Observing movements of the solid Earth with the GRACE satellites

Mass redistributions in the atmosphere, in the ocean, on land and in the Earth's crust act as loads on the Earth's body, under which it deforms. On very long time scales, the rock does not behave elastically, but partly like a viscous fluid. Only by understanding these internal processes we can correctly interpret the gravity changes, which are associated with these deformations and are observed by the GRACE satellites.

Permanent Earth movements

Although the Earth is a deforming planet, our daily experience suggests that it is rigid. Movements of the Earth's surface, apart from earthquakes, are not immediately perceptible to humans. This is because these movements - the most significant is probably plate tectonics - are slow and uniform over large distances. The highest speeds reached at the Earth's surface are in the range of centimetres per year and are almost constant over hundreds to thousands of kilometres. This means that everything, people, houses and cities, move at almost the same velocity. Today, such movements are permanently observed with the help of satellite-based geodetic observation systems such as Global Navigation Satellite Systems (GNSS)Generic term for a class of satellite positioning systems that are operated by different countries or groups of countries. These include GPS of the USA, Galileo of the European Union, the Russian system GLONASS, the Chinese solution BEIDOU, as well a... and flow into a multitude of scientific and technical applications.

As soon as the Earth's surface moves in vertical direction, it also changes the gravitational field, which can be observed with the GRACE satellites. These vertical movements are caused primarily by mass redistributions that exert a pressure on the Earth's surface, which deforms the Earth (Fig. 1). Since these mass displacements, such as air pressure differences, ocean currents and tides, river levels, groundwater level changes or snow cover, also influence the gravity field, the observed change in the gravity field represents a combination of the deformation and the gravitational effect of the deforming load. The relationship between imposed load and deformation for purely elastic deformations of the Earth acting on time scales from hours to decades is very well understood and can be calculated precisely.

Glacial-isostatic adjustment and postglacial land uplift

On much longer time scales over centuries or even millennia, however, the Earth's interior behaves fundamentally different, and the elastic behaviour of the lithosphere has to be separated from the flow behaviour of the mantle rock.

One well-known process is the post-glacial land uplift in northern Scandinavia, which can amount up to one metre per century. This movement, known in science as glacial-isostatic adjustment (GIA), is the still today ongoing reverberation of the last glaciation period 20,000 years ago, northern Europe was covered by an ice sheet of several kilometres thickness. On such a large time scale of thousands of years, the Earth's mantle behaves like a viscous liquid (comparable to a very solid honey) on which the elastic lithosphere floats. At that time, the thick ice sheet had bent the lithosphere downwards by several hundred metres and displaced the underlying mantle material. Due to the melting of the ice at the end of the cold period and the resulting unloading, the lithosphere should rebound immediately. However, it is prevented from doing so by the viscous mantle. The adjustment is delayed and the ground is still rising today. Already 200 years ago, the land uplift in northern Sweden has been interpreted correctly after observing of the associated sea level fall of the Baltic Sea in this region. 

The still ongoing movement is detected today in a multitude of geodetic observations. Due to land uplift, the sea level is falling in the formerly icy regions. The horizontal transport of mantle material towards Scandinavia (Fig. 2), on the other hand, leads to (small) land subsidence in formerly non-glaciated regions of Central Europe, which results there in an additional rise in relative sea level. The associated change in the gravity field is a significant contribution to the gravity signal measured by the GRACE/GRACE-FO satellites. Furthermore, GIA changes the position of the rotation axis of the Earth, which is called polar motion, as well as the position of the centre of mass of the entire Earth with respect to its surface.

Viscosity distribution in the upper mantle

The viscosity of the Earth's mantle is not the same everywhere. Dynamic processes in the Earth's mantle lead to changes in temperature. But since viscosity is strongly dependent on temperature - again, honey can serve as a good example, which also becomes thin when heated - we find viscosities in the Earth's interior that differ by several orders of magnitude. Especially in tectonically active regions, where higher temperatures occur, the viscosity is reduced. If ice caps or glaciers melt above these areas, i.e. they losing mass, this can result in a measurable uplift movement within just a few decades. For example this is observed in West Antarctica and southern Patagonia by GRACE/GRACE-FO. Interpreting this movement, we can understand better how much ice is melting in the respective areas, and also learn a lot about the deforming material in the Earth's interior. For a long time, the analysis of these uplift movements was the only way to directly determine the viscosity of the Earth's mantle.

Postseismic deformations

A dynamically related process is the so-called postseismic deformation, i.e. the continuing deformation of the Earth's crust following a large earthquake. Postseismic deformations are caused by the stresses that build up between crustal blocks and that are then released by a sudden quake and the associated abrupt shift between the crustal blocks (coseismic deformation). Depending on the geometry of the earthquake, this deformation can also manifest itself in an uplift or subsidence of the lithosphere and can, so, be observed with the GRACE/GRACE-FO satellites. Here, also occurs a time-delayed adjustment. This process can last for several years in the vertical movement. Interestingly, this movement occurs faster than expected when considering viscosities derived from GIA. Accordingly it is specified as a transient process, i.e. a transitional behaviour that acts between purely elastic and purely viscous behaviour. Material investigations in the laboratory support this hypothesis, so that geophysical observations and material sciences cross-fertilise each other here.

However, postseismic motions are quite small-scale compared to GIA or loading, so only very large earthquakes can be observed currently with the GRACE/GRACE-FO satellites. With future dual-pair missions such as NGGM/MAGIC, the observation of earthquakes up to magnitude 7 will be possible. Based on historical earthquake catalogues and considering only events with substantial uplift or subsidence, we expect an average of 60 seismic events per year, which should be detectable with a significantly improved gravity field mission from 2032 onwards (Fig. 3).

Text: Dr. Volker Klemann, GFZ