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Seismological data from almost 100 broadband stations (70 < Δ < 420 km) from Croatia, Slovenia, Hungary, Italy, Austria, Bosnia and Hercegovina, Montenegro, and Slovakia have been used in the rupture analysis of the Petrinja (Croatia) MW6.4 earthquake, that occurred on the 29th of December 2020. Several foreshocks and aftershocks have been used as empirical Green’s function (EGF) to isolate source effects from propagation and local soil effects. First, P-wave mainshock seismograms are deconvolved from the EGF seismograms in the frequency domain to obtain the corner frequency (fc). Assuming Brune’s source model, the spectral analysis results in a large stress drop of 25 MPa. Second, using time-domain deconvolution of the Love wave time windows, apparent source time functions (ASTFs) have been computed and indicate an average source duration of 5 seconds. No significant directivity effects can be seen in both the fc values and source durations, whose weak variability suggests a bilateral rupture. Lastly, physical rupture parameters, such as rupture velocity, rupture dimensions, slip model and rise time, have been extracted from the ASTFs by two different techniques: (1) the Bayesian inversion method (Causse et al. 2017) and (2) the backprojection of the ASTFs on the isochrones (Király‐Proag et al. 2019). Both techniques indicate a slow rupture velocity (about 50% of the shear-wave velocity) and a rather short rupture length for an MW6.4 event (about 8 km), consistent with the obtained large seismological stress drop. Such features may be explained by the relatively complex and segmented fault system, typical of immature fault contexts. References:Causse, M., Cultrera, G., Moreau, L., Herrero, A., Schiappapietra, E. and Courboulex, F., 2017. Bayesian rupture imaging in a complex medium: The 29 May 2012 Emilia, Northern Italy, earthquake. Geophysical Research Letters, 44(15), pp.7783-7792.Király‐Proag, E., Satriano, C., Bernard, P. and Wiemer, S., 2019. Rupture process of the M w 3.3 earthquake in the St. Gallen 2013 geothermal reservoir, Switzerland. Geophysical Research Letters, 46(14), pp.7990-7999.

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.

The most seismically active region in Adriatic part of Croatia is the SE coastal region that have numerous cultural heritage sites. Therefore, it is important to estimate seismic risk for cultural heritage sites and to develop procedures for seismic risk assessment. These are the goals of the research project Seismic Risk Assessment of Cultural Heritage Buildings in Croatia (SeisRICHerCRO) funded by the Croatian Science Foundation. Local site effects, a known problem in earthquake engineering, play significant role in earthquake damage distribution. The old town of Dubrovnik is particularly vulnerable in terms of local site effects because the old city is built on three geomorphological and geological entities: southern bedrock ridge, central filled and flattened part (former sea embayment), and northern bedrock ridge. Apart from the last devastating historical earthquake in Dubrovnik area in 1667 of M~7), instrumentally recorded strong earthquake on 15 April 1979 M6.8 with mainshock at the epicentral distance of 105 km from Dubrovnik caused strong damage effects in the wider Dubrovnik area (intensity of VII °MSK) with incalculable damage to cultural and historical objects the old town Dubrovnik The main aim of this study is mapping of local site effects variations in the old city of Dubrovnik using old investigation data as a starting point from 1980’s, after 1979 Reconstruction of Dubrovnik. Data quality and spatial uncertainty brough by older equipment used in 1980’s compared to using novel and sophisticated geophysical research methods followed by new geological investigations will significantly improve planning and reconstruction of Dubrovnik following new microzonation maps. Definition of the seismicity and variations of the local site conditions are one of the inevitable phases of the complex process of repair and strengthening of existing structures in the old town Dubrovnik together with design seismic parameters and the seismic risk level of building inventory as a basis to define the necessary preventive measures against expected strong earthquakes in the future.

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.

This review paper relates original Mohorovičić, A. lecture DJELOVANJE POTRESA NA ZGRADE (1909, 1911) and its English translation EARTHQUAKE EFFECTS ON BUILDINGS (2009) with M5.5 Zagreb 2020 earthquake damage. Mohorovičić said in Introduction of his lecture in 1909: “another strong earthquake is needed to remind people that the building techniques should be further developed and improved…”. Oliver Wendell Holmes (1809-1894) said once: “The young man knows the rules, but the old man knows the exceptions”. This should remined us that going Back to the Future after strong earthquake, we must go Back to the Past, and look after Mohorovičić’s 15 rules how to build earthquake-resistant buildings. He tells us how to build, and unfortunately, we were left unprepared again and didn’t listen wisdom words of an “old man”.

The work of Andrija Mohorovičić (1857–1936) had a large impact on the development of seismology, both in Croatia and world-wide. This paper presents a chronological survey of the development of seismology in Croatia providing context for the discovery of the Mohorovičić discontinuity in 1910. The development of early Croatian seismology was strongly influenced by advances in the field made in both Europe and world-wide. It also was influenced by several strong earthquakes that occurred within its territory, most notably the 1880 Zagreb earthquake, and the 1909 Kupa Valley (Pokupsko) earthquake. By studying the seismograms from the Kupa Valley earthquake, Mohorovičić was able to prove the existence of the boundary layer between the Earth's crust and the mantle (the Mohorovičić discontinuity). After Mohorovičić retired in 1921, seismological research in Croatia lost much of it's momentum, and for 20 years no seismological papers were published. After World War II, the Geophysical Institute was incorporated into the Faculty of Science of the University of Zagreb, and seismologists became active once more. Beginning in the second half of the twentieth century, the scientific interests of Croatian seismologists broadened, and international cooperation intensified. During that time, the number of active seismologists in Croatia varied, but never exceeded twelve, all of whom were affiliated with the Department of Geophysics of the Faculty of Science at the University of Zagreb. This small seismological community bore the responsibility of maintaining high standards in seismological research and education, and of keeping Croatian seismology visible in the world of geophysical sciences.

Trakošćan Castle, built on a rocky peak in the late 13th century, is a cultural heritage site protected as a historical entity by the Republic of Croatia. The Castle is constructed as a highly irregular masonry structure with timber or shallow masonry arches, vaults or dome floors. It was substantially renewed, upgraded and partially retrofitted from the 16th century until the year 2000. The M5.5 (VIII EMS) and M6.2 (VIII-IX EMS) earthquakes, which struck the city of Zagreb on 22 March 2020 and the Pokupsko-Petrinja area on 29 December 2020, strongly shook the Castle’s structure. Earthquake damage was observed and assessed by visual inspection accompanied by ambient vibration measurements. The slight cracks that appeared on masonry arches were found to be critically positioned, and can likely lead to the arches’ collapse if their spreading is not prevented. Ambient vibration measurements, which were compared to pre-earthquake ones, revealed the decrease in the fundamental frequencies of the Castle’s central tower unit and the second floor, thus possibly indicating the loss of structural stiffness as a consequence of the earthquake damage.

One dimensional (1-D) equivalent-linear (EQL) site response analysis is widely used to estimate the amplification of ground motion intensity measures due to local site conditions. The objective of this study is to estimate and compare the site amplification factors (AFs) using two different 1- D EQL analysis approaches: the time series (TS) approach that employs the recorded ground motions that are suitable with the bedrock design spectrum and the random vibration theory (RVT) based method that utilizes the stochastic spectrum for the design earthquake. Random soil profiles with different soil types, groundwater conditions, and EQL soil properties are combined in the analysis with different ground shaking levels. Analysis results showed that the AFs estimated by the TS- approach are systematically higher than the AFs estimated in RVT-based method in the short period range (T

In the morning of 22 March 2020 at 5:24 (UTC) a damaging earthquake struck Zagreb, the capital of Croatia. This shallow local magnitude 5, 5 earthquake was the strongest instrumentally recorded earthquake in the NW part of Croatia since 1908 when the first seismological station was installed in Zagreb. The strongest aftershock of local magnitude 4, 9 happened at 6:01 (UTC) on the same day. Both events were recorded by two strong motion stations in the City of Zagreb, located close to the epicentres (10 km).

The 22 March 2020 Zagreb ML 5.5 earthquake ground shaking resulted in damage to buildings and infrastructure. The most affected buildings were older and cultural heritage buildings (built before 1963) in the old city centre with significant damage extent in the epicentral zone (southeastern foothills of Medvednica Mt.). This study presents site response analysis on the realistic site profiles from the epicentre towards the accelerometric stations QUHS and QARH and comparison with strong motion data recorded during the Zagreb 2020 earthquake. Semi-empirical estimation of the ground motion amplification (i.e., peak ground acceleration at surface) showed that modelled and recorded values are comparable. Moreover, we present 2D model of peak ground acceleration at surface (PGAsurf ) variation for the superimposed site profile from the epicentre towards two accelerometric stations. Ground motion amplification for the Zagreb ML 5.5 earthquake scenario showed that PGAsurf is larger by a factor of 2 than the bedrock value (approx. 0.35 g in the epicentre and 0.20 g on the 12 km distant accelerometric station). This study is a contribution to better understanding of the Zagreb ML 5.5 earthquake effects and significance of local site effects in the damage extent, something that combined with older and heritage buildings resulted in high economic consequences. Therefore, it is important that site-specific ground motion simulation and seismic microzonation of the Zagreb continues with installation of an accelerometric array. This is very important for earthquake retrofitting and resilience of the low, mid- and high-rise buildings with particular care of cultural and historical buildings as well for the further urban planning.

On 29 December 2020 destructive M6.2 earthquake hit well known Petrinja epicentral area, and caused strong damage on many buildings in Petrinja, Sisak, and Glina, as well as on solid modern linear infrastructure (roads, bridges, artificial river embankments, pipelines etc.). The seismic hazard is not depending only on the estimated coseismic ground acceleration that should be used for EUROCODE 8 constructional seismic design, but is also strongly dependent on local soil effects and on the secondary effects of a strong earthquake (landslides, liquefaction, suffosion, etc.). Besides, movement of the crustal blocks along the fault lines that cross the solid objects, in case of surface coseismic rupture such was the Petrinja event, should be evaluated. Local site amplification effects are the results of several physical processes (multiple reflections and diffractions, focusing, resonance, wave trapping) in the overlying superifical deposits and soil, resulting in variable damage distribution that were observed in different local geological units affected by an earthquake. Also, the variable surface topography and various mechanical properties of the terrain such as water table, slopes, presence of heterogeneities, structural discontinuities and cavities, certainly can contribute to the observed damage and increase geological hazard in epicentral area. How many unknown active faults we can identified in Croatia? What could be surface manifestation of a strong earthquake that will occur on a shallow thrust (reverse) fault? Is there any major normal active fault that can surprise seismotectonic experts and civil engineers? The authors published first scientific paper after the Zagreb 22 March 2020 event and are currently working on active tectonics in Kvarner region and Hrvatsko Zagorje. Besides, a new Croatian Science Foundation (HRZZ) project has just been started with special attention on soil dynamic properties and its influence on the seismic hazard of the older cultural buildings in Trakošćan, Šibenik and Dubrovnik.

This paper is devoted to applications of the “Extremum” loss simulation system to two damaging earthquakes which occurred in Croatia in 2020. We provide a calibration procedure of mathematical models used for shaking intensity simulation. The regional macroseismic field parameters, such as the coefficients in the macroseismic field equation; the ratio between the longer (b) and the shorter (a) axes of the higher elliptical isoseismals (the flattening ratio k); the angle that specifies the orientation of the macroseismic field, in particular, the azimuth of the longer axis in the isoseismal ellipse, were all based on extensive macroseismic data acquired for the Balkan region and on the data for an analogous area with similar seismotectonic parameters in the Caucasus. We obtained a fairly good consistency between the results of simulation applied to the impact of the 2020 Croatia earthquakes and observations, confirming that the calibration of the macroseismic model by the Extremum system was both reasonable and effective for enhancing reliability for real time loss estimation.

Seismic bedrock depth represents one of the crucial steps in site response analysis as the input seismic ground motion is propagated from the bedrock level through soil profile column to estimate surface ground motion. In the last two decades, several studies estimated bedrock depth using microtremor Horizontal-to-Vertical- Spectral Ratio fundamental (resonance) frequency and known (mostly boreholes) bedrock depth in different regions through power-law regression. In this study, seismic bedrock (H800 – depth of the bedrock formation identified by shear wave velocity VS ≥ 800 m/s)is estimated using H/V forward modelling routine in which shear wave velocity profile from geophysical measurements was used as an initial soil model. Based on geological, geophysical and microtremor data, empirical relationships between average shear wave velocity in top 30 m, resonance frequency and bedrock depth are derived. Given evaluation of sedimentary thicknesses showed to be within the range of published studies for similar geological and site characteristics if the errors between different methods, approaches and site parameters are considered. The main limitation to validate proposed relationships is the lack of realistic deep borehole data like in similar studies. Further work is required that should include deep data from borehole drilling, seismic refraction and reflection or vertical electrical sounding. The intention is that presented regressions between resonance frequency, bedrock depth and Vs30 VS30 would be implemented into nonlinear site amplification model and further development towards Ground Motion Prediction Equations for Croatia.

During the ten-year period from 2006 to 2015 a total of 36 733 earthquakes were located in Croatia and its surrounding areas, with 37 main events registering magnitudes from 4.0 to 4.9. Seismically the most active was the coastal part of Croatia confined to two seismically distinguished areas. The NW domain was seismically less active, with almost 10000 located events (seven were of magnitude ML ≥ 4.0), among which were the three strongest events that occurred in Croatia during the observed period. Two of them occurred in the Senj picentral area, the first one on 5 February 2007 at 8:30 UTC (ML = 4.9, Imax = VII °MSK) and the second one on 30 July 2013 at 12:58 UTC, (ML = 4.8, Imax = VI °MSK). The third event occurred near Kornati Islands on 18 July 2007 at 10:54 UTC (ML = 4.8). The SE domain experienced the highest number of earthquakes (over 19 000 located events, with 24 events of magnitude ML ≥ 4.0, among which the strongest one was of magnitude ML = 4.9 with the epicentre in Bosnia and Herzegovina near the Croatian border). The seismicity in the continental part of Croatia was weak-to-moderate, with earthquakes of magnitudes ML ≤ 4.1. Focal mechanisms were obtained for 31 earthquakes with magnitudes ML ≥ 4.0, and individual earthquakes have also been macroseismically analysed. Low current moment release rates for both regions (continental and coastal) as compared to long-term averages, indicate the regions are currently in the strain accumulation phase., Tijekom desetogodišnjeg razdoblja od 2006. do 2015. godine u Hrvatskoj i okolnim područjima locirano je 36 733 potresa, od toga 37 glavnih potresa s magnitudama u rasponu od 4,0 do 4,9. Seizmički najaktivniji je bio priobalni dio Hrvatske karakteriziran s dva različita područja seizmičke aktivnosti. U sjeverozapadnom priobalnom odručju, koje je u odnosu na ostatak obalnog područja bilo slabije seizmički aktivno, locirano je gotovo 10000 potresa, od čega je sedam potresa bilo magnitude ML ≥ 4,0, a među njima su i tri najjača potresa koja su se dogodila u Hrvatskoj tijekom promatranog razdoblja. Dva potresa imala su epicentar u Senjskom epicentralnom području. Prvi potres se dogodio 5. veljače 2007. u 8:30 UTC (ML = 4,9, Imax = VII °MSK), a drugi 30. srpnja 2013. u 12:58 UTC, (ML = 4,8, Imax = VI °MSK). Treći potres se dogodio u Kornatskom arhipelagu 18. srpnja 2007. u 10:54 UTC (ML = 4,8). U jugoistočnom obalnom području dogodio se značajno najveći broj zabilježenih potresa (više od 19000 lociranih potresa, od čega 24 potresa magnitude ML ≥ 4.0, među kojima je najjači bio magnitude ML= 4,9 s epicentrom u Bosni i Hercegovini u blizini hrvatske granice). Seizmičnost u kontinentalnom dijelu Hrvatske bila je slaba do umjerena s potresima magnitude ML ≤ 4,1. Žarišni mehanizmi potresa izračunati su za 31 potres s magnitudama ML ≥ 4,0. Potresi su makroseizmički analizirani. U usporedbi s dugoročnim prosjekom, trenutna relativno niska seizmička aktivnost mjerena oslobođenim seizmičkim momentom u jedinici vremena ukazuje da se oba dijela Hrvatske nalaze se u fazi akumuliranja tektonskih deformacija.

The year 2017 marks the 350th Anniversary of the great 1667 Dubrovnik earthquake that caused extensive damage in a wide area around this old Dalmatian town (today in Croatia). This article presents the effects of the 1667 earthquake and examines the first few weeks following the catastrophe. Macroseismic data are reanalysed, for the first time available data are collected of the damage on the territory of Bosnia and Herzegovina (the territory which was in the 17th century under the Ottoman reign) and a new map of macroseismic intensities is presented. This map is in good agreement with the macroseismic field modelled using the SAF (Strong Attenuation at Fault Zones) model. We highlight some problems in the collection of macroseismic information, which are mainly a consequence of the complex political situation in the areas affected by the earthquake. The 6 April 1667 earthquake heavily impacted Dubrovnik and the Dalmatian coast. This event is thought to be the biggest one in the history of Dalmatia and practically defines seismic hazard in the coastal area of Croatia. For this reason, the main goals in this article are the improvement of the epicenter location and the determination of the moment magnitude.

Varaždin is an old baroque city located in the North of Croatia along the Drava River basin (alluvial sediments). Local site effects due to alluvial soft sediments (clays, sands, gravels) can play a crucial role in the amplification of seismic wave ground motions. Ambient noise (microtremors) measurements using the Horizontal-to-Vertical Spectral Ratio (HVSR) method were applied to determine the seismic ground response of the area, expressed by the fundamental/natural frequency ( f 0 ) and the site amplification ( A 0 ) of the sediments. The results of the free-field measurements show variations of the soil fundamental frequencies (0.47–5.19 Hz), site HVSR amplifications (1.95–4.65), Vs ,30 distributions (212–340 m/s) and the sediments cover thicknesses (16–116 m) above bedrock in Varaždin city area. The obtained results are found in good agreement with the geological map, the thicknesses of the sediments from lithological cross sections, existing boreholes data and MASW measurements. It was observed that the directional HVSR analysis and the polarization angles of the microtremors are affected by the horizontal polarization of the ground motion near, along and across the faults and correspond to the earthquake mechanism of the fault-plane solution from the past earthquakes in the area. The potential danger of the soil-structure resonance was estimated by comparison of the microtremor measurements in buildings of different heights (3–11 floors) and nearby free-field locations, using the Standard Spectral Ratio and Floor Spectral Ratio. The fundamental frequency of buildings ranged from 1.78 Hz for taller buildings up to 5.64 Hz for smaller buildings. If the building fundamental frequency is close to the soil fundamental frequency, the seismic ground motion will create possible resonance. Smaller buildings up to 6 floors showed lower potential of soil-structure resonance due to very low soil fundamental frequencies for the entire city area, while taller buildings up to 11 floors showed higher potential of soil-structure resonance. The directional HVSR analysis demonstrated how buildings in the studied area receive majority of the ambient noise energy from the polarization direction which is comparable with the observed polarization angles from free-field measurements. The microtremor HVSR method proved to be useful for estimation of seismic site effects and the potential danger of soil-structure resonance in an area with low seismicity like Varaždin city. Presented results are of great importance concerning monumental heritage and urban planning in Varaždin city.

We present the results of Local Earthquake Tomography (LET) analysis to investigate the crust and uppermost mantle structure in the northern Dinarides and southwestern Pannonian basin. Datasets of P-wave travel times are inverted to recover a three-dimensional P-wave velocity model of the survey area. Two data subsets were used in this study: (1) data recorded on 15 temporary seismic stations, which were deployed in Croatia in the framework of ALPASS-DIPS project, and (2) travel time datasets from the Croatian Seismological Survey and ORFEUS databases. The data enabled to achieve a resolution of less than a hundred kilometres in horizontal directions and a few kilometres in vertical direction in the area with good ray coverage, as is documented by the resolution tests. Velocity variations are computed on a grid using the three-dimensional nonlinear tomographic inversion method. Our study provides the first crustal three- dimensional seismic model of the studied area, and it is correlated with previous results in the survey area allowing us to infer the main crustal structures with high confidence. The velocity model reveals crustal thickening beneath the Dinarides and significant crustal thinning beneath the Pannonian basin. The Moho surface was determined on the basis of the highest velocity gradients in the vertical cross- sections. We find relatively high velocities below the northern Dinarides at shallow depths (

The city of Ivanec is located between valley of the Bednja River and Mt. Ivan&scaron, čica and this area can be prone to significant seismic site amplification due to local site characteristics. This study presents the first assessment of seismic site amplification for the city of Ivanec by the microtremor horizontal-to-vertical-spectral-ratio (HVSR) method and the equivalent-linear (EQL) site response analysis. Based on microtremor measurements and HVSR analysis, fundamental soil frequency and HVSR peak amplitude maps indicate potentially seismic danger zones. The 1-D EQL site response analysis was performed using multiple suites of earthquake ground motions scaled to the 95- and 475-year return periods of peak ground accelerations. Site amplification maps at the predominant peak frequency and ground surface indicate two microzones, one with high amplification in the central part of the city due to soft soil characteristics, and the other with small amplification in the transitional zone from alluvial basin towards the foothills of Mt. Ivan&scaron, čica. HVSR peak amplitudes and site response peak amplifications showed similar spatial distributions with similar predominant peak frequencies but with different amplitude levels. Site amplification maps provided significant information about potential resonance effects for structures of certain heights that can be correlated with the local ground shaking characteristics.

Seismically induced ground motion at a site is generally influenced by the seismic source, the propagation path and the local site conditions. Over the last several decades, researchers have consistently asserted that for near-site attenuation, the spectral parameter κ is subject primarily to the site conditions. In this research, we estimated the parameter κ based on the acceleration amplitude spectrum of shear waves from local earthquakes recorded by seismological stations situated in the western part of Croatia from the slope of the high-frequency part. The spatial distribution of κ values is comparable with seismological, geophysical and geological features, with the published coda-Q values for each station as well as with the isoseismal maps for selected stronger earthquakes in the study area. The complex pattern of longitudinal and transversal major late-orogenic fault zones dissecting early-orogenic thin-skinned tectonic cover in the Kvarner area and the shallow depth to the Moho (Mohorovičić discontinuity) in the Adriatic foreland (southern Istria) are probably responsible for a significant part of wave attenuation and for the anisotropy of attenuation. Regional near-surface attenuation distribution and modelled macroseismic fields point to the conclusion that attenuation properties of rocks in the northwestern External Dinarides are far from isotropic, and the most likely anisotropy sources are the preferential orientations of cracks and fractures under the local tectonic stress field, trapping of waves along major faults (waveguides), and/or attenuation within the fault zones. These results are important for gaining further insight into the attenuation of near-surface crust layers in the northwestern External Dinarides and the associated Adriatic foreland as well as in similar geotectonic settings.

On 28 December 2020, seismic activity in the wider Petrinja area strongly intensified after a period of relative seismological quiescence that had lasted more than 100 years (since the well-known M5.8 Kupa Valley earthquake of 1909, which is known based on the discovery of the Mohorovičić discontinuity). The day after the M5 foreshock, a destructive M6.2 mainshock occurred. Outcomes of preliminary seismological, geological and SAR image analyses indicate that the foreshocks, mainshock and aftershocks were generated due to the (re)activation of a complex fault system—the intersection of longitudinal NW–SE right-lateral and transverse NE–SW left-lateral faults along the transitional contact zone of the Dinarides and the Pannonian Basin. According to a survey of damage to buildings, approximately 15% of buildings were very heavily damaged or collapsed. Buildings of special or outstanding historical or cultural heritage significance mostly collapsed or became unserviceable. A preliminary analysis of the earthquake ground motion showed that in the epicentral area, the estimated peak ground acceleration PGA values for the bedrock ranged from 0.29 to 0.44 g. In the close Petrinja epicentral area that is characterized by the superficial deposits, significant ground failures were reported within local site effects. Based on that finding and building damage, we assume that the resulting peak ground acceleration (PGAsite) values were likely between 0.4 and 0.6 g depending on the local site characteristics and the distance from the epicentre.

The Harmonization of Seismic Hazard Maps in the Western Balkan Countries Project (BSHAP) was funded for 7 years by NATO-Science for Peace Program to support the preparation of new seismic hazard maps of the Western Balkan Region using modern scientific tools. One of the most important outputs of the BSHAP is an updated and unified BSHAP earthquake catalogue that is compiled directly from the datasets of earthquake data providers of the region. The BSHAP earthquake catalogue described here covers the geographic area limited by 38.0° - 47.5°N, 12.5° - 24.5°E and includes 26, 118 earthquakes that occurred in the region between 510 BC and 2012. Details of data compilation efforts including the removal of duplicate events, unification of the magnitude scales, declustering of the catalogue and completeness analysis are presented in this manuscript. New magnitude conversion equations for various local magnitude scales of the data providers are developed with the aim of having homogeneous moment magnitude estimates. Completeness time intervals for the catalogue data are provided as inputs to the seismic source models used to obtain updated seismic hazard of Western Balkan Region. The unified and updated BSHAP catalogue is found to be compatible with the current well-established European and world- wide catalogues and represents a sound basis for analysis of the seismicity of this region.

This manuscript presents the seismic source characterization models that were developed and used for the Western Balkan region in the framework of Harmonization of Seismic Hazard Maps in the Western Balkan Countries Project (BSHAP II) funded by NATO-Science for Peace and Security Program. Relevant knowledge about the geological and seismotectonic structure of Western Balkans and surrounding region was collected and utilized along with the BSHAP focal mechanism database and the BSHAP earthquake catalogue (Markusic et al. in Bull Earthq Eng 14(2):321–343, 2016. doi: 10.1007/s10518-015-9833-z ) to delineate seismic source models for different purposes. The super zone model of large zones bounds the regions with similar seismotectonic characteristics and catalogue completeness levels and was used for calculating the regional b-value of the magnitude recurrence relationship. Additionally, two models of smaller zones that represent the epistemic uncertainty in source geometry, maximum magnitude and style-of-faulting, were developed to be employed in the two-stage (circular and elliptical) smoothing procedure. Sets of sensitivity analyses are performed to support final estimates of some models’ parameters affecting the smoothed seismicity rate. The seismic source models and the logic-tree presented here are to be implemented in the probabilistic seismic hazard assessment for the seismic hazard maps of the Western Balkan region.

This paper presents a major extension of seismic vulnerability research project on the site of Trakošćan Castle based on the initial Horizontal- to-Vertical-Spectral-Ratio (HVSR) results from Stanko et al. (2016). The estimated HVSR site frequencies and HV amplification at Trakošćan Castle can only be used as an indication of the initial soil site frequency and amplification, so- called “natural soil model”, corresponding to the subsoil profile without the influence of an earthquake. The Equivalent-Linear (EQL) site response analysis has been carried out for different earthquake scenarios for a maximum input rock peak ground acceleration (PGAROCK) that corresponds to return periods of 95 (0.08 g), 475 (0.18 g) and 1000 years (0.31 g). The aim of the research is to evaluate structural seismic design responses and to determine type and degree of damage caused by local site effect, which is the results of an alluvial basin and topographic influences. The main objective of this research is the formation of local microseismic zones based on an EQL analysis: surface spectral acceleration and amplification maps at the predominant frequency. Based on the HVSR frequency response of the core structure of Trakošćan Castle and the Tower itself (fundamental and higher frequency modes), maps of surface spectral acceleration and soil amplification at different frequencies (3 Hz, 5 Hz and 10 Hz) are developed for different input PGAROCK levels (0.08 g, 0.18 g and 0.31 g) to evaluate seismic response of the Castle. Observed amplifications are correlated with ground motion polarization and directionality of the ground motion from the alluvial basin to the hilltop. Shortening of predominant frequencies (lengthening of the period), particularly in the alluvial basin, has been observed with higher input PGAROCK in the EQL analysis. This effect is not manifested in the Trakošćan hill and predominant frequencies match HVSR frequencies. The use of certain geophysical survey methods at historical sites is a big problem, because terrain features (e.g. steep hills, mountains, ridges, slopes, cliffs etc.) create lack of space and make it impossible to carry out a research. Microtremor measurements at historical sites can overcome this limitation and provide local seismic response and vulnerability behaviour of historical monuments without destroying their authenticity. Also, computational modelling can greately improve the results. The EQL site response analysis on the site of Trakošćan Castle has confirmed and improved the results of seismic response and vulnerability based on HVSR method.

The Harmonization of Seismic Hazard Maps in the Western Balkan Countries Project (BSHAP) was funded for 7 years by NATO-Science for Peace Program to support the preparation of new seismic hazard maps of the Western Balkan Region using modern scientific tools. One of the most important outputs of the BSHAP is an updated and unified BSHAP earthquake catalogue that is compiled directly from the datasets of earthquake data providers of the region. In the framework of BSHAP, the regional free field strong motion network capacity was increased significantly by the purchased and installed recorders and the BSHAP strong motion database that includes both pre-BSHAP (mostly analog) and post-BSHAP (all digital) recordings was compiled. The BSHAP strong motion database is used for proper selection of the ground motion prediction equations (GMPEs) for the probabilistic seismic hazard assessment (PSHA) by comparing the compiled strong ground motions with the predictions of candidate global, European, and Euro-Med GMPEs in a systematic manner. BSHAP collected relevant knowledge about the geological structure of southwestern Balkans provided a better understanding of the prevailing stress regime in the region. The main output of BSHAP is the new probabilistic seismic hazard maps for Western Balkans, obtained by implementation of the smoothed-gridded seismicity approach. The results are expressed in terms of peak horizontal acceleration (PGA) for 95 and 475 years return periods aligned with Eurocode 8 requirements. The seismic hazard maps derived in this project are a good basis to characterize the seismic hazard of Western Balkans. They will help the national authorities, public and private institutions, civil emergencies agencies, etc., for urban planning, disaster preparedness, and seismic hazard mitigation., Projekt usklađivanja karata seizmičke opasnosti za područje Zapadnog Balkana (BSHAP) financiran je 7 godina od strane NATO programa „Science for Peace“ za pripremu novih karata seizmičke opasnosti za područje Zapadnog Balkana upotrebom modernih znanstvenih alata. Jedan od najvažnijih rezultata BSHAP programa je ažurirani i jedinstveni BSHAP katalog potresa direktno sastavljen iz baze podataka potresa iz pojedinih regija. U okviru BSHAP-a, znatno je povećana mreža „free-field“ uređaja za snimanje jakog gibanja tla kupnjom novih i postojećih uređaja (akcelerografa), te je BSHAP baza podataka zapisa jakih (akcelerografskih) potresa sastavljena od prijašnjih (većinom analognih zapisa) prije BSHAP projekta, i novih digitalnih zapisa nakon i u toku BSHAP projekta. BSHAP baza podataka jakih gibanja koristi se za odgovarajući odabir relacija za procjenu gibanja tla (GMPE) za procjenu vjerojatnosti potresne opasnosti (PSHA) usporedbom prikupljenih zapisa jakih gibanja s GMPE relacijama na globalnoj, europskoj i Euro-Med razini. U okviru BSHAP-a, prikupljeno je relevantno znanje o geološkoj strukturi jugozapadnog Balkana koje je omogućilo bolje razumijevanje prevladavajućeg smjera napetosti u regiji. Glavni rezultat BSHAP-a su nove karte potresne opasnosti za područje zapadnog Balkana dobivene primjenom pristupa tzv. mreže izglađivanja seizmičnosti. Rezultati su izraženi preko vršnih horizontalnih ubrzanja (PGA) za povratne periode 95 i 475 godina usklađeni s zahtjevima Eurocode 8. Izrađene karte potresne opasnosti u ovom projektu dobra su osnova za opis potresne opasnosti područja zapadnog Balkana. One će pomoći nacionalnim vlastima, javnim i privatnim institucijama, civilnim agencijama za hitne slučajeve i sl., te za urbanističko planiranje, pripremu za prirodnu katastrofu i smanjenje seizmičke opasnosti.

This paper presents the empirically based ground truth criteria, or shorter GT criteria, for the estimation of the epicentral location accuracy of the seismic events recorded at network stations within 400 km around the city of Zagreb. The criteria are based only on the network coverage metrics and the GT5 level represents an absolute location error lower than 5 km. They have been developed using a bootstrap resampling method: same earthquakes have been relocated many times but with different, randomly selected seismic stations. We used 330 reference events taken from the pages of ISC (ISC Reference Event Bulletin, 2008) and showed that the location accuracy is most affected by the distance to the farthest station in the seismic network, while not at all influenced by the distance to the nearest. The developed GT criteria for GT595% level of accuracy require that all network stations must be within 125 km from the epicentre and that the secondary azimuthal gap must be less than 200 ° (i.e. the largest gap when any given station is removed from the network) or the network quality metric less than 0.41 (i.e. deviation between the optimal uniformly distributed network and the actual network). The obtained results revealed that the global criteria are too restrictive and unsuitable for the studied area since they require more regular networks. With our criteria, it is possible to achieve higher accuracy for the networks with a bigger secondary azimuthal gap or greater network quality metric. In addition, our criteria limitations are shown for the areas with simpler geological structure.

This paper presents the empirically based ground truth criteria, or shorter GT criteria, for the estimation of the epicentral location accuracy of the seismic events recorded at network stations within 400 km around the city of Zagreb. The criteria are based only on the network coverage metrics and the GT5 level represents an absolute location error lower than 5 km. They have been developed using a bootstrap resampling method: same earthquakes have been relocated many times but with different, randomly selected seismic stations. We used 330 reference events taken from the pages of ISC (ISC Reference Event Bulletin, 2008) and showed that the location accuracy is most affected by the distance to the farthest station in the seismic network, while not at all influenced by the distance to the nearest. The developed GT criteria for GT5_95% level of accuracy require 10 or more network stations, all within 125 km from the epicentre, and the secondary azimuthal gap (the largest gap when any given station is removed from the network) less than 200°, or the network quality metric (the deviation between the optimal uniformly distributed network and the actual network) less than 0.41. The obtained results revealed that the global criteria are too restrictive and unsuitable for the studied area since they require more regular networks. With our criteria, it is possible to achieve higher accuracy for the networks with a bigger secondary azimuthal gap or greater network quality metric. In addition, our criteria limitations are shown for the areas with simpler geological structure., U ovom radu određen je empirijski GT kriterij (engl. ground truth) za područje definirano polumjerom od 400 km oko grada Zagreba. Kriterij koristi mrežnu pokrivenost kao mjeru točnosti lokacije epicentra, a GT5 razina predstavlja apsolutnu pogrešku lokacije manju od 5 km. Primijenjena je metoda ponovnog uzorkovanja, gdje su Monte-Carlo postupkom velik broj puta relocirani isti potresi, ali s različitim slučajno odabranim seizmološkim postajama. Korišteno je 330 referentnih događaja preuzetih sa stranica ISC-a (ISC Reference Event Bulletin, 2008). Pokazali smo da na točnost lokacije najviše utječe udaljenost do najudaljenije postaje mreže, dok uopće ne utječe udaljenost do najbliže. Određeni GT kriterij proučavanog područja za postizanje GT5_95% razine točnosti zahtjeva da se mreža seizmoloških postaja mora nalaziti unutar 125 km od epicentra i da sekundarna azimutalna razlika (tj. najveća razlika između azimuta dviju postaja koja se dobiva uklanjanjem bilo koje postaje iz mreže) mora biti manja od 200° ili mjera kvalitete mreže (tj. srednje apsolutno odstupanje između optimalne uniformno distribuirane mreže postaja i stvarne mreže) manja od 0,41. Dobiveni rezultati pokazuju da su globalni kriteriji previše restriktivni za naše područje budući da zahtijevaju pravilnije mreže. Našim kriterijem je moguće postići višu točnost za mreže s većom sekundarnom azimutalnom razlikom i većom mjerom kvalitete mreže. Također, pokazali smo ograničenost našeg kriterija za područja jednostavnije građe.

The study area covers the Dinarides and southwestern part of the Pannonian basin as the marginal zone between the Adriatic microplate (African plate) and the Pannonian tectonic segment (Eurasian plate). We created a three-dimensional seismic velocity model to 450 km depth using teleseismic tomography. Our travel-time dataset was collected by means of 40 seismic stations from the ORFEUS database and Croatian Seismological Survey database. A set of 90 teleseismic earthquakes were selected in the time range 2014–2015, and relative P-wave travel-time residuals were calculated. For the first time the seismic P-wave velocity model of a relatively high resolution on the entire Dinaridic mountain belt was obtained. Based on this model, a more reliable insight in the relations of the lithosphere plates has been achieved. We imaged a fast velocity anomaly extending underneath the entire Dinaridic mountain belt which indicates cold, rigid materials. The anomaly is steeply sloping towards the northeast and directly indicates the sinking of the Adriatic microplate underneath the Pannonian tectonic segment. In the Northern Dinarides the anomaly extends to the depth of 250 km, whereas in the Southern Dinarides it covers greater depths, up to 450 km. The shallow Adriatic slab extends along the External Dinarides, while the deep Adriatic slab extends beneath the Internal Dinarides and ophiolite zones in the area of central and southern Dinarides. Different slab depths are interpreted as the faster convergence of the plate in the southern Dinarides than in the northern, or the convergence of the plates had started in the southern part and systematically developed to the north.

Spectral parameter kappa κ is used to describe spectral amplitude decay "crash syndrome" at high frequencies. The purpose of this research is to estimate spectral parameter kappa for the first time in Croatia based on small and moderate earthquakes. Recordings of local earthquakes with magnitudes higher than 3, epicentre distances less than 150 km, and focal depths less than 30 km from seismological stations in Croatia are used. The value of kappa was estimated from the acceleration amplitude spectrum of shear waves from the slope of the high-frequency part where the spectrum starts to decay rapidly to a noise floor. Kappa models as a function of a site and distance were derived from a standard linear regression of kappa- distance dependence. Site kappa was determined from the extrapolation of the regression line to a zero distance. The preliminary results of site kappa across Croatia are promising. In this research, these results are compared with local site condition parameters for each station, e.g. shear wave velocity in the upper 30 m from geophysical measurements and with existing global shear wave velocity – site kappa values. Spatial distribution of individual kappa's is compared with the azimuthal distribution of earthquake epicentres. These results are significant for a couple of reasons: to extend the knowledge of the attenuation of near-surface crust layers of the Dinarides and to provide additional information on the local earthquake parameters for updating seismic hazard maps of studied area. Site kappa can be used in the re- creation, and re-calibration of attenuation of peak horizontal and/or vertical acceleration in the Dinarides area since information on the local site conditions were not included in the previous studies.

The Trakoscan 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. Ivancica. Afterwards, it was completely reconstructed. Today the cultural heritage of the Trakoscan 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 Trakoscan 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 Trakoscan 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.

SUMMARY Broad-band seismograms of teleseismic events recorded at the Croatian Seismological Network were used to compute radial receiver functions (RFs) for eight locations in the External Dinarides. Waveform modelling was performed by a multistep matching of the theoretical RFs computed for horizontally layered 1-D isotropic models with the averaged observed RFs. Constraints from existing deep seismic sounding profiles, traveltime curves of regional crustal seismic phases and intuitive inferences gained from interactive forward modelling were used to construct initial 1-D models of the Earth. A non-linear inversion was performed in two steps—a grid search followed by the Monte Carlo search for the model parameters. Concurrently, RFs from different azimuths were stacked to obtain trade-off estimates of crustal thickness versus Vp/Vs ratios. The Moho depths were found in the range from around 40 km for Northern Adriatic stations to over 55 km for stations in the central part of the External Dinarides. Comparing our results with recent maps of the Moho topography inferred from seismic and gravimetric data, we find that for some stations the agreement between our results and the existing Moho maps is very good. For the others, we find the Mohorovicic discontinuity to be considerably deeper, indicating some of the thickest crust in Europe. Although it is plausible that such a deep Moho could be a consequence of a complex tectonic setting of the region (e.g. overlapping of two large tectonic units—the Adriatic microplate and the Dinarides), this result will have to be verified in the future studies using various other geophysical techniques.

Horizontal-to-vertical spectral ratios (HVSR) of ambient vibrations measured in the ancient town of Ston (Croatia) on 99 locations, are shown to be well matched to the theoretical ones computed for body-waves as well as for the surface waves. This match is poorer for sites on the slopes of nearby hills. The ratios of measured peak horizontal ground acceleration during the damaging earthquake in 1996 (M L = 6.0) and the ones obtained using empirical attenuation laws is approximately equal to the mapped value of the dynamic amplification factor determined on the basis of observed HVSR in the vicinity of the accelerometric station. The HVSR of the accelerogram is very similar to the HVSR of the ambient noise. The damage to the building stock in the old town centre caused by the earthquake series of 1996 is closely related to the estimated soil amplification and its fundamental frequency. More measurements in buildings are needed to arrive at confident conclusions about possible soil-structure resonance.

Residuals of the peak horizontal acceleration for site-independent attenuation relations are shown to have weak positive correlation with the average observed peak horizontal/vertical acceleration ratio (q) at the recording site. This ratio systematically increases as the hardness of the soil decreases. Taking the average observed q at the recording station into account reduces the scatter of data slightly in fitting the assumed peak acceleration attenuation law. If the soil classification of the data is available, soil dependence is found to be weaker if the data are corrected for the effect of q prior to regression. This correction reduced the mean value of q = 1.85 for the Dinarides area to q = 1.52, which, on the average, is characteristic of rock sites.

Peak acceleration attenuation relations for horizontal and vertical components are presented for the Dinarides region, based on 145 3-component accelerograms related to 46 earthquakes with local magnitudes of 4.5 or greater and with epicentral distances of less than 200 km as recorded on 39 recording sites in the greater Dinarides region. The attenuation functions were obtained by two-stage stratified regression on the local magnitude and epicentral distance as independent variables. The predicted peak acceleration values within the distance range covered by the data are comparable to the ones obtained for stiff-soil or rock sites when selected reference relations are used. The rather large average residuals are caused mostly by the lack of information on local site conditions and by the use of epicentral distance instead of fault distance.

The rate of aftershock occurrence after the M6 Ston-Slano (Croatia) earthquake is modeled as the Epidemic Type Aftershock Sequence (ETAS). Increase of the modeled cumulative number of aftershocks with time was fitted to observations by the least-squares criterion using the combined grid-search and Monte-Carlo approach. This enabled not only the estimation of the most probable ETAS parameters, but also the determination of their confidence limits, as well as the estimation of the bias between them. It has been found that the bias is significant for some of the parameter pairs, regardless of the threshold magnitude assumed. Residual analyses revealed that all strong aftershocks (M L ≥ 4.5) occurred during the periods of normal to high aftershock activity. There were two periods of quiescence in the sequence, both of which were followed by a strong aftershock.

Some of the elements of regional seismic hazard in Croatia are assessed by computing synthetic accelerograms at a predetermined set of sites. The input dataset consists of structural models, parameters of seismic sources, and an updated earthquake catalog. Synthetic strong-motion time series for frequencies below 1 Hz are computed on a grid of sites using the modal summation technique. The long-period hazard is described by the distribution of estimated peak values of ground displacement, velocity and acceleration, while the short-period hazard is represented by the map of design ground acceleration values (DGA). The highest values of DGA exceeding 0.35 g on the base-rock level are found in the southeastern coastal part of the country, in the greater Dubrovnik area.

The territory of Croatia and neighboring regions is divided into 17 seismic source zones, considering available seismological and geological data. On this basis, seismic hazard elements (seismicity rate, maximum magnitude, b-value, probabilities of exceedance and return periods for a predefined set of magnitudes) are computed using the maximum likelihood method appropriate for treating data-sets with variable completeness thresholds. The values of long term expected peak horizontal acceleration obtained by using a combination of the deterministic and the probabilistic procedure are the highest in the Dubrovnik zone, while the Zagreb zone has the highest earthquake hazard in the continental part of the country.

The paper presents the results of Croatian earthquake catalogue revision for the period 1908–1992. The revised catalogue lists a total of 4853 events (4486 after 1908), of which 3700 are mainshocks. All primary data sources available to us (seismograms, phase onset time readings, macroseismic questionnaires…) were consulted in order to improve the quality and reliability of magnitude, intensity and location. Along with the most important earthquake parameters the catalogue contains entries that enable assessment of the reliability of location and bibliographic references. Using the temporal analysis of the maximum-likelihood estimate of the b-value in the frequency-magnitude relation, the catalogue is estimated to be complete for events with M ≥ 4.0 throughout the investigated period. The magnitude completeness threshold decreases to 3.8 after the middle of the century, and to about 3.6 in the last three decades. It is noted that the b-value variation with time is closely tied to the seismic activity, as well as that at present there seems to be a lack of seismic energy release corresponding to the M = 6.0–6.5 earthquake.

Earth’s magnetic field, geomagnetic field, is a very complex system that varies on a range of scales, both in the space and time domains. The variability of Earth’s magnetic field is the result of various processes, ranging from Earth’s deep interior, its crust, ionosphere, magnetosphere, all the way to the Sun. Geomagnetic field is a vector quantity and is described by geomagnetic elements i.e. mutually orthogonal components X (northerly intensity), Y (easterly intensity) and Z (vertical intensity, positive downwards). The elements describing the direction of the field are declination D and inclination I.