Superplumes - Seismological constraints on the structure of the Core of the Earth

Australia: The Land Where Time Began

Superplumes - Seismological constraints on the structure of the Core of the Earth

The core of the Earth is a difficult entity to study as the mantle prevents the direct sampling of it, and its extreme conditions of temperature, above 3,000 K, and pressure, 135 KPa, are difficult to recreate in the laboratory or by computer simulation. The study of the core can best be achieved by remote observations at the surface of the Earth. The geomagnetic field, e.g., provides information about the dynamics of the fluid outer core, and the elasticity, anelasticity and the density of the fluid outer core and the solid inner core can be discovered by the seismic energy passing through the core. In this paper Ishii reviews seismological constraints on the core, which have been accumulating for nearly 100 years of study. As core properties often have a vital role in understanding the dynamics, composition, and mineralogy, reliability of seismological estimates have been addressed wherever possible.

Summary

Body waves and free oscillations, the 2 types of seismic observations, require different treatment, and should be chosen because of the particular property that is under investigation. An example is data coverage and small-scale variations, which can bias the global average of elastic properties is body-wave data are used. Alternatively, normal-mode data will be of little use in a search for small-scale features. Nonetheless, Ishii says a good model should explain the bulk part of observations of both body-wave and free oscillation once the frequency effects have been considered, as these data sets are sampling the same Earth.

It appears seismic data have well constrained the global average of the outer core wave speed and density. It has been found that these parameters vary smoothly with depth and earlier inferences of discontinuities and layers with low wave speed have been explained by scatterers in the lowermost mantle and the topography on the core-mantle boundary. The compressional wave speed at the top of the core should be slightly lower, compared to the PREM model (Dziewoński & Anderson, 1981), and the gradient of the wave-speed should be slightly higher in the outer core and slightly lower in the lower outer core. It also seems there is good agreement between models, according to the estimates of Ԛ for the whole outer core. Values above 10,000 are preferred by both body-wave and normal-mode studies, which indicates that the outer core is not an efficient attenuator of seismic energy. Studies have been carried out on deviations from the 1-dimensional model, and there is a wide range of strength of lateral heterogeneity estimates. It is argued by one group of studies that the heterogeneities are below, or possibly close to the limit of detection, i.e., within the certainty level. It is argued by another group for a few percent variations. The trade-offs with other parts of the Earth, such structure in the mantle, and anisotropy in the in the inner core, further complicates the resolving of 3-D structure. There is, however, evidence for the top of the outer core being close to being homogeneous with the possible exception of small and thin patches of finite rigidity zones. Finally, Ishii says it is possible to obtain an estimate of viscosity at the base of the liquid core through seismological constraints, though the current observations of the Slichter mode, and therefore the inference of viscosity, is highly controversial.

The transition from liquid to solid core is very sharp with a width less than 5 km, located around 1220 km radius. The boundary was initially believed to have had strong undulations, though it was shown by later studies that this was an artefact that resulted from anisotropy in the inner core. It appears smooth, based on waves reflected from inner-core boundary, while being slightly oblate with 1.5-0.5 km difference between the polar and equatorial radius. The speed of the compressional wave across this boundary is mainly constrained by body wave data, which is between 0.5 and 0.8 km/s. The shear wave change in speed and density are mostly derived mostly from normal-mode data, and they are about 3.5 km/s and less than 1 gm/cm³, respectively.

For compressional and shear wave speeds, as well as density, depth profiles within the inner core is relatively well determined. The speed of the shear wave and density exhibit a small increase with depth. This is mainly constrained by normal mode observations, though they are generally in agreement with the limited number of body wave inferences. The data of the normal mode and the body wave and it increases with depth. Near the inner core boundary is where these parameters are best resolved, and their uncertainties increase with depth.

According to Ishii the peculiar behaviour of inner core sensitive data have been attributed to a variety of parts and properties of the Earth, such as coming from the heterogeneity or topography of the outer core on the core mantle boundary. It is currently understood as an expression of anisotropy of the inner core, with models being proposed that are based on normal mode, absolute and differential travel time data. Material is weakly anisotropic, or even isotropic, near the inner core boundary. Many models favour increasing strength as depth increases. The central part of the inner core has been argued to have a different anisotropic behaviour by some studies, though the form of anisotropy and the axis of symmetry remains the same of the overlying layer. The central inner core has been poorly sampled, however, and further study is required to determine such issues as the width of the transition and the exact form of anisotropy.

Within the inner core lateral variations have been studied with great interest, as there is a distinct signal in the data and as they are essential for deducing the location and symmetry axis and differential rotation of the inner core. The inner core displays a clear dichotomy between the eastern and western hemispheres at large scales. Interpretation of this signal nevertheless ranges from mantle heterogeneity to isotropic or anisotropic variations in the inner and/or outer core. Lateral variations in anisotropy and isotropic wave speeds at smaller scales have been proposed, though it has been difficult to reject or confirm either model as a result of data coverage limitations. Similarly, the location of the symmetry axis has not been determined with confidence. There are some indications that it may be tilted with respect to the rotation axis, though the uncertainty becomes too large to make inferences that are statistically significant once various factors are considered.

A tilted symmetry axis or lateral variation is required to investigate inner core differential rotation; therefore resolvability has been a controversial issue. Estimates vary from 0 to 3^o /yr. Rates much larger than 3^o /yr can be disregarded, as has been shown by careful consideration of biasing factors. Most studies suggest that if the inner core rotates with respect to the mantle the acceptable rate is less than 1^o/yr in an eastwards direction.

Attenuation is the parameter that is least constrained in the inner core. For values obtained from normal mode and body wave observations there is a factor of 10-100 difference between them. It has not been defined clearly what the source of this discrepancy is, and Ishii suggests that it may simply be that attenuation in the inner core is strongly frequency dependent or that there are significant effects of small-scale scatterers. Attenuation may be laterally heterogeneous, anisotropic, depth dependent, and/or layered, based on the body wave data. In order to understand the strength and variation of attenuation and other properties within the inner core a much larger data set is required. Seismic energy can reach the inner core only if the wave leaves the source with less than 6^o take-off angle for a shallow source because of the small size of the inner core, being about 0.7 % of the volume of the Earth. Much has been learned since its discovery, though it is a difficult target to observe.

Sources & Further reading