Seismic response and vulnerability of historical Trakošćan Castle, Croatia using HVSR method

The Trakošćan Castle was built on the rocky peak of the hill in the thirteenth century. Castle structure has weakened with time, wars, inappropriate protection, and earthquakes. On 16 March 1982, the Tower Castle and part of second floor were damaged by an earthquake (M _L  = 4.5) with an epicenter 20 km distance from Mt. Ivančica. Afterwards, it was completely reconstructed. Today the cultural heritage of the Trakošćan Castle is protected as a historical entity by the Republic of Croatia. The Horizontal-to-Vertical-Spectral-Ratio (HVSR) method was applied for the purpose of local seismic response, protection from earthquake, structural seismic vulnerability of weak points, and structural restoration of the Trakošćan Castle related to the 1982 earthquake (microtremor measurements in free-field and in the Tower Castle). The free-field HVSR results (soil fundamental frequencies, amplification factors, shear wave velocity distribution, estimated bedrock depth, and directional effects on ground motion) are compared to the local geology, propagation path, and topographic effects to the estimated local seismic response. The dynamic behavior of the Tower Castle is demonstrated along different axes by the comparison of the directional HVSR analysis to the horizontal components (NS/V and EW/V) and the amplitude spectra (NS, EW and UD). The modal frequency response identifies weak points of the Tower Castle structure. Based on the HVSR analysis, the damage at Trakošćan site, related to the 1982 earthquake, occurred due to the local geology and topographic effects (seismic energy directed through the alluvial basin and trapped into the hill) and the improper way of the Tower Castle construction in the past and the reconstruction after the earthquake.