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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.

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 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.

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.

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.

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.

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.

SUMMARYTo calculate phase-velocity dispersion curves, we introduce a method which reflects both structural and dynamic effects of wave propagation and interference. Rayleigh-wave fundamental-mode surface waves from the South Atlantic Ocean earthquake of 19 August 2016, M = 7.4, observed at the AlpArray network in Europe are strongly influenced by the upper-mantle low-velocity zone under the Cameroon Volcanic Line in Central Africa. Predicting phase-delay times affected by diffraction from this heterogeneity for each station gives phase velocities as they would be determined using the classical two-station method as well as the advanced array-beamforming method. Synthetics from these two methods are thus compared with measurements. We show how the dynamic phase velocity differs from the structural phase velocity, how these differences evolve in space and how two-station and array measurements are affected. In principle, arrays are affected with the same uncertainty as the two-station measurements. The dynamic effects can be several times larger than the error caused by the unknown arrival angle in case of the two-station method. The non-planarity of the waves and its relation to the arrival angle and dynamic phase-velocity deviations is discussed. Our study is complemented by extensive review of literature related to the surface wave phase-velocity measurement of the last 120 years.

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.

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

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

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

The magnitudes (MS, mbP, mbS) of the largest historical earthquakes which occurred in the first half of the 20thcentury, calculated on the basis of records of Wiechert horizontal seismographs in Gottingen (Germany) and Zagreb (Croatia), are compared with one another, as well as with the magnitudes reported in worldwide catalogues. Systematic trends are observed in the data regarding the temporal stability of magnitude estimations in Gottingen, as well as the apparent non-linearity of the instrument responsle in the case of the Wiechert seismograph in Zagreb. We were unable to clearly identify their causes – possible explanations include effects caused by the interaction of the seismometer's frame and mass, as well as local soil conditions, but nonhomogeneity of the reference catalogues cannot be ruled out. The results indicate that a careful re-examination and cross-checking of the reported magnitude figures for the earthquakes from the first half of the 20th century is required.

—The paper presents results of experiments designed to measure the actual dynamic magnification of the Wiechert 1000 kg horizontal seismometer when excited by seismic waves. This is accomplished by comparing 51 digital records of seismic events recorded by the Wiechert and a well calibrated reference seismometer. The results obtained indicate that the magnification of the Wiechert seismometer is influenced by the interaction of its mass and frame, especially for high frequencies. This interaction has been modeled by considering a system of two coupled pendulums, yielding a theoretical dynamic magnification curve which exhibits main features of the observed magnification. The dis crepancy between the nominal and the actual response of the Wiechert seismograph may lead to errors in studies involving spectral analyses of recorded seismograms, and to overestimation of local earth quake magnitudes.

The single backscattering model was used to estimate total attenuation of coda waves (Qc) of local earthquakes recorded on eight seismological stations in the complex area of the western continental Croatia. We estimated Q0 and n, parameters of the frequency dependent coda-Q using the relation Qc = Q0fn. Lapse time dependence of these parameters was studied using a constant 30 s long time window that was slid along the coda of seismograms. Obtained Qc were distributed into classes according to their lapse time, tL. For tL = 20–50 s we estimated Q0 = 45–184 and n = 0.49–0.94, and for tL = 60–100 s we obtained Q0 = 119–316 and n = 0.37–0.82. There is a tendency of decrease of parameter n with increasing Q0, and vice versa. The rates of change of both Q0 and n seem to decrease for lapse times larger than 50–80 s, indicating an alteration in rock properties controlling coda attenuation at depths of about 100–160 km. A very good correlation was found between the frequency dependence parameter n and the Moho depths for lapse times of 50, 60 and 70 s.

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

The paper presents body-wave velocities in the crust and upper mantle of the circum-Adriatic region obtained by analyzing travel times of regional phases of 419 selected earthquakes with epicenters in the central part of the External Dinarides. The studied region was divided into five areas and two kinds of velocity models were determined for each of them. The first kind ( inter-area models ) gives average velocities that characterize the zone between and including the epicentral region and the area in question, while the second kind ( intra-area models ) presents velocities within the respective area. The locations of hypocenters considerably improved when the new models were used instead of the standard one for the Balkan region. The large-scale variation of average velocities obtained by inter-area modeling was found to be relatively small both in the crust and in the upper mantle. On the other hand, the magnitude and distribution of station corrections and the resulting intra-area velocities point to relatively pronounced lateral velocity variation within some of the studied regions. The obtained velocities in most cases do not differ significantly when compared to the values reported in other studies. The most important exceptions occur in the epicentral area (central part of the External Dinarides) where the P-wave velocity in the upper crust is much higher then elsewhere, while the velocity in the upper mantle is found to be lower than that determined by DSS experiments. There is also an indication of pronounced variation of the Poisson ratio in this area.

The attenuation of coda waves is analysed for nine seismic stations in the area of convergent motion of the Adriatic microplate and the Dinarides. The frequency dependent coda quality factor of the form Qc = Q0 fn is estimated for up to seven central frequencies (1.5, 3, 6, 9, 12, 18 and 24 Hz) and for 21 successive 30 s long time windows. Q0 was found to increase from 68–353 for short lapse times of 20–50 s, to 158–373 for lapse times of 90–100 s. Parameter n is observed to vary between 0.46 and 0.89, with a pronounced tendency to decrease with increasing Q0, and vice versa. Both Q0 and n seem to stabilize for lapse times larger than 50–80 s, indicating that a change in rock properties controlling coda attenuation occurs at depths of about 100–160 km. The spatial distribution of observed Q0 is well correlated with observed seismicity and inferred tectonic activity. In particular, we observe significant negative correlation of Q0 with the peak ground acceleration (PGA) estimate for the return period of 475 years. The degree of frequency dependence n, is the smallest for stations on the islands, where the crust is the thinnest. The largest n is observed over the thickest crust in the region, where the Moho lies at depths of over 55 km.

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.

The study of 5514 Ms readings reported to the International Seismological Center and U.S. Geological Survey revealed that Ms values as obtained by the Moscow-Prague formula are significantly distance dependent. Average relative errors of over 0.5 magnitude units are made when using amplitude readings from very near and very distant stations. The new calibrating function for 20 sec Rayleigh waves is proposed in which the amplitudes decrease as distance+1.094, which agrees with results of theoretical studies.

The Kupa Valley (Croatia) earthquake of 8 October 1909 belongs to a group of milestone events in the history of geophysics and seismology. Also known as the Kulpa Valley, Pokuplje or the Pokupsko earthquake, it is often mentioned in textbooks, encyclopedias, and historical overviews of science as the earthquake whose seismograms provided key data for Andrija Mohorovicic's proof of the existence of the crust-mantle boundary that was later named after him. The earthquake occurred at just the right time and place to inspire research that resulted in one of the most-cited seismological papers (Mohorovicic 1910a, 1910b, 1910c) from the beginning of the last century. The paper itself was remarkable, as it contained much more than just the discovery it is mostly known for (see, e.g. , Herak and Herak 2007). As Kanamori (1986) noted in his Seismological Society of America presidential address, “having been motivated by this finding, Mohorovicic made an extensive study on reflection and refraction of seismic waves at a discontinuity to strengthen his conclusion. In fact, his study of this problem seems to have as strong an impact on seismology as his discovery of the discontinuity itself.” The time was right because the theory had been there for a long time, but the instruments had just gotten good enough to record seismograms with the needed detail. In particular, the Kupa Valley earthquake occurred only a year after the first Wiechert seismograph replaced the Vicentini instrument at the Zagreb observatory, which was already equipped with the first-class Riefler clock. The place was right because the earthquake occurred close to Zagreb (about 30 km to the south) where Mohorovicic lived, and it was strong enough to cause some damage in the city. Earthquakes had been of interest in Zagreb for some time, as seismicity around the capital …

We show how by comparing M L SM (computed from strong motion accelerograms within 150 km from the epicenter) and M STA (reported by different seismological stations in Yugoslavia, Italy, Albania, Greece, Austria and Czechoslovakia) it is possible to calibrate amplitude-distance attentuation equations used in M STA definitions. This, more accurate, magnitude determination for regional networks is essential for: (1) better and more accurate integration of the engineering estimates of siesmic risk; and (2) for more accurate and quick estimation of strong motion amplitudes in epicentral region of destructive earthquakes.

article i nfo The seismicity of NW Croatia, seismically the most vulnerable part of the country is presented based on historical records and reanalysed and relocated earthquakes occurring after 1908. The improved picture of the distribution of seismicity shows consistent grouping of foci in space, mostly within eight epicentral areas. The database of fault-plane solutions has been considerably enlarged, and now lists 22 earthquakes. The earthquake mechanisms consistently reveal the subhorizontal to moderately dipping P-axis, predominantly N-S directed in the central part of studied area, to NW-SE and NE-SW directed in the western and eastern parts, respectively. They indicate the prevalence of compressional tectonics with reverse faulting in the central part versus strike-slip motions in the western and eastern sectors. These data are in agreement with stress calculations and kinematics of Quaternary structures obtained by geological studies. The completeness of the catalogue, estimated by comparing the cumulative activity rate for a large number of subcatalogues to the reference, contemporary rate, exhibits large spatial heterogeneity. Taking this into account, we have computed and mapped the parameters of the frequency-magnitude recurrence relation (the b-value and the activity rate).

Recent measurements of ambient vibrations in Croatia include free-field measurements in Zagreb and Ston, as well as those done within buildings. Free-field data at both localities are consistent with the properties of shallow geological structures there. In Zagreb, horizontal-to-vertical spectral ratios (HVSR) of microtremors point to existence of thick alluvial cover (over 100 m) that gradually gets thinner as it reaches the southern slopes of the Mt. Medvednica. In Ston HVSR profiles reveal several tens of meters thick sedimentary cover over the bedrock which gets exposed at the Stovis hill. Over 100 measurements of ambient vibrations in buildings that have recently been performed across Croatia, were analysed by a suite of Matlab routines yielding frequencies and damping for the fundamental and higher modes of oscillation. Applications to real buildings proved that analyses of shaking induced by ambient vibrations in most cases lead to well constrained, reliable, and time independent estimates of frequencies and damping of the building vibrational modes. The measurements done so far form the initial nucleus of the Croatian building inventory, a collection of observed fundamental periods, damping, and spectral shapes, which can prove important in documenting building structural integrity and in assessing the degree of possible damage in future earthquakes.

It is shown that the new definition of strong motion local magnitude M L S M leads to stable estimates of magnitudes for earthquakes in Yugoslavia, with epicentral distances R

A case of seismicity changes prior to the M= 5.5 Knin (Yugoslavia) earthquake has been analysed. In addition to the well-expressed seismic quiescence, significant b-value, strain release and seismicity rate temporal changes have been observed. The estimated precursor time of T = 6.8 yrs is longer than expected on the basis of published log(T)— M relationships. A much better agreement is obtained if log(T) is compared to the longest dimension of the aftershock

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.

We present analyses of one of the strongest earthquake sequences ever recorded within the Adriatic microplate, which occurred near the Jabuka island in the very centre of the Adriatic Sea. The mainshock (29 March 2003, 17:42, ML=5.5) was preceded by over 150 foreshocks, and followed by many aftershocks, over 4600 of which were recorded on the closest station HVAR (about 90 km to the east). As the epicentre was in the open sea and due to the absence of nearby stations, we were able to confidently locate only 597 events. Hypocentral locations were computed by a grid-search algorithm after seven iterations of refining hypocentres and adjusting station corrections. Epicentres lie in a well-defined area of about 300 km 2 , just to the W and NW of the Jabuka island. The vertical cross-sections reveal that hypocentres dip to the NE, closely matching faults from the Jabuka-Andrija fault system, as identified on the available reflection profiles in the area. The fault-plane solution of the main shock based on the first-motion polarity readings agrees well with the CMT solutions and indicates faulting caused by a S–N directed tectonic pressure, on a reverse, dip-slip fault. This is in very good agreement with the seismotectonic framework of the area. These earthquakes are important as they identify the Jabuka-Andrija fault system as an active one, which can significantly influence seismic hazard on the islands in the central Adriatic archipelago and on the Croatian coast between Zadar and Split. Along with several other sequences which occurred in the last two decades, they force us to change our notion of Adria as nearly aseismic, compact and rigid block. In fact, it turns out that recent seismicity of the Central Adriatic Sea is comparable to the seismicity of several well known earthquake-prone areas in the circum-Adriatic region. D 2005 Elsevier B.V. All rights reserved.