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The core-mantle boundary (CMB) is the lower bound on mantle convention, and the lowermost mantle must underlie at leas a thermal boundary layer. There may be a chemical heterogeneity just above the CMB. The lowermost mantle has two types of seismic structure, layering in P and S wave velocities and weak fine-scale heterogeneity from scattered waves, preceding the seismic phase PKP. Observations of scattered PKP waves needing large velocity perturbations close to the CMB are presented. Their position is noted beneath low-velocity lower mantle and close to a patch showing slow basal P velocities.

Geophysical research using data recorded at US and Canadian regional seismographic networks following the May 1992 Chinese nuclear test at Lop Nor reveals a seismically fast layer 130 kilometers (km) thick and 300 km wide at the boundary of the core and mantle. The resolution needed to observe such a mass was obtained by organizing data from 14 regional networks of short-period vertical-component seismometers. The data suggest that the boundary region between mantle and core cannot be viewed as a single homogeneous layer and may be as structurally complex as the crust.

The geometry of an earthquake fault is a function of its depth as observed by the equi-dimensional geometry of the fault which deviates from an elongated structure on increase in depth from the surface of the earth. At shallower depths, the episodic or non-steady faulting geometry is attributed to the increase in the heterogeneity of temperature and composition found near the surface of the earth. A change in the state of faulting from frictional to transformational at intermediate depths affects rupture characteristics such as velocity and geometry. Features of rupture are also affected by transitions, alpha-olivine to beta-phase at a depth of 390 km, beta phase to gamma phase at a depth of 500 km or other phase transitions occurring within the subducting slabs.

The structure of a 350-kilometer-long portion of the core-mantle boundary (CMB) beneath the northeast Pacific was determined by measuring seismic waves that change from shear waves to compression waves after reflecting off the CMB. This section of the CMB is characterized by an amplitude of less than 500 meters, an absence of sharp radial gradients in the 400 kilometers above the boundary and a mantle-to-core transition of less than 1 kilometer in thickness. Hence the mantle in this region is probably not chemically heterogeneous.

Debate over the convection pattern in the mantle takes a new turn from the observation of the submergence of an oceanic lithospheric slab below the west coast of the US into the lower mantle of the Earth. The observation suggests that the submergence, resulting from convection, involves both the upper and lower mantles though it exhibits rate variations between them. One convection cell covers the entire mantle whose variations arise from differences in viscosity.

The discovery of mantle layering that is nearly 920 km deep beneath many subduction zones complicates theories on rocky mantle flow. The theory that the layering is caused by global change in composition with increase in depth cannot be confirmed as it assumes that the depth of the boundary is constant, which is impossible due to the presence of convective currents. The theory also inadequately explains mantle downwellings and the link between global compositional layer and a thermal boundary layer.