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The studied area of the northwestern (NW) Dinarides is located in the northeastern (NE) corner of the Adriatic microplate and is bordered by the Adriatic foreland, the Southern Alps, and the Pannonian basin. Its complex crustal structure is the result of interactions among different tectonic units, the most important of which are the Eurasian plate and the Adriatic microplate. Despite numerous seismic studies in this tectonically complex area, there is still a need for a detailed, small-scale study focusing mainly on the upper, brittle part of the crust. In this work, we investigated the velocity structure of the crust with one-dimensional (1-D) simultaneous hypocenter–velocity inversion using routinely picked P- and S-wave arrival times. Most of the models computed in the combined P and S inversion converged to a stable solution in the depth range between 0 and 26 km. We further evaluated the inversion results with hypocenter shift tests, high- and low-velocity tests, and relocations. This helped us to select the best performing velocity model for the entire study area. Based on these results and the seismicity distribution, we divided the study area into three subregions, reselected earthquakes and stations, and performed the combined P and S inversion for each subregion separately to gain better insight into the crustal structure. In the eastern subregion, the P velocities in the upper 8 km of the crust are lower compared to the regional velocities and the velocities of the other two subregions. The P velocities between 8 and 23 km depth are otherwise very similar for all three models. Conversely, the S velocities between 2 and 23 km depth are highest in the eastern subregion. The NW and southwestern (SW) subregions are very similar in terms of the crustal structure between 0 and 23 km depth, with slightly higher P velocities and lower S velocities in the SW subregion. High vP/vS values were obtained for the layers between 0 and 4 km depth. Below that, no major deviations of vP/vS in the regional model from the value of 1.73 are observed, but in each subregion we can clearly distinguish two zones separated by a decrease in vP/vS at 16 km depth. Compared to the model currently used by the Slovenian Environment Agency to locate earthquakes, the obtained velocity models show higher velocities and agree very well with some of the previous studies. In addition to the general structural implications and the potential to improve the results of seismic tomography, the new 1-D P and S velocity models can also be used for reliable routine earthquake location and for detecting systematic travel time errors in seismological bulletins. [ABSTRACT FROM AUTHOR]

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

The nature of the interaction between Adriatic and Euroasian lithospheric plates in the Dinarides is important for understanding the complex tectonic history of the central Mediterranean. Using the data from permanent and temporary seismic stations in the wider Dinaric region, we imaged the lithospheric and upper mantle structure under this area. Specifically, we focused on mapping the lithosphere asthenosphere boundary (LAB) using the S receiver functions in order to establish boundaries between different tectonic domains present in this region. The lithospheric thickness in the investigated area varies between ∼50 and ∼160 km with high degree of variability between adjacent tectonic realms. Below northwestern Dinarides the LAB depth varies between 100 and 120 km thinning towards Adriatic sea and Pannonian basin, to 90 and 70 km respectively. In the central Dinaric region (Lika region) we find anomalously thin lithosphere with thickness varying between 50 and 70 km and weak velocity gradient defining the LAB. Further south the signal from the LAB is more pronounced with lithosphere getting thicker again with average depths around 90 km. The intriguing observation of thinned lithosphere under central part of the Dinarides coincides with the zone of lower seismicity and with the tomographic images showing the slab gap in this area. [ABSTRACT FROM AUTHOR]

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 V/ V 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 Mohorovičić 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. [ABSTRACT FROM AUTHOR]

In this work, we assess ground shaking in the wider Zagreb area by computing simulated seismograms at regional distances. For the purposes of the simulations, we assemble the 3D velocity and density model and test its performance. First, we compare the low-frequency simulations obtained using deterministic method for both new 3D model and a simple 1D model. We then continue the performance test by computing the full broadband seismograms. To do that, we apply the hybrid technique in which the low frequency (f < 1 Hz) and high frequency (f = 1–10 Hz) seismograms are obtained separately using deterministic and stochastic method, respectively, and then reconciled into a single time series. We apply this method to the MW = 5.3 event and four smaller (3.0 < MW < 5.0) events that occurred in the studied region. We compare simulated data with the recorded seismograms and validate our results by calculating the goodness of fit score for peak ground velocity and shaking duration. Next, to improve the understanding of the strong ground motion in this area, we simulate seismic shaking scenarios for the 1880, MW = 6.2 earthquake. From computed low-frequency waveforms, we generate shakemaps and compare the ground-motion features of the two possible sources of this event, Kašina fault and North Medvednica fault. We conduct a preliminary study to determine which fault is a more probable source of the 1880 historic event by comparing the peak ground velocities and Arias intensity with the observed intensities. [ABSTRACT FROM AUTHOR]

Continental collision zones are structurally one of the most heterogeneous areas intermixing various different units within a relatively small space. A good example of this is the Dinarides, a mountain chain situated in the central Mediterranean, where thick carbonates cover older crystalline basement units and remnants of subducted oceanic crust. This is further complicated by the highly variable crustal thickness ranging from 20 to almost 50 km. In terms of spatial extension, this area is relatively small but covers tectonically differentiated domains making, any seismic or geological analysis complex, with significant challenges in areas that lack seismic information on crustal structure. Presently there is no comprehensive 3D crustal model of the Dinarides (and surrounding areas). Using the compilations of previous studies and employing kriging interpolation, we created a vertically and laterally varying crustal model defined on a regular grid for the wider area of the Dinarides, also covering parts of Adriatic Sea and the SW part of the Pannonian Basin. The model is divided by three interfaces, Neogene deposit bottom, carbonate rock complex bottom and Moho discontinuity, with seismic velocities (P and S waves) and density defined at each grid point. To validate the newly derived model, we calculated travel times for an earthquake recorded on several seismic stations in the Dinarides area. The calculated travel times show significant improvement when compared to the simple 1D model used for routine earthquake location in Croatia. The model derived in this work represents the first step towards improving our knowledge of the crustal structure in the complex area of the Dinarides. We hope that the newly assembled model will be useful for all forthcoming studies (e.g., as a starting model for seismic tomography, as a model for earthquake simulations) which require knowledge of the crustal structure. [ABSTRACT FROM AUTHOR]

We use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method and apply consistent quality control to yield 112 205 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the first-order crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the entire processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study. [ABSTRACT FROM AUTHOR]

The tectonics of the Adriatic microplate is not well constrained and remains controversial, especially at its contact with the Dinarides, where it acts as the lower plate. While the northern part of the Adriatic microplate will be accurately imaged within the AlpArray project, its central and southern parts deserve detailed studies to obtain a complete picture of its structure and evolution. We set up the Central Adriatic Seismic Experiment (CASE) as a AlpArray Complementary Experiment with a temporary seismic network to provide highquality seismological data as a foundation for research with state-of-the-art methods and high-precision seismic images of the controversial area. The international AlpArray-CASE project involves four institutions: the Department of Earth Sciences and the Swiss Seismological Service of ETH Zürich (CH), the Department of Geophysics of the Faculty of Science at the University of Zagreb (HR), the Republic Hydrometeorological Service of the Republic of Srpska (BIH) and Istituto Nazionale di Geofisica e Vulcanologia (I). The established temporary seismic network will be operational for at least 18 months. It combines existing permanent and temporary seismic stations operated by the involved institutions together with newly deployed temporary seismic stations, installed in November and December 2016, managed by ETH Zürich and INGV: five in Croatia, four in Bosnia and Herzegovina and one in Italy. We present our scientific aims andnetwork geometry as well as the newly deployed stations sites and settings. In particular, the new stations show favourable noise level (power spectral density estimates). The new network improves considerably the theoretical ray coverage for ambient noise tomography and the magnitude threshold shown in the Bayesian magnitude of completeness threshold map. [ABSTRACT FROM AUTHOR]

Continental collision zones are structurally one of the most heterogeneous areas intermixing various units within relatively small space. The good example of this are the Dinarides where thick carbonate complex cover overlain older crystalline basement units and remnants of subducted oceanic crust. This is further complicated by the highly variable crustal thickness ranging from 20 to almost 50 km. In terms of spatial extension, this area is relatively small, but covers tectonically very differentiated domains making the analysis complex, with significant challenges in areas with less data coverage. Presently there is no complete 3-D crustal model of the Dinarides (and the surrounding areas). Using the compilations of previous studies, we have created vertically, and laterally varying crustal models defined on a regular grid for the wider area of the Dinarides, also covering parts of Adriatic Sea and SW part of the Pannonian Basin. In addition to the seismic velocities (P- and S-) and density, three interfaces in our model were defined -- Neogene deposits bottom, Carbonate complex bottom and Moho discontinuity. Neogene deposits and the Paleozoic to Eocene Carbonate complex rocks are not present in all areas of the model whereas Moho discontinuity depth is defined for the entire model. The newly derived model has been compared with the simple 1D model used for routine earthquake location in Croatia, and it proved to be a significant improvement. The model derived in this work represents the first step towards improving our knowledge of the crustal structure in the complex area of the Dinarides. We hope that the newly assembled model will be useful for all the forthcoming studies which require knowledge of the crustal structure. [ABSTRACT FROM AUTHOR]

Since the 1980s a number of active and passive seismic experiments have revealed significant information about the Earth's crust in the broader European Alpine region. In this paper, we use seismic waveform data from the AlpArray Seismic Network and three other temporary seismic networks, to perform receiver function (RF) calculations and time-to-depth migration to update the knowledge of the Moho discontinuity beneath the broader European Alps. In particular, we set up a homogeneous processing scheme to compute RFs using the time-domain iterative deconvolution method, and apply consistent quality control to yield 107,633 high-quality RFs. We then perform time-to-depth migration in a newly implemented 3D spherical coordinate system and using a European-scale reference P and S wave velocity model. This approach, together with the dense data coverage, provide us with a 3D migrated volume, from which we present migrated profiles that reflect the firstorder crustal thickness structure. We create a detailed Moho map by manually picking the discontinuity in a set of orthogonal profiles covering the entire area. We make the RF dataset, the software for the full processing workflow, as well as the Moho map, openly available; these open-access datasets and results will allow other researchers to build on the current study. [ABSTRACT FROM AUTHOR]

In the southeast of Bosnia and Herzegovina, the Berkovići earthquake sequence started with the mainshock on 22 April 2022 21:07 UTC at focal depth 22 km with magnitude M L = 6.0 (M w = 5.7). Our preliminary estimation of the mainshock's maximum intensity is VII EMS for Berkovići 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 M L ≥ 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 Berkovići mainshock hypocentre at the horizontal distance of c. 35 km. • The Berkovići earthquake sequence started with the mainshock on 22 Apr 2022 (21:07, M L = 6.0) at the focal depth of 22 km. • The earthquake catalogue (22 Apr 2022–22 Jan 2023) contains 7217 events with a completeness magnitude of 1.3. • Focal depths (15–30 km) are considerably larger than average for the Dinarides. • Focal mechanisms for three (of eight) earthquakes were due to normal faulting that is unexpected in this area. • Seismotectonic analysis hints the mainshock occurred on the blind ramp of the basal thrust of the Dalmatian tectonic unit. [ABSTRACT FROM AUTHOR]

The most recent major earthquake series struck near Petrinja (December 29th 2020 M 6.2), and triggered extensive ground failures in the wider area of Petrinja, Sisak and Glina. Coseismic ground failures including subsidence dolines, liquefaction and landslides have been documented over a large area by various experts and teams. These data are stored in the newly created inventory, which is openly presented in this paper. This inventory is administered and updated by the Croatian Geological Survey, and will be available online via a Web Map Service (WMS) (www.hgi-cgs.hr). The aim of the inventory is to not only provide data for the development of susceptibility maps and more detailed exploration for possible remediation measures, but also to define the priorities for immediate action. The earthquake triggered the rapid development of dropout dolines which endanger the local populations of the villages of Mečenčani and Borojevići. This is still an ongoing process in the vicinity of the houses and therefore in-situ exploration started immediately. Liquefaction related to alluvial sediments of the Sava, Kupa and Glina rivers occurred almost exclusively in loose and pure sands, and was accompanied by sand boils, subsidence and lateral spreading. Liquefaction also presents a greater hazard because settlement of houses and river embankments occurred. Lateral spreading caused failures of river flood embankments and natural river banks. According to the data known to date, the majority of the coseismic landslides were reactivated with minor displacements. Despite that, it has been recognised that houses at the edge, or in landslide colluvium suffered greater damage than other houses located outside the landslide impact zone. [ABSTRACT FROM AUTHOR]

The interaction between the Adriatic microplate (Adria) and Eurasia is the main driving factor in the central Mediterranean tectonics. Their interplay has shaped the geodynamics of the whole region and formed several mountain belts including Alps, Dinarides and Apennines. Among these, Dinarides are the least investigated and little is known about the underlying geodynamic processes. There are numerous open questions about the current state of interaction between Adria and Eurasia under the Dinaric domain. One of the most interesting is the nature of lithospheric underthrusting of Adriatic plate, e.g. length of the slab or varying slab disposition along the orogen. Previous investigations have found a low-velocity zone in the uppermost mantle under the northern-central Dinarides which was interpreted as a slab gap. Conversely, several newer studies have indicated the presence of the continuous slab under the Dinarides with no trace of the low velocity zone. Thus, to investigate the Dinaric mantle structure further, we use regional-to-teleseismic surface-wave records from 98 seismic stations in the wider Dinarides region to create a 3D shear-wave velocity model. More precisely, a two-station method is used to extract Rayleigh-wave phase velocity while tomography and 1D inversion of the phase velocity are employed to map the depth dependent shear-wave velocity. Resulting velocity model reveals a robust high-velocity anomaly present under the whole Dinarides, reaching the depths of 160 km in the north to more than 200 km under southern Dinarides. These results do not agree with most of the previous investigations and show continuous underthrusting of the Adriatic lithosphere under Europe along the whole Dinaric region. The geometry of the down-going slab varies from the deeper slab in the north and south to the shallower underthrusting in the center. On-top of both north and south slabs there is a low-velocity wedge indicating lithospheric delamination which could explain the 200 km deep high-velocity body existing under the southern Dinarides. • Rayleigh-wave phase velocity in the wider Dinarides region using the two-station method. • Uppermost mantle shear-wave velocity model of the Dinarides-Adriatic Sea region. • Velocity model reveals a robust high-velocity anomaly present under the whole Dinarides. • High-velocity anomaly reaches depth of 160 km in the northern Dinarides to more than 200 km under southern Dinarides. • New structural model incorporating delamination as one of the processes controlling the continental collision in the Dinarides. [ABSTRACT FROM AUTHOR]

The basic outline of the Dinarides formation evolves around the interaction between Adria microplate and European mainland due to the north-east movement of Adria. This interaction included many complex tectonic processes such as subduction, collision and extension. The Dinaric mountain belt is one of the most under-researched parts of the Alpine-Mediterranean collision zone. So far, most of the regional seismic studies were focused on the upper-middle crust while the deeper structures remained poorly investigated and the regional geodynamics is still not fully understood. In order to get more details about the regional tectonic history, we use seismic surface wave data and investigate uppermost mantle structures beneath the Dinarides. We analyzed more than 250 teleseismic earthquakes recorded at 74 stations and used a two-station method to get Rayleigh wave dispersion measurement on an inter-station path length larger than one wavelength. We measured surface wave phase velocities at discrete periods in the range from 20 s to 160 s and inverted these results to obtain shear-wave velocity model. Our preliminary results show negative velocity anomaly under the North-West Dinarides and positive anomaly beneath the Pannonian basin and the Adriatic sea, clearly supporting results from previous studies and strengthening the premise of a slab gap existence in the North-West Dinarides. [ABSTRACT FROM AUTHOR]

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 10 000 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 epicentral 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. [ABSTRACT FROM AUTHOR]

This paper presents direct-seismogram inversion (DSI) for receiver-side structure which treats the source signal incident from below (the effective source-time function-STF) as a vector of unknown parameters in a Bayesian framework. As a result, the DSI method developed here does not require deconvolution by observed seismogram components as typically applied in receiverfunction inversion and avoids the problematic issue of choosing subjective tuning parameters in this deconvolution. This results in more meaningful inversion results and uncertainty estimation compared to classic receiver-function inversion. A rigorous derivation is presented of the likelihood function required for unbiased inversion results. The STF is efficiently inferred by a maximum-likelihood closed-form expression that does not require deconvolution by noisy waveforms. Rather, deconvolution is only by predicted impulse responses for the unknown environment (considered to be a 1-D, horizontally stratified medium). For a given realization of the parameter vector which describes the medium below the station, data predictions are computed as the convolution of the impulse response and the maximum-likelihood source estimate for that medium. Therefore, the assumption of a Gaussian pulse with specified parameters, typical for the prediction of receiver functions, is not required. Directly inverting seismogram components has important consequences for the noise on the data. Since the signal processing does not require filtering and deconvolution, data errors are less correlated and more straightforward to model than those for receiver functions. This results in better inversion results (parameter values and uncertainties), since assumptions made in the derivation of the likelihood function are more likely to be met by the inversion process. The DSI method is demonstrated for simulated waveforms and then applied to data for station Hyderabad on the Indian craton. The measured data are inverted with both the new DSI and traditional receiverfunction inversion. All inversions are carried out for a trans-dimensional model that treats the number of layers in the model as unknown. Results for DSI are consistent with previous studies for the same location. The DSI has clear advantages in trans-dimensional inversion. Uncertainty estimates appear more realistic (larger) in both model complexity (number of layers) and in terms of seismic velocity profiles. Receiver-function inversion results in more complex profiles (highly-layered structure) and suggests unreasonably small uncertainties. This effect is likely also significant when the parametrization is considered to be fixed but exacerbated for the trans-dimensional model: If hierarchical errors are poorly estimated, trans-dimensional models overestimate the structure which produces unfavourable results for the receiver-function inversion. [ABSTRACT FROM AUTHOR]