Assessing Topographic Site Effects and Seismic Microzonation in Northern Croatia: Case Study Insights from the 2020 Earthquake Sequence

The 2020 earthquake sequence in Croatia caused significant damage, particularly to cultural assets and older masonry buildings in areas of pronounced topography in Northern Croatia (EMS intensity VI). The observed damage distribution aligns closely with topographical features, with higher intensities recorded in hilly areas—such as Hrvatsko Zagorje, Ivanščica, Kalnik, and Međimurje—compared to adjacent alluvial basins.

To investigate these phenomena, this study presents results from microtremor measurements using the Horizontal-to-Vertical Spectral Ratio (HVSR) method across five localities characterised by distinct geological and morphological configurations. We integrated HVSR fundamental frequencies with local geological data to derive detailed seismic microzonation maps that quantify the terrain's resonance potential. These maps illustrate critical correlations between the slope/height of the dominant hill axis and the measured site frequencies.

Our analysis confirms that topographic site effects are primarily driven by the focusing of seismic waves at ridge crests, a process governed by diffraction, reflection, and wave type conversions. It is observed that amplification is highly frequency-dependent; resonance is strongest when the incoming wavelength aligns with the ridge’s frequency characteristics. Furthermore, the steepness of the topography plays a major role, with the uppermost portions of hills consistently showing stronger resonant motion than lower slopes.

Preliminary site amplification factors calculated for the 2020 earthquake scenarios (Zagreb Mw 5.4 and Petrinja Mw 6.4) reveal complex interactions between topographic irregularities and wave propagation. These findings underscore the necessity of explicitly incorporating topographic site effects into seismic microzonation studies. This approach is essential for producing reliable ground-shaking models and refining the local seismic-hazard assessment, particularly for preserving vulnerable historical structures in seismically active regions of Northern Croatia.