Deep Earth Seismology

Deep Earth Seismology

Lowermost Mantle (2500-2891 km depth)

Lower Mantle  (660-2891 km depth)

Mantle Transition Zone (400-700 km depth)

Outer Core (2891-5150 km depth)

Inner Core (5150-6371 km depth)

Many people know the Earth's interior consists of a set of concentric layers, namely: the solid iron inner core, the liquid iron outer core, a rocky mantle, and the crust we live on. Less people realize how strongly heterogeneous and dynamic the interior is.  Convection in the interior is mainly driven by primordial planetary heat and radioactive heating and is responsible for the plate tectonics and the geodynamo. Variation in composition in the interior is a consequence of the dynamical origin and evolution of our planet.  Understanding the compositions, shapess and dynamics of the heterogeneity in the deep Earth is the main focus of our study. To do this, we use the observations of earthquake waves that propagate deep through the interior. The timing and forms of these waves can be used to map deep variations in physical properties. To understand our observations, we need to combine these with the study of the behaviour of minerals under high pressures and temperatures, and with the study of the dynamics of the deep interior. Understanding the evolution and dynamics of our own planet is also the first step in understanding those of other planets in our solar system and of exoplanets. 

The mantle transition zone is the naming for the region between 400 and 700 km. Within this layer, the dominant minerals in the mantle convert to higher density phases under pressures. The exact depth at which these conversions take place varies laterally depending on temperature and composition. The transition of the mineral phase causes a sharp jump in seismic velocity. This means the depth and nature of these transitions can be mapped with seismic waves that are reflected and converted. Besides being an indicator of temperature and compositions and depth, the thermodynamics of the phase transitions also directly affect the surrounding dynamics. This is especially true for the transition around 660 km, above which cold downgoing slabs appear to stagnate and below which upwelling plumes appear to pond. 

The composition of the lower mantle is unknown. The question is, if it is well-mixed with the upper mantle through convection and therefore has a similar composition, or if the boundary at 660 km impedes convection sufficiently to retain a compositionally distinct lower mantle. We do know there is small-scale heterogeneity in the mantle due to scattered waves. Recently, we mapped a prevalence of these scatterers around 1000 km. This depth range is a new focus of many studies. It appears in tomographic models that downgoing slabs appear to get stuck above this depth and plumes get diverted, similar behaviour to what is described for the 660 km. At 660 km there is a known phase transition, which explains this behaviour, around 1000 km, we do not know what physical processes cause this.

 

The lowermost mantle is the lower thermal boundary layer of the convection in the mantle. There is lots of seismological evidence for heterogeneity just above the core-mantle boundary. In a lot of ways this region appears to have similar complexity to the surface!  The closer we look, using higher-resolution methods, the more we see!

One potential for heterogeneity are the so-called Large Low Velocity Provinces, which are slow velocity features, which extend hundreds to thousands of kilometres above the core-mantle boundary. There are two large provinces (thousands of kilometeres across), one beneath the Pacific and one beneath Africa, and smaller ones in other places, e.g. beneath the Ural Mountains.  While some scientists claim these are compositional distinct, potentially remnants of early Earth's formation, others claim they are sub-imaged clusters of hot upwelling plumes.

Thinner anomalies, only tens of kilometers high, show extremely low velocties, and are thus named Ultra-Low Velocity Zones. These seem to be located at the edges of the Large Low Velocity Provinces, and beneath locations of intraplate volcanism and suspected whole- mantle plumes, e.g. beneath Hawaii, Samoa and Iceland. These zones could be enriched in iron, either due to planetary formation processes, or leaking from the core. Alternatively, they could represent patches of partial melt.

The outer core is a vigorously convecting liquid metal which creates the magnetic field. While we know the outer core is mostly iron, the exact composition is unknown. The other debated question is if the entire outer core convects or if stable layers exist at the top and/or bottom. To determine its composition and stratification, the density and velocity profiles of the outer core need to be determined precisely. The best constraints on this come from whole earth vibrations ('normal modes'), which can be measured after large earthquakes. 

The inner pressures are so high, that the iron alloy turns solid here. As the core cools further, the inner core grows in size. The inner core represents less than 1% of the total Earth volume.  Seismic observations in the inner core are surprisingly complex, showing two hemispheres that show different velocity and attenuation. Additionally, the inner core shows variation in seismic wave speed with direction of propagation. What causes these observations remains unclear, one possibility is that of convection being driven by inner core growth, but this mechanism depends on the conductivity of the iron alloy. 

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