3D Local Earthquake Tomography of England and Wales

For the past three decades crustal studies of the British Isles have been restricted to the interpretation of 2-D seismic reflection and refraction profiles, mostly aquired offshore. The British Geological Survey (BGS) seismic monitoring network has grown substantially over the past twenty years to a density and quality unprecedented for an aseismic region. Recently, this has made it possible to undertake teleseismic studies to image the seismic velocity of the mantle via 3-D tomography and 1-D receiver functions for the crust and uppermost mantle. Whilst the British Isles can be considered an aseismic region by world standards, the BGS network typically records 40 local events of over 2.0 on the local magnitude scale every year. Irrespective of an intra-plate setting, the width of seismogenic zone is exceptional, ranging from the surface to in excess of 30 kilometres depth despite no surface ruptures ever having been observed. For the first time we utilise these locally generated seismic events within the BGS digital catalogue recorded over the past two decades to produce a model of seismic P- and S- velocity to depths of 70 km beneath England, Wales and the Irish Sea at an unmatched resolution. A high quality subset of over 1,000 local events and 18,000 arrival times has been extracted from the entire digital catalogue. This has been used to relocate the events with a 1-D seismic P-velocity model extracted from a regional 2-D model derived by extrapolation of wide-angle refraction profiles. The initial locations and 1-D model have been simultaneously updated and refined using VELEST to produce a consistent set of station corrections for the BGS network which is in good agreement with known geology. The updated locations and 1-D model acts as the reference model for a 3-D tomographic model developed with the SIMULPS inversion code. Our 3-D model will compliment teleseismic and controlled source studies which demonstrate seismic anomalies thought to be associated with the emplacement of magmatic underplate beneath the East Irish Sea. We note that the earthquakes are distributed around the edges of this high velocity and high density body which suggests that they may be related of loading on the crust.