We present an updated empirical relationship between moment magnitude (Mw), epicentral intensity (I0, EMS), and macroseismic focal depth (hm, km) for earthquakes in Croatia and its surroundings. The analysis is based on the updated, revised and uniformly processed Croatian Macroseismic Database and the corresponding Croatian Macroseismic Catalogue, which together provide a homogeneous macroseismic dataset spanning the period 1520–2022. From these, 139 triplets (Mw, ℎm, I0) were selected corresponding to events with independently determined moment magnitudes and reliably derived macroseismic parameters. A three-axes weighted orthogonal regression was performed to obtain a symmetric relationship linking the three variables. The resulting triaxial regression plane is expressed as -0.390 M_w+0.960 log(h_m+1)+0.285 I_0=1 from which closed-form expressions allow consistent estimation of any one parameter given the other two. The new relations exhibit stable predictive performance under out-of-sample conditions, and provide improved consistency across the full range of observed magnitudes, intensities, and focal depths. They also agree well with the most commonly used Croatian macroseismic relation that relates magnitude to epicentral intensity and focal depth. These results enable coherent reassessment of magnitudes for historical earthquakes and rapid post-event estimation of maximum intensity. Adoption of the new relationships would also support the homogenization of macroseismic magnitudes in the Croatian Earthquake Catalogue and provide a solid foundation for future intensity-based seismic hazard assessments in the region.

In regions of low seismicity, such as Baranja in northeastern Croatia, seismic hazard assessments rely heavily on the detailed characterization of the few largest known earthquakes. This study focuses on the two strongest historical earthquakes in the area, macroseismic data from Bosnia and Herzegovina, Croatia, Hungary, and Serbia. The number of intensity observations for the earthquake of 1922 was expanded from 106 to 278, whereas the previously macroseismically not analysed event of 1924 is decribed by 14 data points. Using a modified Kövesligethy–Jánosi model that accounts for intensity anisotropy in the epicentral area, we inverted the macroseismic fields to relocate the epicentres and estimate focal depths and magnitudes. Both events were relocated near the village of Zmajevac, within the Bansko Brdo tectonic unit, close to its boundary with the Drava depression. The 1922 epicentre moved 13 km north-northeast of the original location in the which occurred on 24 November 1922 and 12 August 1924. We re-evaluated these events using newly collected Croatian Earthquake Catalogue (CEC), while the 1924 epicentre shifted 22 km westward. Revised moment magnitudes are Mwm 5.3 and Mwm 4.4 for the 1922 and 1924 events, respectively. Estimated focal depths are shallower than previously listed: 11 km and 8 km, compared to the 18 km and 14 km in CEC. These results indicate that significant seismicity in Baranja is confined to the Bansko Brdo unit, with no evidence of strong earthquakes or faults with sufficient seismogenic potential in the Drava Depression or Northern Baranja–Bačka units. This has important implications for regional seismic hazard estimates. Furthermore, we find no instrumental support for the largest catalogued aftershock of the 1922 event and propose its removal. Finally, we interpret the 1924 earthquake as a late aftershock of the 1922 mainshock, suggesting a dependent relationship between the two.

The medieval town of Ston in southern Dalmatia (Croatia) has experienced several significant earthquakes since the mid-19th century, notably the ones in 1850 and 1996. While the macroseismic parameters as well as classification of damage to buildings in Ston related to the 1996 event are available from previous studies, the 1850 earthquake has only recently undergone detailed macroseismic analysis. It revealed clear similarities between the two events, including their epicenters, focal depths, and epicentral intensities. To enable comparison of the spatial distribution of damage from these twin earthquakes, we combined historical damage reports from 1850 with cadastral records from 1837. This allowed us to geolocate most of the damaged buildings, despite challenges in identifying individual structures due to changes in ownership and building modifications over the intervening years. Our study indicates that both earthquakes caused the most significant damage in the plain area below Bartolomija Hill, characterized by a sedimentary cover 10–30 m thick, with expected ground motion amplification by factors between 3 and 5. In contrast, the hillsides of Bartolomija in the northern part of the city, with their shallow or outcropping bedrock, experienced minimal damage from both events. To our knowledge, this observation of the shaking effects for two strong similar earthquakes in a city that has changed little in the 146 years between them is unique in Croatia. It demonstrates consistency of spatial pattern of earthquake ground motion amplification for comparable input earthquake motion.

The earthquake that occurred near the city of Ston, Croatia, on 13 April 1850 is, together with the one from 1996, the strongest known event in the northwest part of the Dubrovnik epicentral area. This is the region with the highest seismic hazard in Croatia with a rich history of damaging earthquakes. Although listed in the relevant catalogs, this earthquake has never been addressed by a dedicated study. Herewith, we present analyses of a wealth of newly found material related to the damage and postearthquake actions of the authorities of the Province of Dalmatia, then a part of the Austrian Empire. We were able to estimate intensity at five localities, with a further six where the data were sufficient only to constrain the minimum intensity value. By far, most of the data refer to Ston and Dubrovnik. Intensity data points were inverted for the source parameters by two different methods, each of which yielded similar results. The focus is macroseismically located about 7 km east-southeast from Ston, at a depth of 9 km. Estimated epicentral intensity of 8.2 on the European macroseismic scale is equivalent to macroseismic local magnitude M-mL = 6.0 or the moment magnitude M-mw = 5.9. The location of focus and the epicentral intensity are practically identical to those of the Ston-Slano earthquake of 1996. This is why we propose that these two earthquakes share the same composite seismogenic source consisting of a set of imbricated mostly reverse faults related to the basal thrust of the Dalmatian tectonic unit. The reliable location and quantification of the 1850 earthquake should contribute to a better understanding of the active dynamics of the set of large seismogenic faults in the Dubrovnik epicentral area.

In the southeast of Bosnia and Herzegovina, the Berkovic ' i earthquake sequence started with the mainshock on 22 April 2022 21:07 UTC at focal depth 22 km with magnitude ML = 6.0 (Mw = 5.7). Our preliminary estimation of the mainshock's maximum intensity is VII EMS for Berkovic ' i where 29% of buildings were damaged. We analysed the first nine months of this sequence, 22 April 2022-22 January 2023. The earthquakes were located using a guided grid -search algorithm with source -specific station corrections as a mean of solutions for 54 combinations of velocity models and program control parameters. The analysis of aleatory variation and epistemic uncertainty showed that they are very dependent on the station coverage, especially for focal depth. The event catalogue consists of 7217 earthquakes and can be considered complete for ML >= 1.3. Focal depths (15-30 km) are considerably larger than average for the Dinarides, but consistent within the zone of mid -crustal events where the earthquakes occurred. Focal mechanisms were determined with the first -motion polarity method for eight earthquakes: five of them, including the mainshock, were due to reverse faulting on faults striking in the Dinaric direction, with the preferred main fault gently dipping to the northeast. However, three events were due to normal faulting, unexpected for this area. We constructed a regional seismotectonic cross-section to delineate a potential seismogenic source of the mainshock, and it suggests that the mainshock occurred on the NE -dipping blind ramp of the basal thrust of the Dalmatian tectonic unit. Moreover, another NE -dipping and blind ramp of this basal thrust could be responsible for the Ston-Slano 1996 earthquake, located to the SW of the Berkovic ' i mainshock hypocentre at the horizontal distance of c. 35 km.

The small medieval city of Ston in southern Dalmatia (Croatia) has been hit by several strong earthquakes since mid-nineteenth century. The two most important are the ones from the years of 1850 and 1996. Although various aspects of the 1996 event have been well studied so far, the earthquake of 1850 has only recently been macroseismically analyzed. It turned out that the macroseismic epicenters, focal depths, and the epicentral intensities of the two events are virtually the same. As the categories of damage to buildings in Ston caused by the 1996 event were available from previous studies, we here combine details on damage and property ownership from reports in 1850 with the cadastral records from 1837. This allowed us to geolocate most of the damaged houses and thus directly compare the spatial distribution of damage from the two earthquakes. Although the building identification was not straight-forward and unambiguous due to unknown history of each building during the 13 yr between the cadastral survey and the earthquake, the overall damage distributions of both events are found to be similar. They both show the largest damage confined to the plain terrain below the Bartolomija hill characterized by a sedimentary cover 10-30 m thick and expected ground-motion amplification by factors of 3-5. Minimal damage for both events is observed on the hillslopes of Bartolomija in the northern part of the city, where the bedrock is shallow or outcropping. To our knowledge, this observation of the shaking effects for two strong similar earthquakes in a city that has changed little in the 146 yr between them is the only one of its kind in Croatia. It confirms consistency of spatial distribution of earthquake ground-motion amplification for comparable input earthquake motion.

<p>A strong earthquake, <em>M<sub>L</sub></em> = 6.0 (<em>M<sub>W</sub></em> = 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.</p> <p>By 31 October 2022, the DuFAULT project researchers located 6220 aftershocks (39 with <em>M<sub>L</sub> </em>≥ 3.0), with as many as 900 located in the first 12 h of the series. The strongest aftershock, <em>M<sub>L</sub></em> = 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.</p> <p>We will present spatio-temporal analysis of seismicity, resulting focal plane solutions and seismotectonic interpretation.</p>

We present locations of almost 14,000 events from the first six months of the Petrinja (Croatia) earthquakes sequence (mainshock 29 December 2020, Mw 6.4). The catalogue is estimated complete for ML >= 1.20. Initially sparse local seismograph network was densified a week after the mainshock, which reduced uncertainties of focal locations by about 75%. Source-specific station corrections were used in an iterative location scheme. The hypocentres were located with 30 different sets of velocity models and program control parameters in order to gain insight into the epistemic uncertainty of each earthquake location. The bulk of epicentres are located close to, and to the SE of the causative, dextral strike-slip Petrinja fault, but considerable activity has been triggered on smaller faults, up to 25 km away. About a half of the 78 first-motion polarity focal mechanism solutions (FMS) from the full first year of activity were found to have mechanisms similar to the foreshock, the mainshock and the largest of aftershocks, while the rest indicated almost pure reverse faulting. Strike-slip mechanisms occurred more often close to the main fault. The P-axes of FMS for aftershocks were found to be on the average rotated by 16 degrees clockwise with respect to the P-axis of the mainshock, regardless of the style of faulting. The spatial distribution of aftershocks coincides very well with the areas of positive Coulomb stress change caused by the mainshock rupture. The inferred cut-off seismogenic depth in the area (7-10 km), estimated on the basis of published values of the geothermal gradient and the assumption of predominantly granitic upper crust, is considerably shallower than the depth of the deepest reliably located aftershocks (16-18 km). This fact may be explained by considering ophiolite-dominated crust, and by increased strain rate during the sequence, which raises the ability of crustal rocks for brittle failure.

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.

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.

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 countercandidates 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 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 basis of modelled radii of isoseismals V EMS and VI EMS using the regressions derived for a set of 12 earthquakes in 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 its strike.

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.

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.

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 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.

SUMMARY Coda-Q is used to estimate the attenuation and scattering properties of the Earth. So far focus has been on earthquake data at frequencies above 1 Hz, as the high noise level in the first and second microseismic peak, and possibly lower scattering coefficient, hinder stable measurements at lower frequencies. In this work, we measure and map coda-Q in the period bands 2.5–5 s, 5–10 s and 10–20 s in the greater Alpine region using noise cross-correlations between station pairs, based on data from permanent seismic stations and from the temporary AlpArray experiment. The observed coda-Q for short interstation distances is independent of azimuth so there is no indication of influence of the directivity of the incoming noise field on our measurements. In the 2.5–5 s and 5–10 s period bands, our measurements are self-consistent, and we observe stable geographic patterns of low and high coda-Q in the period bands 2.5–5 s and 5–10 s. In the period band 10–20 s, the dispersion of our measurements increases and geographic patterns become speculative. The coda-Q maps show that major features are observed with high resolution, with a very good geographical resolution of for example low coda-Q in the Po Plain. There is a sharp contrast between the Po Plain and the Alps and Apennines where coda-Q is high, with the exception a small area in the Swiss Alps which may be contaminated by the low coda-Q of the Po Plain. The coda of the correlations is too short to make independent measurements at different times within the coda, so we cannot distinguish between intrinsic and scattering Q. Measurements on more severely selected data sets and longer time-series result in identical geographical patterns but lower numerical values. Therefore, high coda-Q values may be overestimated, but the geographic distribution between high and low coda-Q areas is respected. Our results demonstrate that noise correlations are a promising tool for extending coda-Q measurements to frequencies lower than those analysed with earthquake data.

The 30 March 1738 earthquake with an epicenter near Cakovec in Medimurje (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 Medimurje region itself. We present contemporary historical data shedding some light on the effects that the earthquake had on settlements mostly in Medimurje, 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 (I-0 = 7.9 MSK [Medvedev-Sponheuer-Karnik]). The epicenter is found to lie on the hanging wall of the reverse Cakovec fault, about 6 km from its surface trace, and 8 km north-northwest of the town of Cakovec. 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 M-Lm 5.1.

U radu se ukratko opisuje seizmicnost Hrvatske i preliminarne analize zagrebackog potresa od 22. ožujka 2020.

The dense AlpArray network allows studying seismic wave propagation with high spatial resolution. Here we introduce an array approach to measure arrival angles of teleseismic Rayleigh waves. The approach combines the advantages of phase correlation as in the two-station method with array beamforming to obtain the phase-velocity vector. 20 earthquakes from the first two years of the AlpArray project are selected, and spatial patterns of arrival-angle deviations across the AlpArray are shown in maps, depending on period and earthquake location. The cause of these intriguing spatial patterns is discussed. A simple wave-propagation modelling example using an isolated anomaly and a Gaussian beam solution suggests that much of the complexity can be explained as a result of wave interference after passing a structural anomaly along the wave paths. This indicates that arrival-angle information constitutes useful additional information on the Earth structure, beyond what is currently used in inversions.