The subject of this diploma thesis is the investigation of seismo-electromagnetic (SEM) phenomena, i.e. the correlation of seismic activity and electromagnetic signals. Due to the lithospheric, atmospheric and ionospheric coupling, it is in principle possible to detect seismic precursors in the ionosphere. I focus on sub-ionospheric radio wave propagation in the frequency range f=3-50kHz (VLF/LF) between transmitters and receivers, mainly in Europe. The 2009 earthquake (EQ) in L'Aquila (Italy), a 2013 event in Rumania and a 2012 earthquake in Bulgaria are studied in more detail. The sub-ionospheric ground based VLF method, which is based on nighttime amplitude anomalies, and defined as the difference between the observed radio signal and the average of superposed daily signals, is applied.Typical variations of the received radio signal, caused by modifications of the sub-ionospheric waveguide related to seismic activity, were observed. At the very first time the receiving station in Graz was used, based on previous developments mainly in Russia, Japan and Europe. The VLF observations in Graz are special suitable to perform SEM studies in south and south-east Europe. A major challenge is to differentiate seismic from non-seismic influences on the radio path, and therefore special emphasis is on parameter studies of the waveguide.The study confirms results from other authors, as there exists a threshold in EQ magnitude below clear anomalies are difficult to detect.As an outlook, a multi-parameter study including a combination of various SEM methods is desirable and the goal for future scientific investigations.