Investigating D′′ structure beneath the North Atlantic

Data from the GRSN and SPICED array are used to investigate lower mantle structure beneath the North Atlantic Ocean just south of Nova Scotia. We study S waves in an attempt to determine whether a D′′ discontinuity can be observed in this region. The reflected phase SdS cannot be observed unambiguously, so we cross-correlate the traces with synthetic seismograms generated using a number of different 1D Earth models that incorporate positive and negative velocity discontinuities and gradients, as well as thin, high-velocity lamellae. The cross-correlation shows that a discontinuity with an S-wave velocity increase of magnitude 1-3% exists, which exhibits significant topography between 86 and 286 km above the CMB. The study area is located on the edge of a region of anomalously high seismic velocity identified by tomographic modelling, which fits with our observation of a high-velocity discontinuity.