Inner core seismic waves

The shear and compressional acoustic wave velocities for the inner core are the direct output parameters from seismological observations. In order to make a direct comparison between the seismic data and measured physical properties, measurements of the acoustic velocities for iron at core pressures are required. Only very recently has it become possible to measure the elastic constants of s-Fe at high pressures and room temperature (Mao etal., 1999 Lubbers etal., 2000 Fiquet et al., 2001 Anderson et at, 2001). Recent advances in theory and computational methods have also provided new tools for computing the elastic constants of s-Fe at core pressures (Stixrude and Cohen, 1995 Soderhnd et al., 1996 Cohen et al., 1997 Steinle-Neumann and Stixrude, 1999) and core conditions (Laio et al., 2000 Steinle-Neumann et al, 2001 Alfe et al., 2001). There is considerable disagreement on the elastic constants of s-Fe between experimental results and theoretical calculations. The dilferences in the aggregate shear (FJ and compressional (Vp) wave velocities are smaller (Hemley and Mao, 2001 Steinle-Neumann et ai, 2001). Further improvement of theory and experiment is required to resolve the discrepancies. 

Using seismic elastic waves, geoscientists have identified the spherical layers and boundaries in the interior of the Earth. The Earth s radius is 6380 km. Following the radial inward direction and from the ground surface to its center, the Earth s interior has the spherical layers of crust, upper mantle, lower mantle, outer core and inner core. The boundaries between crust and mantle, between mantle and outer core and between outer core and inner core are determined by sudden jump changes in seismic body waves (P-wave and S-wave). The boundary between upper and lower mantle is determined by the deepest focal depth of earthquakes in the mantle. 

The geosphere denotes the mineral part of the Earth it consists of successive concentric layers from the outer crust down to the inner core, (see Figure 13.1). The structure of the interior of the Earth can be zoned by either its physical properties (e.g., density, velocity of P and S seismic waves, and temperature) or its chemical and mineralogical composition. The classification of the geosphere according to its chemical composition identifies three main chemical entities the crust, the mantle, and the core, while physical properties identify five homogeneous entities the lithosphere, the asthenosphere, the mesosphere, the outer core and the inner core. 

Oldham discovered that there are actually two kinds of seismic vibrations, one called P (or "primary," because it travels faster and arrives first) and the other called S (or "secondary," because of its later arrival at the same station). The compressional motion of the P waves can be transmitted through most substances, although the speed at which the wave moves decreases as the stiffness of the medium decreases. In contrast, the transverse motion of S waves cannot be transmitted through a liquid, because the loosely bonded molecules in a liquid slip past each other too easily. S waves are observed to disappear at the top of the core. Then, at a depth of approximately 5100 km, the P wave velocity abruptly increases and there is a hint of the reappearance of an S wave. From such observations, Danish geophysicist Inge Lehman hypothesized in 1936 that the core was stratified, with an outer liquid portion and an inner solid portion. The existence of molten metal at core pressures requires some light element to act as antifreeze. 

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