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Located on the Hellenic Arc, the Christiana‐Santorini‐Kolumbo (CSK) rift zone represents one of the most active and hazardous volcano‐tectonic systems in the Mediterranean. Although this rift zone has been intensively studied, its tectonic evolution and the interplay of volcanism and tectonism are still poorly understood. In this study, we use high‐resolution reflection seismic imagery to reconstruct the opening of the rift basins. For the first time, we relate the activity of individual faults with the activity of specific volcanic centers in space and time. Our analysis shows a pre‐volcanic NNE‐SSW‐oriented paleo basin underneath the CSK volcanoes, representing a transfer zone between Pliocene ESE‐WNW‐oriented basins, which was overprinted by a NE‐SW‐oriented tectonic regime hosting Late Pliocene volcanism that initiated at the Christiana Volcano. All subsequent volcanoes evolved parallel to this trend. Two major Pleistocene tectonic pulses preceded fundamental changes in the volcanism of the CSK rift including the occurrence of widespread small‐scale volcanic centers followed by focusing of activity at Santorini with increasing explosivity. The observed correlation between changes in the tectonic system and the magmatism of the CSK volcanoes suggests a deep‐seated tectonic control of the volcanic plumbing system. In turn, our analysis reveals the absence of large‐scale faults in basin segments affected by volcanism indicating a secondary feedback mechanism on the tectonic system. A comparison with the evolution of the neighboring Kos‐Nisyros‐Yali volcanic field zone and Rhodos highlights concurrent regional volcano‐tectonic changes, suggesting a potential arc‐wide scale of the observed volcano‐tectonic interplay. Plain Language Summary: How do regional tectonic movements and large volcanoes interact? Seismological studies indicate complex volcano‐tectonic feedback links, but, so far, information on the long‐term interactions between tectonics and volcanism is rarely available. The Christiana‐Santorini‐Kolumbo (CSK) rift zone lies in the Aegean Sea and is notorious for its devastating volcanic eruptions, earthquakes, and tsunamis. This region offers the opportunity to study volcano‐tectonic interactions over several million years. In this study, we use high‐resolution seismic imagery to reconstruct the evolution of the rift basins and the CSK volcanoes. We find that all volcanoes lie in a Pliocene transfer zone connecting extensional basins. Volcanism initiated as this older tectonic regime was intersected by a NE‐SW‐directed fault system. Subsequently, all volcanoes evolved parallel to this trend. Several distinct tectonic reorganizations occurred in the Pleistocene, which had a pronounced influence on the CSK volcanoes. In turn, our analysis indicates that the emergence of volcanism also impacted the tectonic evolution of the rift system hindering the evolution of large‐scale normal faults in the volcanic basins. The observed tectonic reorganizations seem to reflect major changes in the stress regime of the Hellenic Arc, potentially also affecting adjacent volcanic centers whose volcano‐tectonic evolution is only poorly constrained so far. Key Points: We reconstruct the volcano‐tectonic evolution of the Christiana‐Santorini‐Kolumbo rift zone using multichannel seismic dataThe overprint of a Pleistocene NE‐SW striking fault system on a Pliocene E‐W oriented system initiated the emergence of volcanismRegional tectonics had a primary control on the volcanic plumbing system, while magmatism had a secondary influence on the tectonic system [ABSTRACT FROM AUTHOR]

Several crustal and lithospheric mechanisms lead to deformation and vertical motion of the upper plate during subduction, but their relative contribution is often enigmatic. Multiple areas of the Hellenic Forearc have been uplifting since Plio‐Quaternary times, yet spatiotemporal characteristics and sources of this uplift are poorly resolved. The remarkable geology and geomorphology of Kythira Island, in the southwestern Hellenic forearc, allow for a detailed tectonic reconstruction since the Late Miocene. We present a morphotectonic map of the island, together with new biostratigraphic dating and detailed analyses of active fault strikes and marine terraces. We find that the Tortonian‐Pliocene stratigraphy in Kythira records ∼100 m of subsidence, and a wide coastal rasa marks the ∼2.8–2.4 Ma maximum transgression. Subsequent marine regression of ∼300–400 m and minor E‐W tilt are recorded in ∼12 marine terrace levels for which we estimate uplift rates of ∼0.2–0.4 mm/yr. Guided by simple landscape evolution models, we interpret the coastal morphology as the result of initial stability or of slow, gradual sea‐level drop since ∼2.8–2.4 Ma, followed by faster uplift since ∼1.5–0.7 Ma. Our findings on‐ and offshore suggest that E‐W extension is the dominant mode of regional active upper crustal deformation, and N‐S normal faults accommodate most, if not all of the uplift on Kythira. We interpret the initiation of E‐W extension as the result of a change in plate boundary conditions, in response to either propagation of the North Anatolian Fault, incipient collision with the African plate, mantle dynamics or a combination thereof. Key Points: ∼100 m of Tortonian‐Pliocene subsidence of Kythira Island was followed by ∼300–400 m of uplift since ∼2.6 MaWe propose that regional E‐W extension, caused by large‐scale tectonic changes ∼1.5–0.7 Ma, is the dominant driver to uplift Kythira IslandWe highlight the general importance of upper crustal faulting as a major process of forearc uplift [ABSTRACT FROM AUTHOR]

The focus on exploiting natural gas resources has been increased in the recent years since it was characterized as the transitional fuel to a net-zero era. Consequently, a reevaluation of the gas resources under a new perspective seems to be logical. Within this context, the gas-bearing Miocene sediments of the Messara basin in Crete (Greece) are discussed in this paper. The gas shows have been first reported during the 1990's, without being thoroughly evaluated. This paper presents the interpretation of eight legacy onshore seismic reflection lines, which led to the construction of the 3D subsurface structural model of the Neogene Messara basin. The seismic data reveals the basin depocenter which accommodates approximately 1500 m of the Miocene sediments. The relationship of the Miocene deposits with the existence of shallow gas is also examined and discussed. New organic geochemical results combined with the published geochemical data are used to discuss the gas generation potential of the Miocene sediments. The porosity and permeability measurements on surface samples are also presented to describe the reservoir characteristics. The integration of the data mentioned above suggests that the Miocene sediments of the Messara basin comprise intervals with adequate organic matter to generate the observed gasses and good reservoir sandstones to accumulate the generated gas. The gas might have been trapped by lateral and vertical facies changes which provide efficient stratigraphic trapping mechanisms. [ABSTRACT FROM AUTHOR]

This study presents new thermochronometric data to constrain the cooling history of the Pindos fold‐and‐thrust‐belt (PFTB) on the NW Peloponnese, and the timing of nappe stacking in the external Hellenides in western Greece. Low‐temperature thermochronology was performed on apatite and zircon minerals extracted from basal exposures of the PFBT in Mt. Erymanthos (NW Peloponnese) using the (U‐Th)/He system. Three bedrock samples collected from a profile across the strike of the PFBT returned thermochronometric ages for the apatite (AHe) between 15 and 9 Ma, while for the zircon (ZHe) the ages are interpreted as unreset (or partially reset). The AHe ages are interpreted as cooling ages related to the Cenozoic geodynamic evolution of the PFTB. Thermal models generated in this research demonstrate two phases of geodynamic evolution of the PFBT during the Cenozoic: an early one associated with the heating of the fold‐and‐thrust belt during 40–20 Ma and a later one associated with fast cooling from 20 Ma onwards. Heating is attributed to the structural thickening of the PFTB while cooling is associated with enhanced uplift and exhumation, which became widespread in Erymanthos by 9 Ma. The initiation of cooling on Mt. Erymanthos echoes the substantial geometric remodelling that underwent the Hellenic arc during the Middle Miocene, associated with slab rollback and arc extension, and is connected to the underplating of the internal segments of the external platform (Gavrovo‐Tripolis unit) by the external metamorphic segments (Mani and Arna units). The current inverse thermal modelling offers the first continuous description of the geodynamic evolution of the most prominent fold‐and‐thrust belt of the Hellenides, from the oceanic stage through basin inversion to uplift and orogen decay. [ABSTRACT FROM AUTHOR]

The 27 November 2019 Mw 6.0 earthquake that occurred in the southwestern part of the Hellenic Arc near Crete Island provided evidence of the high potential for strong earthquakes and active seismicity in the Hellenic Arc. In addition, tsunamis have been reported to occur for the region after major earthquakes in the historical past, so the seismic hazard of the Hellenic Arc should be evaluated in detail. The aim of this study is to evaluate the seismic hazard of the Hellenic Arc more reliably and accurately by estimating the conditional probabilities of a strong earthquake based on Weibull, gamma, log-normal, exponential, Rayleigh, and inverse Gaussian distribution models for the inter-event time of Mw ≥ 6.0 earthquakes that occurred between 1900 and 2019 in the study area. The fit between each model and the data was tested using four different test criteria, namely the log-likelihood value, Akaike information criterion, Bayesian information criteria, and Kolmogorov–Smirnov test. According to the results, the inverse Gaussian distribution was selected as the best, the log-normal distribution as the second best, the Weibull and gamma distributions as intermediate, and the Rayleigh and exponential distribution as the poorest and second poorest model, respectively. The conditional probability of an earthquake with magnitude Mw ≥ 6.0 is estimated to be higher than 0.70 according to all of the models used in this study for the base year te = 0 (te = 2015) and t > 5 years (t > 2020). Moreover, the results obtained based on the inverse Gaussian, exponential, log-normal, and Weibull distribution models are close to each other and are higher than 0.60 for te = 0 and t ≥ 3 years (t ≥ 2018). The outcomes of this study when using the different distribution models will contribute to assessments of the seismic as well as tsunami hazards for the region. [ABSTRACT FROM AUTHOR]

The Nile Delta is the largest clastic accumulation in the Mediterranean Sea and has been deposited within a region of significant Cenozoic tectonic activity. Therefore, it is important to evaluate the crustal thickness and seismic strain rate that impacts the Nile fan deposits; in conjunction with studying the interaction between salt tectonic and plate tectonics in this basin. Bouguer anomalies, crustal thickness and seismic strain were calculated using several equations. The Eastern Mediterranean has gravity anomaly varied from -40 to 210 mGals indicating the changing of the crustal thickness. The crustal thickness of the Hellenic Arc moving above Africa is faster than Turkey towards the Cyprus Arc. The movement variation between them is about 6 mm/y. This displacement distance is assumed to be achieved in about 10 million years. The seismic strain release is abruptly increase started from the Mediterranean Ridge towards the Aegean Sea where the maximum values located along the Hellenic Arc. Also, there are some clusters in the southern and northern Crete, which have high seismic strain release. The deformed fault-block movement is related to the impacts of strike-slip movements, growing salt diapirs. The north-western side of the Nile deep fan deposit characterized by the formation of a series of salt folds and reverse faults above the Messinian evaporites which related to the compressional/subduction tectonics of the adjacent Mediterranean Ridge. These activities are associated with many clusters of mud volcanism along the southern part of the Hellenic and Cyprus arcs which separate both basins. The pre-Messinian sequences structures are related to a classic gravity-spreading deltaic tectonics. The structure reservoir along Nile delta effected and suffers from the subduction of the African plate beneath Eastern Europe and the impact of the salt tectonics. [ABSTRACT FROM AUTHOR]

This study revisits subduction processes at the Hellenic Subduction Zone (HSZ) including tearing, segmentation, and backstepping, by refining the geometry of the Nubian slab down to 150–180 km depth using well-located hypocentres from global and local seismicity catalogues. At the western termination of the HSZ, the Kefalonia Transform Fault marks the transition between oceanic and continental lithosphere subducting to the south and to the north of it, respectively. A discontinuity is suggested to exist between the two slabs at shallow depths. The Kefalonia Transform Fault is interpreted as an active Subduction-Transform-Edge-Propagator-fault formed as consequence of faster trench retreat induced by the subduction of oceanic lithosphere to the south of it. A model reconstructing the evolution of the subduction system in the area of Peloponnese since 34 Ma, involving the backstepping of the subduction to the back-side of Adria, provides seismological evidence that supports the single-slab model for the HSZ and suggests the correlation between the downdip limit of the seismicity to the amount of subducted oceanic lithosphere. In the area of Rhodes, earthquake hypocentres indicate the presence of a NW dipping subducting slab that rules out the presence of a NE-SW striking Subduction-Transform-Edge-Propagator-fault in the Pliny-Strabo trenches region. Earthquake hypocentres also allow refining the slab tear beneath southwestern Anatolia down to 150–180 km depth. Furthermore, the distribution of microseismicity shows a first-order slab segmentation in the region between Crete and Karpathos, with a less steep and laterally wider slab segment to the west and a steeper and narrower slab segment to the east. Thermal models indicate the presence of a colder slab beneath the southeastern Aegean that leads to deepening of the intermediate-depth seismicity. Slab segmentation affects the upper plate deformation that is stronger above the eastern slab segment and the seismicity along the interplate seismogenic zone. [ABSTRACT FROM AUTHOR]

The segmentation and differentiation of subducting slabs have considerable effects on mantle convection and tectonics. The Hellenic subduction zone is a complex convergent margin with strong curvature and fast slab rollback. The upper mantle seismic anisotropy in the region is studied focusing at its western and eastern edges in order to explore the effects of possible slab segmentation on mantle flow and fabrics. Complementary to new SKS shear-wave splitting measurements in regions not adequately sampled so far, the source-side splitting technique is applied to constrain the depth of anisotropy and to densify measurements. In the western Hellenic arc, a trench-normal subslab anisotropy is observed near the trench. In the forearc domain, source-side and SKS measurements reveal a trench-parallel pattern. This indicates subslab trench-parallel mantle flow, associated with return flow due to the fast slab rollback. The passage from continental to oceanic subduction in the western Hellenic zone is illustrated by a forearc transitional anisotropy pattern. This indicates subslab mantle flow parallel to a NE–SW smooth ramp that possibly connects the two subducted slabs. A young tear fault initiated at the Kefalonia Transform Fault is likely not entirely developed, as this trench-parallel anisotropy pattern is observed along the entire western Hellenic subduction system, even following this horizontal offset between the two slabs. At the eastern side of the Hellenic subduction zone, subslab source-side anisotropy measurements show a general trench-normal pattern. These are associated with mantle flow through a possible ongoing tearing of the oceanic lithosphere in the area. Although the exact geometry of this slab tear is relatively unknown, SKS trench-parallel measurements imply that the tear has not reached the surface yet. Further exploration of the Hellenic subduction system is necessary; denser seismic networks should be deployed at both its edges in order to achieve a more definite image of the structure and geodynamics of this area. [ABSTRACT FROM AUTHOR]

A strong (Mw=6.7) intermediate depth earthquake occurred on 8 January 2006 (11:34 UTC) in southwestern Aegean Sea (Greece) causing limited damage to structures on the nearby islands of Kythira and Antikythira, as well as western Crete Island. The epicentral area belongs to the SW segment of the Hellenic Arc, which is known to be associated with the occurrence of large shallow and intermediate depth earthquakes, mainly due to the subduction of the Eastern Mediterranean oceanic lithosphere under the Aegean microplate. The main shock occurred on a dextral strike slip fault at a depth of 75 km, within the descending slab, as it is revealed by both, the spatial distribution of the accurately located aftershocks and its fault plane solution determined in the present study and implying a slab-pull event. The aftershock activity from 8 to 31 January 2006 is distributed in depths ranging from 55 to 75 km, and being comprised in an almost rectangular and vertical plane with a length equal to 28 km and a width of 20 km, which adequately defines the dimensions of the rupture area. The geometry of the Wadati-Benioff zone in this area, namely the southwestern part of the Hellenic Arc, is explored by an exhaustive analysis of all the available phase arrivals gathered from the International Seismological Centre, and the relocation of the earthquakes occurred since 1964 in the South-West Aegean region.

In this paper we investigate the tidal triggering evidence on the earthquakes of the seismic area of the Hellenic Arc using the Hist(ogram)Cum(mulation) method. We analyze the series of the earthquakes occurred in the area which is confined by the longitudes 22° and 28°E and latitudes 34° and 36°N in the time period from 1964 to 2012. In this time period 16,137 shallow and of intermediate depth earthquakes with M L up to 6.0 and 1,482 deep earthquakes with M L up to 6.2 occurred. The result of the this analysis indicate that the monthly variation of the frequencies of earthquake occurrence is in accordance with the period of the tidal lunar monthly variations, and the same happens with the corresponding daily variations of the frequencies of earthquake occurrence with the diurnal luni-solar (K1) and semidiurnal solar (S2) tidal variations. These results are in favor of a tidal triggering process on earthquakes when the stress in the focal area is near the critical level. [ABSTRACT FROM AUTHOR]

Earthquakes exhibit complex correlations in space, time, and magnitude. As such, they present a unique example for a time series exhibiting complexity. To uncover hidden dynamic features of seismicity before the Mw6.4, October 12, 2013 earthquake in the South West part of the Hellenic Arc, we apply the recently introduced methods of natural time analysis along with the ideas of non-extensive statistical physics. Using non-extensive statistics, the analysis of the magnitude distribution of the 2013 seismicity in the South West part of the Hellenic Arc, struck by a strong earthquake ( M w = 6.4 ), was performed for a time period before the main event, which was the strongest one in the South West part of the Hellenic Arc in the last 40 years. The analysis of the frequency magnitude (energy) distribution reveals that the non-extensive parameter q E varies during the last period of the earthquake preparatory phase with a sharp increase a couple of days before the occurrence of the main strong event, indicating an increase in the degree of out-of-equilibrium state before the occurrence of the Mw6.4 earthquake. In addition using the natural time analysis it is shown that foreshock seismicity approaches the critical stage, where the k 1 parameter reaches the critical value of k 1 = 0.070 few days before the occurrence of the main event. [ABSTRACT FROM AUTHOR]

In Fall 2006, in the frame of the European Community SEAHELLARC project, we deployed an "amphibious" seismic array consisting of 17 3-channel stand alone stations onshore and 17 4-channel ocean bottom seismographs in the Kyparissiakos Gulf and surrounding area of western Peloponnese. We observed the microseismic activity for a period of two months. The geometry and location of the array were designed to particularly observe the offshore activity and define the continuation of known onshore active faults offshore. Data evaluation and location of the seismic events were accomplished in three steps, successively improving the location accuracy. First we used the standard procedure for foci location with a 1D velocity model and the Hypoinverse algorithm. Locations were then improved by 3D passive tomography, using P and S arrivals. The optimum solution of the seismic foci location was finally achieved by using a 3D velocity model developed from active seismic observations and 3D gravity modelling. More than 3500 earthquakes over a threshold magnitude of 0.3 MD were identified, using arrivals from a minimum of six stations per event. Shallow seismicity is confined mainly onshore Zakynthos Island, at the continentocean transition to the west, and in the area of Pylos - Messinia. These zones are deforming by westward thrusting of the Alpine napes and collision of the continental backstop of pre-Apulia with the Ionian oceanic lithosphere at the Mediterranean Ridge. Deeper seismicity follows the subduction of the Ionian lithosphere below western Peloponnese with successively increasing hypocentre depths eastwards. A cluster of subcrustal events extending to 90 km depths is located in a pull apart basin NE of Strofades Island in the forearc. These events also generated T-phases. We attribute this "unusual" seismicity to lateral deformation and fracturing of the oceanic lithosphere during its subduction below a continental crust of laterally varying thickness. [ABSTRACT FROM AUTHOR]

Multiple attenuation comprises a very important phase of the seismic reflection processing because it removes undesirable effects occurred during the propagation of the seismic signal in the earth. The present work aims at presenting the effectiveness of modern multiple suppression methods applied on streamer seismic reflection data. The seismic dataset represents an area with moderately dipping seafloor. The specific dataset was chosen in order to present the elimination of the water-column reverberations and internal multiples in dipping layers. Strong seafloor and internal multiples, lateral reflections and coherent noise impose difficulties in discriminating primary reflections in the seismic image of the present work. The prestack demultiple scheme combines wave equation method and deconvolution in order to predict and attenuate water-column reverberations and internal multiple reflections respectively. In particular, the region of moderately dipping seafloor chosen for the application of the proposed scheme images the Frontal Bulge in the outer Hellenic arc. The prestack demultiple method improved the quality of the stacked and migrated sections, by attenuating the majority of multiple reflections and extracting information about the structural units of the study area.

Based on the theory of complex networks, we quantify for the first time correlations between seismic events occurring in the Hellenic arc and Trench system, which is the most seismogenic structure in the European-Mediterranean region. We examine how relatively strong events with their aftershock sequences trigger phase transitions of the underlying network topology of seismic activity from random to scale-free structures. In particular, we show that the network is characterized by a highly clustered spatial structure giving rise to heterogeneous networks exhibiting enhanced small-world attributes. [ABSTRACT FROM AUTHOR]

Abstract: Neither large magnitude nor tsunamigenic earthquakes occur frequently in the eastern Mediterranean, hampering comprehensive study of these events and consequential hazards whenever digital data are required. Using analog seismograms, travel-time catalogs, and hydrodynamic simulations, we reassess here four large (M ~7) historical earthquakes occurring in various regions of the Hellenic Arc: on 6 October 1947, in the Peloponnesus; 9 February 1948, near Karpathos; and a couplet east of Rhodos on 24 and 25 April 1957. Damaging near-field tsunamis are associated with the 1947 and 1948 earthquakes. Results include seismic moments (in units of 1027 dyn·cm) of 1.26 (1947), 0.97 (1948), 0.56 (1957a), and 1.09 (1957b); recovered focal mechanisms and hypocentral locations are consistent with the regional stress field resulting from the ongoing collision between the Nubia plate and Aegea microplate. Seismological reassessments indicate that the sources of the 1947 and 1948 tsunamis involved submarine slumping; hydrodynamic simulations assuming a submarine landslide source for the 1948 tsunami recreate run-up and inundation values consistent with eyewitness accounts presented here. Hydrodynamic simulations also show that the 25 April 1957 earthquake likely generated an unobserved tsunami with a maximum 50cm run-up in Rhodos. [Copyright &y& Elsevier]

Fault population statistics play a key role in the understanding of any statistical seismicity approach. In the present work a non-extensive statistical physics approach is formulated and tested for the local fault length distribution. The approach is composed of the following parts: (i) Tsallis entropy, S, (ii) maximization of the Tsallis entropy under appropriate constrains, and (iii) derivation of the cumulative distribution function (CDF) of the fault length population. This model is tested using fault length data from the Central Crete graben in front of the Hellenic arc and estimated a thermodynamic q parameter equal to 1.16, which supports the conclusion that the fault system in Central Crete graben is a sub-extensive one. [ABSTRACT FROM AUTHOR]

The West Hellenic Arc and Trench (WHA-T) system is seismotectonically one of the most active in the European-Mediterranean region. Data on earthquake and tsunami phenomena occurring in the tectonic segment of the WHA-T from the antiquity up to the present have been updated, critically evaluated, and compiled in the standard format developed for the New European Tsunami Catalog. Most of the tsunamis documented are caused by strong earthquakes occurring in the area offshore Crete Island which is one of the most tsunamigenic areas in the entire Mediterranean Sea region. The major event of AD365 appears to be the largest ever occurred in the Mediterranean Sea at least in the last 2.5 millennia. The mean repeat time of strong tsunamis is calculated from intensity-frequency statistics and probabilistic hazard assessment is performed by assuming random temporal distribution of the events. [ABSTRACT FROM AUTHOR]

The Movri Mountain earthquake (Mw 6.4), western Greece, was likely caused by dextral-slip along a blind high-angle fault, and generated a complex pattern of co-seismic surface ruptures southwest of the Gulf of Corinth. The mapped Nisi, Michoi, and Vithoulkas rupture segments have similar lengths (5-6 km) and vertical offset on the order of 25, 10, and 5 cm, respectively. They are commonly expressed as straight or jagged linear traces with secondary cracks radiating from the main segments. Horizontal slip vector analysis indicates extensional faulting processes for all rupture segments. Although these faults exert some control on the fluvial drainage pattern and at least one of them was ruptured during past events, their escarpments are poorly preserved. The indistinct topographic expression of the studied faults and their complex rupture patterns can be attributed to the distribution of the deformation over a blind fault. [ABSTRACT FROM AUTHOR]

Abstract: Subduction plays a fundamental role in plate tectonics and when interrupted it may trigger a series of geodynamic and sedimentary responses. Synchronous structural modifications recorded across the entire eastern Mediterranean region are dated to a relatively short period — early-to-mid Pleistocene. These deformations are documented within plates (e.g., Arabian, Sinai and African plates), along plate boundaries (e.g., Dead Sea and North Anatolian faults and Cyprus Arc), and in the Mediterranean basin. During the same period the northward subduction of the Sinai plate was interrupted when the Eratosthenes Seamount–Cyprus Arc collision initiated. Subduction–collision processes of the eastern Mediterranean serve as a unique modern analogue for similar settings worldwide. Understanding their association with accompanying neo-tectonic processes is therefore predominantly important. By fostering a detailed and comprehensive approach this research provides a coherent tectonic picture for the eastern Mediterranean early-to-mid Pleistocene tectonic transition in order to explore its triggering mechanisms. Since the Neogene convergence across the eastern Mediterranean was accompanied by Eurasian indentation by Arabia northward motion, westwards Anatolia escape and southwards Aegean propagation. This semi counterclockwise plate motion was temporarily interrupted by the incipient Seamount–Arc collision which is suggested here as a trigger of the early-to-mid Pleistocene tectonic transition. [Copyright &y& Elsevier]

Abstract: We investigate the rupture process of the strongest events of the 14 February 2008 sequence which ruptured ~30 km of the western Hellenic trench. Three were the strongest events of the sequence: the first one (M6.7) occurred on 14 February 2008 at a depth of ~30 km, on a low-angle (10°) thrust fault, followed within 2 h by an M6.1 aftershock at a depth of ~33 km, also on a low-angle thrust fault and the third one on 20 Feb 2008 of M6.0 which is characterized as a strike-slip event at shallower depth (~10 km). We used the M6.1 aftershock as empirical Green''s function (EGF) to invert for the slip distribution of the strongest M6.7 event of the sequence. We were able to identify the low-angle plane (strike 288°, dip 10° and rake 73°) as the fault plane. The slip model is dominated by two distinct slip patches, which extend towards SSE from the hypocenter, thus implying rupture directivity toward the island of Crete, a result which is further supported by our teleseismic waveform inversion results. Maximum slip is estimated at ~270 cm, while average slip on the ruptured area (area of subfaults of non-zero slip) was ~70 cm. Most of the slip (>95% of total) is confined in an area of 34×22 km2. We attribute the occurrence of the shallow strike-slip event, just above the interplate thrusting at depth, to slip partitioning, due to the deflection of slip vectors from the expected plate convergence vector. [Copyright &y& Elsevier]

Earthquake hazard parameters such as maximum magnitude, annual mean seismic activity rate, and the Gutenberg-Richter parameter, have been evaluated for the Hellenic and Cyprean Arc regions of the Eastern Mediterranean. The applied maximum likelihood procedure permits the combination of both historical and instrumental data. The collected data from different sources cover earthquakes with magnitude ≥ 3.5 throughout the last two centuries. The historical part of the catalogue only contains the strongest events, whereas the complete part can be divided into several subcatalogues, each assumed complete above a specific threshold of magnitude. The hazard parameters assessment is performed for the two study regions. The Hellenic Arc region was found to be of higher seismicity level than the Cyprean Arc region. The number of annually expected earthquakes with magnitude ≥ 3.5 is much larger in the Hellenic Arc (56±2) than in the Cyprean Arc (35±2). The maximum magnitude calculated in the Hellenic Arc zone is 7.8±0.4 for the time period equal to the length of the catalogue, i.e., 210 years. For the Cyprean Arc zone, the maximum possible magnitude is 6.8±0.4 for the time span of 330 years. [ABSTRACT FROM AUTHOR]

Abstract: Geochemical and petrographical studies of lavas and ignimbrites from the Quaternary Nisyros–Yali volcanic system in the easternmost part of the Hellenic arc (Greece) reveal insight into magma generating processes. A compositional gap between 61 and 68 wt.% SiO2 is recognized that coincides with the stratigraphic distinction between pre-caldera and post-caldera volcanic units. Trace element systematics support the subdivision of Nisyros and Yali volcanic units into two distinct suites of rocks. The variation of Nd and Hf present day isotope data and the fact that they are distinct from the isotope compositions of MORB rule out an origin by pure differentiation and require assimilation of a crustal component. Lead isotope ratios of Nisyros and Yali volcanic rocks support mixing of mantle material with a lower crust equivalent. However, 87Sr/86Sr ratios of 0.7036–0.7048 are incompatible with a simple binary mixing scenario and give low depleted mantle extraction ages (<0.1 Ga), in contrast with Pb model ages of 0.3 Ga and Hf and Nd model ages of ca. 0.8 Ga. The budget of fluid-mobile elements Sr and Pb is likely to be dominated by abundant hydrous fluids characterised by mantle-like Sr isotope ratios. Late stage fluids probably were enriched in CO2, needed to explain the high Th concentrations. The occurrence of hydrated minerals (e.g., amphibole) in the first post-caldera unit with the lowermost 87Sr/86Sr ratio of 0.7036±2 can be interpreted as the result of the increased water activity in the source. The presence of two different plagioclase phenocryst generations in the first lava subsequent to the caldera-causing event is indicative for a longer storage time of this magma at a shallower level. A model capable of explaining these observations involves three evolutionary stages. First stage, assimilation of lower crustal material by a primitive magma of mantle origin (as modelled by Nd–Hf isotope systematics). This stage ended by an interruption in replenishment that led to an increase of crystallization and, hence, an increase in viscosity, suppressing eruption. During this time gap, differentiation by fractional crystallization led to enrichment of incompatible species, especially aqueous fluids, to silica depolymerisation and to a decrease in viscosity, finally enabling eruption again in the third stage. [Copyright &y& Elsevier]

The western Hellenic subduction zone is characterized by a trenchward velocity of the upper plate. In the Ionian islands segment, complete seismic coupling is achieved, as is predicted by standard plate-tectonic models in which there is no slab pull force because the slab has broken off. The moderate local seismic moment rate relates to a shallow downdip limit for the seismogenic interface. This characteristic may be attributed to the ductility of the lower crust of the upper plate, which allows a décollement between the upper crust of the overriding plate and the subducting plate. Farther south, a deeper downdip limit of the seismogenic interface is indicated by thrust-faulting earthquakes, which persist much deeper in western Crete. A correspondingly larger downdip width of this seismogenic zone is consistent with the suggested larger maximum magnitude of earthquakes here. However, since the seismic moment release rate seems to be moderate in the Peloponnese and western Crete, like in in the Ionian islands, this seismically active interface cannot maintain complete seismic coupling across its larger downdip width. A cause may be the lateral addition of overweight to the part of the slab still attached in Crete, by the free fall of its part that has broken off from the surface further north. This increased slab pull reduces the compressive normal stress across the seismogenic interface and thus causes partial seismic coupling in its shallower part. However, the width of this part may provide an additional area contributing to slip in large earthquakes, which may nucleate deeper on stick-slip parts of the interface. Hints at anomalies in structure and seismicity, which need to be resolved, may relate to the present location of the edge of the tear in the slab. [Copyright &y& Elsevier]

The Anaximander Mountains, occupying an outer-arc position at the junction of the Hellenic and Cyprus arcs, have been studied using a suite of geophysical data including multibeam swath bathymetry, backscatter images, seismic reflection profiles and gravity and magnetic data, which support an improved description of its deformation history. Lithologically, the western mountains can be correlated with the neritic limestones of the Bey Dağlarý unit of SW Turkey. The eastern mountains relate to the ophiolitic Antalya Nappe Complex. In addition to this lithological contrast, a deep-seated crustal difference, which is best reflected in gravimetry, relates to the mid-Tortonian Aksu thrust phase. After the Early–Middle Miocene eastward emplacement of the Lycian Nappes, the Serravallian–Tortonian stage was characterized by the development of an array of grabens with N120°E strikes, which occupied a vast continental area that extended from southern Aegean to southwestern Turkey. During the mid-Tortonian, the last phase of thrusting marked the onset of a different kinematic regime related to the westward rotation of the Anatolian platelet. This Late Miocene change marked the start of differential subsidence that resulted in the formation of the Anaximander Mountains and which is reflected by an unconformity surface between the Lower–Middle Miocene and Plio–Quaternary units. The Messinian–Quaternary period in the western part of the Anaximander Mountains was characterized by distributed sinistral shear parallel to N70°E, which was marked by the onset of an extension on N20°E-striking normal faults that formed long graben-like depressions. During the Pliocene, these basins were transected by N70°E-striking sinistral strike-slip fault zones, although continued crustal extension suggests deformation in transtension. The eastern part of the Anaximander Mountains is instead characterized by N150°E-striking normal and/or oblique normal fault zones, which lack significant evidence of strike-slip deformation. Close examination of newly calculated predicted relative plate motions between Africa (from NUVEL-1A) and Anatolia (from GPS measurements) indicates that plate motion vectors change rapidly at the junction between the Hellenic and Cyprus arcs because of the close proximity to the pole of Anatolian rotation. These calculations indicate that along both the eastern Hellenic Arc and the western Cyprus arcs (the Florence Rise), the relative motion between the Anatolian and African plates is sinistral. On the southern Florence Rise, deformation is characterized by pure left-lateral faulting, whereas farther northwest, in the eastern Anaximander Mountains, the component of strike-slip decreases. Although arc-normal convergence predicts the occurrence of thrust faulting, southwestward trench retreat also causes an extension internal to this outer-arc domain, such that the preexisting N120°E-striking thrust faults of the Aksu phase have been reactivated as normal to oblique normal faults during the Pliocene and Quaternary. The observed sinistral faulting on faults striking N70°E in the western mountains is consistent with the relative plate motion along the eastern Hellenic Arc and merges with the extensional domain in the eastern mountains. [Copyright &y& Elsevier]

The Hellenic arc is the most active seismic region of the transition zone between the African and Eurasian plates. In this rapidly deforming region, the physical state of the crust and upper mantle may vary laterally and vertically. However, a detailed 3D model of the region is lacking. The reliable determination of fault plane solutions is crucial if such a model is to be produced. On this basis, fault plane solutions have been determined from recent microearthquakes and small events to assess their reliability for use in tectonic interpretation. A search algorithm on first motion data and amplitude ratios and also a waveform-matching scheme were employed on data from our temporal network, operated on the western part of the island of Crete. There, a network of 47 three-component digital stations with an aperture of 60 km and an average station spacing of 5 km was operated in 1997. The majority of the shallow microearthquakes under Crete for which fault plane solutions were determined have a near horizontal T-axis oriented essentially E–W. Differences between well determined fault plane solutions from the search algorithm and the waveform-matching technique were significant (strike up to 45°, dip up to 10°, and slip up to 50°). Second, data of small events within the Hellenic subduction zone were obtained from recently installed broadband stations at local and regional distances and inverted for a deviatoric seismic moment tensor. Available solutions from other sources such as the Harvard group, EMSC, or SED show drastic differences in part. Owing to still limited azimuthal station coverage, the resulting fault plane solutions of small events have generally not been well constrained. The observed large variation of fault plane solutions for micoearthquakes and small events poses a caveat to tectonic interpretation in case of using data from a few stations only. [Copyright &y& Elsevier]

The western Hellenic arc has been commonly considered as a largely aseismic subduction zone, from the comparison of a small rate of shortening derived from the seismic moment release, with a large rate of convergence inferred from geology. Complete seismic coupling would instead be expected from models that consider a control by plate tectonic forces, because of the trenchward velocity of the Hellenic–Aegean upper plate now confirmed with GPS measurements. In the region of the Ionian Islands, a subduction interplate boundary has been recently imaged and its seismogenic downdip width suggested to be moderate, from reflection seismic profiling and local earthquake tomography. In the appropriate model for such an earthquake source region, which considers a single interplate fault and takes into account these features, the moderate seismic moment release is found consistent with complete seismic coupling of this subduction. The shallow downdip limit of the seismogenic zone can be interpreted as due to the interplate boundary being overlain there by the ductile deeper crust of the orogenically thickened Hellenides. [Copyright &y& Elsevier]

The Hellenic subduction region displays the same precursory swarm phenomenon as has been found in comparable regions of New Zealand and Japan. In the earthquake catalogue of the Aristotle University of Thessaloniki, 10 past sequences of precursory swarms and related major mainshock events have been identified. These correlate, in respect of location, magnitude and time, with the 9 sequences previously identified in New Zealand, and 9 in Japan, bringing the total of sequences to 28, and the totals of related events (allowing for clustering) to 56 precursory swarms and 42 mainshock events. The results add strength to the hypothesis that swarms are long-range predictors of mainshock events. A close similarity between the swarm and aftershock magnitudes in a given sequence is also confirmed in Greece, supporting the proposal that swarms are an integral part of the seismogenic process in subduction regions. Further, the modelling of swarms as part of an overall increase in seismicity, the onset of which marks the onset of seismogenesis, is well illustrated from past sequences in Greece. Formal tests are being carried out in Greece, in parallel with New Zealand and Japan, to ascertain the performance of the hypothesis as a basis for long-range synoptic forecasting.