Radial structure of the Earth: (II) Model features and interpretations

Abstract

A new reference model is presented for the spherically-averaged profiles of elasticity, density and attenuation, which reflect the bulk composition, temperature profile and dominant processes of the Earth's heterogeneous interior. This study discusses the features of REM1D and geological interpretations while the underlying modeling concepts and reference datasets are described in a companion manuscript. All physical parameters in REM1D vary smoothly between the Mohorovičić and 410-km discontinuity, thereby excluding the 220-km discontinuity in earlier models. REM1D predicts arrival times of major body-wave phases in agreement (±0.8 s, normalized misfit ${\psi }_{\mathrm{pb}}\le$ 0.25 s) with widely used but theoretically incomplete isotropic models optimized for earthquake location. Substantial radial anisotropy is present only in the shallowest mantle (∼250 km) with peak values of shear-wave (a_S = 3.90 %, $\xi$ = 1.08) and compressional-wave anisotropy (a_P = 3.78 %, $\varphi$ = 0.93) between ∼125–150 km, consistent with textures that can form by the alignment of intrinsically anisotropic minerals in this deforming region. The upper mantle (24.4–410 km) is the most dissipative region with a finite bulk attenuation ($Q_{\kappa }\sim$ 386) and strong shear attenuation ($Q_{\mu }\sim$ 60–80) that peaks at a depth of ∼150–175 km in the mechanically weak asthenosphere. An olivine-rich pyrolitic composition is broadly consistent with REM1D structure in the upper mantle and extended transition zone ($\lesssim$ 800 km) with step changes across the 410-km and 650-km discontinuities. Features of the lower mantle can be reconciled with: (i) effects of thermally driven convection throughout the central lower mantle (771–2741 km) leading to an apparent subadiabaticity in the stratification parameter ${\eta }_B$, (ii) effects of spin transitions in iron-bearing minerals that manifest as distinct linear segments in modulus and Poisson's ratios ($\mu /\kappa$, ${\sigma }_P$) on either side of a complex transition region (∼1300–1700 km, 52–73 GPa), (iii) a thermal boundary layer with steeper superadiabatic gradients than near the surface, which ultimately exceed the critical gradients for both $v_P$ and $v_S$ (but not for density $\rho$) at a depth of 2741 km, and (iv) chemical stratification in the bottom ∼500–750 km of the mantle that acts to suppress the thermal effects. Signatures of this thermo-chemical boundary layer are: (i) a gradual increase of density and steep positive gradients with depth $\left(\mathrm{d\rho }/\mathrm{dz}\right)$ in the lower mantle, (ii) large values of the stratification parameter (${\eta }_B>$ 1.03) followed by an abrupt reduction to values below one near the core-mantle boundary (CMB), (iii) variations in bulk modulus with pressure ${\kappa }^{\prime }$ = $\mathrm{d\kappa }$/$\mathrm{dp}$ that are inconsistent with Equations of state (EoS) expectations of a uniform composition, (iv) very steep negative $v_P$ and $v_S$ gradients that form a low-velocity zone in the $D^{″}$ region. The $v_P$ and $\rho$ variations in the outer core have steep gradients and the derivative properties are consistent with a neutrally stable region comprising a well-mixed iron alloy undergoing adiabatic compression (${\eta }_B\simeq$ 1, $N^2\simeq$ 0, negative ${\kappa }^{″}$). REM1D is readily extendable due to its modular construction and represents the average physical properties, features essential for geological interpretations and the construction of a three-dimensional reference Earth model.