The project, titled Earthquake Risk Assessment of the City of Zagreb, was conceived in 2014 but only approved and funded two days after the March 2020 earthquake. Researchers from the University of Zagreb Faculty of Civil Engineering, working with the Croatian Centre for Earthquake Engineering, city departments, seismologists, geologists and geotechnical experts, carried out the work between 2021 and 2023 as part of a broader EU-financed aerial survey programme for disaster risk reduction. The team describes the initiative as a pilot intended to serve as a template for earthquake risk assessments across Croatia and as a foundation for changes to national regulations expected in 2026.

Zagreb’s hazard profile is serious. The city lies along the southeastern edge of Medvednica mountain, and its seismic threat comes mainly from the Medvednica epicentral area, particularly two seismogenic sources: the Reverse Northern Medvednica boundary fault and the nearly perpendicular Kašina strike-slip fault. Historical records recall the Great Zagreb earthquake of 1880, assessed at magnitude 6.1, whose epicentre lay very near the 2020 event and which caused widespread damage and emigration. The official Croatian hazard map assigns the city area a reference peak ground acceleration on rock of 0.20 to 0.28 grams for a 475-year return period, and investigations so far indicate that Eurocode 8 ground types B and C predominate, although a full seismic microzonation of the city remains unfinished.

The greatest technical achievement of the project, according to its authors, was the creation of a comprehensive geographic information system building inventory database. Croatia has no national building registry, so the team overlapped three official data sources: the Digital Cadastral Plan, the Register of Spatial Units and the Existing Land Use 2020 dataset. Of more than 300,000 objects examined, roughly 230,000 proved to be buildings and auxiliary structures, while about 70,000 were stairways and terraces that were excluded. Engineers then collected 35 required attributes for each building, covering geographic position, site category, year of construction, material and type of the lateral load-resisting system, number of storeys, structural regularity, position within a block, interaction with adjacent buildings, floor and roof systems, foundations, occupancy, cultural heritage status and physical condition, among others. Additional attributes consistent with the Global Earthquake Model building taxonomy, including system ductility and seismic code level, were added.

Populating the database proved laborious. Post-earthquake damage assessment databases from Zagreb and Petrinja, cadastral records, aggregated census data, design documentation, and LiDAR recordings from the national aerial survey all fed into the model. Ten experts spent two years collecting, verifying and entering data, combining field surveys with remote desktop work. Public participation, tested through an online questionnaire, yielded fewer than 300 responses, with residents citing privacy concerns, lack of time and anxieties about illegally constructed or modified buildings. The resulting exposure model covers approximately 136,000 residential buildings. More than half of them, 52 percent, are unreinforced masonry structures, 39 percent are confined masonry, 8 percent reinforced concrete and about 1 percent timber. Among unreinforced masonry buildings, roughly three quarters have rigid floor diaphragms. The replacement value of the residential stock is estimated at about 86 billion euros.

The risk model itself rests on three components: hazard, exposure and vulnerability. Two hazard models were used, one developed within the project from eleven seismic area sources within 200 kilometres of the city’s main square, built on the Croatian Earthquake Catalogue, and the European model ESHM20. The project-specific model generated 100,000 stochastic event sets through Monte Carlo sampling, each representing one year of seismicity, and used six ground motion prediction equations in a logic tree. Notably, the local model proved considerably more conservative than the European one for weaker, more frequent earthquakes, producing higher average annual losses. In accordance with the project methodology, the reported results rely on the project-specific hazard model. On the vulnerability side, because locally derived fragility curves were lacking, the team used the Global Earthquake Model database of roughly 500 building classes, applying 98 vulnerability curves to Zagreb’s residential typologies, with consequence models for collapse, fatalities, injuries, displacement and financial loss.

The probabilistic results, computed with the OpenQuake Engine, are sobering. For an earthquake with a 500-year return period, the assessment predicts around 900 collapsed buildings, approximately 600 fatalities, about 2,000 injured people and roughly 79,000 residents requiring temporary shelter, with direct financial losses of about 9.6 billion euros. Around 9,000 buildings would reach complete damage, about 7 percent of the stock, while a further 5,100 would be heavily damaged. Average annual losses amount to roughly 215 million euros, an average annual loss ratio of 0.25 percent that the authors consider high. Financial losses concentrate in the historic centre, where cultural heritage protection raises replacement costs, while the largest numbers of collapsed buildings and fatalities are expected in the northeastern districts, driven mainly by the amount of exposed built-up area. Unreinforced masonry buildings account for about half of all expected financial losses.

Two deterministic scenarios sharpen the picture. A repeat of the 1880 earthquake, modelled at magnitude 6.3 on the Reverse Northern Medvednica boundary fault, would collapse around 1,460 buildings, kill about 1,050 people, injure roughly 3,400 and displace approximately 126,000, with losses near 14.6 billion euros. A repeat of the 2020 magnitude 5.4 event would produce about 300 collapsed buildings, 170 fatalities, 630 injuries, 25,000 displaced residents and losses of about 3.5 billion euros. When the modelled 2020 scenario was compared with actual observed impacts, agreement was good at both city-wide and district scales: the districts that suffered the heaviest losses in reality, led by Donji Grad and Gornji Grad-Medveščak, match the model’s predictions, although the post-earthquake government report estimated higher total financial losses, partly reflecting different unit prices and market changes.

The project’s results have already found practical application. In collaboration with the Zagreb City Office, researchers evaluated the city’s official evacuation routes for potential blockage by debris from collapsed buildings. Following Croatian regulations, which assume debris reaches a distance equal to half the building height, the team created GIS buffers around building footprints, intersected them with road surfaces, and ranked buildings by their probability of collapse in the 1880-type scenario. The analysis identified safer access zones for civil protection forces but showed that problems are expected in the central part of the city, prompting a subsequent detailed study of the Lower Town using discrete element simulations of masonry façade collapse. The authors caution that current fragility models do not capture non-structural elements such as chimneys, parapets and gables, which caused many road blockages in the 2020 earthquake.

The team is candid about the model’s limitations and future needs. There is no microzonation for the entire city, secondary hazards such as landslides and liquefaction were not considered, many building attributes relied on expert judgement, and locally specific fragility and vulnerability functions remain a critical gap, although Croatian research projects are now developing them. Indirect losses and retrofit scenarios are not yet included, and the building database has not been established as an officially maintained data source. Even so, the authors conclude that earthquake risk for Zagreb is high and that the model provides a robust foundation for prioritising retrofitting and preparedness. Masonry buildings, responsible for about 80 percent of expected damage and losses, emerge as the clear priority. With national regulatory changes expected in 2026, the pilot marks a decisive shift from general awareness to data-driven risk management, offering a methodological and data-rich framework that other seismic-prone cities in the region and beyond can adapt.

Subject of Research: Building-level earthquake risk assessment of Zagreb, Croatia, using GIS exposure data and OpenQuake probabilistic modelling

Article Title: Earthquake risk assessment of the city of Zagreb, Croatia: recent advances

Article References: Šavor Novak, M., Uroš, M., Baniček, M., Demšić, M., & Atalić, J. (2026). Earthquake risk assessment of the city of Zagreb, Croatia: recent advances. Bulletin of Earthquake Engineering. https://doi.org/10.1007/s10518-026-02662-y

Image Credits: AI Generated

DOI: 10.1007/s10518-026-02662-y

Keywords: earthquake risk, Zagreb, Croatia, seismic hazard, exposure model, OpenQuake, unreinforced masonry, building inventory, vulnerability, evacuation routes, GIS database, disaster risk reduction