Increased seismicity at the beginning of 20th century in the Czech Republic and adjacent areas

The Czech Republic is situated in an intraplate region with low seismicity. The seismic hazard is relatively low but not negligible. A new map of the seismic hazard of the Czech Republic computed using a probabilistic approach is being compiled and will be released in the second half of 2023. As a part of this project new catalogues of earthquakes, both historical (based on macroseismic observations) and instrumental (based on seismograms) were compiled. These include earthquakes on the territory of the Czech Republic and neighboring areas of Slovakia, Austria, Germany and Poland.
       When analyzing these new enhanced catalogues, we recognized a period of increased seismicity at the beginning of the 20th century. The annual seismicity rate in this time interval for earthquakes with a magnitude 4 and greater is several times higher than at present. One possible explanation is the inconsistent magnitude determination between the beginning of the 20th century and the present time. Therefore, we re-examine historical seismograms at the beginning of the 20th century and verify their magnitudes. We also compared macroseismic observations of historical earthquakes with modern ones having almost the same magnitude.
       We found out that the increased seismicity at the beginning of 20th century is real. During this period, relatively large earthquakes were observed in various source zones in the investigated region. Such earthquakes have not occurred since 1920-1930 to the present.