SEISMIC GROUND MOTION AMPLIFICATION INDUCED BY TOPOGRAPHIC IRREGULARITY IN NORTHERN CROATIA

After the recent 2020 earthquakes ML5.5 in Zagreb and ML6.2 in Petrinja, a considerable part of the reported damage to churches, museums, cultural and older buildings and houses was observed in topographical areas of Northern Croatia. Topographic amplification of seismic ground motion can be affected by the topographic irregularity, inducing phenomena such as the focusing of seismic waves, the diffraction of body and surface waves and the resonance with topographic feature. Over 40 topographical sites were carefully examined within reports of damage observations as a part of Croatian Science Foundation (HRZZ) project IP-2022-10-1296 SIGMATOPCRO. Conducted field investigations included visual damage observations (that still exists) or finished/current renovations, spatial position of topographic features in terms of geometry, size, direction and geological properties correlated with microtremor measurements to determine site effects. An important aspect was also to find a good reference station free of site effects, i.e. located on a stiff, unweathered and rocky outcrop to better understand topographic site amplification and possible directivity effects. Preliminary results of macroseismic damage observations, 2D topographic irregularity model along different directions, geological conditions and site resonance frequencies of topographic features indicate that topographic amplification effects are rather very complex compared to stratigraphic amplification. Seismic ground motion amplification is affected by topographic irregularity shape, directional effects and geological site properties. Also, presence of fractures, water saturations and human interventions in topographic environment can be a relevant factor in enhancing susceptibility of slopes to earthquake induced failures and possible occurrence of higher damage effects at certain sites. Based on preliminary investigation results, detailed multidisciplinary field investigations will be performed to characterize crucial segments along chosen topographical irregularity locations as an input for 1D and 2D complex numerical seismic response modelling. Also, continuous seismic monitoring will be established at these sites to compare empirical data with numerical analysis.