Seismic velocities beneath central and southern Britain from broad-band surface waves

Abstract

Abstract Fundamental mode Rayleigh waves recorded across the UK broad-band network have been analysed to produce phase velocity curves in the period range of 12 to 85 s. An inter-station method has been used for two paths across southern England. The resulting mean phase velocity curve has been compared with a mean curve derived using three stations in central Britain. Lateral refraction characteristics of tripartite phase velocities have been analysed and shown to be less than 5° at periods of greater than 15 s. Forward modelling to S-wave velocity/depth profiles using a Hedgehog search procedure suggests that central Britain is underlain by a thicker crust than southern England, with a steady velocity increase down to the Moho. For southern England, low velocities are apparent either at the base of the crust or in the uppermost mantle. For both regions, sub-crustal lithosphere velocity is contrained to be between 4.6 and 4.7 km s −1 with a layer thickness of less than 140 km for southern England and between 80 and 160 km for central Britain. Both regions appear to exhibit low velocity zone (LVZ) development. However, for central Britain, more data are required to place better constraints on the magnitude of the LVZ channel. Velocities in the lower crust have been used to place constraints on Poisson's ratio which appears to decrease northwards from greater than 0.27, beneath southern England, to between 0.25 and 0.27 beneath central Britain. The distributions of P-wave velocities, S-wave velocities and Poisson's ratio have been interpreted of compositional variation and/or fluid content within the lower crust.