Introduction
IAG Section V "Geodynamics" deals with topics at the frontier of geodesy and geophysics. It is concerned by a number of interdisciplinary research and service activities. The interactions between geodesy and geophysics are both ways: global geodetic results homogeneous in time and space provide essential information for understanding the dynamics of deformations and interactions of the various layers of our planet, while the precise knowledge of the influence of these phenomena on the stations is indispensible for using the full precision capability of the observing techniques. Typical of Section V are also the interactions between the research and service activities: studies performed in Commissions, Special Commissions or Special Study Groups may help in the development of the services, while topical research developped in the latter in order to maintain state of the art performance results in an increased capability for the whole community. The general research themes under Section V can be summarized as follows.
Global references, solid earth deformations :
These topics are basic to ensure the accuracy and consistency of geodetic results that will be used in geophysical studies. A number of Section V-related activities are devoted to them.
The geodetic constants and models imply the maintenance of the international atomic time scale TAI (Temps Atomique International) by the Time Section of the Bureau International des Poids et Mesures (BIPM). The Fundamental Constants in use in geodesy are dealt with by Special Commission 3, while the IERS maintains a set of state of the art models necessary in the estimation and interpretation of the Earth’s rotation and deformations, the IERS Conventions (most recent issue in 1996, next one due in 2000).
The provision of accurate and dense terrestrial references at a precision level that matches the most demanding scientific research is one of the basic responsibilties of the International Earth Rotation Service (IERS), in charge of the maintenance of the International Terrestrial Reference Frame (ITRF), in close cooperation with technique-oriented services currently linked to Section II : the International GPS Service (IGS), and the newly created International Laser Ranging Service (ILRS) and International VLBI Service for Geodesy and Astrometry (IVS, also linked to the International Astronomical Union). The IERS set up a Working Group on the ITRF Datum that clarified the various problems raised by the definition of a conventional terrestrial reference system.
The geodetic monitoring of tide gauges is indeed a key constituent of the study of global sea level changes and their climatic implications. Its is currently performed jointly by the Permanent Service for Mean Sea Level (PSMSL) and the IGS, with the support of Special Commissions 8 on Sea Level and Ice Sheet Variations.
The Earth orientation monitoring is another basic responsibility, performed at the level of accuracy of a fraction of one milliarcsecond (mas) by the IERS.
Earth deformations
Data collection, validation, analysis and distribution of results on Earth tides is the responsibility of the International Center for Earth Tides (ICET), supported by the Earth Tides Commission (Commission V). Studies beyond the Earth tides per se make use of a worldwide network of cryogenic gravimeters to investigate regional phenomena such as tidal loading, atmospheric pressure effects, underground water effects, as well as global modes of the Earth’s core and mantle, in the framework of the Global Geodynamic Project (GGP), whose Data Center is at ICET.
Crustal deformations and the geodetic aspects of natural hazards are intensively studied in the general scientific community. In the last quadrienniums, the implication Section V in these studies was under the Commission on Recent Crustal Movements (CRCM, Commission VII), while a Section II sub-commission, WEGENER, delt with Europe and the Eastern Mediterranean region. Following an examination of means and objectives in this research field by Section V, the IAG Executive Committee decided to merge these two bodies into a new Commission on Crustal Deformations under Section V. Two SSGs have studied geodetic effects associated with earthquakes: gravity changes (SSG 5.172) and plumb line changes (SSG 5.173).
Geodetic implications of atmospheric and oceanic effects
This field is by definition a place for interdisciplinary cooperations between Geodesy, Geophysics, Oceanography, and Atmospheric Sciences. The intent in creating in 1995 a Special Commission 8 on Sea Level and Ice Sheet Variations was to offer a support to these interactions by fostering interdisciplinary studies and projects, with the expectation to provide a connection with the International Geosphere Biosphere Project (IGBP). Due to various circumstances within Section V, the intense activity that took place in this field during the 1995-1999 period was only loosely linked to this Special Commission. Nevertheless, the the IAG Executive Committee decided to persist in this action, where its role of international coordination could be of special importance.
The control of site motions for the validation and interpretation of observed sea level changes is a matter of interest and responsibility for the ICET, the Earth Tides Commission, the IGS and the IERS, while the sea level monitoring is the responsibility of the PSMSL, in cooperation with international projects outside IAG, such as the Global Sea Level Observing System (GLOSS), a project of the Intergovernmental Oceanographic Commission (IOC).
Section V bodies have played a prominent role in the study of a number of global geophysical processes.
The IAU-IUGG Working Group on Nutation of the non rigid Earth completed a comprehensive analysis of all aspects concerned by the phenomenon : seismic input models for computing the Earth’s transfer function for nutations, construction of this transfer function, validation of the rigid Earth nutation theories (at the level of a few tens of microarcseconds), convolution between the transfer function and the rigid Earth nutation theory, and finally the comparison with the VLBI observations.
The geophysical interpretation of temporal variations of the geopotential were studied by SSG 5.174, while the IERS/ITRF Datum WG conducted a very successful Geocenter motion analysis campaign.
SSG 5.173 on ’Interaction of the Atmosphere and Oceans with the Earth’s Rotational Dynamics’ continued the action of a series of SSGs started in 1983 for the study of the atmospheric and oceanic influence on the Earth’s rotation, in close interaction with the IERS. This long term project could provide an inspiring example of specific IAG action and influence in the Earth Sciences through two types of organisational devices, SSGs and Services working in mutual support. The first two SSGs of the series fostered the evolution of knowledge and performance in atmospheric excitation of the Earth’s rotation irregularities that led to the creation of an Atmospheric Angular Momentum sub-bureau (SBAAM) in the newly formed IERS (1988). Then the SSGs in the series served as research support to this service activity, while continuing the extension of the research to other global geophysical fluids (ocean, groundwater, core,…). When IERS organized in 1996 a general review of its activity and performance, time was ripe for recommending an extension of its mission to the monitoring of these phenomena. The establishment of the Geophysical Fluid Centers activity under the IERS, in 1997, now offers a structure to provide data and to further stimulate research in this area. There is common ground with other important geodetic problems, notably the study of time variable gravity from space, variations in the geocenter, and the study of nutation and precession. The 1999-2003 item in the series of SSGs will be an joint IAG/IAPSO Working group which will engage geodetic and oceanographic communities, while strengthening the efforts of the IERS Geophysical Fluid Center for the Oceans.
Summary of the structure, 1995-1999
A total of 13 bodies were connected to Section V during the 1995-1999 term , including 8 permanent ones . The reports and recommendations of these bodies can be found in this volume.
| Commission V: Earth tides, President: H.-G. Wenzel (Germany) |
| Commission VII: Recent Crustal Movements, President: T. Tanaka (Japan) |
| Special Commissions 3: Fundamental Constants, President: E. Groten (Germany) |
| Special Commissions 8: Sea Level and Ice Sheet Variations, President: W.E. Carter (USA), then G. Blewitt (UK) |
| International Center of Earth Tides (ICET). Director: P. Melchior (Belgium) until the end of 1995; B. Ducarme (Belgium) starting in 1996. |
| International Earth Rotation Service (IERS). Chairman of the Directing Board: C. Reigber (Germany). Director of the Central Bureau: M. Feissel (France) until the end of 1997; D. Gambis (France) starting in 1998. |
| Permanent Service for Mean Sea Level (PSMSL). Director: P. L. Woodworth (UK) |
| The Time Section of the BIPM (Bureau International des Poids et Mesures). Director: C. Thomas (France) until the end of 1998 ; acting Director: G. Petit (France) starting in 1999. |
| SSG5.172 Understanding Natural Hazards-The Geodetic Contribution, Chair: S. Okubo (Japan) |
| SSG5.173 Seafloor Interaction of the Atmosphere and Oceans with Earth's Rotational Dynamics, Chair: C. Wilson (USA) |
| SSG5.174 Geophysical Interpretation of Temporal Variations of the Geopotential, Chair: A. Cazenave (France) |
| SSG5.175 Interannual Variations of the Vertical and Their Interpretation, Chair: Z.X. Li (China) |