Geophysics at the University of Michigan

Seismology involves the study of earthquake generated elastic waves that have propagated though the Earth's interior. Ground motion recordings at seismographs around the world carry a wealth of information on the physical process of earthquake faulting, and Earth's internal structure. Research in Jeroen Ritsema's group involves seismic tomography and waveform modeling of broadband body-wave signals. Specific projects include: • the modeling of regional waves with mineralogic models of the upper mantle • interogation of seismic tomography with 3D synthetics • global analysis of wave amplitudes and wave focussing in the heterogeneous mantle • NARS-Baja: seismic analysis of the Gulf of California rift.
An image of shear velocity heterogeneity (percent velocity changes from a radially symmetric reference model) in the upper mantle, illustrating the variable shear wave speed beneath the Mid Atlantic Ridge. Relatively low shear wave speed (regions shaded with "red" colours) extends to about 100-150 km depth, typical for passive mid-ocean ridge spreading. A different upwelling mechanism may be responsible for a low velocity anomaly beneath Iceland, that extends deep into the transition zone.

Seismology involves the study of earthquake generated elastic waves that have propagated though the Earth's interior. Ground motion recordings at seismographs around the world carry a wealth of information on the physical process of earthquake faulting, and Earth's internal structure. Research in Jeroen Ritsema's group involves seismic tomography and waveform modeling of broadband body-wave signals. Specific projects include:
• the modeling of regional waves with mineralogic models of the upper mantle
• interogation of seismic tomography with 3D synthetics
• global analysis of wave amplitudes and wave focussing in the heterogeneous mantle
• NARS-Baja: seismic analysis of the Gulf of California rift.

An image of shear velocity heterogeneity (percent velocity changes from a radially symmetric reference model) in the upper mantle, illustrating the variable shear wave speed beneath the Mid Atlantic Ridge. Relatively low shear wave speed (regions shaded with "red" colours) extends to about 100-150 km depth, typical for passive mid-ocean ridge spreading. A different upwelling mechanism may be responsible for a low velocity anomaly beneath Iceland, that extends deep into the transition zone.

		The research in Peter van Keken's group comprises a dynamical investigation into the large scale thermal and chemical evolution of the Earth's mantle. Of particular interest are the processes that influence how heterogeneity is introduced into the mantle, how it is erased or maintained, and how we find evidence at the surface. Specific projects include: • preservation and mixing of chemical heterogeneity by mantle convection • dynamics of mantle plumes and interaction with the lithosphere • subduction zone dynamics, focusing in particular on the role of water • formation of strain by mantle convection and its relation with seismic anisotropy • dynamics of the Archean earth. The principal tool for investigation is Computational modeling based on finite-element methods. While the geodynamical approach provides independent tests of qualitative hypotheses, it is strongly integrated with observational and experimental work. Most of our research is done in tight collaboration with specialists in geochemistry, seismology, mineral physics and petrology.

In Geodynamics, we are interested in how convective flow in the Earth's interior shape the Earth's surface. Carolina Lithgow-Bertelloni's research is targetted at understanding plate driving forces, great earthquakes, and the structure and deformation of the continental lithosphere. Her goal is to integrate a wide variety of observations, from topography to plate motions to mineral physics and seismology, as constraints on quantitative models of the dynamics of the Earth. The methods range from experimental fluid dynamics of mantle plumes, simplified global flow models of the mantle, numerical models (largely finite elements) of lithospheric deformation and mantle convection, and integration of thermodynamic models of the petrology and physical properties of lithosphere and mantle with mantle convection.
Subducting plates must be pulled towards subduction zones with a force equal to the weight of upper mantle slabs. Hence, slabs in the upper mantle remain physically intact and strong enough to maintain guiding stresses that support the weight of the slab. In the lower mantle, however, slabs must be detached from those in the upper mantle and supported by the viscous mantle, which deforms and flows in response. This flow draws both subducting and overriding plates toward subduction zones at the same speed. These two mechanisms by which mantle slabs drive the surface plates cause subducting plates to move rapidly toward trenches while overriding plates, which are not pulled, move more slowly.

In Geodynamics, we are interested in how convective flow in the Earth's interior shape the Earth's surface. Carolina Lithgow-Bertelloni's research is targetted at understanding plate driving forces, great earthquakes, and the structure and deformation of the continental lithosphere. Her goal is to integrate a wide variety of observations, from topography to plate motions to mineral physics and seismology, as constraints on quantitative models of the dynamics of the Earth. The methods range from experimental fluid dynamics of mantle plumes, simplified global flow models of the mantle, numerical models (largely finite elements) of lithospheric deformation and mantle convection, and integration of thermodynamic models of the petrology and physical properties of lithosphere and mantle with mantle convection.

Subducting plates must be pulled towards subduction zones with a force equal to the weight of upper mantle slabs. Hence, slabs in the upper mantle remain physically intact and strong enough to maintain guiding stresses that support the weight of the slab. In the lower mantle, however, slabs must be detached from those in the upper mantle and supported by the viscous mantle, which deforms and flows in response. This flow draws both subducting and overriding plates toward subduction zones at the same speed. These two mechanisms by which mantle slabs drive the surface plates cause subducting plates to move rapidly toward trenches while overriding plates, which are not pulled, move more slowly.

		Mineral physics seeks an understanding of geophysical processes at the atomic level. By studying the structure, bonding, and energetics of Earth materials at conditions representative of the interior, we gain insight into the origins of the physical structure of Earth as seen by seismology, the forces that generate mantle convection, and reaction and chemical differentiation throughout Earth history. Lars Stixrude uses a variety of methods to study the nature of minerals and melts at high pressure and temperature including first principles quantum mechanical simulations based on density functional theory. Current areas of interest include: • the physics of silicate liquids at elevated pressure and the implications for magma genesis and transport • the elasticity and thermochemistry of mantle minerals and implications for understanding the origin of the seismic structure of the mantle • the state of water in the deep interior, including in hydrous and nominally anhydrous phases • thermodynamic theory of mantle phase assemblages including self-consistent description of phase equilibria and elasticity, and the behavior of materials at the conditions of giant planetary interiors.
Snapshots from first principles molecular dynamics simulations of MgSiO3 liquid at ambient pressure (top) and 125 GPa (bottom) showing the change in structure that accompanies two-fold compression. Silicon-oxygen polyhedra are shown in blue and magnesium ions in yellow. The silicon-oxygen coordination number changes from four-fold at low pressure to six-fold at high pressure, producing a much more efficiently packed structure. It has been predicted, based on these simulations, that silicate melts formed at the base of the mantle would be denser than coexisting solids and would tend to pond there.

Mineral physics seeks an understanding of geophysical processes at the atomic level. By studying the structure, bonding, and energetics of Earth materials at conditions representative of the interior, we gain insight into the origins of the physical structure of Earth as seen by seismology, the forces that generate mantle convection, and reaction and chemical differentiation throughout Earth history. Lars Stixrude uses a variety of methods to study the nature of minerals and melts at high pressure and temperature including first principles quantum mechanical simulations based on density functional theory. Current areas of interest include:
• the physics of silicate liquids at elevated pressure and the implications for magma genesis and transport
• the elasticity and thermochemistry of mantle minerals and implications for understanding the origin of the seismic structure of the mantle
• the state of water in the deep interior, including in hydrous and nominally anhydrous phases
• thermodynamic theory of mantle phase assemblages including self-consistent description of phase equilibria and elasticity, and the behavior of materials at the conditions of giant planetary interiors.

Snapshots from first principles molecular dynamics simulations of MgSiO3 liquid at ambient pressure (top) and 125 GPa (bottom) showing the change in structure that accompanies two-fold compression. Silicon-oxygen polyhedra are shown in blue and magnesium ions in yellow. The silicon-oxygen coordination number changes from four-fold at low pressure to six-fold at high pressure, producing a much more efficiently packed structure. It has been predicted, based on these simulations, that silicate melts formed at the base of the mantle would be denser than coexisting solids and would tend to pond there.