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A strong earthquake, ML = 6.0 (MW = 5.7), occurred on 22 April 2022 at 21:07 UTC with an epicentre near Berkovići in Bosnia and Herzegovina, with focal depth of about 20 km. The earthquake was felt throughout Bosnia and Herzegovina, Montenegro, Croatia (especially Dalmatia), but also in Slovenia, Italy (especially the western coast of the Adriatic), Serbia, Albania and North Macedonia. The maximum intensity of the earthquake was rated as VII–VIII EMS in Berkovići and Ljubinje. A young woman in Stolac lost her life from a rock slide caused by the earthquake. In the wider epicentral area the earthquake caused a number of large or small rock falls, many chimneys were damaged, tiles fell from the roofs, plaster fell off, and there were also large cracks in the walls.By 31 October 2022, the DuFAULT project researchers located 6220 aftershocks (39 with ML≥ 3.0), with as many as 900 located in the first 12 h of the series. The strongest aftershock, ML = 4.9, occurred on 24 April 2022 at 4:27 UTC with focus at a depth of about 25 km and the epicentre also close to Berkovići. The vast majority of earthquakes have their foci relatively deep for this area, at depths between 15 and 28 km. Most of the epicentres form a compact group slightly elongated parallel to the NW-SE Dinaric strike, however two smaller groups northwest and southeast of the main group stand out with extension perpendicular to the Dinaric strike with somewhat shallower foci. The analysis of the focal mechanism and the hypocentral spatial distribution suggest that the mainshock resulted from the NE-SW directed compression and occurred on a reverse fault, on a moderately NE-dipping plane. Interestingly though, this series is also characterized by earthquakes released by a tension along the NE-SW striking and approximately 45° dipping normal faults, documented in the smaller north-western group.We will present spatio-temporal analysis of seismicity, resulting focal plane solutions and seismotectonic interpretation.

In micro tidal settings, processes of bioerosion and bioconstruction can lead to the development of distinct morphological features that define the modern shoreline. When such features are discovered above or below the present-day mean sea level, they reveal relative sea-level (RSL) change. Among the best sea-level indicators on rocky coasts are fixed biological and geomorphological markers – algal rims, Lithophaga lithophaga borehole upper limits, and tidal notches [1, 2]. For instance, the coralline alga Lithophyllum byssoides can build algal rims that are considered one of the most precise RSL markers along rocky coasts [3, 4]. Consequently, studying RSL markers on Lopud, Koločep and Grebeni two elevated palaeoshorelines have been distinguished. Their detailed survey allowed the distinction of seismotectonically uplifted sectors of the coast. The established high-resolution geochronology enables separation of coseismic uplift events from the periods of interseismic subsidence. Studying the local tectonic contribution to RSL change, we approach aspects of neotectonics and palaeoearthquakes offering new insights in a timetable (500 BC up to 1800 AD) where earthquake related data are either missing or incorporate high uncertainties, both regarding their epicentral localities and estimated magnitudes. Our reconstruction provides evidence on two successive major earthquakes which affected approximately 5 km of coasts in the Pelješac- Dubrovnik fault zone, with an uplift amplitude between 40 and 80 ± 15 cm per event (Fig. 1). The earlier, older events, caused larger displacements (60-80 cm), while the later, younger events, revealed on average lower displacements (40-55 cm) corresponding to the 1520 AD quake and the 1667 Dubrovnik earthquake [5, 6]. Here we demonstrate the importance of Lithophyllum rims in the studies of RSL change as they make creation of high-resolution geochronology possible. This new approach refers to algal rims as a possible tool for constraining palaeoseismic events, allowing to supplement the database of instrumental records and historical observations through field-based evidence. Furthermore, the study of algal rims, tidal notches and upper limits of L. lithophaga boreholes allow us to reconstruct the 2.6 ka evolution of Koločep and Grebeni islands and islets coasts, detect seismic events and demonstrate their effects, reconstruct the RSL changes at high resolution and finally distinguish the drivers of RSL change. As the southern part of the eastern Adriatic coast is the area of highest seismic risk in Croatia, the new obtained data will thus be essential for assessment of recurrence rates of the largest earthquakes that the main faults in the area can produce, as well as for understanding of the Late Holocene geodynamics.

Almost 14000 aftershocks have been located in the first six month after the Mw 6.4 Petrinja earthquake of 29 December 2020. Most epicentres lie close to the NW–SE striking right-lateral causative fault, but considerable activity has been recorded in the surrounding area up to about 50 km away. The hypocentres reach depths of over 20 km, with most of activity recorded between the depths of 5 and 16 km. Their spatial pattern reveals activation of a number of smaller faults. The 75 focal mechanism solutions computed using the first motion polarities read from the seismograms of the local and regional seismic networks, indicate that about half of the aftershocks exceeding magnitude M = 3.0 occurred on strike-slip faults, while the large majority of the remaining ones were due to almost pure reverse dip-slip faulting. Analyses of the Coulomb stress change on optimally oriented strike-slip and dip-slip faults following the mainshock rupture, reveals that a large majority of aftershocks occurred in the volumes characterised by the Coulomb static stress increase, whereas the areas where the effective stress decreased remained mostly quiet. The distribution and preferred strike directions of strike-slip and reverse faulting inferred by the Coulomb stress transfer analyses are also in good agreement with individual focal mechanisms.

Analyses of available data (newspaper reports, historical and church chronicles, chronical earthquake overviews, travel books, monographies, research papers, etc.) on effects of the earthquakes that shook the greater Ormož area at the Slovenian–Croatian border in the 1838 and 1839 revealed that one of them, recorded in a number of regional and global catalogues, is in fact a fake – the earthquake of 26 August 1838 never happened. This error creeped into various reports and studies, and then into many relevant catalogues, so this event should by systematically erased from the catalogues used to estimate seismicity rates in the neighbourhoods of north-western Croatia, north-eastern Slovenia, and south-western Hungary. Regarding the earthquake of 31 July 1838, we used important new sources of information that have not been consulted in any previous study. This made inversion of macroseismic parameters more robust. Our estimates of the macroseismic local and moment magnitudes (MLm = 4.8 ; Mwm = 4.8) are mostly higher than the values reported in the available catalogues. Reliable information on the effects of the smaller event of 22 March 1839 were found for two localities only, so its epicentre was placed into the town of Ormož where the maximum intensity was observed. Its estimated magnitudes (MLm = 4.4, Mwm = 4.4) are close to the median of values found in the six consulted catalogues that listed this event. The macroseismic epicentre of the 1838 earthquake lies close to the junction of surface traces of the Donat strike-slip fault and the reverse Čakovec fault. Based on their assumed geometry and the location of the macroseismic hypocentre, we give slight preference to the Donat fault as the seismogenic source.

The relative sea-level (RSL) change is a sum of effects of processes acting on local, regional and global scales. By far the most variable component of RSL change relates to local, non-linear scale processes, that is to active tectonics. The studied islands and islets, Lopud, Koločep and Grebeni are part of Dubrovnik archipelago, situated near the external front of the Dinarides where seismicity attains one of the highest levels in Croatia. The focal mechanisms in the area show that reverse and transpressive faulting prevails.Studying the RSL markers: algal rims, tidal notches and Lithophaga lithophaga borehole upper limits, we identified one elevated palaeoshoreline at Lopud island (today 25-30 cm above the mean sea-level (MSL)) and two elevated palaeoshorelines at Koločep island and Grebeni islets (60-90 cm and 25-40 cm above MSL). The high-resolution geochronology built on radiocarbon dating of algal rims (77 sea-level index points) enabled the distinction of coseismic uplift events from subsidence periods during the last 3300 years. As a result, we provide new, field-based reconstruction of palaeoearthquakes, describe spatial patterns of differential uplift and distinguish the main drivers of RSL change.Two major seismically triggered uplift events with displacements between 40 and 80 ± 15 cm per event have been differentiated along investigated sectors of the coast related to the Pelješac-Dubrovnik fault zone pointing to repeated earthquakes of magnitudes exceeding Mw = 6.5. The earlier, older events, caused larger displacements (60-80 cm) related to 4th to 6th century AD and 750-1100 AD earthquakes, while the later, younger events, revealed on average lower displacements (40-55 cm) corresponding to the 1520 AD quake and the 1667 Dubrovnik earthquake. The cumulative offsets increase from around 0.57 ± 0.15 m at Lopud to 1.00-1.25 ± 0.15 m at Koločep and Grebeni.Interseismic periods allow further insight into regional and global driving mechanisms of RSL change. The distinction of local coseismic displacements of RSL history together with the previously obtained regional glacio- and hydro-isostatic adjustment estimates (between 0.30 and 0.34 mm/yr) allow us to approach the global contribution, which seems to be minimal between ~0 and 1800 cal AD as RSL changed at rates of 0.3 to 0.4 mm/yr. The increased rates of RSL change occurred prior to 0 cal AD, with rates of around 0.7 mm/yr, and go up to 1.4 mm/yr after 1800 cal AD.Overall, here we demonstrate the importance of Lithophyllum rims in the studies of RSL change as they make creation of high-resolution geochronology possible. Moreover, this new approach refers to algal rims as a possible tool for constraining palaeoseismic events, allowing to supplement the database of instrumental records and historical observations through field-based evidence. This research was supported by Croatian Science Foundation project HRZZ-IP-2019-04-9445 – Relative sea-level change and climate change along the eastern Adriatic coast – SEALeveL.

Based on 153 earthquakes (1959–2020) listed in the Croatian Earthquake Catalogue, a conversion relation was obtained between the local magnitude ML,CR and the corresponding moment magnitude Mw as reported by the global and regional agencies. As errors were present in both variables the York regression was used. The best fit line is given by: MwL = (–0.106 ± 0.122) + (1.002 ± 0.027) ML,CR (coefficient of determination R2 = 0.90). The earthquakes considered occurred in Croatia and the neighbouring regions, and their local magnitudes ML,CR ranged between 3.5 and 6.5. Residual analysis suggests that an artificial positive magnitude shift of up to 0.3 magnitude units may have occurred in the early 1980s, when Wiechert mechanical seismographs were replaced by the instruments with velocity proportional recordings without proper recalibration of the magnitude formula. The slope of the regression close to 1.0 indicates that on the average the faults’ aspect ratio (width/length) is about 1/2.

U radu su opisane bitne značajke i glavne posljedice potresa magnitude 5,5 koji je u jeku pandemije virusa COVID-19 zadesio Zagreb i okolicu. Premda je potres, seizmološki gledano, bio umjerene magnitude, prouzročio je gubitak jednoga života i veliku materijalnu štetu. Napravljen je pregled stanja prije trešnje te prikaz lokacije, seizmičke aktivnosti i organizacije pregleda zgrada. Grubo su razvrstani podaci o oštećenjima, s težištem na povijesnoj jezgri i četvrtima blizu epicentra. Na kraju su istaknute nužne aktivnosti koje je odavno trebalo provesti, s nadom da će ih ovaj potres potaknuti., Significant characteristics and main consequences of the 5.5 magnitude earthquake that struck Zagreb and its surroundings in the midst of the COVID-19 pandemic are presented in the paper. Although, from the seismologic aspect, the earthquake was of moderate magnitude, it caused the loss of one life and considerable material damage. An overview of the situation before the quake is given, and information about the location, seismic activity, and organisation of building inspection activity, is presented. The data on damage are roughly classified, with the focus on historic core of the city and districts situated close to the epicentre. A strong emphasis is placed on indispensable activities that should have been carried out a long time ago, in the hope that they will be prompted by this earthquake.

The area of the Vinodol Valley and Bakar Bay represents a NW‒SE oriented valley in the NW Adriatic characterised by prominent historical and instrumentally recorded seismicity. As part of the greater geodynamic domain including the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone, new geological and structural data addressing the tectonic evolution of the area were collected in order to better understand the focal mechanisms of previous earthquakes and to enable identification of potential seismogenic sources. Mapped informal lithostratigraphic units mostly correspond to the Upper Cretaceous, Palaeogene and Quaternary successions described in other parts of the External Dinarides. However, a shorter stratigraphic range of the Gornji Humac fm., the youngest Cretaceous unit in the study area, was determined and suggests that the uplifted area in the central NW part of the Adriatic Carbonate Platform already comprised several thousand km2 (from W and NW Istria to Krk and Vinodol area) at the end of the Turonian. Structural measurements of the fault planes in the study area generally correspond to the existing structural model of the tectonic evolution of the Dinarides. However, in contrast to the SW vergences typical of the Dinarides, NE-vergent reverse structures are common, especially along the SW margin of the Vinodol Valley. Cross-cutting relationships suggest that transpressional (NW–SE and NE–SW striking dextral and sinistral faults) and extensional features (NW–SE and NE–SW striking normal faults) are structurally concurrent or younger than the reverse faults, suggesting a change in the palaeostress field during the Neogene–Quaternary, with prevalent transpression and radial extension. Comparison of results of the palaeostress field analysis and the constructed synthetic focal mechanisms on one side, with available focal mechanism solutions for earthquakes within the Ilirska Bistrica–Rijeka–Senj seismogenic fault zone on the other, shows a favourable orientation of the observed NW‒SE and NE‒SW striking faults with respect to the recent compressional/transpressional stress field (N‒S oriented P-axis), indicating these as potential seismogenic sources within the study area.

The investigated area of the NW Dinarides is located at the NE corner of the Adriatic microplate and is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin. Its complex crustal structure is the result of interactions among different tectonic units, mainly the Eurasian plate and the Adriatic microplate. Despite numerous seismic studies in this tectonically complex area, there is still a need for a detailed, small scale study focusing mainly on the upper, brittle part of the crust. We investigated the crustal velocity structure with 1-D simultaneous hypocenter-velocity inversion using routinely picked P wave arrival times. Most of the computed models converged to a stable solution in the depth range between 0 and 26 km. We further evaluated the inversion results with hypocenter shift tests, high and low velocity tests, and relocations. This helped us to select two best performing velocity models for the whole study area. Based on these results and the seismicity distribution, we further divided the study area into three parts, redefined the earthquake-station geometry, and performed inversion for each part separately to gain better insight into the crustal structure of each subregion. Median velocities in the upper 20 km of the crust in the eastern subregion are lower compared to the regional median and the median of the other two subregions. The northwestern and southwestern subregions are very similar in terms of crustal structure between about 8 and 23 km depth. The largest difference between them is observed in the upper 8 km, with higher median velocities in the southwestern subregion. Compared to the model currently used at Slovenian Environment Agency to locate earthquakes, the velocity models obtained show higher velocities in the upper 30 km depth and agree very well with some of the previous studies. In addition to general structural implications and a potential for improving seismic tomography results, the new 1-D velocity models can also be used for fast routine earthquake location and for detecting systematic travel time errors in seismological bulletins.

The ongoing Petrinja earthquake sequence interests a structurally complex area characterized by the transition between the Dinarides and the Pannonian Basin structural units. The sequence mainshock (December 29, 2020; Mw = 6.4) struck in the vicinity of the Petrinja town and caused significant damage in the human and in the natural environments. The preliminary seismological and geodetic analyses indicated a dextral strike-slip NW-SE oriented fault as the event source. Numerous geologic surface deformation patterns have been identified in the aftermath of the main event, including collapsed sinkholes, liquefaction, different forms of landslides, and surface fractures which nature and causative process require further detailed studies.The aim of our contribution is to apply a multitude of different geophysical, geodetic and geologic methodologies to decipher the Petrinja seismogenic fault geometry in the light of its ongoing earthquake sequence. We will show how the different datasets converge in delineating the fault geometry and discuss their diverging aspects and implications. Our preliminary analyses on the geometric and kinematic characteristics of the mainshock (as well as those of the foreshocks and aftershocks) point to an important structural complexity. This aspect helps us to better understand the seismotectonic framework of the Petrinja seismogenic fault and other regional seismogenic faults of similar geologic and geodynamic setting.

The Department of Geophysics, University of Zagreb and the Italian National Institute of Oceanography and Applied Geophysics (OGS) installed on January 4th 2021, five temporary seismic stations near the town of Petrinja, Croatia, in the aftermath of the 29 Decembre 2020 Mw 6.4 earthquake. The stations equipped with a seismometer and a strong motion sensor, recorded the aftershock sequence beginning six days after the mainshock allowing to augment the permanent seismic network in the area improving the azimuthal coverage and providing additional near‐field observations.In this presentation we summarize the motivation and goals of the deployment; details regarding the station installation, instrumentation, and configurations and observations from the network. The collected data set will be useful for carrying out several seismological studies including the analysis of variability of strong ground motions in near field, the determination of the aftershocks source parameters, the estimation (if any) of rupture directivity of small events, the clustering of events in space and time, the better imaging of the fault zone, the evolution of crustal properties within and outside of the fault zone.

The investigated area of the NW Dinarides is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin at the NE corner of the Adriatic Sea. Its complex crustal structure is the result of interactions among different tectonic units. Despite numerous seismic studies taking place in this region, there still exists a need for a detailed, smaller scale study focusing mainly on the brittle part of the Earth's crust. Therefore, we decided to investigate the velocity structure of the crust using concepts of local earthquake tomography (LET) and minimum 1-D velocity model. Here, we present the results of the 1-D velocity modeling and the catalogue of the relocated seismicity. A minimum 1-D velocity model is computed by simultaneous inversion for hypocentral and velocity parameters together with seismic station corrections and represents the best fit to the observed arrival times.We used 15,579 routinely picked P wave arrival times from 631 well-located earthquakes that occurred in Slovenia and in its immediate surroundings (mainly NW Croatia). Various initial 1-D velocity models, differing in velocity and layering, were used as input for velocity inversion in the VELEST program. We also varied several inversion parameters during the inversion runs. Most of the computed 1-D velocity models converged to a stable solution in the depth range between 0 and 25 km. We evaluated the inversion results using rigorous testing procedures and selected two best performing velocity models. Each of these models will be used independently as the initial model in the simultaneous hypocenter-velocity inversion for a 3-D velocity structure in LET. Based on the results of the 1-D velocity modeling, seismicity distribution, and tectonics, we divided the study area into three parts, redefined the earthquake-station geometry, and performed the inversion for each part separately. This way, we gained a better insight into the shallow velocity structure of each subregion and were able to demonstrate the differences among them.Besides general structural implications and a potential to improve the results of LET, the new 1-D velocity models along with station corrections can also be used in fast routine earthquake location and to detect systematic travel time errors in seismological bulletins, as already shown by some studies using similar methods.

In 2020, central part of Croatia was struck by two major earthquakes: on 22 March in Zagreb and on 29 December near Petrinja. Both earthquakes happened while country was in COVID-19 „lockdown“.Magnitude ML5.5 earthquake occurred on Sunday morning at 5:24 UTC (6:24 CET) with the epicentre at Medvednica Mt., in the Zagreb's outskirts, just 7 km to the north of the centre of the Croatian capital. The intensity in the epicentre and in the historic centre was estimated as VII EMS, and a young girl lost her life. The earthquake struck just a day after public transport was suspended for 30 days, three days after public gatherings of more than five people were forbidden, the restaurants, shops (except for groceries, hygienic and other necessary items) and cultural institutions were closed, and six days after closure of schools and universities. A day after the main event, people were forbidden to leave their city/town/municipality of residence without written permission of local government. At that time, Croatian “lockdown” was described as one of the strictest ones in EU.On Tuesday 29 December 2020 at 11:19 UTC (12:19 CET) a magnitude ML6.2 (MW6.4) earthquake occurred in rural area of central Croatia, near town of Petrinja. It was preceeded by magnitude ML5.0 and ML4.7 events a day before. These events caused significant damage to buildings in Petrinja and Glina and the surrounding villages. The highest intensity was estimated as VIII–IX EMS and seven people lost their lives. This sequence happened also during the “lockdown” due to COVID-19 pandemics with strict measures imposed on 21 December 2020, some of which were cancelled after the mainshock.We will discuss events and processes that followed these strong earthquakes and how having to deal with two damaging events only nine months apart and in the unusual pandemic-related circumstances affected our work as seismologists but also our “everyday” lives.

Analyses of available data (newspaper reports, historical and church chronicles, chronical earthquake overviews, travel books, monographies, research papers, etc.) on effects of the earthquakes that shook the greater Ormož area at the Slovenian-Croatian border in the 1838 and 1839 revealed that one of them, recorded in a number of regional and global catalogues, is in fact a fake - the earthquake of 26 August 1838 never happened. This error creeped into various reports and studies, and then into many relevant catalogues, so this event should by systematically erased from the catalogues used to estimate seismicity rates in the neighbourhoods of north-western Croatia, north-eastern Slovenia, and south-western Hungary. Regarding the earthquake of 31 July 1838, we used important new sources of information that have not been consulted in any previous study. This made inversion of macroseismic parameters more robust. Our estimates of the macroseismic moment magnitude (Mwm = 4.8) is mostly higher than the values reported in the available catalogues. Reliable information on the effects of the smaller event of 22 March 1839 were found for two localities only, so its epicentre was placed into the town of Ormož where the maximum intensity was observed. Its estimated moment magnitude (Mwm) is close to the median of values found in the six consulted catalogues that listed this event. The macroseismic epicentre of the 1838 earthquake lies close to the junction of surface traces of the Donat strike-slip fault and the reverse Čakovec fault. Based on their assumed geometry and the location of the macroseismic hypocentre, we give slight preference to the Donat fault as the seismogenic source., Analizom dostupnih podataka iz novinskih izvješća, povijesnih i crkvenih kronika, raznih kronika potresa, putopisnih knjiga, monografija i znastvenih radova o učincima potresa koji su se dogodili u širem području Ormoža na slovensko-hrvatskoj granici 1838. i 1839. godine, ustanovljeno je da je jedan od njih, zabilježen u brojnim regionalnim i globalnim katalozima, zapravo lažnjak. Radi se o navodnom potresu od 26. kolovoza 1838. koji se nikada nije dogodio. Ova se pogreška uvukla u različite biltene i studije, a zatim i u mnoge relevantne kataloge, pa bi taj potres trebalo sustavno brisati iz kataloga koji se koriste za procjenu razine seizmičnosti u područjima sjeverozapadne Hrvatske, sjeveroistočne Slovenije i jugozapadne Mađarske. Što se tiče potresa 31. srpnja 1838., koristili smo važne nove izvore informacija koji nisu bili korišteni niti u jednoj prethodnoj studiji. To je inverziju makroseizmičkih parametara učinilo robusnijom. Naša procjena makroseizmičke momentne magnitude (Mwm =4,8) uglavnom je veća od vrijednosti zabilježenih u dostupnim katalozima. Pouz-dane informacije o učincima slabijeg potresa od 22. ožujka 1839. pronađene su samo za dvije lokacije, pa je njegov epicentar smješten u grad Ormož gdje je zabilježen maksimalni intenzitet. Njegova procijenjena momentna magnituda (Mwm =4,4) bliska je medijanu vrijednosti pronađenih u šest pregledanih kataloga koji navode ovaj potres. Makroseizmički epicentar potresa iz 1838. godine leži u blizini spoja površinskih tragova Donatskog rasjeda s pomakom po pružanju i reversnog Čakovečkog rasjeda. Na temelju njihove pretpostavljene geometrije i položaja makroseizmičkog hipocentra, dajemo blagu prednost Donatskom rasjedu kao seizmogenom izvoru.

Adriatic microplate (Adria) has been a major component of the central Mediterranean geodynamic history since at least Late Cretaceous time. Although Cenozoic motion of Adria is relatively well established, there remains several open questions regarding its dynamics. This is especially evident when trying to reconstruct the motion of Adria since early Miocene. Although there is a general consensus about the counterclockwise rotation of Adria with respect to Europe, the amount of this rotation is still a matter of vigorous debate. In order to explain various measurements, several models of Adria motion were introduced: 1) Adria moving in conjunction with Africa, 2) Adria moving independently as a single block and 3) Adria moving independently but divided into two fragments.Here we explore the third model by using data from temporary seismic station deployments (AlpArray and AlpArray-CASE) and available permanent stations surrounding the Adriatic Sea. We constructed the tomography image of the Adriatic Sea region using the interstation surface wave dispersion measurements from teleseismic events. Additionally, we test the properties of the Central Adriatic crust by analyzing P-receiver functions from mid-Adriatic island seismic stations. Preliminary results show anomalous lithospheric structure in the Central Adriatic dividing Adria into two sections thus indicating that Adria fragmentation has progressed all the way to the base of the lithosphere.

Following the damaging earthquake of 22 March 2020 (ML = 5.5, Mw = 5.3, Imax = VII EMS) in Zagreb, a question was raised whether this was the largest event after the Great Zagreb earthquake of 1880 (Imax = VIII MSK). The counter-candidates are the events of 17 December 1905 and 2 January 1906, for which relevant earthquake catalogues mostly report larger or comparable magnitudes as for the earthquake of 2020, with their maximum intensities mostly within a narrow margin between VII and VII– VIII in various intensity scales. In order to resolve the question, we have (re)analysed all to us available macroseismic data for the two historical events, collected readings from station bulletins, and analysed available historical seismograms. Macroseismic proxy for the local magnitude (MmR) was estimated on the 11 basis of modelled radii of isoseismals V EMS and VI EMS using the regressions derived for a set of 12 earthquakes in the NW Croatia and the neighbouring areas. Macroseismic magnitude was found to be the largest for the 1906 event (MmR = 5.3), followed by MmR = 5.1 for the 2020 quake. Considering the magnitudes computed after Wiechert seismograms from the Göttingen (GTT) station, and from the amplitude/period readings reported from the German stations JEN and HOH for the earthquake of 1906, as well as the magnitudes calculated from broad-band records of the GTTG station and the stations of the Croatian network for the event of 2020, a unified local magnitude of ML = 5.3 is found for both events. The magnitudes of the 1905 earthquake were consistently the lowest of the three. Taking the uncertainties into account, the events of 1906 and 2020 should be considered approximately equal in size. However, the strongest shaking in the centre of Zagreb was caused by the 2020 event. It occurred on the reverse North Medvednica boundary fault, while the macroseismic epicentres of earthquakes of 1905 and 1906 lie practically on the trace of the nearby strike-slip Kašina fault. That Kašina fault could have been the source of the 1906 earthquake is also hinted at by the elongated region of the strongest shaking along 24 its strike.

Prikazana je cjelokupna povijest inženjerske seizmologije u Hrvatskoj, od vremena A. Mohorovičića do danas.

Following the damaging 2020 earthquake of 22 March 2020 (Mw 5.3), a question was raised if this was the largest event after the Great Zagreb earthquake of 1880. The counter-candidates are the events of 17 December 1905 and 2 January 1906. Relevant earthquake catalogues largely disagree when the magnitudes of the Zagreb earthquakes of 1905 and 1906 are concerned. The reported magnitudes range between 5.1 and 5.6 for the 1905 event, and between 5.3 and 6.3 for the earthquake of 1906. Epicentral intensities for the two earthquakes are stated between VII–VIII and IX. To try to resolve the question, we (re)analysed all available macroseismic data for the two historical events, in order to compare the observed intensity distribution with the one of the 2020 earthquake.

Although strong and damaging earthquakes have hit Zagreb in the past, the 22 March 2020 earthquake (Mw 5.4) is the first one that was recorded by a modern digital local seismic network, and which could be analysed not only by macroseismic methods, but also by microseismic ones. Herewith we used the 3003 carefully analysed and located events from the first year of the aftershock sequence to learn more about the aftershock rate decay, their magnitude distribution, focal mechanisms and hypocentral locations. The aftershock activity rate was found to closely follow the modified Omori law, and fault-plane solutions for 10 events indicated prevailing pure-reverse faulting. Our analyses suggest that the reverse North Medvednica boundary fault (NMBF) was the causative fault, as it fits with the focal mechanisms and with the geometry of aftershock locations. The epicentral area was of a triangular shape with the mainshock in one vertex, and the opposite side of the triangle lying parallel to the surface trace of the NMBF. The hypocentres of aftershocks were predominantly located in the hanging wall of the fault. No surface break was observed, so the rupture is assumed to be buried. These facts were interpreted as a combination of the effect of conservation of mass (seismic flow) requiring some fault-parallel stress redistribution and transfer of material, and the fault loading and activation in the compressive environment controlled by the stress partition at the brittle-ductile transition zone within the crust. The later process involves compression within the hanging wall during the interseismic stage when the fault segment in the brittle crust is locked, followed by sudden dilatation during the rupture phase.

This study, involving remote sensing, seismology, and geology, revealed complex faulting during the mainshock of the Ston–Slano earthquake sequence (5 September, 1996, Mw = 6.0). The observed DInSAR interferogram fringe patterns could not be explained by a single fault rupture. Geological investigations assigned most of the interferogram features either to previously known faults or to those newly determined by field studies. Relocation of hypocentres and reassessment of fault mechanisms provided additional constraints on the evolution of stress release during this sequence. Available data support the scenario that the mainshock started with a reverse rupture with a left-lateral component on the Slano fault 4.5 km ESE of Slano, at the depth of about 11 km. The rupture proceeded unilaterally to the NW with the velocity of about 1.5 km/s for about 11 km, where the maximum stress release occurred. DInSAR interferograms suggest that several faults were activated in the process. The rupture terminated about 20 km away from the epicentre, close to the town of Ston, where the maximum DInSAR ground displacement reached 38 cm. Such a complicated and multiple rupture has never before been documented in the Dinarides. If this proves to be a common occurrence, it can pose problems in defining realistic hazard scenarios, especially in deterministic hazard assessment.

The 30 March 1738 earthquake with an epicenter near Čakovec in Međimurje (Croatia) is the largest known earthquake in the low-seismicity area that includes northernmost Croatia, northeastern Slovenia, southeastern Austria, and southwestern Hungary. So far, it has attracted very little attention in the seismological communities of those countries. It is missing or has wrong source parameters in all of the relevant earthquake catalogs (including the Seismic Hazard Harmonization in Europe (SHARE) catalog, Stucchi et al., 2013), which may influence seismic hazard assessment in this part of Europe, most critically in the Međimurje region itself. We present contemporary historical data shedding some light on the effects that the earthquake had on settlements mostly in Međimurje, but also elsewhere in Croatia, Slovenia, and Hungary. We were able to assign intensities to 12 localities surrounding the epicenter and to resolve the confusion about its date of occurrence. The intensity points were inverted for the location of the macroseismic hypocenter and epicentral intensity (I0=7.9 MSK [Medvedev–Sponheuer–Karnik]). The epicenter is found to lie on the hanging wall of the reverse Čakovec fault, about 6 km from its surface trace, and 8 km north-northwest of the town of Čakovec. The rather small felt area for an earthquake of this maximum intensity implies a shallow macroseismic focal depth of 6 km. These values of intensity and depth correspond to a macroseismic magnitude of MLm 5.1.

Knowledge about the crustal thickness is one of the key elements in the reconstruction of the regional tectonic history. The Dinaric mountain belt is one of the most enigmatic segments of the Alpine‐Mediterranean collision zone, characterized by large variations in crustal thickness and not studied sufficiently. We present a new Moho depth map for the wider Dinarides region which was created using teleseismic earthquake recordings from 87 permanent and temporary seismic stations in the region. Teleseismic data were analyzed using the receiver function method to extract converted P to S waves. The resulting Moho topography fits well within a structural framework comprising a thicker crust under the Dinarides, which gradually becomes thinner toward the Pannonian and Adriatic domains. The profiles crossing the northwestern Dinarides are marked by a relatively sharp decrease in crustal thickness north of the main thrust front. This transition is followed by significant crustal thinning toward the Pannonian basin. The Mohorovičić discontinuity lies the deepest in the central and southern Dinarides, at depths of over 55 km. Here similarly to the northwestern segment we observe a jump in the crustal thickness when transitioning toward the Internal Dinarides, which hints at possible underthrusting (or subduction) of the Adria plate in this region. Moho depths in the transition zone toward the Pannonian basin and in the Pannonian basin proper vary between 25 and 35 km. In the Adriatic domain, we find crustal thickness ranging from 30 km to more than 45 km around the Central Adriatic islands.

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 central part of the External Dinarides (CED) is a geologically and tectonically complex region formed in the collision between the Adriatic microplate and the European plate. In this study, the contributions of intrinsic and scattering attenuation ( Q i - 1 and Q sc - 1 , respectively) to the total S-wave attenuation were calculated for the first time. The multiple lapse-time window analysis (MLTWA method), based on the assumptions of multiple isotropic scattering in a homogeneous medium with uniformly distributed scatterers, was applied to seismograms of 450 earthquakes recorded at six seismic stations. Selected events have hypocentral distances between 40 and 90 km with local magnitudes between 1.5 and 4.7. The analysis was performed over 11 frequency bands with central frequencies between 1.5 and 16 Hz. Results show that the seismic albedo of the studied area is less than 0.5 and Q i - 1 > Q sc - 1 at all central frequencies and for all stations. These imply that the intrinsic attenuation dominates over scattering attenuation in the whole study area. Calculated total S-wave and expected coda wave attenuation for CED are in a very good agreement with the ones measured in previous studies using the coda normalization and the coda-Q methods. All estimated attenuation factors decrease with increasing frequency. The intrinsic attenuation for CED is among the highest observed elsewhere, which could be due to the highly fractured and fluid-filled carbonates in the upper crust. The scattering and the total S-wave attenuation for CED are close to the average values obtained in other studies performed worldwide. In particular, good agreement of frequency dependence of total attenuation in CED and in the regions that contributed most strong-motion records for ground motion prediction equations used in PSHA in Croatia indicates that those were well chosen and applicable to this area as far as their attenuation properties are concerned.

Seismic microzonation maps for Belgrade (Serbia) and its surroundings are presented based on the uniform-hazard-spectrum (UHS) methodology. Such mapping must satisfy the guidelines for performance-based design (PBD), which at present requires specification of two spectral amplitudes, one for which the structure will remain essentially linear, and the other for which it will undergo nonlinear response. These requirements cannot be achieved by specifying the design spectra using only one (same) fixed spectral shape, and those spectra cannot be scaled by the peak ground acceleration alone. Another source of difficulties in the selection of the design amplitudes for PBD occurs when the standard spectrum shape is not capable of describing excitation by large distant earthquakes. Furthermore, scaling the design ground motion only via soil site classification (like A, B, C as in Eurocode 8) ignores the effects of site geology and thus also leads to erroneous results. The maps we present in this paper avoid these shortcomings and include in a balanced way the effects of near and distant large earthquakes, spatial distribution of seismic activity, the site geology and site soil properties.

Seismic anisotropy beneath the greater region of the Central and Southern External Dinarides is estimated from observations of SKS splitting. The area is located in the broad and complex Africa-Eurasia convergent plate boundary zone, where the Adriatic microplate interacts with the Dinarides. We analyzed recordings of 12 broadband seismic stations located in the Croatian coastal region. Evidence of seismic anisotropy was found beneath all stations. Fast axis directions are oriented approximately in the NE-SW to NNE-SSW direction, perpendicularly to the strike of the Dinarides. Average delay times range between 0.6 and 1.0 s. A counter-clockwise rotation in average fast axis directions was observed for the stations in the northern part with respect to the stations in the southern part of the studied area. Fast axis directions coincide with the assumed direction of asthenospheric flow through a slab-gap below the Northern and Central External Dinarides, with the maximum tectonic stress orientation in the crust, and with fast directions of Pg and Sg-waves in the crust. These observations suggest that the detected SKS birefringence is primarily caused by the preferred lattice orientation of mantle minerals generated by the asthenospheric flow directed SW-NE to SSW-NNE, with a possible contribution from the crust.

We present a new crustal thickness map for the wider Dinarides region. The map was constructed from the results of receiver function analyses on more than 90 seismic stations installed in this area, including the newly available AlpArray stations. Moho depth was measured using two methods, simple H − κ stacking and a more complex inversion process based on the neighbourhood algorithm. In the first instance, a simple one layered crust was assumed whereas in the second process a two or three layered crust with the addition of the dipping Moho discontinuity was modelled. The results show thicker crust (> 40 km) stretching under most of the External Dinarides and slightly thinning towards the Adriatic Sea. It is interesting that the results show a significant crustal thickening in the direction of the Panonnian basin with an abrupt change to relatively thinner crust (< 35 km) in the transition zone between the Dinarides and the Pannonian basin proper. In the transition zone the crustal thickness varies between 25 and 35 km and thins to less than 25 km as we enter the Pannonian basin. Furthermore, the new results show significantly thicker crust (> 40 km) in the area of the central Dalmatian archipelago, as compared to the values of less than 35 km reported in this area in all previous studies.

SUMMARYWe infer seismic azimuthal anisotropy from ambient-noise-derived Rayleigh waves in the wider Vienna Basin region. Cross-correlations of the ambient seismic field are computed for 1953 station pairs and periods from 5 to 25 s to measure the directional dependence of interstation Rayleigh-wave group velocities. We perform the analysis for each period on the whole data set, as well as in overlapping 2°-cells to regionalize the measurements, to study expected effects from isotropic structure, and isotropic–anisotropic trade-offs. To extract azimuthal anisotropy that relates to the anisotropic structure of the Earth, we analyse the group velocity residuals after isotropic inversion. The periods discussed in this study (5–20 s) are sensitive to crustal structure, and they allow us to gain insight into two distinct mechanisms that result in fast orientations. At shallow crustal depths, fast orientations in the Eastern Alps are S/N to SSW/NNE, roughly normal to the Alps. This effect is most likely due to the formation of cracks aligned with the present-day stress-field. At greater depths, fast orientations rotate towards NE, almost parallel to the major fault systems that accommodated the lateral extrusion of blocks in the Miocene. This is coherent with the alignment of crystal grains during crustal deformation occurring along the fault systems and the lateral extrusion of the central part of the Eastern Alps.

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 172 Geodesy in Croatia, 2015–2018 . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Geomagnetism and aeronomy in Croatia, 2015–2018 . . . . . . . . ... 181 Hydrology and physical limnology in Croatia, 2015–2018 . . . . . ... 185 Meteorology in Croatia, 2015–2018 . . . . . . . . . . . . . . . . . . . . . . . . 195 Physical oceanography in Croatia, 2015–2018 . . . . . . . . . . . . . . . . 203 Seismology in Croatia, 2015–2018 . . . . . . . . . . . . . . . . . . . . . . . . . 217

The Klana earthquake sequence with the mainshock on 1 March 1870 (Imax = VIII MSK) is one of the most important events that occurred in the Rijeka (Croatia) epicentral region. It is remarkable not just because of its impact to the local community and its significance for the seismic hazard in the Rijeka area, but also because it provided some of important observations on earthquake phenomena during the infancy of seismology. In particular, D. Stur (1871) seems to have been the first to attempt to explain the observed variation in damage by interaction of different local geological units during shaking ; R. Hoernes (1878) heavily relied on Stur’s data from the Klana epicentral region when he proposed that the vast majority of global earthquakes is related to the mountain-building process, and thus named them tectonic earthquakes. In spite of this, the Klana earthquake so far has escaped any detailed consideration by seismologists. In this study we present the historical data sources consulted to extract useful reports on damage and other observed effects, compile macroseismic maps for the five strongest events, and invert the estimated intensity data to compute macroseismic hypocenters and epicentral intensities for the mainshock and the two strongest aftershocks. We also present an earthquake chronicle with data for 92 identified earthquakes in the area for the period January–August 1870. The mainshock’s macroseismic epicenter is located near the active Rasa fault–Klana-Novi Vinodolski fault system, which we thus propose to be the primary source of the sequence. Macroseismic epicenters of the aftershocks lie close to the surface trace of the nearby Skadanscina-Rijeka reverse fault.

The Central External Dinarides are known as a tectonically complex region of moderate seismicity where several strong earthquakes occurred in the last century. In order to gain insight into the attenuation of seismic waves in the area, the extended coda normalization method was applied to band-pass-filtered seismograms of local earthquakes recorded at seven seismological broadband stations. Obtained results indicate strong attenuation of direct body waves: Q 0,P = Q P(1 Hz) is found between 21 and 120 and Q 0,S = Q S(1 Hz) is between 46 and 113, whereas the exponent n in the power law of frequency dependence of the quality factor is found in the range of 0.63–1.52 and 0.65–0.97 for n P and n S, respectively. P-waves are, on the average, attenuated more than S-waves. The three island stations (Dugi Otok (DUGI), Žirje (ZIRJ), Hvar (HVAR)) are distinguished by the strong low-frequency P-wave attenuation and more pronounced frequency dependence of the Q P factor (Q 0,S/Q 0,P > 1.7, Q 0,P n S). The remaining four inland stations (Udbina (UDBI), Morici (MORI), Kijevo (KIJV), Cacvina (CACV)) all exhibit similar qualitative attenuation properties for P- and S-waves (n P ≈ n S ≈ 1 and Q 0,S ≈ Q 0,P), although individual values of the Q-factors vary notably within this group. Low-frequency attenuation of direct S-waves in the crust is stronger than mean attenuation of scattered coda waves in the lithosphere, especially for long coda lapse times. The results are also qualitatively in agreement with the thermal regime in the area.

Seismic microzonation maps for Skopje (Macedonia) are presented based on the Uniform-Hazard-Spectrum (UHS) methodology. UHS satisfies the guidelines for performance-based design (PBD), which require specification of two sets of spectral amplitudes, one for which the structure will remain essentially linear, and the other for which it will undergo a nonlinear response. The UHS method also considers the contributions from large distant earthquakes and includes simultaneous effects of site geology and site soils. The maps presented include the effects of near and distant large earthquakes in a balanced way, spatial distribution of seismic activity, site geology, and site soil properties.

Seismic microzonation maps for Stip (Macedonia) and its surroundings are presented based on the uniform-hazard-spectrum (UHS) methodology. Such mapping satisfies the guidelines for performance-based design (PBD), which requires specification of two sets of spectral amplitudes—one for which the structure will remain essentially linear, and the other for which it will undergo a nonlinear response. The UHS method also enables us to include contributions from excitation by large distant earthquakes as well as the simultaneous effects of site geology and site soils. Thus, the maps we present include, in a balanced way, the effects of near and distant large earthquakes, spatial distribution of seismic activity, the site geology, and the site soil properties.

We present macroseismic analyses of three historical earthquakes that occurred in 1750, 1838, and 1904 in the Bakar epicentral area (Rijeka region, Croatia). Using various historical sources, we were able to compile intensity maps and assess macroseismic parameters for each of these events. The estimated epicenters lie close to known active fault systems, and the epicentral intensities are lower than those listed in catalogs. We have also identified a number of strong foreshocks and aftershocks which are currently missing from all catalogs. The three events analyzed, together with the Klana earthquake of 1870, are the largest known events to have occurred in these tectonically active parts of the northwest Dinarides. As such, they are important in characterizing seismic hazard in the vicinity of the main Croatian port, the city of Rijeka.

Earthquake design codes, which are based on the performance-based design principles, aim to satisfy both the operational continuity and safety requirements for the designed structures. This calls for the design for at least two very different levels of strong earthquake shaking, and thus requires the amplitudes and the shapes of the elastic acceleration spectra to vary with geographic coordinates, and with the probabilities of exceedance. The traditional code procedures, which scale the design spectra in terms of peak ground acceleration and fixed shape spectra cannot meet these objectives. In this paper we show how the performance based design principles can be satisfied if the Uniform Hazard Spectral (UHS) method is used for scaling the design spectra. We illustrate this method for seismic zoning of the northwestern segment of the Bosnia and Herzegovina area, centered at Banja Luka in the Republic of Srpska. The method we present in this paper can be implemented in essentially all seismic regions of the world.

Prof. Dr. Dragan Hadžievski passed away on October 20, 2015 in Skopje (Macedonia). He was prominent Macedonian seismologist and one of the founders of the seismological observatory in Skopje.

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.

Based on a large set of arrival times of the Pg phase reported by local and regional stations, we estimate azimuthal anisotropy of the Pg-wave velocity in focal volumes of the upper crust in NW Croatia. The method is based on analyses of the azimuthal dependence of ratios of cumulative differences of arrival times and travel paths between foci of earthquake pairs, computed for rays propagating within narrow azimuthal windows. The results clearly indicate the presence of anisotropy of 3.3% with the direction of fast velocity (approximately NNE-SSW) coinciding with the direction of the maximum tectonic pressure as revealed by 23 available focal mechanisms and previous geological investigations. Although a large part of observed anisotropy can be explained assuming that focal volumes are pervaded by a system of vertical extensive-dilatancy anisotropy (EDA) cracks aligned under the influence of local tectonic stress field, there is indication that — to a smaller extent — some role was also played by alignment of structural features in the region.

Continuous broad-band measurements of vibrations on the 3rd floor of the building of the Department of Geophysics, Faculty of Sciences and Mathematics (DGFSM) in Zagreb provided high quality data that enabled monitoring of fluctuations of the building’ s basic dynamic parameters (fundamental frequencies and the corresponding damping). It was found that nonlinear behaviour is present even for the small strains induced by ambient vibrations, which was manifested in measurable reduction of fundamental frequency and increase of damping for larger excitation levels. These effects were also observed in response of the structure to two small local earthquakes which occurred during the observation period. Correlation of the observed long-term wander of dynamic parameters with the change of meteorological elements revealed notable correspondence of the variations of the fundamental frequency and the relative air humidity (which was related to soil saturation, thus causing variability of soil-structure interaction). The damping was found to vary synchronously with the air temperature. The results point to the importance of considering all aspects of nonlinearity of building response even for small strains.

Andrija Mohorovicic, a world-renowned seismologist and the greatest Croatian scientist of all time, was born 150 years ago on 23 January 1857 in Volosko near Rijeka, Croatia. Although his name is mostly associated with the discovery of the crust-mantle boundary, his other achievements in seismology, as well as those in other geophysical disciplines, certainly deserve to be remembered.⇓ ![][1] Mohorovicic enrolled in Prague University to study mathematics and physics in 1875. The very solid foundations in science that he obtained from professors such as Ernst Mach and Heinrich Durege are clearly reflected in his later work and scientific attitude. An overview of his days in Prague is given by Orlic (1998). After graduation he taught at high schools in Zagreb and Osijek and at the Nautical School in Bakar. In 1892 he became the director of the Meteorological Observatory in Zagreb. The following year Mohorovicic obtained the doctorate of philosophy at Zagreb University, where he was later elected an adjunct associate university professor to teach courses on geophysics and astronomy. He became a corresponding member of the Yugoslav Academy of Science and Arts in 1893 and a full member in 1898. Mohorovicic retired in 1922. An excellent and thorough account of his life and scientific accomplishments was published in a monograph by Skoko and Mokrovic (1982, 1998) in Croatian and English. At the beginning of his career Mohorovicic focused on meteorology. His scientific interests lay in the explanation of various meteorological phenomena—atmospheric dynamics and observations of rare events ( e.g. , a tornado near Novska or the whirlwind near Cazma). He also studied the climate of Zagreb and the decay of temperature with altitude. Looking to learn more about the atmospheric circulation but lacking proper instruments, he built his own nephoscope, a camera obscura- like instrument for observation of clouds. … [1]: /embed/graphic-1.gif

Along the Periadriatic Lineament in the Alps and the Sava-Vardar Zone of the Dinarides and Hellenides, Paleogene magmatic associations form a continous belt, about 1700 km long. The following magmatic associations occur: 1) Eocene granitoids ; 2) Oligocene granitoids including tonalites ; 3) Oligocene shoshonite and calc-alkaline volcanics with lamprophyres, and 4) Egerian-Eggenburgian (Chattian) calc-alkaline volcanics and granitoids. All of these magmatic associations are constrained by radiometric ages, which indicate that the magmatic activity was mainly restricted to the time span between 55 and 29 Ma. These igneous rocks form, both at surface and in the subsurface, the distinct linear Periadriatic-Sava-Vardar magmatic belt with three strikes that are controlled by indentation of Apulia and Moesia and accompanying strike-slip faulting. The geology, seismicity, seismic tomography and magnetic anomalies within this belt suggest that it has been generated in the African-Eurasian suture zone. Based on published analytical data, the petrology, major and trace element contents and Sr, Nd and O isotopic composition of each magmatic association is briefly defined. These data show that Eocene and Oligocene magmatic associations of the Late Paleogene Periadriatic-Sava-Vardar magmatic belt originated along a consuming plate margin. Based on isotopic systems, two main rock groups can be distinguished with: 1) 87Sr/86Sr=0.7036-0.7080 and d18O=5.9-7.2‰ indicating basaltic partial melts derived from a continental mantle-lithosphere, and 2) 87Sr/86Sr=0.7090-72131 and d18O=7.3-11.5‰ indicating crustal assimilation and melting. The mantle sources for the primary basalt melts are metasomatized garnet peridotites and/or spinel lherzolites and phlogopite lherzolites of the upper mantle wedge origin. The geodynamic evolution of the plutonic and volcanic associations of the Periadriatic-Sava-Vardar magmatic belt was related to the Africa-Eurasia suture zone that was dominated by break-off of the subducted lithospheric slab of Mesozoic oceanic crust, at depths of 90-100 km. This is indicated by their contemporaneity along the 1700 km long belt.

A hybrid technique consisting of modal summation and subsequent finite differences modelling is applied for the computation of synthetic accelerograms along a profile crossing the city of Zagreb, the capital of Croatia. Assuming the source geometry is known, the amplification properties of the underlying soil may be determined by comparison of synthetics and their response spectra computed for a bedrock model with the ones obtained under the assumption of a realistic laterally varying local model. The peak ground acceleration is larger by a factor of up to 3.5 than the value obtained for the bedrock model. The amplification of the response spectra is most prominent for frequencies below 2 Hz, and increases sharply to the SW from the mapped fault running through the centre of the city.