Preliminary Observations on Historical Castle Trakošćan (Croatia) Performance under Recent ML ≥ 5.5 Earthquakes

Trakošćan Castle, built on a rocky peak in the late 13th century, is cultural heritage site protected as historical entity by Republic of Croatia. The Castle constructed highly irregular masonry structure with timber or shallow arches, vaults dome floors. It was substantially renewed, upgraded and partially retrofitted from 16th century until year 2000. M5.5 (VIII EMS) M6.2 (VIII-IX earthquakes, which struck city Zagreb 22 March 2020 Pokupsko-Petrinja area 29 December 2020, strongly shook Castle’s structure. Earthquake damage observed assessed visual inspection accompanied ambient vibration measurements. slight cracks that appeared arches were found to be critically positioned, can likely lead arches’ collapse if their spreading not prevented. Ambient measurements, compared pre-earthquake ones, revealed decrease fundamental frequencies central tower unit second floor, thus possibly indicating loss structural stiffness consequence earthquake damage.