The 2020 earthquakes in Croatia caused significant damage, particularly to churches, museums, cultural assets, and older buildings located in areas of pronounced topography in Northern Croatia (intensity VI, EMS). This study aims to derive insights from available macroseismic data on historical and recent seismic events, with the objective of characterising localised topographic damage patterns. Our specific inquiry is whether local ground effects, especially irregular topography, contribute significantly to damage beyond the influence of a building's inherent structural condition. This research also highlights gaps in empirical evidence, historical data, and methodologies for analysing topographic effects.

Accurate seismic hazard assessment requires region-specific ground motion models. Croatia currently lacks a comprehensive, regionally calibrated spectral Ground Motion Model (GMM) covering a wide range of magnitudes, distances, and site conditions. The 2020 Croatian earthquakes highlighted critical gaps in instrumental network coverage and the absence of a regional GMM. Because strong-motion data for high-magnitude events remain too sparse to develop empirical model, this study presents a new stochastic Ground Motion Model for Croatia (CRO-GMM). The model is developed using point-source and finite-source modelling calibrated with region-specific seismological parameters - stress drop, frequency-dependent quality factors Q(f), and the high-frequency attenuation parameter kappa (κ) derived primarily from regional weak-motion records. The model utilises 69,120 earthquake scenarios simulated across moment magnitudes 4.5–7.1, distances of 1–160 km, focal depths of 8–18 km, and frequencies of 0.3–100 Hz (periods 0.01–3 s), accounting for variations across regional tectonic zones: the transition of the Pannonian Basin toward the Internal and External Dinarides and the Southern Dinarides. The resulting GMM, applicable to rock conditions, is integrated with a nonlinear site amplification model for Croatia. Validation against European and global GMMs shows that CRO-GMM performs comparably to established benchmarks while better capturing local seismotectonic characteristics. Comparison with recorded strong motions from the Ston (1996, Mw6.0), Zagreb (2020, Mw5.4), and Petrinja (2020, Mw6.4) earthquakes demonstrates reasonable agreement within ± 2σ uncertainty bounds. The model provides an essential tool for advanced probabilistic seismic hazard analysis and engineering design in Croatia. Future validation using an expanded Croatian strong-motion database will be essential for further refinement and evaluation of model performance for a variety of local site conditions.

The 2016 earthquake in Central Italy caused varying degrees of damage across Camerino's historic centre (Italy). Two medieval masonry buildings, despite their similar construction characteristics and close proximity, exhibited notable differences in damage extent, highlighting the role of seismic amplification in historic urban areas. This study aims to refine site-specific seismic response analyses by developing a detailed engineering-geological model that integrates litho-stratigraphic, geotechnical, and geophysical data. In-situ mechanical analyses with Equotip on outcropping rocks beneath the buildings, combined with borehole and geophysical data, enabled the identification of key lithological and geotechnical contrasts. These findings revealed significant stratigraphic heterogeneity between the two sites. The first exhibited alternating layers of varying stiffness and velocity, while the second presented a more homogeneous stratigraphy, yet weathered in its shallower portion. These differences influenced site amplification, correlating with greater structural damage at the first site and less deterioration at the second. Variations in litho-mechanical properties, including lower Equotip hardness values and distinct seismic velocities in weathered units, contributed to these discrepancies. This research emphasises the importance of integrating detailed stratigraphic reconstructions with seismic analysis to enhance the resilience of historic structures. The developed methodology provides a transferable framework for seismic risk assessment and retrofitting strategies in other heritage sites. By enhancing subsurface analysis, this approach contributes to cultural heritage preservation in seismically active regions.

Here we analyze the rupture process of the 29 December 2020 M(W)6.4 Petrinja earthquake (Croatia), the largest event instrumentally recorded in this area characterized by a moderate strain-rate intraplate setting. We use foreshocks and aftershocks, recorded at more than 80 broadband stations located 70-420 km from the earthquake, as empirical Green's functions (EGFs) to separate source effects from propagation and local site effects. First, we deconvolve the mainshock P-wave time windows from the EGFs in the frequency domain to obtain the corner frequency (f(c)). Spectral analysis based on the Brune's source model reveals a large stress drop of 24 MPa. Next, by deconvolving the Love waves in the time domain, we calculate the Apparent Source Time Functions (ASTFs). We find that the average duration of the source is similar to 5 s, with no significant directivity effects, indicating a bilateral rupture. To extract physical rupture parameters such as rupture velocity, slip distribution and rise time, we deploy two techniques: (a) Bayesian inversion and (b) backprojection onto isochrones of ASTFs. Both techniques show a low rupture velocity (40%-50% of the shear wave velocity) and a rupture length of less than 10 km, that is, much less than would typically be expected for a magnitude 6.4 earthquake. This apparent anticorrelation between stress drop and rupture velocity may be attributed to the complex and segmented fault system characteristic of immature intraplate settings.

Recent significant seismic events, namely the Zagreb MW5.3 and Petrinja MW6.4 earthquakes in 2020, have highlighted the critical need for enhanced seismic hazard assessment. To facilitate a more accurate assessment of seismic hazard, it is imperative to refine and adjust input parameters. Ground Motion Prediction Equations (GMPEs) assume a pivotal role in this aspect. However, the accurate allocation of GMPEs for specific regions necessitates an extensive database comprising strong motion (SM) recordings. This proves challenging for areas characterized by moderate seismic activity, such as Croatia. In response to this challenge, we have established the first systematic SM digital database, continually updated to address this gap. While the BSHAP database (Salic et al., 2017) was formally recognized as the initial Croatian strong motion database, it primarily contained analogue waveforms, often falling short of satisfactory quality standards. Our database confines its scope to the geographical boundaries of 41.2°N – 47.7°N and 12.5°E – 20.5°E. Comprising over 150 good-quality recordings from 2020 to the present day, with magnitudes equal to or surpassing 3.5, this database serves as a valuable resource. In this study, we tested various widely used Ground Motion Prediction Equations (GMPEs) to define the most suitable models. The findings of this investigation lay the foundation for further GMPE development tailored to the Croatian SM database, leveraging the Hybrid Empirical Method (HEM). References:Salic, R., Sandikkaya, M.A., Milutinovic, Z. et al. BSHAP project strong ground motion database and selection of suitable ground motion models for the Western Balkan Region. Bull Earthquake Eng 15, 1319–1343 (2017). https://doi.org/10.1007/s10518-016-9950-3

Tectonic deformation along the External Dinarides fold-and-thrust belt is slow, with transpressional crustal strain redistributed along multiple faults. Some of these faults reach the surface along the NE Adriatic coast as part of the strike-slip Dinaric Fault System (DFS). This work uses new high-resolution sub-bottom seismic data to characterize the geodynamic significance of active surface deformation in the offshore sector of the Kvarner area, Croatia. Seismic profiles reveal gentle folding in early Quaternary strata that is associated with contractional deformation in the Rijeka Bay; yet, active faults are documented only in narrow zones along the DFS, and specifically in the Vinodol and Velebit channels. Fault Mechanism Solutions (FMS) confirm that a strike-slip tectonic regime exists in the study area, but the strike of surface faults and FMS data are discrepant, probably as a result of strain dissipation along (creeping? ) faults whose geodynamic response differs from deep-rooted seismogenic structures. This differing geodynamic response causes important caveats when linking surface deformation to deeper seismogenic structures which are, offshore Kvarner, either blind structures or currently deforming under distinct stress conditions to near-surface faults. Crucially, this work presents the first geological evidence for active faulting along the DFS in Croatia, a piece of information deemed critical for future geohazard assessments.

The aim of this study was to create a preliminary seismic microzonation map of the administrative area of the City of Zagreb (Croatia). Geophysical measurements collected in the southern and eastern part of Zagreb were used together with geophysical data available for the northern part of the city. Their analysis and comparison with the geological data enabled the distinction of eight areas with homogeneous seismic behavior - HSB zones. Northern and southwestern parts of the city, assigned to HSB zones 1 to 5, are characterized with shallow bedrock and mostly compacted Miocene and Pliocene sediments. Central and southeastern parts of Zagreb, assigned to HSB zones 6 to 8, are characterized with deep bedrock overlayed by thick unconsolidated Quaternary deposits. These materials show low average shear wave velocity in the upper 30 meters below the surface (Vs30), and their thickness enables amplification of seismic waves and the emergence of local site effects.

Kvarner area belongs to the External Dinarides fold-and-thrust belt that is characterized by intensive tectonic deformations of a few kilometers thick sedimentary cover of the central part of the Adriatic microplate. The main deformations occurred predominantly during Eocene to Oligocene thin-skinned tectonics, while late-orogenic thick-skinned deformations and wrenching resulted in the exhumation of the orogenic belt. The latter tectonic mechanism is supposed to be still active, but there is no reported evidence of active faults on the surface of predominantly karstic terrain. Nevertheless, seismological data reveal subsurface activity along various fault plane solutions, and crucial evidence of possible active deformations on the surface is expected within stratified superficial deposits in the area. However, stratified Quaternary sediments are rare onshore and on the Kvarner islands but are widespread on the bottom of the surrounding Adriatic Sea. During the targeted high-resolution sub-bottom geoacoustic seismic survey we focused on the zones that are characterized by earthquakes and on the previously arbitrarily recognized regional seismogenic sources. However, only shallow seismic profiles along and across the Vinodol and the NW part of the Velebit channel revealed clear evidence of fault-related deformations of the youngest Quaternary sediments. The fault zone is up to hundreds of meters wide, limited by parallel sub-vertical fault planes, and characterized by deformations of the strata between the planes either in a positive (uplifted) or a negative (downthrown) manner along the strike, which are typical for strike-slip faults. Besides, the disturbed layering of the uppermost well-stratified unit (Late Pleistocene) resembles fluid/sediment escape structures that could be related to strong shaking during prehistorical earthquakes. The fault zone is also tentatively recognized in the onshore bedrock along the strike of the submerged fault, where it appears as an indistinct fractured zone that is more corroded than surrounding bedrock carbonates. Therefore, sub-bottom profiling has been proven to be a useful tool for identifying active faults and should be used as a key method in future seismotectonic research of submerged seismogenic zones.

In order to produce an input for more accurate Ground motion prediction equations (GMPE) that will include site effects, an array of geophysical measurements was done in Dalmatia region in Croatia, as part of CRONOS Project – Investigation of seismically vulnerable areas in Croatia and seismic ground motion assessment. Research methods used were Horizontal/vertical spectral ratio (HVSR) and Multichannel analysis surface waves (MASW), measured at 21 locations with different site effects across this seismically active region. That included over 230 microtremor HVSR measurements, enabling local soil conditions characterization, alongside seismic microzonation mapping of town of Sinj. Around 40 MASW profiles were studied, providing site Vs30 estimation. Also, continuous borehole and surface accelerographs were installed at 14 locations to assemble strong motion database for the region. Based on this research, an interactive and open access Geographic information system (GIS) map was constructed, showing locations of measurements and all related geophysical and geological data. The regional extent of map displays locations of all measurements, combining those that are close. The local map extent shows more detail about every measurement site, in form of a label, symbol classification or a pop-up window. Additionally, in case of dense measurement data at one location (e.g. microzonation of Sinj), interpolation map of area was created, offering a fast and visually intuitive way of understanding results. Adding geological layers to the map allowed for correlation with geological site conditions, which facilitated analyses results interpretation and detection of sites that required additional measurements.

The earthquake of 9 November 1880 was one of the most important moments in the seismic history of Zagreb (Croatia). It is the strongest earthquake to have occurred in the greater Zagreb area, and as such it defines the seismic hazard in northwestern Croatia, the most populated part of the country. The main objective of this study was to reanalyze the location and magnitude of the earthquake, the input parameters which are crucial for a better assessment of seismic hazard, as there were macroseismic indications that the previous assessments should be revised. In addition, the strongest aftershock occurred two days after the main event, so it can be assumed that the observed intensities were caused by the cumulative effect of these two events. Therefore, a new isoseismal map was created, synthetic macroseismic modelling was performed and additional geophysical and microtremor measurements were taken. Based on all the information collected, the attempt to separate the effects of the strongest aftershock from the effects of the mainshock (to avoid a cumulative effect), a new assessment of the location of the epicentre of the main 1880 earthquake in Zagreb and its magnitude was made. When it comes to historical earthquakes, from a seismological point of view, even small improvements in the definition of the main seismological parameters of an earthquake significant for a given area are very important - for a better understanding of the geodynamics of the area, the earthquake mechanism and the spatial distribution of damage after the earthquake, as well as for the consistent assessment of seismic hazard and thus risk.

UNESCO World Heritage Site Old City of Dubrovnik (Croatia) has been devastated by historical earthquake in 1667 of M similar to 7 as well 1979 M6.8 Montenegro earthquakes causing strong damage effects. The aim of this research is the evaluation of the geological structure and local site effects under the Old City, using non-invasive seismic survey methods to define the areas that can be subjected to stronger damage in the case of an earthquake. The results present basis for seismic hazard and seismic risk assessment of cultural heritage - which are necessary activities for their preservation as an important witness of the past.

This study presents a first developed site amplification model for Croatia using random vibration theory (RVT)based site response analysis and measured shear wave velocity (Vs) profiles for different site parameters (VS30 - average shear wave velocity in the top 30 m, fundamental site frequency/period; H800 - depth to the bedrock layer with reference to shear wave velocity VS = 800 m/s). The impedance between the topsoil layers and the engineering bedrock in the measured Vs profiles is more prominent than that in the soft transition modelled in the theoretical soil profiles. The results indicate that (i) the site amplification factors (AFs) are greater than that of the code-based AFs (current and revised Eurocode 8) for stiff soil/soft rock sites and (ii) significant nonlinearity in soft soils occur compared to that of recent ground motion models because of difference in variability between the measured and theoretical Vs profiles. These observations imply significant deviations from the site factors computed based on the site response models in this study and recent models, as well as from those provided in the present Eurocode 8 and its revised version. The presented site AF model based on different site parameters (VS30, frequency/period or bedrock depth) can be integrated into existing or future developed regional ground motion models to characterize the influence of site effects on design response spectra following the variability in soil profile properties on weak and strong seismic responses. This is especially important when used in seismic hazard studies in areas with sparse empirical ground motion database.

Cultural heritage buildings generally refer to the ancient structures having high cultural and historical significance. These buildings are constructed using obsolete practices and require special considerations with respect to the lateral resistance, especially in moderate and high seismic regions. This study focuses on the earthquake performance assessment of the Episcopal Seminary Building and Classical Gymnasium (Jesuit College) located in the Old City of Dubrovnik, Croatia (UNESCO World Heritage Site). The construction dates back to 1662 and was developed in different stages until 1765. During this period, Jesuit College suffered damages from two major earthquakes i.e., M7.6 Dubrovnik in 1667 and M6.9 Montenegro in 1979. The material composition, structural drawings, and fundamental frequencies of the building were previously obtained in the framework of the research project “Seismic Risk Assessment of Cultural Heritage in Croatia – SeisRICHerCRO”. The material is predominantly composed of irregular stone blocks laid in lime mortar. The structural details such as floor vaults, arches, flexible diaphragms, and spatially irregular openings are numerically modelled using the finite element method. The analytical model is calibrated by performing eigenvalue analysis to compute the material parameters i.e., elastic modulus and density (supported by extensive literature review) that allows the modal frequencies to match with the values obtained from the ambient vibration testing. The seismic performance is then evaluated using the linear analysis procedure in accordance with the current guidelines of the Eurocode 8 and the corresponding Croatian National Annex. For the design earthquake, critical damage zones are identified and recommendations for retrofitting measures are proposed.

The overall objective of the project “Investigation of seismically vulnerable areas in Croatia and seismic ground motion assessment” – CRONOS – is to make Croatian society more resilient to the impact of destructive earthquakes. The aim of the CRONOS project, funded by Norwegian Financial Mechanism, is to facilitate this through the development and modernization of seismic hazard assessment in Croatia and stimulate the development of seismic risk reduction policies through scientific infrastructure and capacity building, knowledge transfer and international research cooperation. The first step towards the presented goal is to analyse and understand the past seismicity of the given area. Therefore, for the chosen area (42.5 – 44.5 °N, 14.75° – 17.75 °E) an earthquake catalogue CEC-Cronos has been prepared. The chosen area includes the wider area of central and southern Croatian coastal area (Dalmatia) – one of the seismically most prone areas in Croatia. The given catalogue contains almost 48000 earthquakes which occurred between the years 306 and 2020. Those earthquakes, magnitudes up to 6.7 and intensity in the epicentre up to IX °MSK, have been statistically processed and will be presented. Croatia is characterized by a moderate-to-high level of seismicity, highest in its northwestern and coastal parts. More than 145.000 earthquakes from the period before Christ till the end of 2020 are contained in the Croatian Earthquake Catalogue (CEC). There were more than 100 stronger earthquakes, whose computed or estimated magnitudes were more than 5. The majority of the earthquakes on Croatian territory are the result of the strain accumulation caused by the rotation of the Adria microplate towards the Eurasian tectonic plate. Additionally, central Croatia is in a contact zone of three big geological units: The Alps, the Dinarides (or The Dinaric Alps), and the Pannonian Basin.

MauerwerkVolume 26, Issue 2 VorschauFree Access Vorschau: Mauerwerk 3/22 First published: 16 May 2022 https://doi.org/10.1002/dama.202270204AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Share a linkShare onFacebookTwitterLinked InRedditWechat No abstract is available for this article. Volume26, Issue2May 2022 RelatedInformation

In the wake of recent 2020 M L ≥ 5.5 earthquakes in Croatia, Zagreb M L 5.5 and Petrinja M L 6.2, the insufficient instrumental network as well as the lack of regional ground motion prediction equation (GMPE) were identified as the drawbacks of our engineering community. The former is related to the quality definition of active seismicity (most of the instruments are installed in the southern part of Croatia with fewer installed around Zagreb in the northwestern part of Croatia), and the latter is related to the proper number of strong motion recordings. In Croatia, there is a sparse database of ground motion recordings for moderate earthquakes which makes a well-designed ground motion selecting procedure hardly achievable. Following this, strong motion BSHAP database for empirical estimation of the response spectrum based on Fourier amplitude spectrum and the ground motion duration using Random Vibration Theory approach adjusted to source, propagation, and local site conditions was used. Regionally adjusted ground motion model estimations for the M L 6.2 Petrinja 2020 earthquake scenario are comparable with the previously published GMPEs models for this part of Europe and for the Western part of North America. However, model-to-model variability and uncertainties in local GMPE exceeded those of global GMPEs and are influenced by statistically less stable and more limited datasets. Model is applicable for magnitudes up to M w 6.5 and Joyner-Boore distances up to 200 km with usable frequency range between 0.4 and 33 Hz. The presented model is a step forward toward performing hybrid-empirical seismic hazard studies in areas with sparse ground motions such as the region of Croatia.

In 2019 and 2020, the Balkan region was hit by two earthquakes of magnitude M-W = 6.4 On November 26, 2019, an earthquake struck Northwestern Albania, It was the strongest to hit Albania in more than 40 years Cities such as Thumane, Tirana, and Durres suffered damage, but Durres was the hardest hit with several building collapsed. On December 29, 2020, an earthquake occurred in the Sisak-Moslavina county of Croatia, located approximately 50 km south of Zagreb. Comparable to Albania, it was the strongest to hit in the Pokupsko-Petrinja seismic zone since the October 8, 1909 Pokupsko earthquake and the largest earthquake in the region in 140 years. It caused extensive damage in the cities of Petrinja, Glina, and Sisak as well as in numerous neighboring small towns and small settlements in the region. The shaking effects and building damage of both earthquakes could be investigated and documented during field operations. In Durres (Albania), the most affected buildings by the earthquake damage were reinforced concrete (RC) frame buildings with infill walls. Whereas, the Croatia earthquake caused major damage on the older and modern unreinforced masonry buildings. The paper provides an overview of the earthquake-induced damage in different types of buildings and their variations. The causes of the damage as well as the consequences for rapid response to earthquake are discussed in close relation to the standardization in low to moderate seismic regions in Europe.

SUMMARYOn 29 December 2020, a shallow earthquake of magnitude Mw 6.4 struck northern Croatia, near the town of Petrinja, more than 24 hr after a strong foreshock (ML 5). We formed a reconnaissance team of European geologists and engineers, from Croatia, Slovenia, France, Italy and Greece, rapidly deployed in the field to map the evidence of coseismic environmental effects. In the epicentral area, we recognized surface deformation, such as tectonic breaks along the earthquake source at the surface, liquefaction features (scattered in the fluvial plains of Kupa, Glina and Sava rivers), and slope failures, both caused by strong motion. Thanks to this concerted, collective and meticulous work, we were able to document and map a clear and unambiguous coseismic surface rupture associated with the main shock. The surface rupture appears discontinuous, consisting of multi-kilometre en échelon right stepping sections, along a NW–SE striking fault that we call the Petrinja-Pokupsko Fault. The observed deformation features, in terms of kinematics and trace alignments, are consistent with slip on a right lateral fault, in agreement with the focal solution of the main shock. We found mole tracks, displacement on faults affecting natural features (e.g. drainage channels), scarplets and more frequently breaks of anthropogenic markers (roads, fences). The surface rupture is observed over a length of ∼13 km from end-to-end, with a maximum displacement of 38 cm, and an average displacement of ∼10 cm. Moreover, the liquefaction extends over an area of nearly 600 km2 around the epicentre. Typology of liquefaction features include sand blows, lateral spreading phenomenon along the road and river embankments, as well as sand ejecta of different grain size and matrix. Development of large and long fissures along the fluvial landforms, current or ancient, with massive ejections of sediments is pervasive. These features are sometimes accompanied by small horizontal displacements. Finally, the environmental effects of the earthquake appear to be reasonably consistent with the usual scaling relationships, in particular the surface faulting. This rupture of the ground occurred on or near traces of a fault that shows clear evidence of Quaternary activity. Further and detailed studies will be carried out to characterize this source and related faults in terms of future large earthquakes potential, for their integration into seismic hazard models.

In this study, we analyzed the near-field seismic records of two moderate sized earthquakes in the Western Balkan region: the September 2016 Skopje earthquake, magnitude M(L)5.3 and the March 2020 Zagreb earthquake, magnitude M(L)5.5. Such recordings at close epicentral distances are rare and are thus very useful for testing some of the theoretical assumptions used in modeling earthquake risk. Firstly, response spectra were computed using the digital time histories for the three closest stations to the Skopje 2016 earthquake and the two closest stations to the Zagreb 2020 earthquake. Their characteristics were examined in terms of frequency and peak amplitude ranges. Secondly, the Nakamura method was applied to the records from the selected five stations coded SKO, FCE, IZIIS, QUHS, and QARH. The results of the spectral analysis were compared with interpretations from the geological and geotechnical maps at each location. Our findings support the idea that these combined methods can be used to categorize the underlying structural profile to a first approximation and can be used to derive velocity models.