Bert Vermeersen

Reassessment of the Potential Sea-Level Rise from a Collapse of the West Antarctic Ice Sheet

Collapse and Rise The West Antarctic Ice Sheet (WAIS) is thought to be inherently unstable and susceptible to rapid collapse if it reaches a certain warming threshold. Although such an event is considered unlikely, to predict the consequences of collapse it is important to know how much sea level would rise in such a case. The WAIS is thought to contain enough ice to raise sea level by 5 to 7 meters were it to collapse. Bamber et al. (p. 901, see the cover; see the Perspective by Ivins) have reassessed that number, on the basis of better data on the geometry of the WAIS, and conclude that its sudden collapse would raise sea level by about 3.2 meters, on average, with large and important regional variations. Although this is only about half as much as previously thought, its impact on coastal areas would still be devastating. A collapse of the West Antarctic Ice Sheet would raise global sea level by 3.2 meters, but with large regional variations. Theory has suggested that the West Antarctic Ice Sheet may be inherently unstable. Recent observations lend weight to this hypothesis. We reassess the potential contribution to eustatic and regional sea level from a rapid collapse of the ice sheet and find that previous assessments have substantially overestimated its likely primary contribution. We obtain a value for the global, eustatic sea-level rise contribution of about 3.3 meters, with important regional variations. The maximum increase is concentrated along the Pacific and Atlantic seaboard of the United States, where the value is about 25% greater than the global mean, even for the case of a partial collapse.

Ice sheets, sea level and the dynamic earth

Published by the American Geophysical Union as part of the Geodynamics Series, Volume 29. In this monograph, we present recent progress in geophysical modeling and observational tools related to the process of glacial isostatic adjustment (GIA). Rather than a retrospective view, however, we have been led by one over arching mission: to gather significant contributions that present the state-of-the-art in the field and beyond, just as it is being reshaped by new space-geodetic technologies. In this light, the monograph includes discussion on new progress in a number of long-standing problems: the modeling of the Earths viscoelastic response; the prediction and analysis of sea-level changes and anomalies in the Earths rotation and gravity field; and the inference of mantle viscosity. Such contributions are complemented by papers that focus on results obtained by GPS and constraints expected from impending satellite missions, as well as predictions of geophysical observables (e.g., present-day 3D deformations, gravity signals and fault instability) related to these efforts. In these many applications it is important to understand recent progress in GIA research and the limitations that currently impact that research.

Postseismic GRACE and GPS observations indicate a rheology contrast above and below the Sumatra slab

More than 7 years of observations of postseismic relaxation after the 2004 Sumatra-Andaman earthquake provide an improving view on the deformation in the wide vicinity of the 2004 rupture. We include both Gravity Recovery and Climate Experiment (GRACE) gravity field data that show a large postseismic signal over the rupture area and GPS observations in the back arc region. With increasing time GPS and GRACE show contrasting relaxation styles that were not easily discernible on shorter time series. We investigate whether mantle creep can simultaneously explain the far-field surface displacements and the long-wavelength gravity changes. We interpret contrasts in the temporal behavior of the GPS-GRACE observations in terms of lateral variations in rheological properties of the asthenosphere below and above the slab. Based on 1-D viscoelastic models, our results support an (almost) order of magnitude contrast between oceanic lithosphere viscosity and continental viscosity, which likely means that the low viscosities frequently found from postseismic deformation after subduction earthquakes are valid only for the mantle wedge. Next to mantle creep, we also consider afterslip as an alternative mechanism for postseismic deformation. We investigate how the combination of GRACE and GPS data can better discriminate between different mechanisms of postseismic relaxation: distributed deformation (mantle creep) versus localized deformation (afterslip). We conclude that the GRACE-observed gravity changes rule out afterslip as the dominant mechanism explaining long-wavelength deformation even over the first year after the event.

DynaQlim – Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas

The isostatic adjustment of the solid Earth to the glacial loading (GIA, Glacial Isostatic Adjustment) with its temporal signature offers a great opportunity to retrieve information of Earth’s upper mantle to the changing mass of glaciers and ice sheets, which in turn is driven by variations in Quaternary climate. DynaQlim (Upper Mantle Dynamics and Quaternary Climate in Cratonic Areas) has its focus to study the relations between upper mantle dynamics, its composition and physical properties, temperature, rheology, and Quaternary climate. Its regional focus lies on the cratonic areas of northern Canada and Scandinavia.

Normal Mode Theory in Viscoelasticity

In modeling a particular geophysical phenomenon, the choice of the rheology used depends on 1) mathematical difficulty, 2) the quality of the geophysical data which the calculations of the model are required to match and 3) our knowledge of the rheological behavior of the medium at hand. Over the last few decades a considerable amount of knowledge has been gained about mantle rheology in terms of the values of rheological parameters and deformation mechanisms. For instance, what is most important, as far as mantle convection is concerned, is clearly the strong temperature dependence of the viscosity which the laboratory-derived values of the activation energy and volume seem to suggest. This intense interest in understanding convection in a fluid with markedly temperature-dependent viscosity is attested by the recent fundamental studies by geophysicists using analytical, numerical and experimental methods. In what follows, however, rather than discussing topics of mantle rheology and mantle convection, for which we refer to the book by Ranalli (1995), we will try to address the main questions that are at issue in attempting to study transient and long time scale geodynamic phenomena in a wide arc of time scales, ranging from years, characteristic of post-seismic deformation, to hundreds of millions of years as in the case of true polar wander driven by subduction, making use of the analytical normal mode theory in viscoelasticity with different models of mantle rheology. In Figure 1.1 we sketch the entire geodynamic spectrum spanning the whole range of phenomenological time scales. One of the key questions is whether one can devise a constitutive law which can satisfactorily model all these phenomena, from the anelastic transient regime to the steady-state domain.

Global Dynamics of the Earth: Applications of Viscoelastic Relaxation Theory to Solid-Earth and Planetary Geophysics

This chapter dealswith the expansion in spheroidal and toroidal harmonics of themomentumandPoisson equations, for spherical, self-gravitating, stratified, viscoelastic planets. For the linear viscoelastic Maxwell rheology, the Correspondence Principle is considered for obtaining the viscoelastic solution from the equivalent elastic problem. Both normal mode and complex contour integration techniques are used for anti-transforming the field from the s-domain to the time domain. Boundary conditions at the surface of the planet, at the core-mantle boundary and at the internal interfaces between layers of different elastic and density characteristics, are obtained. Point sources for loads and dislocations, the latter limited to the spheroidal component for applications to gravitymodeling, are expanded in spherical harmonics for Green function derivation. 1.1 Rheological Models In modeling a particular geophysical phenomenon, the choice of the rheology used depends on (1) mathematical difficulty, (2) the quality of the geophysical data which the calculations of the model are required to match and (3) our knowledge of the rheological behavior of themedium at hand. Over the last few decades a considerable amount of knowledge has been gained about mantle rheology in terms of the values of rheological parameters and deformation mechanisms. For instance, what is most important, as far as mantle convection is concerned, is clearly the strong temperature dependence of the viscosity which the laboratory-derived values of the activation energy and volume seem to suggest. This intense interest in understanding convection in a fluidwithmarkedly temperature-dependent viscosity is attested by the recent fundamental studies by geophysicists using analytical, numerical and experimental methods. In what follows, however, rather than discussing topics of mantle rheology and mantle convection, for which we refer to the book by Ranalli (1995), we will try to address the main questions that are at issue in attempting to study transient and long time scale geodynamic phenomena in a wide arc of time scales, ranging from years, characteristic of post-seismic deformation, to hundreds of millions

Geophysical Impact of Field Variations

For low harmonics (up to about degree and order 30), the earth’s gravity field is predominantly determined by two solid-earth processes: mantle convection, and glacial isostatic adjustment (GIA) due to Pleistocene deglaciation after the last great Ice Age. The relative importance of these two processes is not always as obvious as one might think. For example, it is tempting to attribute the deep geoid-low above Canada solely to GIA. However, from comparisons of forward GIA and convection models with geoid data it is nowadays generally assumed that both GIA and mantle convection must be responsible for this conspicuous feature.

Modelling and Observing the Mw 8.8 Chile 2010 and Mw 9.0 Japan 2011 Earthquakes Using GOCE

Earthquakes change the gravity field of the area affected by the earthquake due to mass redistribution in the upper layers of the Earth. In addition, for sub-oceanic earthquakes deformation of the ocean floor causes relative sea-level changes and mass redistribution of water that has again a significant effect on the gravity field. Two such recent, large sub-oceanic earthquakes are the 27 February 2010 Chile Maule earthquake with a magnitude of Mw 8.8 and the 11 March 2011 Japan Tohoku earthquake with a magnitude of Mw 9.0. The goal of ESA’s satellite GOCE—launched in March 2009—is to map the Earth’s gravity field with unprecedented accuracy and resolution. To this end, GOCE carries a gravity gradiometer. Although the mean gravity field is to be mapped, the sheer size of both earthquakes and associated mass redistribution make them both potential candidates for detecting the co-seismic gravity changes in the GOCE gradiometer data. We assess the detectability of gravity field changes in the GOCE gravity gradients by modelling these earthquakes using a forward model. Furthermore, we analyse the GOCE data before and after the respective earthquakes and assess their quality. Based on these analyses we conclude that despite its small signal size at GOCE altitude the Japan earthquake may be visible in the gravity gradients when more post-earthquake data become available. Because of the short data period before the Chile earthquake this signal will probably not be visible.

TPW Driven by Subduction: Non-linear Rotation Theory

This chapter deals with the development of a non-linear rotation theory, driven by internal density anomalies, as for those due to mantle convection, for a stratified, viscoelastic, incompressible Earth. We show how mantle convection TPW represents a very powerful constraint for the mantle viscosity profile, and our finding is that the lower mantle has to be definitively stiffer than the upper mantle.

Rotational Dynamics of Viscoelastic Planets

The rotation of the Earth is not regular. It changes on virtually every time scale we know in both position of the rotation axis and rotation rate. Even in our daily lives we sometimes experience the consequences of such changes, such as the second that is subtracted or added to clocks at the beginning of a new year. Although this second is not much more than a curiosity for most of us, the rotational changes it implies can influence our lives in a more fundamental sense. There are indications that the emergence of the great ice ages some two million years ago was triggered by a gradual shift of the rotation axis over the Earth’s surface, combined with wandering of the continents and associated changes in ocean currents (note that we are talking here about the onset of ice ages — the period of the 100,000 year cycles of ice build-up and decay is determined by astronomical causes). In the 19th century, both the rate of rotation and the position of the rotation axis were shown to be variable. Nowadays we know that these changes occur on all time scales: from shorter than a day to geological ones of hundreds of millions of years. The changes in position of the rotation axis can be divided into two main categories: those in which the position of the axis changes with respect to the distant stars but not with respect to the Earth’s surface, and vice versa. For the latter category, it looks to a hypothetical observer in space as though the Earth shifts underneath its rotation axis as a solid unit while the rotation axis itself remains fixed with respect to the stars, while for an observer on Earth it looks as though the rotation axis is wandering over the Earth’s surface. Displacements of the axis of rotation with respect to the fixed stars (changes in which the whole planet is moving rigidly as one unit) are mainly due to external forces, notably the gravitational interactions between the Earth and the Sun, the Moon and the other planets of the solar system. The astronomically well-known precession and nutation are examples of this. The external forces exert a net torque on the equatorial bulge of the Earth, as a consequence of which the rotation axis spins. The most important periods are about 26,000 years (precession) and 18.6 years (nutation)