Structure of the Earth: Mantle and core

Abstract

The deep interior structure of the earth has been extensively analyzed using a wide variety of seismic phases and techniques during the last four years. Most studies have emphasized quantitative three‐dimensional mapping of the lateral velocity heterogeneity of the mantle and core. These aspherical velocity variations are believed to be direct manifestations of thermal and compositional heterogeneity associated with convective processes. A first generation of global models for the lateral velocity variations in the deep earth has been produced, providing tantalizing images of large scale structures suggestive of a non‐steady state thermal convection system.The upper 400 km of the mantle has the strongest lateral velocity variations, of up to ±10% for shear velocity. Surface‐wave analyses that do not require a priori regionalizations have demonstrated that there is a strong association between surface tectonic provinces and the uppermost mantle velocity variations. Thus, the thermal and convective state of the upper 200 km of the mantle can be reliably interpreted in the context of plate tectonics. The upper mantle models support the contention that continents have deep roots, with differences in velocity structure from oceanic and active tectonic regions extending as deep as 400 km. Very long‐wavelength lateral velocity variations of a few percent have been detected in the transition zone at depths from 400 to 670 km, as well as throughout the lower mantle. These deep‐seated variations have little correspondence to surface tectonics, and efforts to interpret their nature are just beginning. The lowermost 200 km of the mantle (D″ region) has lateral velocity fluctuations comparable to those in the upper mantle, and evidence has been presented for the presence of a sizable velocity discontinuity at the top of the D″ layer. A combined thermal and compositional boundary layer, roughly mirroring the lithosphere, is a likely explanation for this anomalous zone. The core‐mantle boundary appears to have significant (10 km) long‐wavelength topography, presumably sustained by dynamic stresses from deep mantle convection. The inner core may have strong lateral heterogeneity or axially symmetric anisotropy, suggesting a complex thermal and compositional state.Significant progress has been made in characterizing the frequency dependence of anelastic attenuation in the mantle in the short‐period body‐wave band. Models for teleseismic P‐wave attenuation operators have converged, with t* values of 0.7–1.0 s appropriate at 1 Hz, and t* values of 0.4–0.6 s appropriate at 4 Hz. Regional variations of attenuation are slowly being mapped out as well.