Geodetic and Geophysical Repeated Measurements in Geodynamic Monitoring Networks of Estonia

Abstract

Repeated measurements were started in the first Estonian geodynamic network — the Põltsamaa-Lelle geodynamic line in 1961. Three more geodynamic networks were established in the 1970s: in Navesti (Central Estonia), in Viru-Nigula (North—East Estonia) and in Paluküla (West Estonia). The aim of these networks was a detailed study of local crustal movements in the regions where tectonic faults are located. Repeated levellings as well as gravimetric, electrometric and magnetometric observations were made on those networks. On Viru-Nigula network the measurements of baseline lengths for the determination of horizontal movements were performed. Measurements on the networks were stopped at the end of 1980s due to the lack of financial resources. Repeated high precision gravity measurements on Põltsamaa-Lelle geodynamic line are still carried out nowadays.

The present paper gives an overview of the repeated measurements, mainly of repeated levellings and gravity measurements. Based on the repeated levellings, the vertical displacements of benchmarks are calculated and the possible reasons for benchmark displacements, like the change in groundwater level, subsidence of building, fault activity and postglacial rebound, are discussed. In Põltsamaa-Lelle geodynamic line the connection between the vertical movements of benchmarks and observed gravity change is brought out.