Seismic Hazard Assessment of the East Thuringian Region/Germany – Case Study

East Thuringia/Germany, especially the region Gera-Ronneburg, is part of the large Kyffhäuser-Jachymov-Fault-Zone and displays moderate seismicity. However, its seismic hazard is significantly higher than that of the surrounding area including the Vogtland/Northern Bohemian region. The earthquake catalogue of Germany contains for this region besides the well-investigated Central German Earthquake (March 1872, I ₀ =VII-VIII) entries of up to I ₀ =VIII (14th century). Epicentral intensities and coordinates of these historical earthquakes are considered as uncertain. In seismic hazard analysis historical events which are uncertain are often neglected. But, especially in regions of moderate seismicity and infrequent larger earthquakes, the time window considered should be extended as far as possible. Apart from the necessity to study the historical sources of the strongest 14th century earthquakes, we investigate the influence of these events on the seismic hazard, taking into account the uncertainties of their size and location. Generally, the investigations clearly reveal the importance of defining source regions on the one hand and the significance of the local relevant attenuation function on the other hand. A further important point in seismic hazard assessment is the strong influence of the geological site conditions on seismic hazard (amplification or damping phenomena). For both points the well-known Central German Earthquake (1872) supplies important information.     

The East Thuringian Seismic Network

The East Thuringian Seismic Network (OTSN) was installed in 1997. It started its operation with five and now consists of six seismic stations, the GRSN (German Regional Seismic Network) station MOX and a control and analysis centre. All stations are equipped with 3-component GÜRALP and short-period seismometers, RefTek 24-bit data acquisition systems (dynamic range 23.5 bit), hard disks, GPS-receivers, modems and communication computers for dial-up purposes. The seismic signals are sampled at 100 Hz and stored on the hard disk. Simultaneously, the signals are processed by a STA/LTA detector which generates an extended event list. The central station calls these event lists once per day, analyses them, produces a list of real seismic events and calls the waveform data for these events only from the single stations. All stations operate completely autonomously and the whole system works automatically, but all operations can also be carried out interactively. The event analysis is performed manually using common seismic analysis programs. The main purpose of installing the seismic network is to investigate the local seismicity, its relation to recent tectonics, the stress field and structure of the upper crust in order to render more precisely the seismic hazard of East Thuringia. A further aim of the network is to improve the seismic monitoring situation for the neighbouring regions, especially the Vogtland/Northern Bohemia and the Western Saxony area.     

Local seismic hazard assessment in areas of weak to moderate seismicity—case study from Eastern Germany

Generally the seismic hazard of an area of interest is considered independent of time. However, its seismic risk or vulnerability, respectively, increases with the population and developing state of economy of the area. Therefore, many areas of moderate seismic hazard gain increasing importance with respect to seismic hazard and risk analysis. However, these areas mostly have a weak earthquake database, i.e., they are characterised by relative low seismicity and uncertain information concerning historical earthquakes. In a case study for Eastern Thuringia (Germany), acting as example for similar places in the world, seismic hazard is estimated using the probabilistic approach. Because of the lack of earthquakes occurring in the recent past, mainly historical earthquakes have to be used. But for these the actual earthquake sources or active faults, needed for the analysis, are imprecisely known. Therefore, the earthquake locations are represented by areal sources, a common practice. The definition of these sources is performed carefully, because their geometrical shape and size (apart from the earthquake occurrence model) influence the results significantly. Using analysis tools such as density maps of earthquake epicentres, seismic strain and energy release support this. Oversizing of areal sources leads to underestimation of seismic hazard and should therefore be avoided. Large location errors of historical earthquakes on the other hand are represented by several alternative areal sources with final superimposition of the different results. In a very similar way information known from macroseismic observations interpreted as source rather than as site effects are taken into account in order to achieve a seismic hazard assessment as realistic as possible. In very local cases the meaning of source effects exceeds those of site effects very likely. The influence of attenuation parameter variations on the result of estimated local seismic hazard is relatively low. Generally, the results obtained by the seismic hazard assessment coincide well with macroseismic observations from the thoroughly investigated largest earthquake in the region.