Deep origin of mid-ocean-ridge seismic velocity anomalies

ZHANG and Tanimoto have used the results of a new high-resolution three-dimensional model of shear-wave velocities in the Earth's upper mantle to conclude that, whereas hotspots are underlain by low-velocity anomalies extending to ∼200 km depth, the corresponding anomalies under mid-ocean ridges are shallower (∼100 km) in extent. Here we show that such a shallow origin for the mid-ocean-ridge anomalies is inconsistent, by a factor of 3-5, with the observed travel-time residuals for the seismic phase SS (ref. 3). A three-dimensional model describing shear velocities in the entire mantle, derived independently of the SS observations but based on a large set of waveforms (15,000 seismograms ) and differential travel-time data (8,200 measurements ), shows the velocity anomalies associated with all mid-ocean ridges as continuous features down to 300 km depth. Some of these low-velocity anomalies continue throughout the upper mantle, and may extend into the lower mantle. © 1992 Nature Publishing Group.