ALBERT

Description: Aftershock identification plays an important role in the assessment and characterization of large earthquakes. Especially, the length of the aftershock sequence is an important aspect of declustering earthquake catalogues and therefore impacts the frequency of earthquakes in a certain region, which is important for future seismic hazard assessment. However, in intraplate regions with low deformation rates and low to moderate seismicity, it is still questionable if aftershocks after a major event may continue for much longer time. In this study, we use one of the earliest instrumentally recorded earthquakes, the 1906 Dobrá Voda earthquake (Ms/Imax=5.7/VIII-IX), to compare different approaches of aftershock determination and their suitability for understanding the recorded earthquake sequence. The Dobrá Voda segment of the Vienna Basin Transfer Fault System is one of the seismically most active zones in Slovakia with the 1906 earthquake as the strongest recorded earthquake. We first assess the epicentral intensity of the earthquake according to the Environmental Intensity Scale (ESI2007) using contemporary descriptions of earthquake effects. This additional information leads to constrain the maximal intensity to IESI2007=IX. This result agrees well with first the assessment of Imax in 1907 and indicates the reliability of this intensity data. In the second step, earthquake data are plotted for two spatial windows extending 13 km and 26 km from the epicenter of the mainshock, respectively. Despite uncertainties regarding the completeness of data due to war times and lack of nearby seismic stations, the overall temporal evolution of seismicity can apparently not be described as an Omori-type aftershock sequence following the event in 1906. Instead, earthquake occurrence within 13 km of the mainshock shows elevated earthquake activity right after the 1906 event that only decays to a lower level of activity within decades after the mainshock. The decline of seismicity therefore occurs over time scales which are much longer than those predicted by the Omori relation. We conclude that today’s seismic activity may still be affected by the 1906 earthquake.