Geological researches revealed vertical crustal movements of several thousand meters and horizontal movements of tens of km in comparatively recent geological time such as the last 10-20 million years by faulting, bending, folding or overthrusting. Such changes as may now be in progress are considered to be detected and measured by geodetic means. However, the actual rates of changes per year are extremely small, and since it was doubtful that the conventional geodetic surveys may contribute much to clarify the tectonic movements in a reasonable period of years.
Of geodetic surveys, precise levelling have, in principle, remarkable high precision (better than 1×10-6) and contributed till now to the detectians of features of crustal deformations accompanied by large earthquakes. However, we have yet only insufficient data to trace the progressing tectonic activities, because the time intervals between successive resurveys were too long (20-30 years). Recently, under the Project of Earthquake Prediction, the Geographical Survey Institute has started to repeat precise levelling along the first order levelling routes all over the land of Japan every five years. It is expected that such works may be able to reveal the details of tectonic deformations and accumulation of strain in the land of Japan.
The precision of horizontal positionings by conventional precise triangulations is far inferior to that of precise levelling. It is estimated to be somewhat better than 1×10-5. If horizontal deformations are assumed to be of the order of 10-7 per year, this means that one century or more years are necessary to trace the movements. This might be the reason why reliable records were not obtained by triangulation except when sudden destructions were caused by large earthquakes as in Japan and California. Recently, the accuracy of distance measurement with an electrooptical instrument (such as Geodimeter) has attained to about 1×10-6, and we have now the back ground of detecting and tracing the horizontal deformations in a reasonably short period (say, in several tens of years).
The direct evidence of the theory of continental drift may be provided by changes in astronomical positions, but it was unfortunate the suggested rapid changes are in longitude. Until very recently, the reliability of longitude observation was not sufficient owing to the inaccuracies of observing instruments and the inaccurate corrections to the time of propagation of wireless time signals. Now, the standard time signals (UTC) in the world are synchronized in the accuracy of better than 1ms (=1/1000sec), and high precision instruments (PZT and precise astrolabe) have also been developed. Relative movements of continents of the order of several cm per year may be detected in the near future. Furthermore, artificial satellites have become promising means of connecting geodetic nets in accurate and pure three dimentional geometrical ways.
In conclusion, although conventional geodetic survey did not contribute so much until recently, we may expect in near future newly developed geodetic means will afford useful data which clarify the tectonic movements of the crust and reveal the internal activities of the earth.
