expected stresses from first order plate motions. By contrast, the inversion results for northwest Scotland show near east-west extension with possible σ1 and σ2 directions lying in a north south band, and that the magnitudes of σ1 and σ2 are similar. The relative magnitude of the principal stresses, R, determined for England and Wales suggests that the intermediate stress σ2 is close to the average value of σ1 and σ3 .\ud \ud The clear difference in the stress inversion results between northwest Scotland and England and Wales suggests that the principal stress directions expected from first order plate motions have been modified in Scotland by local stress conditions due to glacio-isostatic adjustment

Publisher: Elsevier
Year: 2010
DOI identifier: 10.1016/j.tecto.2009.10.006
OAI identifier: oai:nora.nerc.ac.uk:9574

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