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ORFEUS (Observatories and Research Facilities for European Seismology, orfeus-eu.org) is a non-profit foundation that coordinates and promotes seismology in the Euro-Mediterranean area and beyond, through harmonized collection, archival and distribution of seismic waveform data, metadata, alongside offering services and products managed at national level by more than 60 participating seismological Institutions. ORFEUS is one of the founding members of EPOS Seismology (www.epos-eu.org/tcs/seismology) and a fundamental partner of EC-funded projects. ORFEUS services are largely integrated in the EPOS Data Access Portal (www.ics-c.epos-eu.org). The key goals of ORFEUS (Bylaws, Article I.3: orfeus-eu.org/documents//ORFEUS_Bylaws_September_2022.pdf) are achieved through the development and maintenance of data services targeted to a broad community of seismological data users. ORFEUS comprises: (i) the European Integrated waveform Data Archive (EIDA; orfeus-eu.org/data/eida); (ii) the European Strong-Motion databases (orfeus-eu.org/data/strong); and iii) the recently established group representing the community of European mobile pools, including amphibian instrumentation (orfeus-eu.org/data/mobile). Products and services for computational seismology are also considered for integration in the ORFEUS domain. Currently, ORFEUS services provide access to the waveforms acquired by ~18,000 stations in the Euro-Mediterranean region, including dense temporary experiments (e.g., AlpArray, AdriaArray), with strong emphasis on open, high-quality data. Access to data and products is ensured through state-of-the-art information and communication technologies, with strong emphasis on federated web services, clear policies and licenses, and acknowledging the crucial role played by data providers. Significant efforts are underway, by ORFEUS participating institutions, to enhance the existing services to tackle the challenges posed by the Big Data Era, and to actively encourage interoperability and integration of multidisciplinary datasets in seismological and Earth Science workflows. ORFEUS also implements Community services that include software and travel grants, webinars, workshops and editorial initiatives. ORFEUS data and services are assessed and improved through the technical and scientific feedback of a User Advisory Group, which comprises European Earth scientists with expertise on a broad range of disciplines.

The 30 October 2020 Samos earthquake (Mw 7.0) ruptured an east–west striking, north dipping normal fault located offshore the northern coast of Samos Island, previously inferred from the bathymetry and regional tectonics. This fault, reported in the fault-databases as the North Samos and/or Kaystrios Fault, ruptured with almost pure dip-slip motion, in a region where both active extension and strike-slip deformation coexist. Historical information for the area confirms that similar ~ Mw7 events had also occurred in the broader Samos area, though none of the recent (last ~ 300 years) mainshocks appears to have ruptured the same fault. The spatial and temporal distribution of relocated aftershocks indicates triggering of nearby strike-slip and normal fault segments, situated in the areas where static stress has increased due to the mainshock generation. The relocated aftershocks and the slip model indicate that the sequence ruptured the upper crust (mainly the depth range 3–15 km). The top of the rupture plane nearly reached the sea bottom, located at a depth of

We explore a hypothetical zero-latency earthquake early warning (EEW) system in Greece, aiming to provide alerts before warning thresholds of the intensity of ground motion are exceeded. Within the seismotectonic context of Greece, both shallow- and intermediate-depth earthquakes (along the Hellenic subduction zone) are plausible and, thus, examined. Using regionally applicable attenuation relations, we combine and adjust the methodologies of Minson et al. (2018) and Hoshiba (2020) to examine what are the minimum magnitudes required to invoke the warning thresholds at the user site. With simple modeling, we examine how fast an alert can be issued and what is the available warning time when taking into account delays due to finite-fault rupture propagation, alongside other delays. These computations are merged with delays introduced due to the present-day configuration of the Greek national monitoring network (varying spatial density of permanent monitoring stations). This approach serves as a tool to assess the feasibility of an EEW system at specific sites and to redesign the national permanent monitoring network to serve such a system more effectively (we provide results for four sites.). Warning times for on-land crustal earthquakes are found to be shorter, whereas for intermediate-depth earthquakes in Greece an EEW system is feasible (provides warning times of several tens of seconds at large cities, e.g., on Crete Island) even with the current configuration of the national monitoring network, which is quite sparse in the southern part of the country. The current network configuration also provides sufficient early warning (e.g., of the order of 10 s for a warning threshold of 0.05g) at the center of Athens from earthquakes of the eastern Gulf of Corinth—a zone posing elevated hazard in the broader area of the Greek capital. Several additional assumptions and factors affecting the operability of an EEW system in Greece (i.e., source process complexity and uncertainty in attenuation laws) are also discussed.

The National Observatory of Athens data center for the European Integrated Data Archive (EIDA@NOA) is the national and regional node that supports International Federation of Digital Seismograph Networks and related webservices for seismic waveform data coming from the southeastern Mediterranean and the Balkans. At present, it serves data from eight permanent broadband and strong-motion networks from Greece and Cyprus, individual stations from the Balkans, temporary networks and aftershock deployments, and earthquake engineering experimental facilities. EIDA@NOA provides open and unlimited access from redundant node end points, intended mainly for research purposes (see Data and Resources). Analysis and quality control of the complete seismic data archive is performed initially by calculating waveform metrics and data availability. Seismic ambient noise metrics are estimated based on power spectral densities, and an assessment of each station’s statistical mode is achieved within each network and across networks. Moreover, the minimum ambient noise level expected for strong-motion installations is defined. Sensor orientation is estimated using surface-wave polarization methods to detect stations with misalignment on particular epochs. A single data center that hosts the complete seismic data archives with their respective metadata from networks covering similar geographical areas allows coordination between network operators and facilitates the adhesion to widely used best practices regarding station installation, data curation, and metadata definition. The overall achievement is harmonization among all contributing networks and a wider usage of all data archives, ultimately strengthening seismological research efforts in the region.

On 3 March 2021, the Mw6.3 Tyrnavos earthquake shook much of the Thessalia region, leading to extensive damage in many small towns and villages in the activated area. The first main shock was followed in the next day, on 4th of March 2021, by an “equivalent” main shock with Mw6.0 in the adjacent fault segment. These are the largest earthquakes to strike the northeastern part of Thessalia since the M6.3, 1941 Larissa earthquake. The main shocks triggered extensive liquefaction mainly along the banks of the Titarisios tributary where alluvial flood deposits most probably amplified the ground motions. Our seismic monitoring efforts, with the use of recordings of the regional seismological network along with a dense local network that was installed three days after the seismic excitation initiation, led to the improved understanding the geometry and kinematics of the activated faults. The aftershocks form a north–northwest–trending, east–northeast–dipping, ~40 km long distribution, encompassing the two main ruptures along with minor activated structures, consistent with the rupture length estimated from analysis of regional waveform data and InSAR modeling. The first rupture was expanded bilaterally, the second main shock nucleated at its northern tip, where from this second rupture propagated unilaterally to the north–northwest. The focal mechanisms of the two main shocks support an almost pure normal faulting, similar to the aftershocks fault plane solution determined in this study. The strong ground motion of the March 3 main shock was computed with a stochastic simulation of finite fault model. Coseismic displacements that were detected using a dense GPS / GNSS network of five permanent stations located the Thessaly region, have shown an NNE–SSW extension as expected from the nature and location of the causative fault. Coulomb stress changes due to the coseismic slip of the first main shock, revealed that the hypocentral region of the second main shock was brought closer to failure by more than 10 bars.

The 30 October 2020 M7 Samos earthquake occurred offshore the Greece-Turkey cross border region, and will be recalled as among the deadliest (118 fatalities) that affected both countries. It genera...

The focus of this study is Anthemountas basin in northern Greece, where land subsidence phenomena had been previously identified. The basin has a NNW-SSE orientation, which is optimal to the currently acting ~ N-S extension field in the back-arc Aegean area and is bordered to its north and south edge by two, well-pronounced faults. The region of study is located close to the greater metropolitan city of Thessaloniki and comprises a number of critical facilities including the city’s international airport. The research object is the monitoring of the evolution of the displacement at Anthemountas graben, using InSAR time-series and the investigation of the dominant driving mechanism. We show new surface displacement results from the ENVISAR satellite, and overall, an enhanced dataset of SAR time-series is presented, spanning the period 1992 to 2010, by using the satellites ERS1, ERS2 and ENVISAT. Results indicate a continuously deforming environment in both decades, with an increasing magnitude. The detailed study of the deformation pattern together with the analysis of in situ data defines aquifer overpumping as the main cause of the detected displacement of both decades. Critical regions are examined in detail, as for example: an area close to Thermi, the town of Perea and the Thessaloniki’s international airport, a major hub of the Balkans. The latter was subjected to an increasing deforming velocity during the monitoring period. Overall, we conclude that a water management plan should be of high priority for the area.

Accurate VS30 estimates are increasingly important in seismically active areas due to their efficacy in site effect analysis for seismic hazard assessment as well as microzonation evaluation and shake maps development. A comparison between different approaches for the time-averaged shear-wave velocity (VS) in the upper 30 m (VS30) is presented. In this study of VS30 a site-to-site or averaged zonation value technique comparison is assessed for proxy-based and measured or assigned values, respectively. Three urban areas of Edessa, Grevena and Xanthi in Northern Greece were selected for this study. The merit of this selection was based on the historical seismicity, geology and geomorphology of these areas. This paper highlights the strong correlation between the proxy-based approach and the assigned average values in zones of common geophysical and geotechnical characteristics within the frame of EC8 site characterization. In addition, on site-by-site comparison proxy-based VS30 values, agree with the ones derived from measured geophysical data. With the exception of areas where shear wave velocity has a high rate increase with depth, as well as, in shallow bedrock above 30 m the proxy tends to underestimate the VS30 value. Higher disagreements are reported when stiffer formations are denoted through geotechnical mapping that are not clarified in the geological map.

On 12 June 2017 (UTC 12:28:38.26) a magnitude Mw 6.3 earthquake occurred offshore Lesvos Island in SE Aegean Sea, which was widely felt, caused 1 fatality, and partially ruined the village of Vrisa on the south-eastern coast of the island. I invert broad band and strong motion waveforms from regional stations to obtain the source model and the distribution of slip onto the fault plane. The hypocentre is located at a depth of 7 km in the upper crust. The mainshock ruptured a WNW-ESE striking, SW dipping, normal fault, projecting offshore and bounding the Lesvos Basin. The strongest and most aftershocks clustered away from the hypocentre, at the eastern edge of the activated area. This cluster indicates the activation of a different fault segment, exhibiting sinistral strike-slip motions, along a plane striking WNW-ESE. The slip of the mainshock is confined in a single large asperity, WNW from the hypocentre, with dimensions 20 km × 10 km along fault strike and dip, respectively. The average slip of the asperity is ~ 50 cm and the peak slip is ~ 1 m. The rupture propagated unilaterally towards WNW to the coastline of Lesvos island at a relatively high speed (~ 3.1 km/s). The imaged slip model and forward modelling was used to calculate peak ground velocities (PGVs) in the near-field. The damage pattern produced by this earthquake, especially in the village of Vrisa is compatible with the combined effect of rupture directivity, proximity to the slip patch and the fault edge, spectral content of motions, and local site conditions.

For routine practice, we need simple tools to reliably identify earthquakes with large isotropic (ISO) components. This study aims to highlight a possible indicator. Non‐double‐couple (non‐DC) components of moment tensors (MTs) play a key role in our understanding of faulting earthquake processes and/or in identifying explosions. As opposed to DC components of the calculated seismic source model, the non‐DC components (compensated linear vector dipole and ISO) are more vulnerable to errors in location, inaccurate velocity modeling, and noise. Methods for analyzing resolvability of ISO are relatively complicated. We propose a simple procedure to identify an earthquake with a strong ISO component. Recent MT determinations include space and time grid search of the centroid position, mainly the depth and time. The centroid is identified with a trial source position that maximizes correlation between real and synthetic waveforms. In synthetic tests with varying ISO percentage, we compare the correlation‐depth dependence for two types of MT inversion: full and deviatoric. We show that in the inversion of data with a significant ISO component under the deviatoric assumption (i.e., when ISO is neglected), we might obtain an inaccurate centroid depth. However, when we make the grid search twice, under the deviatoric‐MT and full‐MT assumptions, and compare the results, we can obtain an indication of the significant ISO and avoid depth bias. This straightforward method is applied to two shallow earthquakes in Greece (the 27 January 2012 M w 5.3 Cretan Sea earthquake and the 26 June 2009 M w 4.9 Santorini earthquake).

We use teleseismic data to calculate the source model of the 24 May 2014 earthquake and regional catalogues to examine the spatial-temporal characteristics of the sequence. The sequence started in Saros Basin but almost simultaneously aftershocks spread along a ∼200 km zone, activating the entire North Aegean Trough. The aftershock sequence was rich in moderate (M 19 Nm (Mw6.8). This earthquake highlighted the fact that strike-slip faulting in the North Aegean Sea, can attain large lengths and activate very wide zones, reaching densely populated regions.

The Mw6.4 earthquake sequence of 2015 in western Greece is analyzed using seismic data. Multiple point source modeling, nonlinear slip patch, and linear slip inversions reveal a coherent rupture image with directivity toward the southwest and several moment release episodes, reflected in the complex aftershock distribution. The key feature is that the 2015 earthquake ruptured a strong asperity, which was left unbroken in between two large subevents of the Mw6.2 Lefkada doublet in 2003. This finding and the well-analyzed Cephalonia earthquake sequence of 2014 provide strong evidence of segmentation of the major dextral Cephalonia-Lefkada Transform Fault (CTF), being related to extensional duplex transform zones. We propose that the duplexes extend farther to the north and that the CTF runs parallel to the western coast of Lefkada and Cephalonia Islands, considerably closer to the inhabited islands than previously thought. Generally, this study demonstrates faulting complexity across short time scales (earthquake doublets) and long time scales (seismic gaps).

We combine probabilistic and deterministic approaches in an attempt to tackle a challenging topic in engineering seismology, that of the prediction of strong ground motion in urban areas located close to active faults, with sparse modern seismicity. The case we study is the city of Xanthi in northern Greece and we generate bedrock synthetic time series to be used as input motions into site-specific site effect analysis. We re-assess the seismic hazard using the probabilistic method and then perform hazard deaggregation in order to define realistic earthquake scenarios, which we examine in a deterministic way. The scenarios examined are: (a) a reference earthquake of M6.7, return period 100 years and at ~30 km distance from the city and (b) a reference earthquake of M5.8, return period 475 years and at ~3 km distance from the city of Xanthi. To compute synthetic time series for the distant source, we apply the stochastic method for finite faults. For the near-fault earthquake scenario we adopt a hybrid deterministic-stochastic approach that includes the semi-analytical method implemented in the COMPSYN code for the low frequency (~ 3 Hz). The results of the probabilistic and the deterministic approaches converge to a satisfactory degree.

Two case studies, in northern Greece, that monitor surface deformation were presented. Both are related to aquifer overexploitation and its impact on the earth’s surface. Using the archive of ERS and ENVISAT satellites, radar time series were performed applying the PS and SBAS techniques. For the justification of the remote sensing results and the interpretation of the physical mechanism behind the detected deformation, in situ data were also exploited. The areas of focus are: the village of Kalochori, a significant industrial hub, and the Anthemountas basin, an active tectonic region, both located in the vicinity of the metropolitan area of Thessaloniki. The Kalochori case indicated that the aquifer activity is directly affecting the surface movements. A low underground water level in the 90’s caused a subsidence of more than 20 mm/year and the subsequent recharge of the aquifers in the 2000s caused a surface rebound with a rate of up to +12 mm/year. Regarding Anthemountas basin, the subsidence has a maximum rate of −18 mm/year in the 90’s, and the deformation appears to exist over the whole basin. In conclusion both cases reveal that northern Greece is suffering from a hazard imposed by anthropogenic causes.

We study the 26 January and 3 February, 2014 (~ Mw6) events in Cephalonia, combining weak and strong motion waveforms from regional and local stations. The hypocenter of the January 26 event is located at the southernmost tip of the Paliki Peninsula, at a depth of ~ 15 km. The centroid moment tensor (CMT) solution indicates rupture along a N20°E dextral strike-slip fault, dipping to the east. The hypocenter of the February 3 event is 10 km NNE of the first, at shallower depth (~ 5 km). The CMT solution of this event is highly uncertain. The kinematic slip model for the January 26 event indicates that the rupture was mainly confined to shallow depths, and it propagated upwards and towards NE. The major slip patches, when projected to the surface, cover the western part of the Paliki Peninsula and include the areas where surface ruptures were observed. Our preferred slip model for the event of February 3 is based on a published two-segment fault model. Although this is our preferred slip model, it is worth noting, that the single segment inversion provided a similar slip pattern. The rupture propagated predominantly southwards along both segments. The main slip episode on both segments occurred almost simultaneously. Total duration of the rupture propagation did not exceed 9 and 6 s, respectively. The 2014 Cephalonia doublet did not rupture the Cephalonia Transform Fault (CTF). The diffuse pattern of the aftershocks implies the activation of a network of faults on-shore the Paliki Peninsula, in accordance with the local stress field derived from aftershocks. The 2014 sequence has implications for the seismic hazard assessment: active faults in western Cephalonia exist on-shore; some have gentle dip angles; the strike-slip motions can be combined with thrust components; and the segmented ruptures may introduce time delays that increase the duration of strong ground shaking.

How continental areas deform has been a subject of intense research, especially in the prism of plate tectonics. The distribution of seismicity in continental areas is not confined to a single fault, but earthquakes are distributed over wide zones, hundreds or even thousands wide, which contain many faults. Within these deforming zones, as for example the Alpine-Himalayan Belt, there are blocks, which are relative aseismic, as the region of central Turkey, and are commonly referred to as microplates.

The 8 January 2013 Mw5.8 earthquake sequence south of Lemnos Island (East Aegean Sea) is investigated. The focal mechanisms of the strongest events, obtained by waveform modelling, together with the HypoDD relocated epicentres, clearly show the activation of a dextral strike-slip fault, no more than 14 km in length, trending N60°E, and extending in depth from approximately 3 to 25 km, with the best determined focal depths in the range 3 to ~ 13 km. The distribution of slip onto the fault plane shows a single patch of 7 × 8 km 2 , where the average and the peak slip are equal to 30 cm and 130 cm, respectively. The locus of the peak slip is well resolved to be ~ 2.5 km away from the hypocentre, towards SW. The slip model was used to forward model the distribution of Peak Ground Velocity (PGV in cm/s) in the near source. The PGV contours imply that if any directivity is associated with this event, this is towards SW. The sequence in study, is located in the eastward extension of the Agios Efstratios Fault Zone, which was the locus of the February 19, 1968 Mw7.2 earthquake, which produced surface rupture on the island itself. The Agios Efstratios Fault Zone terminates off shore, south of Lemnos Island. The 2013 sequence is not connected, in a strict sense, with the Agios Efstratios Fault. It indicates rupture of another fault, approximately parallel to it, and the characteristics of the sequence are in accordance with the regional tectonics and previous knowledge. The Coulomb stress change associated with the mainshock, is also examined to evaluate any significant enhance of stresses along the Agios Efstratios Fault Zone.

We propose a new approach to resolve the isotropic component of the seismic moment tensor and its uncertainty. In linearized inversion problems, where the earthquake or explosive-source location and origin time are fixed (e.g., assumed to be known), the uncertainty of the moment tensor can be studied through the eigenvalues and the eigenvectors of the design matrix, which allows the representation of the theo- retical misfit by means of a 6D error ellipsoid. Because the design matrix depends only on the structural model and receiver source geometry, the analysis can be performed using recorded seismic waveforms, or without. In the nonlinear inversion problems, where the free parameters are eight (e.g., the six elements of the moment tensor, depth, and origin time), we propose a waveform-inversion scheme in which the trace of the moment tensor varies systematically and the remaining seven free parameters are opti- mized for each specific value of the trace. In this way, a 1D experimental probability density function of the moment tensor trace is constructed. To demonstrate the appli- cability of the method, we apply it to two shallow earthquakes (Mw 4.9 and 4.7) with epicenters close to the Columbo volcano, located 20 km northeast of the island of Santorini, Aegean Sea, Greece. We use 15 near-regional (60-310 km) records at frequencies below 0.1 Hz and two alternative crustal models. We conclude that the main uncertaintiesare attributed to the crustal modeland tothe trade-off betweenthe isotropic component and the source depth.

The available data set of focal mechanisms in Albania and adjacent countries is enriched with the determination of source parameters for 27 earthquakes of moderate-size. The broad band waveforms recorded by the regional networks were used to do time-domain moment tensor inversion and also to invert for the slip distribution onto the fault plane for two events: one (11 February 2010) in the proximity of Lake Scutari (Albania-Montenegro borders) and the other (10 March 2010) in Serbia. Forward modelling is additionally applied, which incorporates synthetic Green's functions calculated for a 1D velocity model, to get a first-order evaluation of the distribution of strong ground shaking around the hypocenters. Forward modeling of strong ground shaking based on event-specific moment tensor and slip distribution indicates low levels of expected ground motion in accordance with observations. The new data confirm previous knowledge for the active tectonics of the region as for example the thrust/reverse faulting, along the low elevation areas of the foreland, and the sub-parallel normal faulting along the high elevation areas in the mountains.

The distribution of seismicity, faulting pattern and its effect on local geomorphology is examined for the islands of Lemnos, Aghios Efstratios, Lesvos, Chios, Samos and Ikaria of north-eastern Aegean Sea, Greece. The main active faults on each island are described in terms of their geometrical characteristics and geomorphology. Faults that comply with specific criteria (geological age, effect on relief, their geometrical relationship to the active stress field) have been characterized as active. We evaluated and reviewed published information, augmented with new field data for onshore faults, while the effects of faulting on the sea-floor and their probable association with recorded earthquakes were used to determine offshore faulting. The relation of active faulting to the stress pattern has been examined as well. It is shown that as the deformation changes gradually from transtensional in the north to extensional in the south, so does the active faulting pattern. The effect of the westernmost splays of the North Anatolian Fault Zone, the largest of which is the ~ 300 km long, North Aegean Trough, is profound due to their close vicinity, causing shearing in good agreement with the modeled principal displacement zone deformation pattern. Faulting in the area is controlled by the distance from the main dextral principal displacement zones: the northern part of the area is directly affected by the North Anatolian Fault Zone and its splays, while this effect gradually weakens in the central and southern areas. The geomorphology responds to this faulting, causing the formation of fault-parallel gulfs near Lesvos and Lemnos and fault-defined shorelines in the rest of the islands. Ikaria exhibits a notable fault-controlled tilted topography as the result of footwall uplift.

The industrial area of Oreokastro, NW of the city of Thessaloniki, is monitored using radar interferometry to determine the spatial evolution of the underlying ground deformation trends. Previous studies, using SAR data acquired between 1992 and 1999, have revealed subsidence; however, the driving mechanism has not been, so far, solidly explained. Here, SAR satellite data from ERS 1, 2 and ENVISAT missions, acquired between 1992 and 2010, are analysed to enhance our understanding of the ground displacement trends and provide a thorough interpretation of the phenomena. The analysis confirms a subsiding displacement pattern from 1992 to 1999, whereas the recent data indicate that after 2003 the motion direction has changed to uplift. This whole monitoring of subsidence and the subsequent uplift is a rarely documented phenomenon, and in the case of Oreokastro is not reflecting a natural process; on the contrary, the driver is anthropogenic, related to the regional aquifer activity. Our study also highlights the fact that the local faults act as groundwater barriers and captures the existence of a possible previously unknown tectonic structure.

The January 2012 moderate-size (Mw5.3) sequence in Cretan Basin is studied, in an attempt to shed light to the deformation pattern along the central part of the southern Aegean Sea, south of the Hellenic Volcanic Arc. The two strongest events of the earthquake sequence, occurred on January 26 (UTC 04:24 Mw5.0) and on January 27 (UTC 01:33; Mw5.3), with epicenters lying 25 km and 50 km SW of the Christiani Isles and Santorini volcanic fields, respectively. Moment tensor inversions revealed the operation of pure strike–slip motions, along N–S and E–W trending vertical planes. The deviatoric moment tensor solutions, in a number of cases, involved more than 30% non-double couple component. The seismicity was confined in a vertical column of small ( ~ 5 km) horizontal extent, whereas the best constrained depths were in the range of 5 to 14 km. The good station coverage and the striking similarity of the waveforms permitted directivity effects to be resolved, from the shape and amplitude of the apparent Source Time Functions (STFs), obtained through an empirical Green's function approach. The rupture directivity towards south, resolved the N–S trending plane as the fault plane for both of the two strongest events. This sequence provided evidence that the WNW–ESE trench-parallel extension accommodated within the overriding Aegean plate, which so far was well documented in the western and eastern parts, it is also operating in the central part of the southern Aegean Sea. The slip models for the two strongest events were simple, single patched and the calculated static stress drop was of the order of 35 bars, in agreement with the average value in the back-arc Aegean Sea region.

International audience; The January 2010 Efpalio earthquake sequence provides some key elements to enhance our view on the western Corinth Gulf tectonics. The sequence lasted almost six months, and included two Mw>5 strong events, both exhibiting normal faulting along approximately E-W trending planes. This paper attempts to construct a unified seismotectonic model of the sequence jointly interpreting earthquake locations, moment-tensors and slip inversions in terms of the possible activated fault planes. Previous studies have connected the prevailing microseismic activity to a major low-angle, north-dipping structure under the Corinth Gulf and the Efpalio sequence favors such a general trend. Moreover, it clearly shows the significance of the shallow activity, so far less recognized, and possibly connected to the relatively steep faults outcropping on the northern coast. The first 18 January 2010 Mw>5 event had almost no on-fault aftershocks and most likely it occurred on a 55 degrees south-dipping nodal plane. The early off-fault aftershocks formed two clusters roughly E-W trending, both of which are connected with normal faulting. Cross-sections revealed that the northernmost cluster is connected with a north-dipping structure, where the second 22 January 2010 Mw>5 event occurred. In addition, the very shallow parts of the faults (

We present a catalog of moment tensor (MT) solutions and moment magnitudes, Mw, for 119 shallow (h ≤ 40 km) earthquakes in Greece and its surrounding lands (34°N–42°N, 19°E–30°E) for the years 2006 and 2007, computed with the 1D Time-Domain Moment Tensor inversion method (TDMT_INV code of Dreger, 2003 ). Magnitudes range from 3.2 ≤ Mw ≤ 5.7. Green's functions (GF) have been pre-computed to build a library, for a number of velocity profiles applicable to the broader Aegean Sea region, to be used in the inversion of observed broad band waveforms (10–50 s). All MT solutions are the outcome of a long series of tests of different reported source locations and hypocenter depths. Quality factors have been assigned to each MT solution based on the number of stations used in the inversion and the goodness of fit between observed and synthetic waveforms. In general, the focal mechanisms are compatible with previous knowledge on the seismotectonics of the Aegean area. The new data provide evidence for strike-slip faulting along NW–SE trending structures at the lower part of Axios basin, close to the heavily industrialized, and presently subsiding, region of the city of Thessaloniki. Normal faulting along E–W trending planes is observed at the Strimon basin, and in Orfanou Gulf in northern Greece. A sequence of events in the east Aegean Sea close to the coastline with western Anatolia sheds light on an active structure bounding the north coastline of Psara–Chios Islands about 20–25 km in length exhibiting right lateral strike-slip faulting.

On 24 May 2009 (GMT 16:17) a moderate size (Mw5.2) earthquake sequence burst north of Lake Doirani along the Greek-FYROM borders and ruptured a N282°E striking normal fault of ∼ 6 km in length dipping at an angle of ∼ 60° to the north. The focal mechanisms for 25 of the stronger events, obtained through time-domain moment tensor inversion, reveal the prevalence of normal faulting along ∼ WNW–ESE striking planes, sometimes dip slip motions are combined with a considerable strike-slip component. Broad band waveforms from the Hellenic Unified Seismological Network (HUSN) and neighbouring networks were used to invert source time functions (STFs) and calculate the slip model for the strongest event (GMT 16:17). STFs were obtained through an empirical Green's function approach. No directivity could be clearly identified for this moderate magnitude event. Slip was released in a single patch of dimensions 6 km × 8 km, maximum slip is estimated at 15 cm, while average slip on the ruptured area (area of subfaults of non-zero slip) was 3 cm. One characteristic of the slip model was the large rise time, for a moderate size event, which was estimated to be 1.0 s, implying a slow slip-rate function. Brune's static stress drop is calculated ∼ 5 bars. It must be noted that the north dipping plane is identified as the fault plane, (slip vector trending N332°E) based on the model variance of the STFs inversion.

Περίπου 2000 μηχανισμοί γένεσης σεισμών που εκδηλώθηκαν στο Αιγαίο και τη γύρω περιοχή κατά το χρονικό διάστημα 1912-2006, με μεγέθη 1.5, Nearly 2,000 earthquake focal mechanisms in the Aegean Sea and the surroundings for the period 1912- 2006, for 1.5

The densely populated city of Thessaloniki (Northern Greece) is situated in the vicinity of active seismic faults, capable of producing moderate to strong earthquakes. The city has been severely affected by such events several times during the last 15 centu- ries. The most recent event occurred on 20 June 1978 (M6.5) in the Mygdonian graben, with an epicentral distance of about 30 km, causing extended damage in the city, with macroseismic intensities between MSK V+ and VIII+. The majority of buildings affected by the earthquake were of reinforced-concrete typology, typical to many southern European metropolitan areas. The source properties of the normal-faulting causative event and the source-to-city propaga- tion path are well known from previous studies. The soil structure under the metropolitan area of Thessaloniki is assigned NEHRP categories B, C, D on the basis of geotechnical and geologic information and single-station ambient-noise measurements. A finite source model and various rupture scenarios of the June 1978 earthquake are used to perform forward sto- chastic modeling of strong ground motion in terms of peak ground and spectral acceleration. Rock motion is assessed under the city and it is transferred to the surface in accordance with the respective soil category. A GIS tool is employed to compare the estimated strong-motion parameters with the observed detailed damage pattern induced by the 1978 earthquake. For selected natural periods, a satisfactory correlation is established between macroseismic inten- sity and peak ground and spectral acceleration, thus encouraging the application of stochastic modeling for generating realistic ground-shaking scenarios in metropolitan areas.

The July 2003 sequence in the Gulf of Saros (Northeastern Aegean Sea) is investigated, in terms of accurate event locations and source properties of the largest events. The distribution of epicenters shows the activation of a 25-km long zone, which extends in depth between 9 and 20 km. The major slip patch of the 6 July 2003 M-w 5.7 mainshock is confined in a small area (similar to 45 km(2)), which coincides with the deeper (12-20 km) part of the activated zone. The epicenters of the sequence follow the northern margin of the Saros depression. This observation supports recent studies, according to which the continuation of the Ganos fault in the Gulf of Saros does not coincide with the fault along the northern coast of the Gelibolu peninsula, but it is located at the northern boundary of the Saros depression. This is further supported by the fact that the focal mechanisms of the mainshock and of the largest aftershocks of the 2003 sequence imply almost pure dextral strike-slip faulting, whereas the fault bounding the Gulf of Saros to the south appears as a normal fault on seismic sections. Thus, we infer that the principle deformation zone consists of a major strike-slip fault, which lies close to the northern margin of the Saros depression and this fault could be regarded as the continuation of the northern branch of the North Anatolian Fault into the Saros Gulf and North Aegean Trough as suggested by regional tectonic models. The northeastern extent of the 2003 sequence marks the western termination (at similar to 26.3 degrees E) of a long-term seismic quiescence observed in the period following the 1912 Ganos earthquake, which may be associated with the extend of the rupture of the particular earthquake. (c) 2005 Elsevier B.V. All rights reserved.

Broadband data from the Greek National Seismological Network are used to study the moderate size (M5.5) earthquake, which occurred on 2 December 2002 near the town of Vartholomio, in western Peloponnese (Greece). Time domain moment tensor inversion applied to retrieve the focal mechanism of the mainshock and of three of the larger aftershocks of the sequence, revealed almost pure strike-slip faulting along NW–SE or NE–SW trending nodal planes. The relative source time functions for the mainshock, obtained from an empirical Green’s function analysis, do not reveal any clear directivity to any of the stations. A careful observer might suggest directivity towards NW, if any. Optimum values are 0.4 s for the rise time and 2.7 km/s for the rupture velocity. The spatial and temporal distribution of fault slip showed that the major part of the resolved slip occurred beneath the mainshock’s epicenter, ~20 km underneath the western coast of Peloponnese. This probably accounts for the considerable damage observed to the nearby towns. The resolution between the two nodal planes does not permit an identification of the fault plane; however the statistics on the slip distribution model, the preliminary analysis of aftershock locations and macroseismic data favour the NW–SE trending plane as the fault plane, which is connected with sinistral strikeslip motions. These are the first implications for sinistral strike-slip motions in this area and more data are needed in the future to get more reliable resolution of the motions. D 2004 Elsevier B.V. All rights reserved.

The stochastic method for finite faults is applied to simulate strong ground motion from the 7 September 1999, moment magnitude M 5.9 Athens earthquake. The method includes descritization of the fault plane into a certain number of subfaults, each of which is assigned an ω -2 spectrum. A slip-distribution model, derived from previous studies of this earthquake, is used to specifically account for the source effect. Contributions from all subfaults are then empirically attenuated to the observation sites, where they are summed to produce the synthetic acceleration time history. The method is first calibrated against its ability of reproducing the recordings at 19 strong-motion stations, at epicentral distances ranging from 16 to 61 km. The calibrated model is then applied to calculate synthetics at a large number of grid points covering the area around the fault plane. Simulated peak values are subsequently used to produce synthetic peak ground acceleration and spectral acceleration maps at hard rock. Both peak ground acceleration and spectral acceleration maps imply energy directivity toward the east, where most of the damage was concentrated. The directivity effect is most prominent at large periods (>2 sec) and in the period range 0.2 to 0.3 sec. Independent geotechnical studies showed considerable site effect at periods

Stochastic finite-fault simulation is used to simulate the acceleration time histories of two intermediate depth (h∼90 km) earthquakes, which occurred on 30 May 1990 in Vrancea (Romania) and on 22 January 2002 near Karpathos island in Greece. Site amplification functions were used for all stations included in the simulation process. For the Romanian earthquake, peak acceleration values were observed at the more distant from the epicentre stations. In Romania, strong anisotropy in Q-values was observed along the NE–SW direction (along strike of the fault) compared to the normal to the strike of the fault direction. Specific site corrections were required for the station in Bucharest, employing the H/V ratio technique, to refine the site-specific amplification function and achieve better match of the simulated spectra. In the Aegean Sea region, average values for attenuation and site amplification functions were used for all stations with overall satisfactory results. Any unmatched discrepancies between observed and simulated spectra would require further accurate knowledge of the site characteristics.

The empirical Green's functions technique is applied to simulate strong ground motion records from the September 7, 1999, Athens earthquake. Information on the fault parameters from previous independent studies has been used and several scenarios were examined, in regard to the location of the starting point of the rupture, by comparing the synthetic records with the corresponding observed ones, through a residual function and a correlation function. The results show that the rupture started at the deepest, � 4-5 km, part of the fault from its western edge. This hypocenter was then used, in combination with the initial fault model, to stochastically simulate the strong ground motion during the Athens main shock, in terms of peak-ground acceleration at hard rock. The results show that directivity might have significantly contributed to the destructiveness of this earthquake at specific parts of the Athens Metropolitan area.

Source parameters for 40 earthquakes with epicentres in the Aegean Sea and the surrounding lands have been determined using P and SH body waveform inversion of digital waveforms from the Global Seismograph Network (GSN). These results are combined with 145 previous solutions, 41 of which were determined by first motion polarities and 104 by waveform modelling. Thus, a new updated database of source parameters for earthquakes for the period 1953–1999 is available. These fault plane solutions confirm our previous knowledge about the seismotectonics of the Aegean Sea and the surrounding lands. They also provide strong evidence that the western part of the Peloponnese (Gulf of Patras to Gulf of Kyparissia) is mainly deforming by strike-slip faulting, parallel in strike to the Cephalonia Fault and are in favour of the models that suggest a connection of the North Anatolian Fault (NAF) zone with the transform zone of the Ionian Islands. The sedimentary arc (inner part of the Hellenic trench) is deforming by normal faulting with the T-axes trending ∼E–W. We did not determine any fault plane solutions at the eastern part of the Hellenic arc indicating pure strike-slip motions as expected from the presence of the Pliny and Strabo trenches. However, we did determine solutions that indicate normal faulting combined with a considerable strike-slip component. In the central Aegean Sea the newly determined focal mechanisms support the conclusion that the area, extending from the eastern coasts of Evia up to the coasts of Turkey, is exhibiting a clear strike-slip character. Thus, the effect of the propagating tip of the North Anatolian fault into the Aegean is very clearly pronounced. The focal mechanisms in the back-arc Aegean area confirm the existence of normal faulting with T-axes trending NNE–SSW in western Anatolia and NNW–SSE in Greece. A zone showing NNE–SSW extension that runs parallel to Northern Aegean branch of the NAF zone up to the Corinth Gulf is depicted from the mechanisms of few earthquakes.

Routine catalog phase data from the permanent stations of the Greek National Seismological Network were used to relocate 445 aftershocks of the 26 July 2001 M 6.5 Skyros (North Aegean Sea) earthquake, using the double-difference (DD) algorithm of Waldhauser and Ellsworth [Bull. Seism. Soc. Am. 19 (2000) 1353]. The relocated epicenters define two zones: (a) a main NW–SB cluster of ∼27 km length and (b) a secondary less pronounced NE–SW cluster of ∼15 km length, which was activated within 24 h after the mainshock occurrence. Redistribution of stresses due to the occurrence of the mainshock certainly played a role in this activation. The Skyros mainshock occurred in a region controlled by the activity of the Aegean Sea strands of the dextral strike-slip North Anatolian Fault (NAF). The main aftershock zone is almost perpendicular to the main tectonic NAF strands, and confirms that the causative fault strikes NW–SE associated with sinistral strike-slip motion. The 2001 Skyros earthquake is the first instrumentally recorded event that indicates re-activation of the old tectonic NW–SE trending structures of continental Greece under the presently acting stress field in a way sketched by Kiratzi [Geophys. J. Int. 151 (2002) 360].

Qβ for shear-waves is determined for the inner part of the Hellenic arc, the back-arc area, as a function of frequency in the range 0.6–16 Hz. We used 314 digital records from 32 earthquakes with magnitudes (Mw) ranging from 3.9 to 5.1. Epicentral distances ranged from 65 to 515 km. The data were obtained in 1997 during a 6-month operation of a digital portable network in Greece. The Qβ estimates were made for five frequency bands centred at 0.8, 1.5, 3.0, 6.0 and 12.0 Hz and the Qβ values obtained were 47, 79, 143, 271 and 553, respectively. The results show that Qβ for S-waves increases with frequency taking the form Qβ=55f 0.91 (or Qβ−1≃0.018f−0.91). The high attenuation and the strong frequency dependence found, which is close to the frequency dependence of coda Q for Greece, are characteristic of an area with high seismicity, rapid extension, and in agreement with other similar studies in Greece.

Data from a large-scale experiment which took place in Greece during the period January–July 1997 have been used to investigate the structure of the Aegean area using surface waves. During this experiment, 30 seismic broadband instruments were deployed throughout the whole Greek area. Additional data during the period 1996–2000 from other temporary networks have been included in the dataset. One hundred eighty-five events with magnitudes 4.0VMwV5.5 recorded by these stations have been collected and processed. The individual dispersion curves of the group velocity of Rayleigh waves for each source-station path have been calculated, producing more than 700 paths covering the studied region. These curves have been used to determine Rayleigh group velocity maps using a 2D-tomography method. On the basis of a regionalization of the dispersion measurements, local averaged dispersion curves have been obtained and non-linearly inverted to obtain models of shear-wave velocity versus depth. Since the dispersion curves in the period range 5 sVTV30 s are mostly affected by the crustal structure, the model velocities are estimated down to a depth of approximately 35–45 km. The results from the non-linear Hedhehog inversion as applied to a few local dispersion curves show a crustal thickness of approximately 32 km for the Northern Aegean Sea, and a relatively thin crust of approximately 22–24 km for the Southern Aegean Sea. D 2002 Elsevier Science B.V. All rights reserved.

SUMMARY Gephart & Forsyth’s method has been applied to estimate stress orientations from earthquake fault-plane solutions of the westernmost North Anatolian Fault zone (NAFZ), its westward continuation in the Northern Aegean Sea, where it interacts with the normal faulting of central Greece, and of the western coastal parts of Anatolia. In total, 163 fault-plane solutions were used to invert for the stress tensor of 9 seismotectonic subregions. Magnitudes range from 1.1 to 7.4. All events have shallow focal depths; most occurred in the uppermost 15 km and very few in the 15 to 40 km depth range. The region studied is characterized by strike-slip (NAFZ), by transtension (Central Greece and western Anatolia) and by a strong signature of transpression in the Marmara area. From NAFZ to central Greece, the σ3 axis changes, continuously and smoothly, its orientation from N35 ◦ E in the westernmost NAFZ to nearly North‐South in continental central Greece, and the maximum horizontal stress, σHmax changes from WNW‐ESE to ∼E‐W. The total angle of lateral variation in the direction of either σHmax or σ3 is ∼40 ◦ . In central western Anatolia the orientation of σ3 is NNE‐SSW (N10 ◦ E) while in southern western Anatolia it is NNW‐SSE (N33 ◦ W). In both cases the development of the major grabens (Simav, Gediz, Menderes) is in agreement with the resolved stress tensor. At the termination of the NAFZ against central Greece the regime is complex. The dextral shear motion transferred from the east crosses the Pilion peninsula and northern Evia Island where it interacts with the major normal faults of continental Greece. These faults have variable orientations. The E‐W trending Quaternary fault system and the NW‐SE trending fault system, which is inherited from the past. The latter faults under the presently acting stress field are reactivated either as strike-slip faults with sinistral strike-slip component (when oriented at small angle in respect to σ3) or as normal faults with small sinistral strike-slip component (when oriented at high angle in respect to σ3). The occurrence of the recent 2001 July 26 Skyros event, which was clearly connected to a NNW‐SSE fault with sinistral strike-slip motion, provides strong seismological evidence that the old structures can equally well be reactivated under the presently active stress field.

Acceleration time histories, recorded during the destructive 15 April 1979 ( M 7.1) Montenegro earthquake, have been simulated using a stochastic modeling technique for finite faults proposed by Beresnev and Atkinson (1997, 1998a). In this approach, the ground-motion amplitudes are simulated as a summation of stochastic point sources. The length of the fault was taken as 70 km and its width as 25 km, and the fault plane was divided into 13 × 5 elements. The applied methodology is tested against its ability to predict site-specific strong-motion records by the incorporation of mean frequency-dependent site-amplification factors, based on a gross characterization of the site class. The results show that the overall agreement between stochastic and recorded waveforms and spectra is quite satisfying. Nevertheless, significant discrepancies exist at certain stations, implying that site-amplification functions play an important role in the simulation process. A repetition of the simulations combined with the use of site-specific amplification function estimated by the horizontal-to-vertical ratios technique improved the fit to the observed time histories and spectra. Manuscript received 4 July 2001.

Instrumental magnitudes in Greece have been reported as: a) M magnitudes based on the records of the Wiechert or Mainka seismographs, b) MLGR magnitudes based on the records of the Wood-Anderson (WA) seismographs (To = 0.8 sec, Veffective ∼ 1000) or other short period seismographs calibrated against WA records and, c) MLSM magnitudes based on strong motion records (accelerograms). Comparison of such magnitudes with moment magnitudes, Mw, for 329 earthquakes, with epicenters in the broader Aegean area, performed in this study, showed that M, MLGR+0.5 and MLSM are practically equal to Mw, with a small overall standard error (σ = 0.23). Therefore, equivalent moment magnitudes, Mw ∗ , estimated from these magnitudes and reported in the catalogues of the Geophysical Laboratory of the University of Thessaloniki are equal to moment magnitudes for all practical purposes with reasonable uncertainties. It has been further shown that surface wave magnitudes, Ms ,f or M s

Broad band seismic waveforms are used to determine the source model of the 16 April 2015 (UTC 18:07:44) earthquake, Mw6.1, which occurred 14 km SW of Kasos Island, in the eastern Hellenic subduction zone. The mainshock is connected with oblique leftlateral motion on a reverse fault, dipping to SE. Most of the aftershocks are compatible with strike-slip or oblique normal faulting, with the T-axes showing along arc extension. A finite fault slip inversion was performed, allowing for the rake angle to vary across the fault, to capture the variation in the slip vectors. The rupture initiated in the lower crust, at a centroid depth of 23 km, and propagated mainly towards SW. The slip is confined in depth within ~17km and 27km, mainly in a single asperity, with the peak slip of the order of 60 cm. The slip model provided synthetic seismograms which matched satisfactory the observed, and with forward modelling the ShakeMap was calculated. The 2015 Kasos earthquake sequence is compatible with shear motions parallel to the strike of the subduction zone. It provides evidence that part of the deformation in the eastern Hellenic subduction is taken up by the simultaneous operation of reverse faulting and of minor strike-slip and oblique normal faulting, with slip vectors aligned ~ parallel to the Pliny and Strabo Trenches and the long axis of the local bathymetry.

Thessaloniki, the second largest city in Greece, is located at the northeastern shore of Thermaikos Gulf, a shallow water body and remnant of the Axios valley sedimentary system that has received massive sedimentation. The broader region is surrounded by neotectonic fault zones, several of which can be considered as active, based on their kinematic data compatibility with the active stress field and the frequent microseismicity. Part of the city is built on basement rocks of primarily Mesozoic age, but most of it is founded on the sedimentary filling of the Axios basin that has been active since Eocene. Historical records indicate that Thessaloniki, has suffered several earthquake disasters, both from sources close to the city, as well as from more distant sources (e.g. 1829 M7 Drama and 1904 M7 Kresna events). The event of 1759 (MMI IX; M6.5) is reported as the most destructive for Thessaloniki with a postulated epicenter at the Axios River estuary in Thermaikos Gulf. We combine the most updated tectonic and seismological data for the broader Thessaloniki area to postulate a scenario of a moderate-magnitude earthquake and model the expected strong shaking and off-fault coseismic surface effects, with emphasis on liquefaction phenomena that match the damage patterns described in historical archives. In particular, we delineate zones prone to liquefaction occurrence and evaluate the potential of the geological units. The latter is achieved based on data provided by in situ tests and the former based on information provided by geological maps regarding the age and depositional process of the sediments.

The 1980 Volos (Almyros) seismic sequence, the last of an unusual clustering of earthquakes with surface magnitude 6.0-7.0, which hit Thessaly (central Greece) between 1930 and 1980, is among the least studied recent earthquakes in Greece. Repeated measurement of three first-order spirit leveling traverses crossing the meizoseismal area of the seismic sequence indicates that between 1970/1971 and 1988/1989 a subsidence of about 35 cm is observed at the Almyros Neogene- Quaternary graben. The observed displacements are significant against random and systematic errors and can be assigned to the 1980 earthquake sequence. Inversion of leveling data using an elastic dislocation model, constrained by structural and seis- mological data indicates that the mainshock (Mw 6.5) and the largest aftershock (Mw 6.1) were probably associated with reactivation of two subparallel, homothetic nor- mal faults marking the northern margin of the Almyros Neogene-Quaternary graben.

— Source parameters for thirteen earthquakes in the SE Adriatic region have been determined using P and SH body-waveform inversion. The results of this modeling are combined with eleven other earthquakes with M ≥ 5 whose focal mechanisms have been determined mainly by waveform modeling. The results confirm that movement on mainly low-angle reverse faults causes the deformation in coastal southern Yugoslavia through Albania up to the Lefkada Island in NW Greece. This zone of thrusting has a NW–SE trend (N34°W), follows the coastline, and dips towards the continent. The slip vectors of these events trend at ∼N229° along the Dalmatian coasts, to ∼N247° along Albania and NW Greece. The deformation is attributed to the continental collision between the Adriatic block to the west and Eurasia to the east. Along the mountain line in eastern Albania (Albanides Mts.) and in NW Greece (Hellenides Mts.), E–W extension is occurring. The E–W extension associated with the orogenic belt could be attributed to a variety of models such as: gravity, internal deformation of the thrust wedge, a probable down bulge of the dense lithosphere of the Adriatic block beneath the Eurasian lithospheric plate in combination with the compressional stresses applied along the collision belt.