Anomalous seaward dip of the lithosphere–asthenosphere boundary beneath northeastern USA detected using differential-array measurements of Rayleigh waves

S U M M A R Y Rayleigh wave phase velocities and azimuth anomalies in the period range of 30–100 s are measured for a set of four triangular arrays of broad-band seismometers in coastal northeastern USA. This is a region in which a strong upper mantle slow shear velocity anomaly (a ‘New England Anomaly’), crosses the continental margin. Earthquakes from a wide range of directions are used to detect the variation of parameters with azimuth, θ , of propagation. No lateral heterogeneity in phase velocity is detected at these periods between stations at the centre and the edge of the Anomaly. However, large (10–20 per cent) azimuthal variations occur, and have a cos(1θ ) dependence, which is indicative of a dipping structure in the upper mantle. Corresponding azimuth variations, with a magnitude of ±5◦, are also detected. This behaviour is consistent with a southeasterly (N150◦E) dip of the lithosphere–asthenosphere boundary beneath New England. This dip is associated with the shoaling of the New England Anomaly beneath the Adirondack mountains, west of the array. It is opposite to the dip associated with lithospheric thickening toward the interior of the craton.