THE 2014 MW 6.9 NORTH AEGEAN TROUGH (NAT) EARTHQUAKE: SEISMOLOGICAL AND GEODETIC EVIDENCE

A strong earthquake (Mw 6.9) on 24 May 2014 ruptured the North Aegean Trough (NAT) in Greece, west of the North Anatolian Fault Zone (NAFZ). In order to provide unbiased constrains of the rupture process and fault geometry of the earthquake, seismological and geodetic data were analyzed independently. First, based on teleseismic long-period P- and SH- waveforms a point-source solution yielded dominantly right-lateral strike-slip faulting mechanism. Furthermore, finite fault inversion of broad-band data revealed the slip history of the earthquake. Second, GPS slip vectors derived from 11 permanent GPS stations uniformly distributed around the meizoseismal area of the earthquake indicated significant horizontal coseismic slip. Inversion of GPS-derived displacements on the basis of Okada model and using the new TOPological INVersion (TOPINV) algorithm permitted to model a vertical strike slip fault, consistent with that derived from seismological data. Obtained results are consistent with the NAT structure and constrain well the fault geometry and the dynamics of the 2014 earthquake. The latter seems to fill a gap in seismicity along the NAT in the last 50 years, but seems not to have a direct relationship with the sequence of recent faulting farther east, along the NAFZ

Multi-sensor measurement of dynamic deflections and structural health monitoring of flexible and stiff bridges

We investigated the response of bridges of different types to controlled and to wind and traffic-induced excitations; the emphasis was on deflections,derived from recordings of geodetic sensors and accelerometers (output-only analysis). Our focus was to push the limits of the existing experimental techniques, in order to cover not only flexible, but also stiff structures, and to present independently validated results. Our study focused on a 700m long, thin-deck cable-stayed bridge, a stiff steel pedestrian bridge, a historic composite (masonry/steel) train bridge and a 30m long, gradually decaying, currently swaying pedestrian timber bridge. Our basic strategy was first to develop data measurement and processing techniques using controlled (supervised learning) experiments, and then, (1) use collocated, redundant and distributed geodetic sensors (GPS/GNSS and Robotic Total Stations, RTS), as well as accelerometers, in order to record bridge excitations, especially con-trolled excitations leading to free attenuating oscillations;(2) develop techniques to denoise recordings of various sensors based on structural/logical constraints and sensor fusion, compensating for the weaknesses inherent in each type of sensor), validate results and avoid pitfalls;(3) monitor the episodic and gradual decay of a pedestrian bridge, through repeated surveys under similar loading and environmental conditions and using similar instrumentation.The output of our studies is to confirm the potential of modern sensors to measure, under certain conditions, reliable mm-level dynamic deflections even of stiff structures (3-6Hz dominant frequencies) and to provide firm constraints for structural analysis, including evidence for changes of first modal frequencies produced by structural decay, even to identify dynamic effects such as foundations response to dynamic loading

Slow Slip Triggers the 2018 Mw 6.9 Zakynthos Earthquake Within the Weakly Locked Hellenic Subduction System, Greece

Slow slip events (SSEs) at subduction zones can precede large-magnitude earthquakes and may serve as precursor indicators, but the triggering of earthquakes by slow slip remains insufficiently understood. Here, we combine geodetic, Coulomb wedge and Coulomb failure-stress models with seismological data to explore the potential causal relationship between two SSEs and the 2018 Mw 6.9 Zakynthos Earthquake within the Hellenic Subduction System. We show that both SSEs released up to 10 mm of aseismic slip on the plate-interface and were accompanied by an increase in upper-plate seismicity rate. While the first SSE in late 2014 generated only mild Coulomb failure stress changes (≤3 kPa), that were nevertheless sufficient to destabilize faults of various kinematics in the overriding plate, the second SSE in 2018 caused stress changes up to 25 kPa prior to the mainshock. Collectively, these stress changes affected a highly overpressured and mechanically weak forearc, whose state of stress fluctuated between horizontal deviatoric compression and tension during the years preceding the Zakynthos Earthquake. We conclude that this configuration facilitated episodes of aseismic and seismic deformation that ultimately triggered the Zakynthos Earthquake

Fault-model of the 2017 Kos-Bodrum (east Aegean Sea) Mw 6.6 earthquake from inversion of seismological and GPS data – Preliminary Report

The 20 July 2017 Kos-Bodrum Mw 6.6 normal fault earthquake (AFAD, 2017) at the NW edge of the Quaternary Gökova Bay graben, was a destructive earthquake associated with a small tsunami (Yalciner et al., 2017). In addition, it is the first normal faulting earthquake in the Aegean covered by a dense array of continuous GPS stations which permit a detailed finite fault modeling (FFM). The preliminary seismological evidence (epicenters, hypocenters of the main shock and of the main aftershocks, and focal mechanism of the main shock) deriving from various agencies, data and techniques, indicate a shallow, nearly E-W striking normal faulting, but its details, including its dip (northerly or southerly) are not resolved. On the basis of independent analysis of seismological and geodetic data we obtained Finite Fault Models (FFM), which are very similar and hence describe the “true” fault

Forecasting magma-chamber rupture at Santorini volcano, Greece

How much magma needs to be added to a shallow magma chamber to cause rupture, dyke injection, and a potential eruption? Models that yield reliable answers to this question are needed in order to facilitate eruption forecasting. Development of a long-lived shallow magma chamber requires periodic influx of magmas from a parental body at depth. This redistribution process does not necessarily cause an eruption but produces a net volume change that can be measured geodetically by inversion techniques. Using continuum-mechanics and fracture-mechanics principles, we calculate the amount of magma contained at shallow depth beneath Santorini volcano, Greece. We demonstrate through structural analysis of dykes exposed within the Santorini caldera, previously published data on the volume of recent eruptions, and geodetic measurements of the 2011–2012 unrest period, that the measured 0.02% increase in volume of Santorini’s shallow magma chamber was associated with magmatic excess pressure increase of around 1.1 MPa. This excess pressure was high enough to bring the chamber roof close to rupture and dyke injection. For volcanoes with known typical extrusion and intrusion (dyke) volumes, the new methodology presented here makes it possible to forecast the conditions for magma-chamber failure and dyke injection at any geodetically well-monitored volcano

Geodetic and seismological analysis of the 2017 Kos-Bodrum Mw 6.5 earthquake (SE Aegean Sea) provides evidence for the evolution of normal faulting in Gökova graben

The epicentral area of the 2017, Mw 6.5 destructive Kos-Bodrum normal faulting earthquake, associated with a small tsunami in a touristic region, is located within the active Gökova graben and represents a case of a previously unrecognized, shallow normal fault without clear geomorphological signature. Analysis of teleseismic body-waveform data and geodetic data, both in the near- and far- field, indicate a well-constrained 25km long and 10km deep south-dipping normal fault which produced small uplift in the Bodrum peninsula. Modeled fault marks the NW edge of the actively extending part of the Gökova graben and was associated with two clusters of aftershocks, while only its eastern part correlates with known active faults. The 2017 earthquake seems to reflect an immature normal fault between Kos Island and the Anatolia mainland, and expansion of the Gökova graben, perhaps through echelon fractures, as is observed in seismic profiles farther east. Activation of normal faults with modest, if any geomorphic signature, hence not easily recognized before the earthquakes seems not unusual in the Aegean area, and represents a major issue for earthquake hazard analysis

Finite seismic fault modeling based on geodetic observations and the TOPological INVersion (TOPINV) algorithm

Various geophysical processes are described by differential equations which may lead to systems of redundant, highly non-linear systems of ordinary equations. In the latter, constant terms derive from measurements and represent stochastic variables, in addition, no approximate values of the solutions of these equations are available. An example is faults reactivated during earthquakes which, in their near-field, produce permanent ground displacements which may be measured by geodetic techniques, usually GPS. Each fault is approximated by a planar surface in an elastic half-space and is defined by 9 variables.No formal methods to solve such problems exist, and various techniques, mostly optimization methods are adopted. The most frequently used methods are (1) to a priori constrain some variables and optimize only 2 or 3, usually through search techniques and using a simple cost function, the mean weighted misfit. And (2) to use a small sample of points in a search space, so that these points and their misfits can be easily treated as 3-D data, and then optimize the search in the vicinity of points with minimum misfit, i.e. identify a point solution. These techniques may lead to solutions trapped in local minima, to correlated solutions or to solutions with poor error control.To overcome these problems, a numerical-topological, exhaustive grid search-based technique in the Rn space is proposed (n the number of unknown variables). This technique is in fact a generalization and refinement of techniques used in lighthouse positioning and in some cases of low-accuracy 2-D positioning. The basic concept is to identify the optimal n-dimensional compact grid space(s) containing the unknown “true” solution(s). This is possible first by transforming the model space into a discrete n-dimensional Grid G and the observation equations into inequalities on the basis of an optimization parameter k and of their standard errors. Then, starting from an objectively selected value of k and using a Boolean approach all gridpoints of grid G are tested, and those satisfying the observations inequalities are identified. If these points define compact spaces, they represent potential solutions approximating the “true” solution. The process is repeated for gradually smaller values of k until the optimal minimum n-dimensional volume containing the true solution is obtained. From the corresponding optimal set of gridpoints a stochastic solution is computed as the first and second statistical moment of the population of the gridpoints.The overall method has been tested with synthetic data using and accuracy-oriented approach, in which the “true” solution is a priori known, on the basis of the precision, accuracy and compactness of the solution.Also, the method was applied in the 2003 Leucas earthquake, Mw 6.2, for modeling a double seismic fault (i.e. inversion of 18 unknowns) providing unbiased results and consistent with the seismological evidence.Οι σημειακές θραύσεις των πετρωμάτων κατά τη διάρκεια ενός σεισμού προκαλούν μόνιμες παραμορφώσεις στη γύρω βραχόμαζα και περιγράφονται από διαφορικές εξισώσεις. Συνήθως θεωρείται ότι τα ρήγματα μπορούν να προσομοιωθούν από μία ή περισσότερες επίπεδες επιφάνειες θραύσης (πεπερασμένα ρήγματα) σε ελαστικό χώρο, και έχουν προταθεί συστήματα συνήθων εξισώσεων που συνδέουν τα χαρακτηριστικά του κάθε ρήγματος που προσδιορίζονται από 9 μεταβλητές με επιφανειακές παραμορφώσεις (elastic dislocation models). Εάν, οι μόνιμες παραμορφώσεις επεκτείνονται μέχρι και την επιφάνεια του εδάφους και έχουν μετρηθεί με γεωδαιτικές μεθόδους, συνήθως GPS, είναι δυνατόν να δημιουργηθεί σύστημα εξισώσεων παρατήρησης που συνδέει τα χαρακτηριστικά των σεισμικών ρηγμάτων με γεωδαιτικές παρατηρήσεις. Η επίλυση τω συστημάτων αυτών (αντιστροφή) μπορεί να προσδιορίσει τη γεωμετρία και την κινηματική ενός σεισμικού ρήγματος (ή περισσότερων σεισμικών ρηγμάτων). Η αντιστροφή δεν είναι συνήθως δυνατή με τυπικές αλγεβρικές μεθόδους επειδή οι εξισώσεις οδηγούν σε υπερστατικό σύστημα παρατηρήσεων, είναι γενικά εξαιρετικά μη γραμμικές, δεν υπάρχουν διαθέσιμες προσεγγιστικές εκτιμήσεις των χαρακτηριστικών των ρηγμάτων και οι σταθεροί όροι των εξισώσεων (μετρήσεις) χαρακτηρίζονται από αβεβαιότητες (σφάλματα).Για το λόγο αυτό έχουν προταθεί διάφορες αριθμητικές τεχνικές αντιστροφής, συνήθως δειγματοληπτικές (sampling search) βασισμένες στη μέθοδο Monte Carlo, οι οποίες εστιάζονται στην ελαχιστοποίηση μίας αντικειμενικής συνάρτησης (συνάρτηση κόστους, cost function) για συγκεκριμένα πεδία τιμών (χώρο αναζήτησης n διαστάσεων για σύστημα n αγνώστων). Τα βασικά μειονεκτήματα των μεθόδων αυτών είναι a priori δεσμευμένες τιμές αγνώστων, εκτιμήσεις εγκλωβισμένες σε τοπικά ακρότατα, υποτίμηση της αβεβαιότητας των παρατηρήσεων, λύσεις με υψηλό βαθμό συσχέτισης και ελλιπής προσδιορισμός των στατιστικών τους χαρακτηριστικών.Με στόχο να αντιμετωπιστούν τέτοιου είδους προβλήματα, αναπτύχθηκε μια εναλλακτική μέθοδος τοπολογικής/γεωμετρικής αντιστροφής (αλγόριθμος TOPological INVersion, TOPINV) με πηγή έμπνευσης την παραδοσιακή πλοήγηση με χρήση φάρων. Η βέλτιστη λύση δεν βασίζεται σε μία σημειακή ακρότατη λύση, αλλά στον προσδιορισμό ενός n-διάστατου χώρου που περιέχει την «αληθή» λύση. Αυτό επιτυγχάνεται αφενός με τη μετατροπή των εξισώσεων παρατήρησης σε ανισώσεις συναρτήσει των σφαλμάτων των μετρήσεων και μιας παραμέτρου βελτιστοποίησης k. Και αφετέρου με τη μετατροπή του n-διάστατου (υπερ-)χώρου των πιθανών λύσεων σε διακριτό χώρο σημείων. Για επιλεγμένη τιμή της παραμέτρου k και με λογική Boole προσδιορίζονται τα σημεία που ικανοποιούν τις ανισώσεις παρατήρησης (ομοίομορφη αναζήτηση, uniform search), και κατά πόσο ορίζουν συμπαγή χώρο (ή χώρους) n διαστάσεων, ο οποιος εξ ορισμού περιέχει την αληθή λύση του συστήματος των εξισώσεων. Από τον πληθυσμό των σημείων αυτών υπολογίζεται η βέλτιστη λύση (ή λύσεις) και ο αντίστοιχος Πίνακας Μεταβλητότητας-Συμμεταβλητότητας ως πρώτες και δεύτερες στατιστικές ροπές, αντίστοιχα.Η αποτελεσματικότητα της μεθόδου τεκμηριώνεται με την επίλυση προβλημάτων με βάση συνθετικά δεδομένα, όπου είναι γνωστή η αληθής λύση, και με ανάλυση ευαισθησίας αναφορικά με την συμπαγότητα (compactness) της λύσης, την ακρίβειά της (precision) και την απόκλισή της από την αληθή τιμή (αξιοπιστία, accuracy).Η μέθοδος εφαρμόστηκε σε αντιστροφή γεωδαιτικών παρατηρήσεων για τον προσδιορισμό δύο σεισμικών ρηγμάτων (18 συνολικά άγνωστες μεταβλητές) που ενεργοποιήθηκαν κατά το σεισμό του 2003 της Λευκάδας, μεγέθους 6.2, του πρώτου σεισμού στον Ελληνικό χώρο που καλύφθηκε από εκτενή δεδομένα GPS, προσφέροντας ακριβή λύση χωρίς δεσμεύσεις και συμβατή με ανεξάρτητα σεισμολογικά δεδομένα

Strong motion displacement waveforms using 10-Hz precise point positioning GPS: an assessment based on free oscillation experiments

The recently developed precise point positioning (PPP) technique permits to compute instantaneous coordinates of a GPS station relatively to distant reference stations and waveforms of ground displacements during strong motions at the 1 Hz level. This is another application of GPS, different from the computation of static coseismic movements or of accurate monitoring of dynamic displacements of structures using a static receiver and a nearby moving receiver (DGPS). Recently, earthquake ground displacement waveforms using 10-Hz GPS data have also been calculated, but no independent evidence to assess their quality exists. To overcome this problem, we evaluated the output of 10-Hz PPP results on the basis of supervised learning experiments. Semistatic and dynamic displacements (damped harmonic oscillations) of known characteristics of the order of a few centimeter were produced and were recorded by GPS, an accelerometer, and a robotic total station. Time series of instantaneous displacements were analyzed using different PPP techniques and were compared with reference (true) values derived from DGPS and the other sensors. Our analysis revealed that the PPP-derived coordinates are contaminated by long-period noise but they can display the details of semistatic displacements, while their short-period component describes well the pattern of waveforms and spectra (at least up to 4 Hz) of dynamic displacements, with up to 20 mm accuracy for isolated points. These results indicate that 10-Hz PPP-GPS is useful for earthquake engineering and can safely be used to reconstruct waveforms of deflections of the ground and of various points on structures during strong motions.Published1853 – 18661T. Deformazione crostale attivaJCR Journa