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Good question posed in the comment after my last entry by Hypo-theses blogger Ian Simpson. Did St. Patrick chase the earthquakes out of Ireland? That would be a nice story indeed, but maybe we can come up with a scientific reason.I won't claim to be an expert, but a Google search (shhh, don't tell my students) came up with a very nice PowerPoint from Dr. Brian Baptie of the Earthquake Seismology team, Earth Hazards and Systems Programme, British Geological Survey (excellent web site). The paper is on Seismo-tectonics and state of stress of the British Isles, presented at the 8th International Symposium on Upper Rhine Graben Evolution and Neotectonics. You can find and download the presentation here.
Baptie does indeed note a strong variation in spatial distribution of seismicity throughout the United Kingdom. Earthquakes occur in a north-south band along the length of Britain. But there is an absence of seismicity in northeast Britain, the northwest Atlantic margin and Ireland (as we pointed out yesterday). The rather striking pattern is shown in the figure below.
The figure above (click on it for an enlarged version) shows historical seismicity of the UK (yellow) from 1832 to 1970 for earthquakes of magnitude above 3.0 and instrumental seismicity (red) from 1970 to present for earthquakes with magnitudes > 2.0 (from Earthquakes in the UK).Baptie concludes that a prevailing stress field due to northwest compression is dominant, suggesting a combination of plate boundary forces plus post-glacial rebound may be controlling re-activation of old faults.
The stress field does explain the focal mechanisms of the events but since the sources (MAR and Alpine collision) are distant it doesn't really explain the distribution GB vs Ireland.
ReplyDeleteI have heard it suggested that the main west coast of GB distribution might relate to an area of mantle thinning / delamination in the Tertiary but can't track a reference to it at the moment.