Mapping fine-scale heterogeneities within the continental mantle lithosphere beneath Scotland: Combining active- and passive-source seismology

Abstract

We analyze new observational evidence for seismic velocity discontinuities in teleseismic receiver functions in comparison to well-documented discontinuities observed in marine reflection profiles and wide-angle reflection-refraction profiles in northern Scotland. Our study establishes the viability of mapping small amplitude P to S (Ps) converted phase arrivals from the upper mantle generated in the P-wave coda of teleseismic earthquakes using well-known receiver function methods. Teleseismic earthquakes recorded by a small array of portable broadband stations and permanent short-period stations are used to ascertain the lateral extent of velocity discontinuities within the continental mantle lithosphere beneath Scotland. Radial receiver functions contain distinct Ps converted phases at ∼3.1–3.2 s (Moho) and at 4.5–5.2 s (upper mantle depths) after the direct P wave. We suggest that the upper mantle Ps phase originates from a high-velocity and/or anisotropic layer within the upper mantle. At two stations, ORE and BACA, located along the northern shoreline of Scotland, these upper mantle phases can be correlated with the W reflector, a bright, regionally extensive seismic reflector previously observed on marine deep seismic reflection and wide-angle refraction-reflection profiles. However, the variability in physical characteristics (depth, thickness, azimuthal variations in the vertical structure, velocity contrast, and anisotropic properties) suggests the possibility that there may be multiple layered reflectors in the upper mantle beneath northern Scotland and raises doubt about the global significance of some of these reflectors.