Seismic observation of a high‐velocity slab 1200–1600 km in depth

A slablike high‐velocity region 1200–1600 km beneath the east coast of North America apparently has a half‐width of 450 km and a lateral extent of 3000 km and a depth extent of more than 800 km. This slab lies beneath the boundary where the Farallon Plate had been subducting for up to 120 MA. The high‐velocity anomaly has been inferred previously from travel‐time studies; this study uses body‐waveforms to verify and refine its structure. The anomalous region has a peak shear wave velocity 1.5% greater than the surrounding region and compressional velocity 0.6% greater. The width and smoothness of this anomaly is constrained by the complexity observed in SH waveforms from deep earthquakes beneath South America observed at GDSN stations in North America. This slab is about 5 times broader and 5 times weaker in its velocity anomaly than most slabs observed in the upper mantle. This observation implies that if this slab has passed through the 650 km discontinuity, which is the most likely explanation for the observation, it has been spread by more than simply thermal conductivity. The possibility that there is two‐layer convection with thermal coupling is not ruled out, however, as thermal coupling could lead to a similar pattern.