Introduction - Geodesy: Introduction to Geodetic Datum and Geodetic Systems - page 7

been powerful in support of the research of mobilism and can provide more
abundant and accurate information for current research in the geosciences. The
contributions of modern geodesy are chiefly:
1. It provides not only precise geodetic information for the study of plate move-
ment and crustal deformation but also provides new methods for establishing
accurate kinematic models of recent plate movement and crustal deformation.
Very long baseline interferometry (VLBI), satellite laser ranging (SLR), and
GPS are able to measure the precise and relative velocity of plates with an
approximate speed of 1 mm/year so as to calculate directly the Euler vector of
the relative plate movement from actual data. In the past 20 years, a massive
amount of data on plate movement has been obtained using geodetic techniques;
the correctness of the modern plate movement model NUVEL-1 derived from
geophysical and geological data has been tested and observational models have
been established. At present, geodesy is determining global, regional, and local
crustal movements with unprecedented space-time resolution, according to
which the stress-strain model inside the plates can be established to test the
truthfulness of the rigid plate hypothesis, to deduce the deformation inside the
plates, and to provide the basis for the explanation of faulting, seismicity, and
other tectonic processes. Some geology and structures cannot be explained
currently by plate tectonics, which awaits further improvement. Geodesy will
presumably make new contribution to this.
2. The variations in polar motion and the Earth's rotation velocity are linked to
information about the Earth's structure and diverse geodynamic processes. The
precision of the Earth's rotation parameters determined by space geodesy has
been the most effective tool in extracting and differentiating such information.
Based on certain models (circle structure hypothesis, elasticity and viscoelastic-
ity hypothesis of the Earth's mantle and core, etc.) of the Earth's structure, the
corresponding rotation equations can be established to study the precession,
nutation, and polar motion of the Earth's three axes (axis of rotation, axis of
figure, and axis of angular momentum); the model of the Earth's structure can be
verified and modified by comparing the observed values and the values of
theoretical inference. One example of this is the correction proposed by VLBI
observed data to the nutation series IAU1980, which impelled restudy of the
Earth model. Polar motion includes the free motion (Chandler wobble) deter-
mined by the Earth's elasticity that lasts for 410-440 days, the superimposed
forced oscillation lasting for a year, the minor swing lasting nearly a day, and the
low-amplitude swing that lasts for a long period of 25-30 years. The factors
causing these wobbles in different periods are a central subject of modern
geophysics and involve a series of very important issues such as the exchange
of angular momentum between the solid Earth, the atmosphere, the sea, and
Earth's core; tidal friction and dissipation; the change in rotation angular
momentum caused by seasonal climate variations; the viscoelastic core-mantle
structure; nuclear magnetic fluid dynamics (geomagnetic dynamo); and core-
mantle electromagnetic coupling. The motivating factor for the length of day