1st Croatian Conference on Earthquake Engineering 1CroCEE 2021

Proximity of the active faults in Croatia is being studied for evidence of past earthquakes by archaeoseismology. Excavations of the Roman city of Siscia (Sisak, Croatia) yielded large chunks of thick brick walls (considered to be the city wall), collapsed in the adjacent ditch in their entirety. The wall is made of brick masonry on both sides and a thick concrete infill between them. Much of the collapsed portion is entire, there are not even joints between adjacent bricks. Wall chunks are found in various orientation: the original horizontal layering of masonry is now mostly vertical, even overturned. The remaining foundation displays features of twisting and shearing. The construction level is underlain by sandy clay. Significant recycling of construction material occurred in later centuries, so original dimensions are estimates only. We suggest that a major earthquake damaged the city wall of Siscia. Excitated by site effects of loose soil, high peak ground acceleration caused the wall to be removed from its foundation, landing it ultimately in the ditch nearby. Presumed intensity of the earthquake was IX. Fault activity within a couple of kilometres distance was responsible for this collapse. Rebuilding of the city wall in the late antique period suggests that the first wall collapsed between the beginning of the 3rd and the middle of the 4th century. This earthquake between ~200 AD and ~350 AD are missing from historical catalogues. We suggest that fault activity in Croatia is more widespread than previously considered. Archeoseismological features – even if lacking proper dates – must be added to the catalogues of historical earthquakes of the region.