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The deep inner Earth Global Geophysics 2015 Deep interior of the Earth David Stevenson (nature, 2003): 2003 1864 Seismic velocity-depth model Discovering the inner structure of Earth • • • • Crust Mantle Outer core Inner core Ray-path coding within the Earth P S K I J c i p s LR LQ P-wave in the mantle S-wave in the mantle P-wave through the outer core P-wave through the inner core S-wave through the inner core reflection from the mantle-outer-core boundary reflection from the outer-core-inner-core boundary P-wave reflected from the surface of the Earth close to the earthquake focus S-wave reflected from the surface of the Earth close to the earthquake focus Rayleigh wave Love wave Shadow zones Crust-Mantle boundary (Moho) Andrija Mohorivicic - 1909 A chemical boundary – and seismic boundary Crust-mantle transition in Kohistan (Pakistan), exhumed oceanic island arc Mantle - outer core boundary Beno Gutenberg Richard Oldham Shadow zone (103° – 143°) Note diffracted waves at core-mantle boundary, in dashed lines Outer-core-inner-core boundary Inge Lehmann - 1936 Refractions of waves at outer-core-inner-core boundary (PKIKP), appear in „shadow zone“. Density and elastic moduli for the whole Earth α= K + 4 / 3µ ρ β = µ/ρ However, even if we know α and β as a function of depth, we have unknowns of K, µ and ρ. The Adams-Williamson equation to determine density as a function of radius: dρ GM r ρ (r ) GM r ρ (r ) =− =− 2 4 2 φ dr r 2 2 r (α − β ) 3 Starting at Earth’s surface, work inwards, and apply equation successively to shells of uniform composition, and a=R G – gravitational constant M r = M E − 4π ∫ ρ (a )a 2 da a =r r – radius ME – mass of Earth R – radius of Earth Mr – Mass at radius r Elastic moduli and density with depth Attenuation of seismic waves: the quality factor (Q) Q=Q=− 2π × elastic energy stored in the wave energy lost in one cycle or wavelength 2πE TdE / dt 2πE dE =− d QT Perfect elastic material – Q inf Totally dissipative material – Q=0 E – energy t – time T – period of the seismic wave Through integration, E = E0 e −2πt /( QT ) Alternatively written, A = A0 e − wt /( 2Q ) w – angular frequency A0 – amplitude of wave time t ago Gravity, pressure and quality factor with depth 361.7 GPa Whole line Qp Dotted line Qs The mantle structure Notable features • The ”lid” above 220 km (low-velocity zone for S waves), occur mainly beneath continents, bounded at bottom by Lehmann discontinuity (220±30 km). • Mantle transition zone 400 – 670 km (5-7 % increase at each of these depths). • Lower mantle at depth down to 2700 km, D’. • Lowermost 150-200 km of mantle (2700-2900 km), D”. Long-wavelength perturbations of S-wave Cross-section from Aegean (left) to Japan (right) P-wave mode S-wave mode Q-factor as function of depth Seismic anisotropy • • Directional velocity dependence in a crystal or material Hooke’s law in anisotropic form: σ ij = cijkl ε kl P-wave anisotropy Avp = Vpmax-Vpmin/Vpmean Shear wave splitting and anisotropy dVs = Vsmax – Vsmin Olivine single crystal - (Mg,Fe)2Si04 Composition of upper mantle Ca(Mg,Fe)SiO3 (Mg,Fe)2Si04 (typical 91-94% Mg) MgSiO3 (enstatite) FeSiO3 (ferrosilite) Mineral physics High pressure, high temperature experiments to simulate conditions and processes within the Earth. Percy Bridgeman Ahrens (1980: Science) Phase transitions in the mantle transition zone Exothermic reactions, 10 % density increase β, γ-spinels Pyroxene to garnet Endothermic reaction, 10 % density increase (Mg,Fe)SiO3 (bridgmanite) (Mg,Fe)O Phase transitions continued γ-spinel to Mg-silicate perovskite (~660 km) γ-spinel -> ringwoodite (~520 km) β-spinel -> wadsleyite (~400 km) α-spinel -> olivine (upper mantle) D” Analogue for Earth‘s lithosphere Very heteogeneous Ultra-low velocity layers: 5-40 km vertical, 10% Vp, Vs reductions Slab graveyards? Still plenty of conjecture and research Composition of the inner core: Seismic parameter as a function of density for metals φ = K / p = α 2 − 4 / 3β 2 Shock wave experiments (i.e., Ahrens, 1980) Anisotropic structure of the inner core Combining lab experiments and seismic data Shock wave experiments (generating P and T), Measuring sound velocity and density Up to 8 % light elements in the core (S, O, Si, C, H) Twice the light element content in outer core, compared to inner core O-rich compositions do not fit seismic constraints, and indicate O poor core (i.e., not oxidizing conditions) Duffy (2011: nature); Huang et al. (2011:nature)