Macroseismic Constraints on Deep Crustal Structures in the Bohemian Massif

Abstract. We analysed two moderate earthquakes that occurred near Mirotice in southern Bohemian Massif (Czechia) in March 2024 (M_L 3.5) and April 2025 (M_L 3.1), integrating seismological analyses with crowdsourced macroseismic observations. Both events originated at unusually large focal depths for the region (23–24 km) and were widely felt across southern and central Bohemia, attracting significant public and scientific attention. Macroseismic effects were documented through extensive citizen participation, with over 1,500 questionnaires for each event collected by the Institute of Geophysics of Czech Academy of Sciences. Maximum macroseismic intensities reached IV–V; however, the spatial distribution of observations was distinctly non–circular, forming a pattern controlled by major tectonic structures. Areas underlain by granitoids of the Central Bohemian Pluton exhibited dense and far reaching macroseismic responses, consistent with low seismic energy attenuation of the Moldanubian, whereas regions dominated by early Palaeozoic sedimentary rocks of the Teplá–Barrandian showed sparse observations, indicating higher attenuation. Focal mechanisms indicated normal faulting on a NW–SE striking fault plane, consistent with regional stress field. The unusually large focal depths may be attributed to low geothermal gradient in the area suggesting deep seated structural controls on earthquake nucleation. The results delineate two subvertical structures within the Bohemian Massif and demonstrate that crowdsourced macroseismic data provide valuable constraints on earthquake effects and deep geological controls on seismic wave propagation, even in regions with low seismicity, and effectively complement instrumental observations.