Hellenic Arc Research Articles

Chronology of Quaternary shoreline progradational sequences related to eustatic sea-level changes: Sedimentation and subsidence in Saronikos Gulf, Greece

The stacking pattern of shoreline progradational sequences in subsiding basins have been used as a chronologic tool in the Mediterranean region. Most previous studies have used a few key seismic lines seaward of deltas, where the record is complicated by deltaic distributary switching. Southeastern Saronikos Gulf, in the western Aegean Sea at the northwestern end of the South Aegean Arc, lacks large river input of sediment. The ENE-WSW and NW-SE trending neotectonic Aegina and Methana basins lie between the tectonically active Gulf of Corinth and the relatively stable Cyclades plateau. New high-resolution seismic profiles from southeastern Saronikos Gulf have been interpreted according to the principles of seismic and sequence stratigraphy. Alpine basement and marine Pliocene sediments have been recognized in the surrounding basin margins and more than 300 m and 150 m of unconsolidated sediments have been mapped in Aegina and Methana basin respectively. Progradational clinoforms that toplap against transgressive surfaces, have been traced along strike, leading to the establishment of a robust chronostratigraphic framework further controlled by sedimentation rates as extracted from cores. Southeastern Saronikos Gulf poses an outstanding example of development of progradational wedges formed within a microtidal, wave dominated regime of low sedimentation rates, between 2.8 cm/ka and 3.2 cm/ka, within the framework of low subsiding basins with rates between 0.03 and 0.31 m/ka. In this environment, more than 180 well preserved shoreline progradational units, with attributes similar to the ones reported from the Aegean and Eastern Mediterranean seas, have been mapped and correlated to sea level lowstands back to 866 ka. Deeper Mid-Pleistocene progradational units are also recognized, tentatively back to 1.4 Ma. Subsidence rate shows a dramatic decrease in the last 1 Ma and significant numbers of faults became inactive during the last 130 ka. In Early-Middle Quaternary, Methana basin was an isolated lake and extended terrestrial areas were shaped in the Poros-Agios Georgios plateau during sea level lowstands. Aegina basin was continuously connected to the Aegean through the Agios Georgios fault valley. This study demonstrates the robustness of chronology based on stacked shoreline progradational sequences and its applicability to determining rates of tectonic subsidence.

Tsunami Propagation and Flooding in Sicilian Coastal Areas by Means of a Weakly Dispersive Boussinesq Model

This paper addresses the tsunami propagation and subsequent coastal areas flooding by means of a depth-integrated numerical model. Such an approach is fundamental in order to assess the inundation hazard in coastal areas generated by seismogenic tsunami. In this study we adopted, an interdisciplinary approach, in order to consider the tsunami propagation, relates both to geomorphological characteristics of the coast and the bathymetry. In order to validate the numerical model, comparisons with results of other studies were performed. This manuscript presents first applicative results achieved using the weakly dispersive Boussinesq model in the field of tsunami propagation and coastal inundation. Ionic coast of Sicily (Italy) was chosen as a case study due to its high level of exposure to tsunamis. Indeed, the tsunami could be generated by an earthquake in the external Calabrian arc or in the Hellenic arc, both active seismic zones. Finally, in order to demonstrate the possibility to give indications to local authorities, an inundation map, over a small area, was produced by means of the numerical model.

Effusive-explosive transitions of water-undersaturated magmas. The case study of Methana Volcano, South Aegean Arc

Most arc volcanoes erupt intermediate to silicic magmas that have sufficient volatile contents to behave explosively. Despite this explosive potential, low-energy effusive eruptions of such viscous water-rich magmas are common occurrences. Hence, predicting the style of the next eruption, with its obvious importance on hazard mitigation, remains one of the main challenges of volcanology. Here, we investigate the changes in eruptive styles at Methana volcano, from the South Aegean Arc. This volcano has generated multiple andesite-to-dacite eruptions, with the last event occurring in historical times (~2250 BP). We focus on 14 eruptive events, 3 being explosive. We compare the petrology and geochemistry of the deposits to reconstruct the magma chamber events that preceded both types of eruptions. We estimate the thermo-chemical conditions that characterized each eruptive event, and highlight the causes that promoted or inhibited magma fragmentation: temperature, pressure, composition, relative volumes of different magma batches, dissolved water content, crystallinity, melt and bulk viscosities. The results indicate that Methana harbors an upper-crustal silicic reservoir stored at ~2 kbar (likely dacitic to rhyodacitic in composition) that is in a highly crystalline state (>50 vol% crystals, mostly uneruptible). All the eruptions are triggered by deeper magma recharge ascending from storage pressures of ~4–5 kbar, leading to some rejuvenation of the shallow silicic mush. The most mafic recharges (~55 wt% SiO2, ~1000 °C, >3 wt% H2O) promote effusive eruptions after mixing and hybridizing the upper-crustal reservoir (hybrids of ~57–64 wt% SiO2, ~900–950 °C, generally 3–4 wt% H2O). Slightly colder, wetter and more differentiated recharges (~62 wt% SiO2; ~900 °C, ~4 wt% H2O) trigger explosive events after having minimal interaction with the upper-crustal reservoir. However, the dissolved water content is not the only factor influencing explosivity, as some of the wettest magmas (>4.5 wt% H2O) generate lava flows. Our data indicate that a key control on effusive-explosive transitions at Methana is the crystallinity of the erupted material. A high crystallinity (40–55 vol%) increases the bulk-viscosity, which results in the magma having a slower ascent velocity. In addition, crystallinity enhances the formation of permeable pathways for the gas. Longer ascent timescales and enhanced permeability in the conduit improve the outgassing potential of the magmas and lead to effusive behavior. The opposite occurs for explosive events. Here, the lower crystallinity of the magmas (30 vol%) translates into lower bulk-viscosities and faster ascent rates, which inhibit outgassing. This case study highlights the importance of obtaining better constraints on the state of subvolcanic magma reservoirs, in particular concerning crystallinity and volatile concentrations, which has to become a priority in the years to come.

The 25 October 2018 Mw = 6.7 Zakynthos earthquake (Ionian Sea, Greece): A low-angle fault model based on GNSS data, relocated seismicity, small tsunami and implications for the seismic hazard in the west Hellenic Arc

We present a joint analysis of seismological and Global Navigation Satellite System (GNSS) data of the seismic sequence that started on October 25, 2018 with a shallow Mw = 6.7 earthquake offshore Zakynthos (Ionian Sea, Greece). The inversion of the GNSS data shows the activation of a N-S striking thrust/oblique-slip fault at the African-Aegean plate interface with a length of ∼26 km, and depth shallower than 15 km. The fault-plane geometry is well constrained by GNSS with a low-dip angle (23°), and dip-direction towards east. This is consistent with the distribution of the relocated aftershocks (1811 events). Our analysis indicates that the October 25, 2018 event ruptured the Hellenic megathrust. This event highlights the high degree of seismic coupling in the western region of the Hellenic Arc. It also highlights the “strong” nature of the subducting slab with the occurrence of “locked” patches under the Ionian seafloor that fail during large, reverse/oblique-slip earthquakes. The latest, shallow seismic events of the western Hellenic arc (1976 Zakynthos, 1997 Strofades, 2018 Zakynthos) point to a sequence of low-angle events along the plate interface with most of co-seismic deformation taken up by the upper (Aegean) plate. The GNSS data also point to a similar pattern between the co-seismic strain release and the long-term (interseismic) strain accumulation along the west Hellenic Arc.

Melt and fluid evolution in an upper-crustal magma reservoir, preserved by inclusions in juvenile clasts from the Kos Plateau Tuff, Aegean Arc, Greece

Pre- to syn-eruptive fluids released by the magmatic system forming the non-welded rhyolitic Kos Plateau Tuff ignimbrite, Greece, were characterized using fluid inclusions entrapped in quartz from granitic clasts entrained from the roof or walls of the feeding magma reservoir. This fluid was initially of intermediate density and relatively low salinity (3–11 wt% NaClequivalent). Following significant pressure drop(s), it separated into immiscible brine and CO2-bearing vapor occurring as texturally less mature inclusions. The transition from single- to two-phase-fluid occurred at magmatic conditions, as supported by high fluid inclusion homogenization temperatures (ca. 700 °C) and simultaneous trapping of fluid inclusions with silicate melt inclusions. The large recorded pressure drop together with texturally immature fluid inclusion shapes suggests a possible direct relation between the fluid phase change and the incipient 161 ka caldera-forming eruption. All inclusion types occur predominantly along (pseudo-) secondary trails in the granitic clasts, indicating efficient entrapment in microfractures formed at high crystallinity, as attested to by elevated concentrations of incompatible elements in the silicate melt inclusions.Fast quenching of the inclusions due to eruption prevented significant post-entrapment chemical modification, providing pristine samples of magmatic fluid compositions. Element concentrations in co-existing fluids and silicate melt were analyzed using laser ablation inductively-coupled plasma mass spectrometry and compared in view of element transporting capabilities and magmatic-hydrothermal ore formation. Economically important metals such as Cu, Zn, Mo, and W were extracted from the silicate melt by the fluid. In particular, Cu concentrations in the single-phase fluid in the range of 100–500 ppm agree well with previous experimental and modeling studies and, therefore, represent values expected for primary magmatic fluids. Most elements got further enriched in the brine upon phase separation, except Li, B, and As, which may have contributed significantly also to the vapor phase. Considering the similarity between vapor and intermediate-density fluid in terms of salinity, however, the metal-enriched brine phase only comprised a minor fraction of the bulk fluid.

Active crustal deformation and rotations in the southwestern Balkans from continuous GPS measurements

The western limb of the Hellenic Arc defines the boundary of a large intracontinental active extensional domain covering the Aegean and the southwestern part of the Balkan peninsula. Along this boundary a transition from collision in the north to subduction in the south is associated with post-Miocene clockwise rotations of ∼50∘. We present a new GPS velocity field that, with new permanent station velocities in Albania, Bulgaria, Kosovo, Montenegro, and Northern Macedonia, provides insights into previously unresolved aspects on the large-scale dynamics of continental lithosphere and on the relation between long and short-term kinematics. In particular we address (1) the kinematic description of the collision/subduction transition, (2) the relation between long-term finite rotations with geodetically-measured instantaneous rotations, (3) the forces maintaining and resisting the deformation of the extensional domain and (4) the extent of its northern boundary. We use the analogy of rigid elongate inclusions in the velocity field to predict the senses and approximate rates of rotation of crustal blocks in the deforming continental region. Across the collision/subduction transition zone a large rotational pattern in the velocity field is found whose spatial pattern and sense of rotation is compatible with the observed paleomagnetic rotations through the occurrence of fault-bounded block rotations. Geodetic observations show that clockwise vertical axis rotation of the western limb of the Hellenic margin is active today, with distribution and rates that are essentially the same as that defined by the paleomagnetic data for the last 5 Ma. The maximum rates of the rotational component of the velocity field is found along the wide arrangement of fault-bounded blocks in central Greece that accomodate the NE-SW dextral shear transferred from the northern Aegan trough. The northern limit of the rotating margin defines a hinge (Scutari-Pec transverse zone) that has remained stationary throughout the Middle Miocene (relative to the upper plate of the collision/subduction system) with important implications for the distribution of strength of the western margin of the Hellenic Arc. The distribution and style of deformation along the western boundary of the extensional domain is controlled by the relation between gravitational forces, driving the flow of crustal material towards the low-lying adjacent regions sea floor, and heterogeneous resistive forces along the collision/subduction boundary.

Chemical Evolution of Calc-alkaline Magmas during the Ascent through Continental Crust: Constraints from Methana, Aegean Arc

AbstractQuaternary calc-alkaline andesitic to dacitic lavas effusively erupted on top of about 30 km thick accreted continental crust at Methana peninsula in the western Aegean arc. We present new data of major and trace element concentrations as well as of Sr–Nd–Pb isotope ratios along with mineral compositions of Methana lavas and their mafic enclaves. The enclaves imply a parental basaltic magma and fractional crystallization processes with relatively little crustal assimilation in the deep part of the Methana magma system. The composition of amphibole in some mafic enclaves and lavas indicates deeper crystallization at ∼25 km depth close to the Moho compared with the evolved lavas that formed at <15 km depth. The presence of amphibole and low Ca contents in olivine suggest high water contents of ∼4 wt% in the primitive magmas at Methana. The compositions of andesitic and dacitic lavas reflect fractional crystallization, assimilation of sedimentary material, and magma mixing in the upper 15 km of the crust. The Methana magmas have fO2 of FMQ + 1 to FMQ + 2 (where FMQ is the fayalite–magnetite–quartz buffer) at temperatures of 1200 to 750 °C and the fO2 does not vary systematically from mafic to felsic compositions, suggesting that the mantle wedge was oxidized by sediment subduction. Amphibole is an important fractionating phase in the more evolved Methana magmas and causes significant changes in incompatible element ratios. Although xenocrysts and mineral compositions indicate magma mixing, the major and trace element variation implies only limited mixing between dacitic and basaltic melts.

Geochemistry of lavas from Aegina and Poros (Aegean Arc, Greece): Distinguishing upper crustal contamination and source contamination in the Saronic Gulf area

Pliocene (Phase 1) deposits from the volcanic centres of Aegina and Poros (western Aegean Arc) consist of andesitic-dacitic lava flows and domes with mafic enclaves, whereas basaltic-andesitic and rhyolitic lava flows were emplaced during the early Pleistocene Phase 2 and its precursor activity. Fifty-nine new whole rock analyses of lavas and enclaves show geochemical characteristics in keeping with their continental arc setting, with a significant spread in Sr-Nd-Hf-Pb isotope compositions and a strong crustal imprint (e.g. 87Sr/86Sr 0.7041–0.7076). Unlike other Aegean volcanic centres, the Phase 1 deposits show high Ba concentrations and high Ba/Th ratios. These are unlikely to be a signature of fluid-mediated metasomatism of the mantle wedge; subduction of barite-enriched sediments appears a more plausible option. The amount of sediment added to the mantle wedge is ca. 4%. High 208Pb/204Pb relative to 206Pb/204Pb, especially for the Poros lavas, points towards contamination with continental crust similar to the basement of Ios. First-order modelling shows that up to 35% crustal contamination took place. Additionally, crystal fractionation, magma mixing and crystal accumulation shaped the whole rock geochemistry. Similar to the neighbouring island of Methana, the widest isotopic variation is seen in the early Pleistocene deposits, reflecting remobilisation of magma from the previous volcanic episode by isotopically more primitive mafic magmas after a period of volcanic quiescence. The isotopic contrast between the volcanic centres of the Saronic Gulf and those of Santorini and Kos-Nisyros further west is likely related to the geometry and physical characteristics of the subducting slab and mantle wedge, rather than the actual amount of sediments subducted or their composition.

Seismicity and geodynamics of western Peloponnese and central Ionian Islands: Insights from a local seismic deployment

The tectonic setting of western Peloponnese and central Ionian Islands, Greece, is characterized by the subduction of the oceanic African plate beneath the Aegean micro-plate. The transition from subduction to continental collision in northwestern Greece is accommodated by the right-lateral Cephalonia transform fault. In this work, we exploit the recordings of a temporary seismic network composed of 15 stations operating from July 2016 until May 2017 to investigate the complex deformation of this region.Our local network fills in a major observational gap in one of the most tectonically active regions of the Hellenic arc. We detected and located more than 1200 local earthquakes and constrained five 1D optimum local velocity models. The relocated seismicity (including the aftershock sequence that followed the October 2018 Mw 6.7 earthquake offshore Zakynthos) and associated focal mechanisms constrained for the major earthquakes point out a complex crustal deformation.We propose a clockwise rotation of the Ionian Akarnania Block accommodated by major marginal strike-slip fault zones that appear segmented along their strike. Additionally, left-lateral motion is observed on the Kyllini-Cephalonia fault along a north-west direction. Finally, the seismicity recorded in north Cephalonia (offshore Myrtos and Fiskardo) suggests that the Cephalonia transform fault is a large deformation zone where secondary WNW-striking sinistral strike-slip faults occur.

The 2018 Mw 6.8 Zakynthos (Ionian Sea, Greece) earthquake: seismic source and local tsunami characterization

SUMMARYWe investigated the kinematic rupture model of the 2018 Mw 6.8 Zakynthos, Ionian Sea (Greece), earthquake by using a non-linear joint inversion of strong motion data, high-rate GPS time-series and static coseismic GPS displacements. We also tested inversion results against tide-gauge recordings of the small tsunami generated in the Ionian Sea. In order to constrain the fault geometry, we performed several preliminary kinematic inversions by assuming the parameter values resulting from different published moment tensor solutions. The lowest cost function values were obtained by using the geometry derived from the United States Geological Survey (USGS) focal solution. Between the two conjugate USGS planes, the rupture model which better fits the data is the one with the N9°E-striking 39°ESE-dipping plane. The rupture history of this model is characterized by a bilateral propagation, featuring two asperities; a main slip patch extending between 14 and 28 km in depth, 9 km northeast from the nucleation and a slightly shallower small patch located 27 km southwest from the nucleation. The maximum energy release occurs between 8 and 12 s, when both patches are breaking simultaneously. The maximum slip is 1.8 m and the total seismic moment is 2.4 × 1019 Nm, corresponding to a Mw value of 6.8. The slip angle shows a dominant right-lateral strike-slip mechanism, with a minor reverse component that increases on the deeper region of the fault. This result, in addition to the observed possibility of similar mechanisms for previous earthquakes occurred in 1959 and 1997, suggests that the tectonic deformation between the Cephalonia Transform Fault Zone and the northern tip of the Hellenic Arc Subduction zone may be accommodated by prevailing right lateral low-dipping faults, occurring on re-activated structures previously experiencing (until Pliocene) compressional regime. Comparison of predicted and observed tsunami data suggests the need of a better characterization of local harbour response for this type of relatively short-wavelength events, which is important in the context of tsunami early warning. However, the suggested dominantly strike-slip character would in turn imply a reduced tsunami hazard as compared to a dominant thrust faulting regime from this source region.

The Tsunami Inundation Hazard of the Maltese Islands (Central Mediterranean Sea): A Submarine Landslide and Earthquake Tsunami Scenario Study

The tsunami hazard for the Maltese Islands is poorly defined, and historic records are available for only two recent events. Most of the population and touristic infrastructure of the archipelago is concentrated along the eastern low-lying coastline, which is exposed to tsunamis from near-field and far-field sources. In this study we present a scenario-based tsunami inundation study to assess the impact of potential significant cases. We simulated four scenarios—two submarine landslide sources (outer Malta Plateau slide and Gela Basin slide) and two earthquake sources mimicking events comparable to the 365 A.D. western Hellenic Arc event and the 1693 south-east of Sicily event. We find that all scenarios cause inundation in densely populated low-lying bays or rias of Mellieha Bay, Xemxija, Salini, Gzira, Msida, Marsaskala, St Thomas Bay, Marsaxlokk and Birzebbuga. The largest inundation extents and flow depths (> 10 m) are produced by the two landslide sources and the western Hellenic Arc earthquake. Of interest is the role of the Malta Escarpment and Sicily in amplifying and reflecting tsunami waves, and in generating consistent hot spots along the eastern coastline of Malta.

Tsunami hazard and risk assessment for Alexandria (Egypt) based on the maximum credible earthquake

Throughout its history, the northern coast of Alexandria has experienced devastating earthquakes and tsunamis impacts from tsunamigenic sources in the Eastern Mediterranean. The most hazardous tsunami events for Northern coast of Egypt were related to the earthquakes of 365 in Crete with Mw8.5, 1222 in Cyprus with Mw 7–7.5, and 1303 in Rhodes Island with Mw8.0.The 365 and 1303 earthquakes were accompanied by tsunamis that resulted in widespread destruction and subsequent fatalities along the coast of Alexandria as evidenced by the available historical reports for the Mediterranean region, geomorphologic and paleo-tsunami investigations accomplished recently.In this work, for each tsunamigenic source, i.e., West Hellenic Arc, East Hellenic Arc, and Cyprian Arc, the maximum credible earthquake (MCE) is defined then modeled with NAMI-DANCE. The comparison of the tsunami inundation maps of MCE scenarios computed for each tsunami source shows that the East Hellenic arc (EHA) is the most hazardous source for the Alexandria coast, with a tsunami wave height of 5.5–6.0 m, while the Cyprian arc (CA) is the least hazardous source.The aggregated tsunami inundation map for Alexandria defines the furthest boundary between inundated and non-inundated lands and is associated with hazard levels based on water heights. This map shows that the coasts of Al Amereya district and Borg Al Arab city are expected to be more affected than the others districts. It also indicates that the west side of Alexandria, called Al Sahel Al Shamally, is expected to have less impact than the eastern side, because it is naturally protected by a barrier of carbonate material parallel to the shoreline.The exposure analysis shows that more than 85,000 people are located in the tsunami hazard zone (representing 15.8% of Alexandria governorate's population). Regarding buildings and infrastructure, the most exposed district is Al Amereya with 65.8% of total exposed objects. Among the buildings, the residential ones are the most exposed and about 21% can be classified as highly vulnerable.In terms of risk assessment, the Al Amereya district is identified as the area with the highest tsunami risk.

Tectonism and Its Relation to Magmatism Around Santorini Volcano From Upper Crustal P Wave Velocity

AbstractAt extensional volcanic arcs, faulting often acts to localize magmatism. Santorini is located on the extended continental crust of the Aegean microplate and is one of the most active volcanoes of the Hellenic arc, but the relationship between tectonism and magmatism remains poorly constrained. As part of the Plumbing Reservoirs Of The Earth Under Santorini experiment, seismic data were acquired across the Santorini caldera and the surrounding region using a dense amphibious array of >14,300 marine sound sources and 156 short‐period seismometers, covering an area 120 km by 45 km. Here a P wave velocity model of the shallow, upper‐crustal structure (<3‐km depth), obtained using travel time tomography, is used to delineate fault zones, sedimentary basins, and tectono‐magmatic lineaments. Our interpretation of tectonic boundaries and regional faults are consistent with prior geophysical studies, including the location of basin margins and E‐W oriented basement faults within the Christiana Basin west of Santorini. Reduced seismic velocities within the basement east of Santorini, near the Anydros and Anafi Basins, are coincident with a region of extensive NE‐SW faulting and active seismicity. The structural differences between the eastern and western sides of Santorini are in agreement with previously proposed models of regional tectonic evolution. Additionally, we find that regional magmatism has been localized in NE‐SW trending basin‐like structures that connect the Christiana, Santorini, and Kolumbo volcanic centers. At Santorini itself, we find that magmatism has been localized along NE‐SW trending lineaments that are subparallel to dikes, active faults, and regional volcanic chains. These results show strong interaction between magmatism and active deformation.

Reconciling the stratigraphy and depositional history of the Lycian orogen-top basins, SW Anatolia

Terrestrial fossil records from the SW Anatolian basins are crucial both for regional correlations and palaeoenvironmental reconstructions. By reassessing biostratigraphic constraints and incorporating new fossil data, we calibrated and reconstructed the late Neogene and Quaternary palaeoenvironments within a regional palaeogeographical framework. The culmination of the Taurides in SW Anatolia was followed by a regional crustal extension from the late Tortonian onwards that created a broad array of NE-trending orogen-top basins with synchronic associations of alluvial fan, fluvial and lacustrine deposits. The terrestrial basins are superimposed on the upper Burdigalian marine units with a c. 7 myr of hiatus that corresponds to a shift from regional shortening to extension. The initial infill of these basins is documented by a transition from marginal alluvial fans and axial fluvial systems into central shallow-perennial lakes coinciding with a climatic shift from warm/humid to arid conditions. The basal alluvial fan deposits abound in fossil macro-mammals of an early Turolian (MN11–12; late Tortonian) age. The Pliocene epoch in the region was punctuated by subhumid/humid conditions resulting in a rise of local base levels and expansion of lakes as evidenced by marsh-swamp deposits containing diverse fossil mammal assemblages indicating late Ruscinian (late MN15; late Zanclean) age. A second pulse of extension, accompanied by regional climatic changes, prompted subsequent deepening of the lakes as manifested by thick and laterally extensive carbonate successions. These lakes, which prevailed c. 1 myr, later shrank due to renewed progradation of alluvial fans and eventually filled up and dried out, reflected by marsh-swamp deposits at the top of a complete lacustrine succession that contains diverse micro-mammal assemblages indicating a latest Villanyian (MN17; Gelasian) age. A third pulse of tectonic reorganisation and associated extension dissected the basins into their present-day configuration from the early Pleistocene onwards under warm/humid climatic conditions. The new age data provide means to correlate deposits across various basins in the region that help to place the basin development into a regional tectonic framework, which can be attributed to the consequence of the well-articulated regional phenomena of slab-tear/detachment-induced uplift followed by crustal extension and basin formation (late Tortonian), the outward extension of the Aegean arc (early Pliocene) and eventually accompanied by westward extrusion of the Anatolian Plate (early Pleistocene).

Tsunami Potential of Moderate Earthquakes: The July 1, 2009 Earthquake (Mw 6.45) and its Associated Local Tsunami in the Hellenic Arc

On July 1, 2009, a Mw 6.45 earthquake ruptured south of Crete Island (Greece) along the Hellenic Arc triggering a local tsunami. Eyewitness reported the tsunami from Myrtos and Arvi (south-eastern Crete) and from the north of Chrisi Islet, located to the southeast of Crete. The earthquake occurred offshore, about 80 km south of Crete, where routine earthquake locations are poor. The hypocentre is relocated using a 2-D velocity model and several local 1-D velocity models. Epicentral locations obtained by using the different velocity models show very minor variations. Instead, relocated hypocentres can be grouped into two sets of solutions: (1) those with a shallower depth (depth 25 km) either in the upper-plate or along the plate interface. Models accounting for either homogeneous or heterogeneous slip on the causative fault are tested, with the heterogeneous one better reproducing the observations in terms of number of tsunami waves reaching the shoreline and duration of the sea disturbance. The short travel time, about 10 min, of the first tsunami arrival at the southern coast of Crete represents a big challenge for tsunami early warning systems operating in the area.

The Kos–Nisyros–Yali Volcanic Field

The Kos–Nisyros–Yali volcanic field has produced a range of volcanic products over the last 3 million years. Volumetrically, silicic magma dominates, and activity includes one of the largest known explosive eruptions of the Aegean arc, the >60 km3 (dense-rock equivalent), 161 ka rhyolitic Kos Plateau Tuff. The Kos–Nisyros–Yali volcanic field is situated within an area of active crustal extension, which has greatly influenced magmatic processes and landscape development in the region. Recent seismic unrest, surface deformation and intense geothermal activity indicate that the system remains active, particularly around the Nisyros and Yali edifices. These signs of magmatic activity, together with the fact that the most recent eruptions have become increasingly silicic, would justify detailed monitoring of the area.

Eastern Mediterranean volcanism during marine isotope stages 9 to 7e (335–235 ka): Insights based on cryptotephra layers at Tenaghi Philippon, Greece

Tephra layers preserved in distal sedimentary archives represent chronicles of explosive volcanism that can complement the often more fragmentary information from near-source volcanic deposits to establish complete volcanic histories. With regard to these aspects, the Middle Pleistocene of the Eastern Mediterranean region stands out as it has a complex and diverse, but as yet largely unexplored record of volcanic eruptions. Here we present the first distal tephra record for the Eastern Mediterranean region spanning from 335 to 235 ka (corresponding to Marine Isotope Stages [MIS] 9 to 7e); our record has been derived from peat cores from the iconic terrestrial climate archive of Tenaghi Philippon (NE Greece). We have identified twenty-seven cryptotephra layers that represent eruptions from diverse Mediterranean sources. Six cryptotephra layers can be linked to Campanian volcanoes, and another six layers are tentatively correlated to Aeolian Arc volcanism. Of the ten cryptotephra layers that we have identified as deriving from the Aegean Arc, eight originate from Santorini volcano and two are tentatively attributed to either Kos or Milos. Five cryptotephra layers have yet unknown origins.Most of the identified cryptotephras represent previously undocumented eruptions. We provide age estimates for all cryptotephras and, by extension, for the underlying eruptions based on orbitally tuned pollen data from the same cores. The only cryptotephra layer in the 335–235 ka record from Tenaghi Philippon that represents a previously known eruption has a palynostratigraphically derived age of c. 289 ka and can be tentatively linked to the Seiano Ignimbrite from the Campanian Volcanic Zone; this represents the first time that this eruption can be traced beyond its proximal area. The documentation and geochemical characterization of tephra layers from different Mediterranean sources in the Tenaghi Philippon peat cores for MIS 9–7e is an important step towards the integration of regional Mediterranean tephrostratigraphic information for the Middle Pleistocene.

Dramatic Pliocene–Quaternary subsidence of the southern Rhodes Basin and concomitant north-tilting and uplift of the Anaximander Mountains, the junction of Hellenic and Cyprus arcs, eastern Mediterranean Sea

The Anaximander Mountains and the Rhodes Basin at the junction of Cyprus and Hellenic arcs are one of the tectonically most complex regions in the eastern Mediterranean. Harmonically-tilted Messinian and the uppermost Messinian–Pliocene successions across the northern and eastern margins of the Rhodes Basin showed that these regions experienced dramatic subsidence exceeding ~1500–2500 ms (~1125–1875 m @ 1500 m s−1) during the upper Pliocene–Quaternary. Tilted upper Pliocene–Quaternary growth strata wedges showing apparent north-directed downlap along the northern foothills of the Anaximander Mountain suggested that this region experienced ~1000–2500 ms (~750–1125 m) concomitant uplift. The absence of faults showing extensional stratigraphic separations and the relative subsidence rates calculated using amount of tilting collectively suggest that the Rhodes Basin experienced sagging of its deeper basinal regions, which is attributable to the loading effect of the building of the Tauride Mountains modified by the vertical axis rotations of the Beydağları Block and the Island of Rhodes, and the horizontal axis rotation of the Anaximander Mountain.Messinian and uppermost Messinian–Pliocene were tectonically relatively quiet as shown by the harmonically structured and isopachous successions across the Rhodes Basin and Anaximander Mountain. This interval coincides with the paleomagnetically-determined 9° counterclockwise vertical axis rotation of the Island of Rhodes. The dramatic subsidence observed within the Rhodes Basin and the concommitant uplift of the Anaximander Mountain and its western extension occurred broadly coincident with southeast tilting of the Island of Rhodes and its associated 500–600 m subsidence between 2.5 and 1.8 Ma, the northwest tilting between 1.5 and 1.1 Ma, and the associated re-emergence of the southeastern coast. The large counterclockwise rotation of the Island of Rhodes since 0.8 Ma corresponds with the enormous subsidence observed across the northeastern sector of the Rhodes Basin, which brought the shelf-edge deltas from the near-sea level to their present-day depth of 1200–1500 m.

The volcanic history of Pyrgousa—volcanism before the eruption of the Kos Plateau Tuff

The islet of Pyrgousa, 8 km west of Nisyros, has andesites overlain by proximal deposits of the Kos Plateau Tuff (KPT). These andesites record the evolution of the Kos-Nisyros volcanic centre prior to the large KPT eruption at 161 ka. This precursory activity had previously been recorded only in dacites and rhyolites in the Kefalos Peninsula, farther from the volcanic centre. This study investigated the age, geochemistry, mineralogy, and petrogenesis of the andesite domes, probable flows, and associated talus breccias on Pyrgousa. Analysed samples are basaltic andesite and andesite, with strong enrichment in Ba and Sr and low values of Ti and Zr. An 40Ar/39Ar date of 1.9 ± 0.1 Ma on biotite is similar to dates from dacite and rhyolite stocks and domes in the Kefalos Peninsula, and like those dates is the maximum age due to excess 40Ar. There is no evidence on Pyrgousa for a stratovolcano precursor of the KPT eruption. The andesite domes geochemically resemble Pliocene domes in Methana and Aegina that mark the onset of magmatism in the northwestern South Aegean Arc, with an important magma component derived from subcontinental lithospheric mantle. These early andesites heated the upper crust, thus facilitating the growth of upper crustal magma chambers, which were filled by felsic crystal mush derived by differentiation of hydrous intermediate magmas with a more asthenospheric signature.

Turkey and adjacent area seismicity forecasts from earthquake inter-event time mean ratio statistics

The alarm-based forecasting model for earthquakes called moment ratio (MR) is retrospectively tested on Turkey and adjacent area seismicity. This model uses the ratio of the mean inter-event time over the variance as a precursory alarm function to forecast future earthquakes in a given region. In a former study, the MR model was successfully tested in forecasting large earthquakes with magnitude M ≥ 7, occurred in Japan. In this study, it is tested on Turkey and adjacent area seismicity using lower magnitude thresholds, namely by learning from M ≥ 5 events, to forecast earthquakes with magnitude M ≥ 6. For this purpose, a composite earthquake data file is compiled using Kandilli Observatory and Earthquake Research Institute Regional Earthquake and the Tsunami Monitoring Center (KOERI-RETMC) provided catalogs, for the period 1900–2016, and the SHARE European Earthquake Catalog (SHEEC) for the historical period 1000–1899. In this catalog, earthquakes are listed using surface magnitude scale Ms. The time periods used in training and testing are selected by taking into consideration the completeness of the magnitude. Finally, Molchan error diagrams are used to evaluate the forecasting performance of the MR method in practice using a retrospective test. Obtained results are presented as standard MR forecasting maps showing the overall forecasts and optimal maps showing high alarm areas with minimal miss and alarm rates. In addition, the relative intensity (RI) forecasting method is applied to compare different results. Results show MR forecasts outscoring random guessing with good performance compared to RI forecasts. The forecasting maps point to a small high alarm area situated along the Hellenic arc subduction zone east of Crete Island.