Slab top dips resolved by teleseismic converted waves in the Hellenic subduction zone

International audienceThe variations of the arrival times and polarities with backazimuth and distance of teleseismic P-to-S converted waves at interfaces bounding the slab crust under the upper plate mantle are used to constrain the depth, dip angle and azimuth of the slab of the Hellenic subduction zone. A grid search is designed to estimate the model parameters. Dip values of 16-18°, with an azimuth of 20° to 40°, are thus derived at 3 sites aligned over 50 km along the eastern coast of Peloponnesus. They are consistent with the variation from 54 to 61 km of the slab top depths constrained below each receiver. North of the Gulfs of Corinth and Evvia, a similar depth for the top of the slab is found at a distance from the subduction at least 100 km larger. This suggests flatter subduction of a different slab segment. Such a variation in slab attitude at depth across the region from south of the eastern Gulf of Corinth to north of Evvia is a candidate for the control of the recent or active localized crustal thinning of the upper plate we documented in earlier work, and of the surface deformation

Towards the new Thematic Core Service Tsunami within the EPOS Research Infrastructure

Tsunamis constitute a significant hazard for European coastal populations, and the impact of tsunami events worldwide can extend well beyond the coastal regions directly affected. Understanding the complex mechanisms of tsunami generation, propagation, and inundation, as well as managing the tsunami risk, requires multidisciplinary research and infrastructures that cross national boundaries. Recent decades have seen both great advances in tsunami science and consolidation of the European tsunami research community. A recurring theme has been the need for a sustainable platform for coordinated tsunami community activities and a hub for tsunami services. Following about three years of preparation, in July 2021, the European tsunami community attained the status of Candidate Thematic Core Service (cTCS) within the European Plate Observing System (EPOS) Research Infrastructure. Within a transition period of three years, the Tsunami candidate TCS is anticipated to develop into a fully operational EPOS TCS. We here outline the path taken to reach this point, and the envisaged form of the future EPOS TCS Tsunami. Our cTCS is planned to be organised within four thematic pillars: (1) Support to Tsunami Service Providers, (2) Tsunami Data, (3) Numerical Models, and (4) Hazard and Risk Products. We outline how identified needs in tsunami science and tsunami risk mitigation will be addressed within this structure and how participation within EPOS will become an integration point for community development

Towards the new Thematic Core Service Tsunami within the EPOS Research Infrastructure

Tsunamis constitute a significant hazard for European coastal populations, and the impact of tsunami events worldwide can extend well beyond the coastal regions directly affected. Understanding the complex mechanisms of tsunami generation, propagation, and inundation, as well as managing the tsunami risk, requires multidisciplinary research and infrastructures that cross national boundaries. Recent decades have seen both great advances in tsunami science and consolidation of the European tsunami research community. A recurring theme has been the need for a sustainable platform for coordinated tsunami community activities and a hub for tsunami services. Following about three years of preparation, in July 2021, the European tsunami community attained the status of Candidate Thematic Core Service (cTCS) within the European Plate Observing System (EPOS) Research Infrastructure. Within a transition period of three years, the Tsunami candidate TCS is anticipated to develop into a fully operational EPOS TCS. We here outline the path taken to reach this point, and the envisaged form of the future EPOS TCS Tsunami. Our cTCS is planned to be organised within four thematic pillars: (1) Support to Tsunami Service Providers, (2) Tsunami Data, (3) Numerical Models, and (4) Hazard and Risk Products. We outline how identified needs in tsunami science and tsunami risk mitigation will be addressed within this structure and how participation within EPOS will become an integration point for community development.publishedVersio

Towards the new Thematic Core Service Tsunami within the EPOS Research Infrastructure

publishedVersio

Quaternary evolution of the eastern Gulf of Corinth

The Gulf of Corinth is the second most active continental rift in the world and thus a much-studied natural laboratory for analyzing details of rift history. A new detailed offshore seismic survey combined with previously acquired data in its least studied part, the Lechaion Gulf, shed light on the tectono-sedimentary evolution of the eastern end of the Corinth rift. This study shows that: (i) the Lechaion Gulf is the submerged northern part of the onshore Corinth–Nemea basin, (ii) they are both bounded to the south by the north dipping Klenia and Kenchreai faults, which are considered at present inactive, (iii) both the Corinth–Nemea basin and the Lechaion Gulf were formed at around between 3.6 and 4 Ma BP (middle to late Pliocene), at the same time with the Megara basin, and (iv) the Lechaion Gulf was submerged and took its present shape at around between 0.7 and 1.7 Ma BP, at the same time with the Gulf of Corinth and the Alkyonides Gulf. Furthermore, sequence stratigraphy interpretation of seismic profiles from the Lechaion Gulf revealed: (i) a total post-alpine sediment thickness of almost 3 km below the Lechaion Gulf, (ii) at least 400 m of sediments accumulated during the last 245 ka, corresponding to a mean sedimentation rate of 1.3 m/ka for the last 245 ka and 2.3 m/ka for the Holocene, and (iii) differential vertical movement, in the order of 4.5 km, between the bedrock under the Lechaion Gulf and the adjacent mountains yields an accumulative average slip rate of 0.9 m/ka or less, over the last 4 Ma. The slip rate that was estimated for the Holocene is about 4.5 m/ka for the southern part and 2.5 m/ka for the northern part, while the extensional rate is similar with those calculated by geodetic surveys, about 7.7 mm/yr ± 1.0. The earthquake recurrence interval over the eastern end of the Corinth rift for the majority of the faults varies between ~100 and ~300 years, for magnitudes between 6.1 and 6.4. Finally, the high possibility of tsunami generation is investigated, while the tsunami propagation and its results on the coast are analyzed through modeling. Therefore, for estimating more accurately the slip rates, the uplift rates, the extensional rates and the earthquake recurrence interval over the whole Corinth rift, the presently mentioned tectono-sedimentary evolution of the Lechaion Gulf should be taken into consideration. The results of this research aim to contribute to the improvement of the existing models for the creation and evolution of the Corinth rift and the understanding of similar geological environments. They also hope to provide the necessary tools for emergency planners to be able to assess the risk and to develop tsunami risk reduction plans for the inhabitants of the coastal areas around the Gulf of Corinth, which in the future could be hit by a tsunami.Ο Κορινθιακός Κόλπος είναι η δεύτερη πιο ενεργή ηπειρωτική τάφρος στον κόσμο και ως εκ τούτου ένα ιδιαίτερο φυσικό εργαστήριο για την ανάλυση της ιστορίας διάνοιξης ηπειρωτικών τάφρων. Μια νέα λεπτομερής θαλάσσια σεισμική έρευνα σε συνδυασμό με δεδομένα που είχαν αποκτηθεί στο λιγότερο μελετημένο τμήμα του κόλπου, τον Κόλπο του Λέχαιου, έριξε φως στην τεκτονο-ιζηματογενή εξέλιξη του ανατολικού άκρου της Κορινθιακής τάφρου. Η μελέτη αυτή δείχνει ότι: (i) ο Κόλπος του Λέχαιου είναι το βυθισμένο βόρειο τμήμα της χερσαίας λεκάνης Κορίνθου-Νεμέας, (ii) και οι δύο, προς τα νότια οριοθετούνται από τα ρήγματα Κλένια και Κεχριές που κλίνουν βόρεια, τα οποία σήμερα θεωρούνται ανενεργά, (iii) τόσο η λεκάνη της Κορίνθου-Νεμέας όσο και ο κόλπος του Λέχαιου σχηματίστηκαν γύρω στα 3.6 - 4 Ma πριν από σήμερα (μέσο με ανώτερο Πλειόκαινο), ταυτόχρονα με τη λεκάνη των Μεγάρων και iv) ο κόλπος του Λέχαιου βυθίστηκε και πήρε το σημερινό του σχήμα από περίπου 0.7 έως 1.7 Ma πριν από σήμερα, ταυτόχρονα με τον Κορινθιακό Κόλπο και τον Κόλπο των Αλκυονίδων. Επιπλέον, η στρωματογραφική ερμηνεία των σεισμικών προφίλ από τον κόλπο του Λέχαιου αποκάλυψε: (i) το συνολικό πάχος των ιζημάτων κάτω από τον κόλπο του Λέχαιου είναι σχεδόν 3 km, (ii) τουλάχιστον 400 m ιζημάτων που συγκεντρώθηκαν κατά τη διάρκεια των τελευταίων 245 ka, δίνοντας μια μέση ταχύτητα ιζηματογένεσης 1.3 m/ka για τα τελευταία 245 ka και 2.3 m/ka για το Ολόκαινο και iii) διαφορική κατακόρυφη κίνηση, της τάξης των 4.5 km, μεταξύ του υποβάθρου κάτω από τον κόλπο του Λέχαιου και των γειτονικών βουνών, δίνουν ένα αθροιστικό μέσο ρυθμό ολίσθησης 0.9 m/ka ή λιγότερο, κατά τη διάρκεια των τελευταίων 4 Ma. Ο ρυθμός ολίσθησης που εκτιμήθηκε για το Ολόκαινο είναι περίπου 4.5 m/ka για το νότιο τμήμα και 2.5 m/ka για το βόρειο τμήμα, ενώ ο συντελεστής επέκτασης είναι παρόμοιος με εκείνους που υπολογίζονται από γεωδαιτικές έρευνες, περίπου 7.7 mm/yr ± 1.0. Η περίοδος επανάληψης για σεισμούς που λαμβάνουν χώρα στο ανατολικό άκρο της Κορινθιακής τάφρου, για την πλειονότητα των ρηγμάτων κυμαίνεται μεταξύ ~ 100 και ~ 300 έτη, για μεγέθη μεταξύ 6.1 και 6.4. Τέλος, μελετάται η μεγάλη πιθανότητα εκδήλωσης tsunami, ενώ η διάδοση του κύματος και τα αποτελέσματά του στην ακτή αναλύονται μέσω συνθετικών μοντέλων. Επομένως, για να εκτιμηθούν ακριβέστερα οι ρυθμοί ολίσθησης, οι ρυθμοί ανύψωσης, οι ρυθμοί επέκτασης και η περίοδος επανάληψης σεισμικών γεγονότων σε ολόκληρη την Κορινθιακή τάφρο, θα πρέπει να ληφθεί υπόψη η σημερινή τεκτονο-ιζηματογενής εξέλιξη του κόλπου του Λέχαιου. Τα αποτελέσματα της παρούσας έρευνας φιλοδοξούν να συνεισφέρουν στη βελτίωση των υπαρχόντων μοντέλων για την δημιουργία και εξέλιξη της Κορινθιακής τάφρου και την κατανόηση αντίστοιχων γεωλογικών περιβαλλόντων. Επίσης ευελπιστούν να δώσουν τα απαραίτητα εργαλεία στους σχεδιαστές μέτρων έκτακτης ανάγκης, προκειμένου να μπορέσουν να αξιολογήσουν την επικινδυνότητα και να καταρτίσουν σχέδια για τη μείωση του κινδύνου από τσουνάμι για τους κατοίκους στις παράκτιες περιοχές γύρω από τον Κορινθιακό κόλπο, οι οποίες στο μέλλον θα μπορούσαν να πληγούν από την εκδήλωση ενός τσουνάμι

Fault models for the Bodrum–Kos tsunamigenic earthquake (Mw6.6) of 20 July 2017 in the east Aegean Sea

We investigate a fault model for the shallow, strong (Mw6.6) tsunamigenic earthquake of 20 July 2017 with source area in the east Aegean Sea between Bodrum peninsula (Turkey) and Kos Isl. (Greece). Fault plane solutions are consistent with the regional active tectonics indicating normal faulting striking about E–W. A relocated hypocenter of the main shock was obtained and the rupture time-space history for two alternative fault solutions dipping either to the south or to north was approached by inverting P-waves recorded at teleseismic (30°–90°) distances. In both fault solutions the seismic moment calculated corresponds to magnitude Mw=6.6 while the earthquake had total rupture duration of less than 10 s. The maximum co-seismic slip was close to the hypocenter ∼1.6m and ∼2.2m for the south and north dipping faults, respectively. Close to the surface coseismic slip of ∼0.3m was calculated. Our analysis of Sentinel satellite InSAR images showed ground deformation fringes indicating ∼13 cm vertical displacement change between the North and South coasts of the Karaada islet. This result is in line with our preference of the south dipping fault, supported primarily by GPS displacements, regional tectonics, aftershock activity and tsunami observations, although results of InSAR inversion fit equally well both solutions.Published1016463T. Sorgente sismicaJCR Journa

Methoni Mw 6.8 rupture and aftershocks distribution from a dense array of OBS and land seismometers, offshore SW Hellenic subduction

International audienceAlong the southwestern offshore Hellenic subduction zone, the overriding Aegean upper plate above the Mediterranean oceanic lithosphere generates uncommon large earthquakes on the offshore megathrust fault. The largest subduction thrust event, for half a century, has been the 14 February 2008 Methoni earthquake (Mw = 6.8) that occurred offshore of the southwest coast of Peloponnesus. We conducted micro-seismicity experiments around the rupture area and forearc domain-between Peloponnesus and Crete-using ocean bottom seismometers (OBS) jointly with land-based seismological stations. Our first experiment in 2006, had revealed an association of the Matapan Trough, a 400-km-long forearc basin, with local seismicity clustering and a possible gap in activity over the later Methoni rupture area. Here we present new data of post-Methoni seismic activity, recorded during a time-span of 11 months, beginning in October 2008 within the period of proposed afterslip on the megathrust, by an extended and dense seismic array consisting of up to 33 OBS. A minimum 1D velocity model was constructed for the region to provide better constraints on absolute locations and double-difference relocation was applied to produce an enhanced image of the spatial distribution of hypocenters. The high resolution earthquake locations confirm correlation of the Matapan Trough with local seismicity as a regional feature, also filling up the previously observed gap. Over the Methoni rupture area, we constrain seismicity to be located mainly within the upper plate. Hypocenters are also resolved above the updip and downdip edges of the rupture area, respectively. Seismic activity provides hints of upper plate structures which were activated in response to post-seismic deformation spreading within the forearc crust. Our findings highlight the characteristics of a megathrust domain which is related with a highly deformable overriding plate and controlled by a segmented lower plate topography