Search Results (12)

The advanced determination of the type (foreshock–aftershock–swarm) of an ongoing seismic cluster is quite challenging; only retrospective solutions have thus far been proposed. In the period of January–March 2025, a seismic cluster, recorded between Santorini volcano and Amorgos Island, South Aegean Sea, caused considerable social concern. A rapid increase in both the seismicity rate and the earthquake magnitudes was noted until the mainshock of M_L = 5.3 on 10 February; afterwards, activity gradually diminished. Fault-plane solutions indicated SW-NE normal faulting. The epicenters moved with a mean velocity of ~0.72 km/day from SW to NE up to the mainshock area at a distance of ~25 km. Crucial questions publicly emerged during the cluster. Was it a foreshock–aftershock activity or a swarm of possibly volcanic origin? We performed real-time discrimination of the cluster type based on a daily re-evaluation of the space–time–magnitude changes and their significance relative to background seismicity using earthquake statistics and the topological metric betweenness centrality. Our findings were periodically documented during the ongoing cluster starting from the fourth cluster day (2 February 2025), at which point we determined that it was a foreshock and not a case of seismic swarm. The third day after the M_L = 5.3 mainshock, a typical aftershock decay was detected. The observed foreshock properties favored a cascade mechanism, likely facilitated by non-volcanic material softening and the likely subdiffusion processes in a dense fault network. This mechanism was possibly combined with an aseismic nucleation process if transient geodetic deformation was present. No significant aftershock expansion towards the NE was noted, possibly due to the presence of a geometrical fault barrier east of the Anydros Ridge. The 2025 activity offered an excellent opportunity to investigate deciphering the type of ongoing seismicity cluster for real-time discrimination between foreshocks, aftershocks, and swarms. Full article

This study presents the living coccolithophore communities and the morphological variability of Emiliania huxleyi in the South Aegean Sea from three sampling regions during winter-early spring (March 2017, March 2019) and summer (August 2019). Emphasis is given to March 2017 to monitor the variations in coccolithophore assemblages after an exceptionally cold event in December 2016, which resulted in newly produced dense waters that ventilated the Aegean deep basins. The assemblages displayed distinct seasonality with the predominance of E. huxleyi and Syracosphaera molischii during winter-early spring, associated with the water column mixing. By contrast, summer assemblages were featured by holococcolithophores and typical taxa of warm, oligotrophic upper waters. It seems that the phytoplanktonic succession as well as the nutrient supply to the upper euphotic layers were affected by the water column perturbation during the extreme winter of 2016–2017, which led to strong convective mixing and dense water formation. The decreased coccosphere densities during March 2017, accompanied by the notable presence of diatoms, were most probably associated with a prolonged diatom bloom, causing delay in the development of the coccolithophore community and resulting in a nitrogen-limited setting. Emiliania huxleyi morphometry showed the characteristic seasonal calcification trend of the Aegean, with the dominance of smaller coccoliths in the summer and increased coccolith length and width during the cold season. The intense cold conditions and wind-induced mixing during the winter of 2016–2017 possibly increased the absorption of atmospheric CO₂ in surface waters, causing increased acidity and the subsequent presence of etched/undercalcified E. huxleyi coccoliths and other taxa, most probably implying in situ calcite dissolution. Full article

The dispersion of microplastics in the sea is an emerging and crucial environmental problem. In this preliminary study, the hydrodynamics of microplastics transferred from flooded agricultural areas to the sea was assessed. The Daniel storm in 2023 in region of Thessaly, Greece, initiated the transfer of plastic debris via the Pinios River, which subsequently discharged to the coastal basin at the south area of Thermaikos Gulf (NW Aegean Sea). Field sampling and laboratory measurements of microplastics collected at the mouth of the Pinios were conducted. The dispersion of microplastics discharged by the Pinios River is subject to the dominant wind conditions over the area, which in turn determines the water circulation in the NW Aegean Sea. Thus, a hydrodynamic model was initially applied, followed by a transport model for the study of the dispersion of the microplastics. The models were applied for SW and NE winds and indicated that the majority of microplastics with a settling velocity 0.1 m/s accumulate in areas close to the river’s mouth or lateral coastal zones; however, under the influence of SW winds, minor quantities tend to reach the east coasts of the Thermaikos Gulf, while massive quantities are transported away from the river’s mouth in case of microplastics floating on the sea’s surface. Full article

The objective of this study is to compare the most common methods for forecasting wave characteristics (height and period) for the maximum in height 1/3, 1/10, and 1/100 of the waves, using available wind and wave datasets. The testing marine region is the Central Aegean Sea and, in particular, the east coast of Evia Island that is exposed to N, NE, E, and SE incoming offshore waves. The estimated values of wave characteristics resulting from three different wind datasets and the two wave datasets present differences of 2–60%. These variations are attributed to the different methodological approaches followed, to the different types of raw data in terms of space and time periods, and to the fetch limitations. The proposed methodology for the calculation of wave characteristics for the highest (1/10) and (1/100) waves appears to yield reasonable outcomes when compared to the corresponding significant (highest 1/3) wave heights and periods. Finally, mean values of wave height and period calculated from the five available datasets, considering their standard deviation, seem to be more representative than the values calculated separately from each dataset. Full article

This study demonstrates a rich complexity of the time–frequency ionospheric signal spectrum, dependent on the measurement type and platform. Different phenomena contributing to satellite-derived and ground-derived geophysical data that only selected signal bands can be potentially sensitive to seismicity over time, and they are applicable in lithosphere–atmosphere–ionosphere coupling (LAIC) studies. In this study, satellite-derived and ground-derived ionospheric observations are filtered by a Fourier-based band-pass filter, and an experimental selection of potentially sensitive frequency bands has been carried out. This work focuses on band-pass filtered ionospheric observations and seismic activity in the region of the Aegean Sea over a two-year time period (2020–2021), with particular focus on the entire system of tectonic plate junctions, which are suspected to be a potential source of ionospheric disturbances distributed over hundreds of kilometers. The temporal evolution of seismicity power in the Aegean region is represented by the record of earthquakes characterized by M ≥ 4.5, used for the estimation of cumulative seismic energy. The ionospheric response to LAIC is explored in three data types: short inspections of in situ electron density (Ne) over a tectonic plate boundary by Swarm satellites, stationary determination of three Ne density profile parameters by the Athens Digisonde station AT138 (maximum frequency of the F2 layer: foF2; maximum frequency of the sporadic E layer: foEs; and frequency spread: ff), and stationary measure of vertical total electron content (VTEC) interpolated from a UPC-IonSAT Quarter-of-an-hour time resolution Rapid Global ionospheric map (UQRG) near Athens. The spectrograms are made with the use of short-term Fourier transform (STFT). These frequency bands in the spectrograms, which show a notable coincidence with seismicity, are filtered out and compared to cumulative seismic energy in the Aegean Sea, to the geomagnetic Dst index, to sunspot number (SN), and to the solar radio flux (F10.7). In the case of Swarm, STFT allows for precise removal of long-wavelength Ne signals related to specific latitudes. The application of STFT to time series of ionospheric parameters from the Digisonde station and GIM VTEC is crucial in the removal of seasonal signals and strong diurnal and semi-diurnal signal components. The time series formed from experimentally selected wavebands of different ionospheric observations reveal a moderate but notable correlation with the seismic activity, higher than with any solar radiation parameter in 8 out of 12 cases. The correlation coefficient must be treated relatively and with caution here, as we have not determined the shift between seismic and ionospheric events, as this process requires more data. However, it can be observed from the spectrograms that some weak signals from selected frequencies are candidates to be related to seismic processes. Full article

Shoreline configurations are a complex outcome of the dynamic interplay between natural forces and human actions. This interaction shapes unique coastal morphologies and affects sediment transport and erosion patterns along the coastline. Meanwhile, ephemeral river systems play a vital role in shaping coastlines and maintaining ecosystem sustainability, especially in island settings. In this context, the present study seeks to develop a holistic approach that views coast and watershed systems as a continuum, aiming to investigate their relationships in an island environment, while accounting for human interventions in the river regime. For this task, the empirical USLE method was employed to quantify sediment production and transport from the catchment area to the coast, while hydraulic simulations using HEC-RAS were conducted to assess sediment retention within flood-affected areas. Moreover, coastal vulnerability to erosion was evaluated by applying the InVEST CVI model in order to identify areas at risk from environmental threats. The coastal zone of Petra–Molyvos, Lesvos, Greece, was selected as the study area due to ongoing erosion issues, with particular emphasis on its interaction with the Petra stream as a result of significant human intervention at its mouth. According to the study’s findings, the examined coastal zone is highly vulnerable to combined erosion from wind and waves, while the river’s mouth receives only a small amount of sediment from water fluxes. Evidently, this leads to an increase in beach retreat phenomena, while highlighting the necessity for integrated coastal–watershed management. Full article

The present study examines the recent evolution of a cliff coast along the Aegean Sea, considering its geotectonic context, oceanographic factors, sediment dynamics, and human impact. Initially, the formation of this coastal stretch was influenced by neotectonic faults, oriented both semi-parallel and diagonally relative to the present coastline orientation (NE–SW). Subsequently, the delivery of terrestrial sediment from ephemeral rivers and cliff erosion, along with nearshore wave-induced hydrodynamics have played a secondary role in shaping its current configuration, which includes a beach zone along the base of the cliff. This secondary phase of coastal evolution occurred over the past 4–5 thousand years, coinciding with a period of slow sea level rise (approximately 1 mm/year). Evidence such as uplifted notches and beachrock formations extending to around 5 m water depth suggests intervals of relative sea level stability, interrupted by episodic tectonic events. Anthropogenic interventions, related to both changes in coastal sediment budget and coastal engineering projects, have caused beach erosion, particularly in its central and northern sectors. Full article

Satellite geodesy, an indispensable modern tool for determining upper-crust deformation, can be used to assess tectonically active structures and improve our understanding of the geotectonic evolution in tectonically active regions. A region fulfilling these criteria is the North Aegean, part of the Eastern Mediterranean. It is one of the most tectonically, and hence, seismically, active regions worldwide, which makes it ideal for applying a satellite geodesy investigation. Although many regional studies have been carried out across the entire Aegean region, there are three more focused case studies that provide better resolution for different parts of the North Aegean. The synthesis of these case studies can lead to an overall geodynamic assessment of the North Aegean. The North Aegean Sea case study is characterized by the North Aegean Trough (NAT), which is directly associated with the westward prolongation of the North Anatolian Fault (NAF). Both NE–SW normal and strike-slip faulting have been documented in this offshore region. Geodetic analysis considers geodetic data, derived from 32 permanent GPS/GNSS stations (recorded for the 2008–2014 time period). This results in the estimation of the Maximum (MaHE) and Minimum (MiHE) Horizontal Extension, Maximum Shear Strain (MSS) and Area Strain (AS) parameters, based on triangular methodology implementation; the same strain parameters have similarly been estimated for the Strymon and Thessalian basins, respectively. The Strymon basin (first case study) is located in the central part of the northern Greek mainland, and it is dominated by NW–SE (up to E–W) dip-slip normal faults; this area has been monitored by 16 permanent GPS/GNSS stations for seven consecutive years. Regarding the Thessalian basin case study, E–W, dip-slip and normal faults are noted at the basin boundaries and within the Thessalian plain. This region has also been monitored for seven consecutive years by 27 permanent GPS/GNSS stations. However, this case study is characterized by a strong seismic event (Mw6.3; 3 March 2021), and thus all strain parameters depicted the pre-seismic deformation. Analysis of these three different case studies confirmed the current tectonic setting of the North Aegean region, while revealing new aspects about the geodynamic evolution of the wider region, such as highlighting areas with significant tectonic activity and the crucial role of strike-slip faulting in the broader Aegean region. Full article

The present contribution aims to give an insight into the main terrestrial and marine processes leading to delta flooding in the case of the transboundary Evros delta, located at the microtidal NE Aegean Sea, on the basis of recorded flood events in the Evros deltaic plain. The prevailing weather conditions at the onset of the event, along with sea-level rise above the mean state, portray the mechanism for the development of compound flood events and subsequent riparian flooding. This system blocks the riverine water’s seaward exit, resulting in the flooding of the lower deltaic plain. The river discharge is recognized as a secondary factor acting mainly toward the persistence of the events. Several limitations restrict the quantification potential of the relative contribution of the key factors to the development, onset, and duration of a flood. Mitigation of the impacts of such flood events requires intercountry cooperation and a management plan based on a network of environmental monitoring. Full article

A hydrographic survey of the southwestern coastal margin of Lesvos Island (Greece) was conducted by the Naftilos vessel of the Hellenic Hydrographic Service. The results have been included in a bathymetric map and morphological slope map of the area. Based on the neotectonic and seismotectonic data of the broader area, a morphotectonic map of Lesvos Island has been compiled. The main feature is the basin sub-parallel to the coast elongated Lesvos Basin, 45 km long, 10–35 km wide, and 700 m deep. The northern margin of the basin is abrupt, with morphological slopes towards the south between 35° and 45° corresponding to a WNW-ESE normal fault, in contrast with the southern margin that shows a gradual slope increase from 1° to 5° towards the north. Thus, the main Lesvos Basin represents a half-graben structure. The geometry of the main basin is interrupted at its eastern segment by an oblique NW-SE narrow channel of 650 m depth and 8 km length. East of the channel, the main basin continues as a shallow Eastern Basin. At the western part of the Lesvos margin, the shallow Western Basin forms an asymmetric tectonic graben. Thus, the Lesvos southern margin is segmented in three basins with different morphotectonic characteristics. At the northwestern margin of Lesvos, three shallow basins of 300–400 m depth are observed with WNW-ESE trending high slope margins, probably controlled by normal faults. Shallow water marine terraces representing the last low stands of the glacial periods are observed at 140 m and 200 m depth at the two edges of the Lesvos margin. A secondary E-W fault disrupts the two terraces at the eastern part of the southern Lesvos margin. The NE-SW strike-slip fault zone of Kalloni-Aghia Paraskevi, activated in 1867, borders the west of the Lesvos Basin from the shallow Western Basin. The Lesvos bathymetric data were combined with those of the eastern Skyros Basin, representing the southern strand of the North Anatolian Fault in the North Aegean Sea, and the resulted tectonic map indicates that the three Lesvos western basins are pull-aparts of the strike-slip fault zone between the Skyros Fault and the Adramytion (Edremit) Fault. The seismic activity since 2017 has shown the co-existence of normal faulting and strike-slip faulting throughout the 90 km long Lesvos southern margin. Full article

On 30 October 2020, a strong earthquake of magnitude 7.0 occurred north of Samos Island at the Eastern Aegean Sea, whose earthquake mechanism corresponds to an E-W normal fault dipping to the north. During the aftershock period in December 2020, a hydrographic survey off the northern coastal margin of Samos Island was conducted onboard R/V NAFTILOS. The result was a detailed bathymetric map with 15 m grid interval and 50 m isobaths and a morphological slope map. The morphotectonic analysis showed the E-W fault zone running along the coastal zone with 30–50° of slope, forming a half-graben structure. Numerous landslides and canyons trending N-S, transversal to the main direction of the Samos coastline, are observed between 600 and 100 m water depth. The ENE-WSW oriented western Samos coastline forms the SE margin of the neighboring deeper Ikaria Basin. A hummocky relief was detected at the eastern margin of Samos Basin probably representing volcanic rocks. The active tectonics characterized by N-S extension is very different from the Neogene tectonics of Samos Island characterized by NE-SW compression. The mainshock and most of the aftershocks of the October 2020 seismic activity occur on the prolongation of the north dipping E-W fault zone at about 12 km depth. Full article

The assessment of seawater quality in coastal areas is an important issue as it is related to the welfare of coastal ecosystems, a prerequisite for the provision of the related ecosystem services. During the last decades, marine eutrophication has become an important problem in coastal waters as a result of nutrient inputs increase. Consequently, there is need for appropriate methods and tools to assess the eutrophication status of seawater which should be user-friendly to coastal managers and support the adoption of effective plans for the protection and sustainable development of the coastal environment. In this framework, a user-friendly webGIS application has been developed and the Strait of Mytilene at the southeastern part of the Island of Lesvos in the NE Aegean Sea, Greece, was used as a case study. The methodology includes, as a first step, the evaluation of the accuracy of spatial interpolators widely applied in oceanographic studies for assessing the spatial distribution of relevant variables. The most appropriate interpolator revealed for each variable is subsequently applied for the production of the representative thematic layer. The second step involves the integration of the information from the optimal thematic layers representing the spatial distributions of the variables under study; as a result, a new thematic layer illustrating the eutrophication status of the study area is produced. The webGIS application is fully available via a web browser and provides a number of geoprocessing modules developed in Python which implement the user interface, the application of the interpolation analytical tasks, the statistical evaluation toolset and the integration of the optimal interpolated layers. Suggestions for further improvement of the proposed webGIS application are discussed. Full article