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Chapter 5. Solid-Earth Sciences
Investigations of the Earth's Interior
Studies of the structure, composition, and processes in the interior of the Earth have included investigations from the crust and upper mantle to the deep mantle and the core. The tools have included the deployment of portable broadband instruments in both passive and active field experiments, and studies of body waves, surface waves, and free oscillations using data from the global seismographic observing system. The CIRES personnel engaged in this research are now housed in the Benson Earth Sciences Building, so that the work is closely integrated with other related work in the Department of Geological Sciences.
Crust and Upper Mantle. The recent work on the structure and dynamics of the crust and upper mantle has been led by Anne Sheehan and Craig Jones, both in the Department of Geological Sciences and CIRES fellows. Their geophysical studies are strongly supplemented by the geochemical research of Farmer, who applies radiogenic isotope systematics to problems of the origin and evolution of continental crust and tectonic modifi- cation of continental mantle lithosphere.
Experiments in which Sheehan and Jones have participated include the Rocky Mountain Front experiment, Colorado Plateau/Great Basin seismic experiment, Sierran Paradox I and II, Snake River Plain experiment, and the Continental Dynamics of the Rocky Mountains experiment. Sheehan and former CIRES research associate Ken Dueker have developed high-resolution imaging techniques using phases converted at subsurface seismic discontinuities and have applied these to dense PASSCAL deployments. Other studies include seismic anisotropy and applications of seismic tomography.
The efforts of this group also extend to the sea floor, with participation in an ocean-bottom seismograph experiment on the East Pacific Rise, which explored the structure and dynamics of this mid-ocean ridge, and the Lau Basin experiment in the western Pacific.
The Deep Interior. The late Ned Benton carried out some of the earliest work at CU on the deep interior. His primary interest was the physics of the geodynamo and other problems in fluid dynamics related to core processes and the generation of the geomagnetic field. He was one of the creators of the IUGG program Study of the Earth's Deep Interior (SEDI).
Seismic array analysis by Bob Engdahl and co-workers permitted the recovery of clear observations of previously undetected or unexplained seismic phases. The source of these phases was convincingly associated with major boundaries in the Earth. Reflected core phases were used to fix accurately the radii of the inner and outer core and to describe the nature of these boundaries. These data are still used as a standard to test Earth models.
Some of the first CIRES work on the analysis of surface waves and normal modes was performed by Martin Smith, a former CIRES fellow also associated with the Department of Physics.
John Wahr and his co-workers have made significant contributions to studies of the interior, with emphasis on the applications of geodetic information (Earth rotation, gravity), in addition to the analysis of seismic wave travel-times. He and graduate student Artie Rodgers tried to use standard earthquake travel-time tables to infer core-mantle boundary (CMB) topography and concluded that the data were inadequate to provide accurate maps. He and Juergen Neuberg examined wave reflections from a small portion of the CMB to derive topography and in the process noted a reflection that is presumably from the top of the transition layer at the bottom of the mantle.
This group also examined the possibility of laterally-varying structure in the core caused by density anomalies in the mantle. They were able to quantify the possible size of this variability and compute its effects on various observable quantities. With co-workers from Belgium, Wahr examined how internal density anomalies would perturb the geoid and the shapes of all boundaries, whether at the surface or internal.
Wahr's work on post-glacial rebound encompassed extension of the computational formalism and quantification of the sources of error in inferring mantle viscosity from rebound observations. His group carried out numerous studies of the use of new types of geodetic observations in rebound investigations and estimated the deformation of the outer surface and internal boundaries caused by the rebound. His work on tides in the solid body of the Earth and nutations led to an effective formalism for studying the shape of the CMB, as well as a method for using earth tides to infer wave attenuation in the mantle at very low frequencies.
Work on the deep interior is also led by Michael Ritzwoller, Department of Physics and affiliate of CIRES. In addition to his Earth-oriented research, he contributes to efforts to understand how large-scale convection in the solar convection zone affects helioseismic oscillations. Some of his work with numerous colleagues, including J. Resovsky and E. Lavely, has focused on the theory and applications of normal mode analysis. They have developed three-dimensional models of mantle structure, including a recent (1999) model of mantle shear wave velocity structure. They are now working on the density structure of the mantle from normal modes. Ritzwoller contributed some of the earliest work leading to discovery of inner core anisotropy.
This group, with major contributions from A. Levshin, has completed group velocity tomography to estimate continental crustal and uppermost mantle structure in Antarctica, Eurasia, South America and the Arctic. Levshin has contributed to the development of a new technique for measurement of surface wave characteristics (dispersion, polarization, spectral amplitudes) that is efficient and convenient for analyzing large volumes of data. Ritzwoller and co-authors were among the first to invert globally orbiting long-period surface waves for lateral attenuation models of the upper mantle.
The laboratory studies of mineral physics of the kind done by Spetzler and Getting, as well as Joseph Smyth of geological sciences, provide essential background for the interpretation of seismic observations in terms of internal composition and processes. Since the 1990s Spetzler has worked on the chemical and physical effects of fluid flow in undersaturated porous rocks, with emphasis on seismic wave attenuation, and studies of mantle rheology through experimental determinations of shear anelasticity in candidate mantle minerals. He has also carried out geophysically important high-pressure, high-temperature equation-of-state measurements in a diamond anvil cell, using ultrasonic (GHz) interferometry and X-rays. Getting developed a unique apparatus for measuring attenuation and dispersion in mantle minerals at seismic frequencies and small strains. The attenuation observed in these and other experiments is much more frequency-dependent than indicated by field observations and this raises some crucial questions for the science. Getting also developed a number of devices for high-pressure experiments that have been installed in several of the premier laboratories in the United States.
