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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.

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.

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. 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.

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.

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.

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.

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.

During the 2002–2005 period a total of 3459 earthquakes were located in Croatia and its neighbouring areas with 15 main events registering magnitudes from 4.0 to 5.5. Seismically the most prominent were the two strongest earthquake sequences recorded in the central part of the Adriatic Sea, near Jabuka Island (the first one with the mainshock on March 29, 2003, 17:42, ML = 5.5, and the second, weaker, with the mainshock on November 25, 2004, 6:21, ML = 5.2). In the epicentral area W and NW of the Jabuka Island 781 earthquakes were confidently located (28 events with magnitudes equal to or larger than 4.0). Seismically active coastal part of Croatia, especially its southern part exhibited the seismicity within well-known epicentral areas. The earthquake with the magnitude ML = 5.5, recorded in the Imotski–Grude area, on May 23, 2004 at 15:19 (Imax = VI–VII °MSK) was the second strongest event during the studied period. Continental part of Croatia experienced moderate seismicity during the observed period, with earthquakes of magnitudes ML ≤ 3.9., U Hrvatskoj i susjednim područjima locirano je 3459 potresa u razdoblju od 2002 do 2005 godine. Zabilježeno je 15 glavnih potresa s magnitudama od 4.0 do 5.5. Seizmički najaktivnije bilo je područje u blizini otoka Jabuka u sredi{njem djelu Jadranskog mora gdje su zabilježene dvije velike serije potresa (prva s glavnim potresom koji se dogodio 29. ožujka 2003. u 17:42, ML = 5.5, i druga slabija s glavnim potresom koji se dogodio 25. studenog 2004., u 6:21, ML = 5.2). U epicentralnom području zapadno i sjeverozapadno od otoka Jabuka lociran je 781 potres (28 potresa s magnitudama većim od 4.0). Seizmička aktivnost obalnog područja Hrvatske bila je ograničena na do sada poznata epicentralna područja. Potres koji se dogodio 23. svibnja 2004. u 15:19, u seizmičkom području Imotski–Grude, magnitude ML = 5.5 (Imax = VI–VII °MSK) bio je drugi najja~i potres zabilježen u promatranom razdoblju. U kontinentalnom djelu Hrvatske zabilježena je umjerena seizmičnost u promatranom razdoblju, s potresima M ≤ 3.9.

During the 1997 - 2001 period seismic activity of Croatia was confined to the previously identified seismically active areas. All together 1925 earthquakes were located. Seismically the most active was the coastal part of Croatia, especially its southernmost part where the Ston-Slano epicentral area exhibited the continuation of the great earthquake sequence after the September 5, 1996 main shock. The strongest aftershock was recorded on April 26, 1997 at 07:30 (ML = 4.5, Imax = VI °MSK). The earthquake with the same magnitude ML = 4.5, recorded in the Zrmanja river valley, near Obrovac, on November 9, 2000 at 03:01 (Imax = VI °MSK). These two events were the strongest ones recorded in Croatia during the studied period., Tijekom razmatranog petogodišnjeg razdoblja seizmičnost je u Hrvatskoj bila ograničena na poznata epicentralna područja. Ukupno je locirano 1925 potresa. Najaktivniji je bio obalni dio Hrvatske, posebno njegov najjužniji dio, gdje je nastavljena pojačana aktivnost u epicentralnom području Ston-Slano nakon velikog potresa iz 1996. godine. Njegov najjači naknadni potres zabilježen je 26. travnja 1997. u 7:30 (ML = 4.5, Imax = VI °MSK). Potres jednake magnitude dogodio se i u dolini rijeke Zrmanje blizu Obrovca 9. studenog 2000. godine u 03:01 (Imax = VI °MSK). Ova su dva potresa ujedno i najjača u Hrvatskoj u razmatranom razdoblju.

The study presents (re)evaluation of fault-plane solutions for 40 earthquakes that occurred in the period 1956-1995 in Croatia and neighboring regions. All events were analyzed in the same manner, using the best available velocity models and the most recently updated earthquake catalogue for that region. For the most important earthquakes our solution is briefly discussed and compared to the results of previous studies when they were available. The results are presented with all related data thus enabling estimation of each solution’s reliability.

The seismicity of Croatia and its surrounding areas in 1989 was analysed on the basis of the earthquake catalogue consisting of 361 earthquakes. Its completeness threshold was estimated to be MLC ≥ 3.0. Seismically the most active was the coastal part of Croatia, where the strongest earthquake in 1989 occurred on December 6 with the focus beneath the hill-sides of the Kamešnica Mt. The fault plane solution for this event indicates the presence of a tectonic stress-field directed approximately SW-NE, which is compatible with the assumed anticlockwise rotation on the Adriatic microplate around the pole in Northern Italy, and the associated subduction of the Adriatic plate under the Dinarides. The aftershocks of the Kamešnica Mt. event were numerous, with hypocenters at depth up to 20 km. Macroseismic investigations confirm the frequently observed fact that seismic energy is much more efficiently absorbed perpendicularly to the direction of the Dinaric belt than along it.

Seismic activity in Croatia and surrounding areas in the 1993-1996 period was mostly confined to the previously identified seismically active areas. Nine events (excluding aftershocks) with the magnitude equal to or exceeding 4.5 occurred during that time. The most important earthquake sequence is the one that started on September 5, 1996 (Ml=6.0, Imax=VIII MSK) in the Ston-Slano area (greater Dubrovnik region). Microseismic locations of hypocentres of thousands of aftershocks, as well as the best double-couple CMT solution for the main-shock indicate that the earthquakes occurred on the NW-SE striking reverse fault system dipping towards NE.

On 22 March 2020, Zagreb was struck by an M5.5 earthquake that had been expected for more than 100 years and revealed all the failures in the construction of residential buildings in the Croatian capital, especially those built in the first half of the 20th century. Because of that, extensive seismological, geological, geodetic and structural engineering surveys were conducted immediately after the main shock. This study provides descriptions of damage, specifying the building performances and their correlation with the local soil characteristics, i.e., seismic motion amplification. Co-seismic vertical ground displacement was estimated, and the most affected area is identified according to Sentinel-1 interferometric wide-swath data. Finally, preliminary 3D structural modeling of the earthquake sequence was performed, and two major faults were modeled using inverse distance weight (IDW) interpolation of the grouped hypocenters. The first-order assessment of seismic amplification (due to site conditions) in the Zagreb area for the M5.5 earthquake shows that ground motions of approximately 0.16&ndash, 0.19 g were amplified at least twice. The observed co-seismic deformation (based on Sentinel-1A IW SLC images) implies an approximately 3 cm uplift of the epicentral area that covers approximately 20 km2. Based on the preliminary spatial and temporal analyses of the Zagreb 2020 earthquake sequence, the main shock and the first aftershocks evidently occurred in the subsurface of the Medvednica Mountains along a deep-seated southeast-dipping thrust fault, recognized as the primary (master) fault. The co-seismic rupture propagated along the thrust towards northwest during the first half-hour of the earthquake sequence, which can be clearly seen from the time-lapse visualization. The preliminary results strongly support one of the debated models of the active tectonic setting of the Medvednica Mountains and will contribute to a better assessment of the seismic hazard for the wider Zagreb area.