For the past three decades, deep crustal studies of the British Isles have been restricted to the interpretation of 2-D seismic reflection and refraction profiles, mostly acquired offshore. During this period, the British Geological Survey (BGS) seismic monitoring network has developed to an unrivalled density for a region of low intraplate seismicity. In an average year, the modern network records approximately 40 earthquakes in the crust beneath the British Isles with local magnitudes > 2. Statistical tests show the modern and historical pattern is not random. Understanding of the tectonic processes behind the pattern are hindered by the sparseness of onshore deep crustal studies where the majority of earthquakes are concentrated.\ud For the first time local earthquake tomography, a method more commonly applied to tectonically active regions, is used to produce high resolution 3-D images of seismic P-wave velocity (Vp) and the P- to S-wave velocity ratio (Vp/Vs) in the crust beneath England, Wales and the Irish Sea. To account for low seismicity, over 1,000 earthquakes are utilised from the past 25 years of monitoring. The existing BGS digital catalogue is enhanced by a two-fold increase in seismic arrival time picks, significantly reducing earthquake location errors in the input dataset.\ud The tomographic models establish a strong and previously undemonstrated link between Palaeocene magmatism and more widespread earlier phases of Caledonian magmatism. A regional Vp anomaly (> 7.2 km/s) in the lower crust centred on the East Irish Sea Basin is inferred as Palaeocene magmatic underplate with seismicity concentrated around its eastern and southern margins. In the mid- and lower-crust earthquake clusters are evident around the edges of local Vp/Vs anomalies (> 1.80), most significantly beneath the Ordovician volcanic centre in Snowdonia. The models are supplemented by the inversion of 185 independently determined focal mechanisms to consider the influence of local variations in far-field intraplate stresses alongside lithostatic stress from overburden pressure
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