Geodynamische Interpretation von Höhenänderungen mit gravi- metrischen Methoden

The comparison of repeated precision levelling in Austria shows an interesting pattern of recent crustal movements, which is characterized by uplift rates of 1-2 mm/a in the area of the main Alpine ridge. A geodynamical interpretation of these elevation changes and the associated mass transfers makes it necessary to gather information about changes in height as well as in gravity. The tasks of this project are high precision measurements of height and absolute gravity, which may serve as an initial measurement for a long series of repeated measurements. An area covering the periadriatic suture S of Klagenfurt was selected for these investigations. In 1996 the high precision levelling line Klagenfurt - Ferlach - Kleiner Loiblpass - Loibl was measured and compared with the results from 1958 and 1978. Local changes in elevation relative to Klagenfurt were observed ranging from - 20 mm (3 km S of Klagenfurt) to +18 mm (Ferlach - Unterloibl). A significant general trend could not be found. Furthermore, GPS stations with a connection to the levelling net were installed at 11 locations N and S of the periadriatic suture. First, the JILAg-6 instrument available for the absolute gravity measurement was upgraded to the most recent level. Thereafter, absolute gravity measurements were made twice at each of the 4 newly installed stations (Kleiner Loiblpass, Loibltunnel, Ebriach and Trögern) and connected to the Austrian absolute gravity net. Additional comparative measurements were possible at Graz, Wettzell and Jozefoslaw within the scope of contemporaneous international projects like UNIGRACE. An accuracy of +/- 1 Gal was achieved. The gravity changes between two repeated measurements vary from 5 Gal (Ebriach and Trögern) to 25 Gal (Kleiner Loiblpass). Microgravity nets were installed around the absolute gravity stations in order to secure them in their local environment. Relative gravity measurements with up to 5 instruments (type LCR D, LCR G, Scintrex CG-3) and precision levelling were carried out twice at these micronets. The standard deviations within one measuring campaign ranged from 7 - 20 Gal. With one exception (Trögern) the repeatability was 5 Gal. In spite of the lower accuracy of the relative gravity measurements compared to the absolute measurements, the repeatability of both methods is in the same range. The changes in elevation within the micro nets were < 2 mm. From the analysis of the initial measurements carried out within the scope of this project we conclude that repeated precision levelling along the lines in the project area should always be combined with absolute gravity measurements. Regarding the expected changes in elevation of around 1 mm/a, repeated measurements should be made every 3 - 5 years. Even if this interval is far too short to permit a geodynamical interpretation, we can learn about the true accuracy of absolute gravity meters. The observation of the local geological, hydrological or man- made changes and the modelling of the corresponding changes in gravity is the weakest link in the whole system. By research work in this area the detection limit of gravity changes caused by geodynamical processes could be reduced below 10 Gal.