A Review of the Nature and Variability of the Anelastic Properties of the Upper Mantle Beneath North America and Eurasia

All reasonable mechanisms for producing low-velocity zones in the upper mantle should also produce zones of low omega. The available studies of upper mantle omega and velocity for the same regions suggest that a coincidence of low velocity and low omega zones does indeed occur. Seismic body- and surface-wave data indicate a substantial low velocity, low omega zone in the upper mantle beneath western North America. The zone appears to be 150 km thick or more, the velocities for both P and S waves being lower than typical upper mantle velocities for stable regions. The available evidence for eastern North America indicates that a low velocity zone is either absent for that region or more poorly developed than it is in western North America. Most interpretations for P waves in eastern North America include no low velocity zone. Surface wave studies of the shear wave velocity structure beneath eastern North America indicate the possibility of a low velocity zone for S waves, at least in some regions. The available data for northern Europe and Asia indicate that it is a stable region with velocity and attenuative properties much like those of eastern North America.