2020

GROUND DEFORMATION FROM GNSS DATA FOLLOWING STRONG IONIAN SEA EARTHQUAKES IN 2014, 2015 AND 2018: Co-Seismic offsets and station baseline changes

ELENI I. PARTHENIOU

NATIONAL AND KAPODISTRIAN UNIVERSITY OF ATHENS | Department of Geology and Geoenvironment

ΕΘΝΙΚΟ ΚΑΙ ΚΑΠΟΔΙΣΤΡΙΑΚΟ ΠΑΝΕΠΙΣΤΗΜΙΟ ΑΘΗΝΩΝ

ΣΧΟΛΗ ΘΕΤΙΚΩΝ ΕΠΙΣΤΗΜΩΝ ΤΜΗΜΑ ΓΕΩΛΟΓΙΑΣ ΚΑΙ ΓΕΩΠΕΡΙΒΑΛΛΟΝΤΟΣ ΠΡΟΓΡΑΜΜΑ ΜΕΤΑΠΤΥΧΙΑΚΩΝ ΣΠΟΥΔΩΝ: ΓΕΩΦΥΣΙΚΗ

ΜΕΤΑΠΤΥΧΙΑΚΗ ΔΙΑΤΡΙΒΗ ΕΙΔΙΚΕΥΣΗΣ

Μελέτη σεισμικής παραμόρφωσης Περιοχής Κεντρικών Ιονίων Νήσων περιόδου 2014 - 2018 με χρήση δορυφορικών γεωδαιτικών δεδομένων (GNSS)

ΕΛΕΝΗ Ι. ΠΑΡΘΕΝΙΟΥ

Α.Μ.: 21526

Επιβλέποντες: • Βασίλειος Σακκάς, Ε.ΔΙ.Π. (Επιβλέπων). • Νικόλαος Βούλγαρης, Καθηγητής • Αθανάσιος Γκανάς, Διευθυντής Ερευνών, Γεωδυναμικού Ινστιτούτου, Εθνικού Αστεροσκοπείου Αθηνών.

ΑΘΗΝΑ

ΟΚΤΩΒΡΙΟΣ 2020

ΔΗΛΩΣΗ ΠΕΡΙ ΜΗ ΠΡΟΣΒΟΛΗΣ ΠΝΕΥΜΑΤΙΚΗΣ ΙΔΙΟΚΤΗΣΙΑΣ

Προσβολή πνευματικής ιδιοκτησίας θεωρείται η ολική ή η μερική αναπαραγωγή του έργου άλλου προσώπου ή η παρουσίαση του έργου κάποιου άλλου ως προσωπικού του γράφοντος. Το Τμήμα Γεωλογίας και Γεωπεριβάλλοντος λαμβάνει πολύ σοβαρά υπόψη και καταδικάζει την προσφυγή σε τέτοιου είδους πρακτικές από τους Μεταπτυχιακούς Φοιτητές. Σε περιπτώσεις πρόδηλης ή εκ προθέσεως προσβολής πνευματικής ιδιοκτησίας, τα αρμόδια όργανα του Τμήματος δύνανται να επιβάλουν ως κύρωση έως και την οριστική διαγραφή από το ΠΜΣ. Κατά την εκπόνηση Διπλωματικής Εργασίας Ειδίκευσης οι Μεταπτυχιακοί Φοιτητές οφείλουν να τηρούν τις ακόλουθες κατευθυντήριες οδηγίες: Η Διπλωματική Εργασία Ειδίκευσης πρέπει να αποτελεί έργο του υποβάλλοντος αυτήν φοιτητή. Η αντιγραφή ή η παράφραση έργου τρίτου προσώπου αποτελεί προσβολή πνευματικής ιδιοκτησίας και συνιστά σοβαρό αδίκημα. Στο αδίκημα αυτό περιλαμβάνεται τόσο η προσβολή πνευματικής ιδιοκτησίας άλλου φοιτητή όσο και η αντιγραφή από δημοσιευμένες πηγές, όπως βιβλία, εισηγήσεις ή επιστημονικά άρθρα. Το υλικό που συνιστά αντικείμενο λογοκλοπής μπορεί να προέρχεται από οποιαδήποτε πηγή. Η αντιγραφή ή χρήση υλικού προερχόμενου από το διαδίκτυο ή από ηλεκτρονική εγκυκλοπαίδεια είναι εξίσου σοβαρή με τη χρήση υλικού προερχόμενου από τυπωμένη πηγή ή βάση δεδομένων. Η χρήση αποσπασμάτων από το έργο τρίτων είναι αποδεκτή εφόσον, αναφέρεται η πηγή του σχετικού αποσπάσματος. Σε περίπτωση αυτολεξεί μεταφοράς αποσπάσματος από το έργο άλλου, η χρήση εισαγωγικών ή σχετικής υποσημείωσης είναι απαραίτητη, ούτως ώστε η πηγή του αποσπάσματος να αναγνωρίζεται. Η παράφραση κειμένου, αποτελεί προσβολή πνευματικής ιδιοκτησίας. Οι πηγές των αποσπασμάτων που χρησιμοποιούνται θα πρέπει να καταγράφονται πλήρως σε πίνακα βιβλιογραφίας στο τέλος της εργασίας. Η προσβολή πνευματικής ιδιοκτησίας επισύρει την επιβολή κυρώσεων. Κατά την απόφαση επί των ενδεδειγμένων κυρώσεων, τα αρμόδια όργανα του Τμήματος θα λαμβάνουν υπόψη παράγοντες όπως το εύρος και το μέγεθος του τμήματος της εργασίας που οφείλεται σε προσβολή πνευματικής ιδιοκτησίας. Οι κυρώσεις θα επιβάλλονται σύμφωνα με το Άρθρο 7 Παράγραφος 7 του Κανονισμού Σπουδών. ΒΕΒΑΙΩΣΗ Βεβαιώνω ότι η Διπλωματική Εργασία Ειδίκευσης με τίτλο «Μελέτη σεισμικής παραμόρφωσης Περιοχής Κεντρικών Ιονίων Νήσων περιόδου 2014 - 2018 με χρήση δορυφορικών γεωδαιτικών δεδομένων (GNSS)» την οποία υποβάλλω, δεν περιλαμβάνει στοιχεία προσβολής πνευματικής ιδιοκτησίας, όπως αυτά προσδιορίζονται από την παραπάνω δήλωση, τους όρους της οποίας διάβασα και αποδέχομαι. Παρέχω τη συναίνεσή μου, ώστε ένα ηλεκτρονικό αντίγραφο της διπλωματικής εργασίας μου να υποβληθεί σε ηλεκτρονικό έλεγχο για τον εντοπισμό τυχόν στοιχείων προσβολής πνευματικής ιδιοκτησίας.

Ημερομηνία Υπογραφή

6/10/2020

Αφιερώνεται στη μνήμη του πατέρα μου

Ευχαριστίες

Από την θέση αυτή, αισθάνομαι την ανάγκη να εκφράσω τις ευχαριστίες και την ευγνωμοσύνη μου σε όλους όσοι συνέβαλαν στην προσπάθειά που κατέβαλα για την εκπόνηση της παρούσας μεταπτυχιακής διατριβής.

Ευχαριστώ ολόψυχα τον επιβλέποντα και πανεπιστημιακό μου δάσκαλο κ. Βασίλη Σακκά, για την εμπιστοσύνη που μου έδειξε με την ανάθεση την παρούσας διατριβής και την ευκαιρία που μου έδωσε μέσω αυτής, να ασχοληθώ με την ανάλυση και ερμηνεία δορυφορικών Γεωδαιτικών δεδομένων (GNSS). Τον ευχαριστώ επίσης για την απλόχερη παροχή των δεδομένων αυτών, χωρίς τα οποία δε θα είχαν προκύψει τα αποτελέσματα της έρευνας αυτής. Τα όσα μου δίδαξε σχετικά με την μεθοδολογία επεξεργασίας και ανάλυσης των δεδομένων αυτών και τα αντίστοιχα λογισμικά, αποτελούν για μένα πολύτιμα εφόδια όχι μόνο για την εκπόνηση της διατριβής μου αλλά και για την περεταίρω ενασχόλησή μου με το συγκεκριμένο αντικείμενο. Η αμέριστή στήριξη, η εμψύχωση και οι συζητήσεις που κάναμε τόσο σε επιστημονικό όσο και σε ανθρώπινο επίπεδο, ήταν για μένα καθοριστικής σημασίας, ιδιαίτερα σε δύσκολες στιγμές που προέκυψαν κατά τη διάρκεια της προσπάθειάς μου αυτής.

Τη βαθιά μου ευγνωμοσύνη θέλω να εκφράσω στον συν-επιβλέποντα της διατριβής μου, διευθυντή ερευνών του Γεωδυναμικού Ινστιτούτου του Εθνικού Αστεροσκοπείου Αθηνών, κ. Αθανάσιο Γκανά, για την αμέριστη συμπαράσταση, τη βοήθεια, και την καθοδήγηση του, σε όλα τα στάδια της προσπάθειάς μου για την εκπόνηση της μεταπτυχιακής μου διατριβής. Είναι εξάλλου εκείνος ο οποίος μου έδωσε την ιδέα να ασχοληθώ με την επεξεργασία και ανάλυση δορυφορικών Γεωδαιτικών δεδομένων (GNSS). Οι υψηλές απαιτήσεις και οι καίριες επισημάνσεις του, αποτέλεσαν κίνητρο για την αύξηση της απόδοσης μου και ήταν καθοριστικές για την ποιοτική αναβάθμιση και την ολοκλήρωση της παρούσας εργασίας. Τον ευχαριστώ θερμά για τον χρόνο που μου αφιέρωσε και για τις πολύτιμες γνώσεις που μου μετέδωσε, όχι μόνο σχετικά με το αντικείμενο της διατριβής μου αλλά και σε ότι αφορά τη γεωφυσική και γενικότερα τη γεωλογία. Η καθημερινή επικοινωνία τόσο σε επιστημονικό όσο και σε επίπεδο ανθρωπίνων σχέσεων ήταν καταλυτική για την διεκπεραίωση της παρούσας διατριβής.

Οφείλω να ευχαριστήσω ιδιαιτέρως τον καθηγητή του Τμήματος Γεωλογίας και Γεωπεριβάλλοντος κ. Νικόλαο Βούλγαρη, για την συμμετοχή του στην τριμελή επιτροπή μου, τη στήριξη και τις πολύτιμες παρατηρήσεις του σχετικά με τη συγγραφή της παρούσας διατριβής.

Ευχαριστώ επίσης τα ελληνικά δίκτυα για τα δεδομένα GNSS. Οφείλω επίσης να αναγνωρίσω την υποστήριξη αυτής της έρευνας από το έργο "HELPOS - Hellenic Plate Observing System" (MIS 5002697) το οποίο υλοποιείται στο πλαίσιο της δράσης "Ενίσχυση των υποδομών Έρευνας και Καινοτομίας", χρηματοδοτούμενη από το επιχειρησιακό πρόγραμμα "Ανταγωνιστικότητα, Επιχειρηματικότητα και Καινοτομία" ( NSRF 2014-2020) που συγχρηματοδοτείται από την Ελλάδα και την Ευρωπαϊκή Ένωση (Ευρωπαϊκό Ταμείο Περιφερειακής Ανάπτυξης).

Σε αυτή μου την προσπάθεια, ουσιαστικό ρόλο διαδραμάτισαν οι φίλοι και οι συνάδελφοί μου, τους οποίους δεν θα ήθελα να ξεχάσω.

Από καρδιάς θα ήθελα να ευχαριστήσω και να εκφράσω τη βαθιά μου εκτίμηση για τους εξαιρετικούς συναδέλφους γεωλόγους και φίλους, Βαρβάρα Τσιρώνη και Γιάννη Καραμήτρο. Η άψογη συνεργασία, η εμψύχωση και η αμέριστη επιστημονική και ηθική τους συμπαράσταση καθ’ όλη τη διάρκεια της προσπάθειάς μου αυτής, ήταν για μένα εξαιρετικά σημαντική.

Ευχαριστώ θερμά τη συνάδελφο, συμφοιτήτρια και αγαπητή μου φίλη, γεωλόγο Χρύσα Δόξα για όσα μοιραστήκαμε τόσο σε επιστημονικό όσο και σε ανθρώπινο επίπεδο κατά τη διάρκεια των μεταπτυχιακών μας σπουδών. Η αμέριστη βοήθεια και η συμπαράστασή που απλόχερα μου προσέφερε μου έδωσαν δύναμη σε δύσκολες για εμένα στιγμές.

Ένα μεγάλο ευχαριστώ οφείλω στις καρδιακές μου φίλες, των οποίων η αμέριστη στήριξη, η συνεχής ενθάρρυνση και η κατανόηση αποτελεί για μένα κινητήρια δύναμη στην προσπάθεια εκπλήρωσης όλων μου των ονείρων

Τέλος, δεν θα μπορούσα να μην ευχαριστήσω τα μέλη της οικογένειάς μου που είναι οι ακρογωνιαίοι λίθοι στην προσπάθειά μου αυτή. Στάθηκαν στο πλευρό μου πραγματικοί συνοδοιπόροι καθ’ όλη τη διάρκεια των σπουδών μου και όχι μόνο, στηρίζοντας με, με κάθε τρόπο, ενθαρρύνοντας και εμψυχώνοντας κάθε μου προσπάθεια. Χωρίς την αγάπη και την υπομονή τους δεν θα τα είχα καταφέρει Το μεγαλύτερο ευχαριστώ οφείλω στον πολυαγαπημένο μου πατέρα Γιάννη, στη μνήμη του οποίου αφιερώνεται η παρούσα μεταπτυχιακή διατριβή. Ήταν εκείνος που μου εμφύσησε την αγάπη του για τις θετικές επιστήμες από πολύ μικρή ηλικία. και υπήρξε για μένα πρότυπο επιστήμονα και ανθρώπου. Η αγάπη του και οι αξίες που μου μετέδωσε, μου δίνουν δύναμη, με καθοδηγούν και με κάνουν να θέλω να γίνομαι καλύτερη για να του μοιάσω.

ΠΕΡΙΛΗΨΗ

Σκοπός της παρούσας μελέτης είναι η μέτρηση και ερμηνεία της συν- και μετα-σεισμικής παραμόρφωσης του γήινου φλοιού λόγω ισχυρών σεισμών στην περιοχή των κεντρικών Ιωνίων Νήσων, κατά την περίοδο 2007-2018 με τη χρήση δεδομένων του Παγκόσμιου Δορυφορικού Συστήματος Πλοήγησης (GNSS). Το Ιόνιο Πέλαγος είναι μια περιοχή του Δυτικού Ελληνικού τόξου με υψηλή σεισμική δραστηριότητα λόγω της παρουσίας του δεξιόστροφου ρήγματος της Κεφαλονιάς, ενός ρήγματος μετασχηματισμού κατά μήκος του οποίου πραγματοποιείται η σχετική κίνηση μεταξύ των τεκτονικών πλακών της Ευρασίας και της Αφρικής. Τα δεδομένα που χρησιμοποιήθηκαν προέρχονταν από ημερήσιες παρατηρήσεις σταθμών GNSS του Εθνικού Αστεροσκοπείου Αθηνών (ΕΑΑ) και του Εθνικού και Καποδιστριακού Πανεπιστημίου Αθηνών (ΕΚΠΑ). Οι περισσότεροι από αυτούς τους σταθμούς εγκαταστάθηκαν κατά την περίοδο 2006-2010 και παρείχαν χρήσιμα δεδομένα (ημερήσιες θέσεις) για τη μέτρηση των σεισμικών μετατοπίσεων μετά τους ισχυρούς σεισμούς του 2014 (νησί της Κεφαλονιάς δύο ρηχά σεισμικά γεγονότα) και του 2015 (νησί της Λευκάδας). Στην παρούσα μελέτη, χρησιμοποιήσαμε τις ημερήσιες συντεταγμένες σταθμών GNSS για την α) παρατήρηση των μεταβολών στις χρονοσειρές λόγω σεισμικών κινήσεων (offsets) και β) την παρακολούθηση του ρυθμού μεταβολής της οριζόντιας απόστασης (baseline) μεταξύ των σταθμών για να ανιχνεύσουμε προ-σεισμική παραμόρφωση του φλοιού. Εντοπίσαμε σημαντικές σεισμικές μετατοπίσεις (offsets) σε όλες τις περιπτώσεις τις οποίες παρουσιάζουμε τόσο σε πίνακες όσο και σε γραφικές παραστάσεις. Προσδιορίσαμε επίσης σημαντικές αλλαγές του ρυθμού μεταβολής της οριζόντιας απόστασης (baseline) μεταξύ των σταθμών GNSS κατά τη διάρκεια χρονικών περιόδων που προηγούνται ισχυρών σεισμών. Τα αποτελέσματα θεωρούνται αξιόπιστα λόγω της μεγάλης διάρκειας δεδομένων (π.χ. σχεδόν έξι χρόνια στην περίπτωση σταθμών στην νήσο Λευκάδα) και της καλής συσχέτισης (R2 > 0,70). Αυτό το αποτέλεσμα δείχνει ότι τα δεδομένα GNSS μπορούν να ανιχνεύσουν προ-σεισμική παραμόρφωση.

EXTENDED SUMMARY

The purpose of this study is to measure the seismic offsets and the crustal deformation on the central Ionian Sea during the period 2007-2018, by use of Global Navigation Satellite System (GNSS) data, originate from fifteen (15) permanent GNSS stations. The stations are equipped with dual-frequency geodetic receivers and belong to the networks of NOANET (National Observatory of Athens; Ganas et al., 2008; 2013), HxGN SmartNet Greece, EUREF and NKUA (National and Kapodistrian University of Athens; Sakkas and Lagios, 2015, 2017). Most of these stations were installed during the period 2006-2010 and are located on the Central Ionian Islands, northwestern Peloponnese and southwestern central Greece. These daily data are provided by the network administrators in the form of RINEX files (version 2.11) and were processed using Bernese 5.2 software (Dach et al. 2015) following the double-difference method. Daily position observations were obtained for each station in IGb08 Cartesian coordinates (X, Y, Z). The positions were then converted into EGSA 1987 system (Greek projection system). We used these data to a) observe the time series for changes due to seismic motions (offsets) and b) monitor changes in the baselines between the stations, in relation to time. The later, was performed for the period 2006 to 2019, emphasizing on how the baselines were affected by the large seismic events that occurred in the study area during 2014-2018.

In the period 2014-2018, four (4) strong seismic events occurred in the broader area of the Central Ionian Islands. These earthquakes occurred on January 26, 2014 13:55 UTC (magnitude of seismic moment Mw = 6.1), and on February 3, 2014 03:08 UTC (Mw = 5.9), November 17, 2015 07:10 UTC (Mw = 6.4) and October 25 2019 22:54 UTC (Mw = 6.6). The earthquakes of 2014 were onshore Cephalonia island, the earthquake of 2015 occurred onshore Lefkada island and the earthquake of 2018 occurred offshore in the sea area south of Zakynthos island (Romano et. al, 2019). We detected significant offsets in all cases and we report it in both tabular and graphic forms. We also detected significant changes in baseline rates between GNSS stations during periods preceding strong earthquakes. It is interesting to study whether these changes can be detected before and after strong earthquakes in order to investigate if GNSS data could be used to predict earthquakes. The Ionian Sea is an area with high seismic activity due to the presence of the Cephalonia Transform Fault (CTF), a horizontal strike-slip fault accommodating the relative motion between the tectonic plates of Eurasia and Africa. As a result the study of GNSS data is critical for monitoring ground movements before, during and after significant earthquakes.

The results of this study are robust because of the long data duration (e.g. nearly six years for the stations located on Lefkada island) and the goodness of fit (R2>0.70). This result demonstrates that GNSS data may detect pre-seismic deformation.

Table of Contents ΠΕΡΙΛΗΨΗ ...... 7 EXTENDED SUMMARY ...... 8 1. CHAPTER 1: INTRODUCTION ...... 1 1.1 Tectonic Setting ...... 2 1.2 Hellenic Arch-Trench System ...... 4 1.3 Seismicity of the Central Ionian Sea ...... 8 1.4 The Cephalonia Transform Fault (CTF) ...... 11 1.5 Geological Settings ...... 15 1.5.1 Lefkada Island ...... 15 1.5.2 Cephalonia Island ...... 17 2. CHAPTER 2: DATA PROCESSING AND ANALYSIS ...... 19 2.1 GNSS Theory ...... 19 2.2 GNSS networks in Western Greece - Station Features ...... 19 2.3 Data Processing ...... 23 2.4 Seismic Offsets ...... 25 2.4.1 Offsets caused by the January 26 2014, M6.1 Cephalonia event ...... 43 2.4.2 Offsets caused by the February 3 2014, M5.9 Cephalonia event ...... 44 2.4.3 Offsets caused by the November 17, 2015 M6.4 Lefkada event ...... 46 2.4.4 Offsets caused by the October 25, 2018 M6.6 Zakynthos event ...... 49 2.5 Diagrams showing the position change in three components for each station ...... 50 3. CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES ...... 51 3.1 Method of processing ...... 51 3.2 Cephalonia events (26/01/2014 and 03/02/2014) ...... 54 3.2.1 Baseline VLSM-PONT ...... 54 3.2.2 Baseline VLSM-KARA ...... 56 3.3 Lefkada event (17/11/2015) ...... 58 3.3.1 Baseline SPAN - PONT ...... 59 3.3.2 Baseline PONT-VLSM ...... 61 3.3.3 Baseline PONT-KARA ...... 63 3.3.4 Baseline PONT-AGRI ...... 65 3.4 Zakynthos M6.6 event 25/10/2018 ...... 68 4. CHAPTER 4: DISCUSSION AND CONCLUSIONS ...... 71 REFERENCES ...... 74 ANNEX I: List of focal mechanisms plotted in Fig. 7 ...... 86 ANNEX II: Time Series Plots (E, N, Up) of GNSS stations in the Ionian Sea ...... 88

List of figures

Figure 1: Tectonic map of the Eastern Mediterranean (after Jolivet et al. 2013). Thick black lines are main plate boundary faults with ticks/arrows indicating relative sense of movement...... 2 Figure 2: 3-D visualization of the Hellenic subduction beneath Eurasia (after Ganas and Parsons, 2009)...... 5 Figure 3: N-S compiled lithospheric-scale cross-section of the Aegean region from the Rhodope massif to the African passive margin across Greece and the Cyclades (after Jolivet and Brun 2010). This figure highlights the most significant parts of the present-day Hellenic Arc-Trench system, such as the accretionary complex, the back-arc extension, the volcanic arc (Santorini) and others...... 6 Figure 4: Schematic location map of the Africa-Europe convergence in the Greek region. The map shows the Hellenic Arc-Trench system starting to the northwest in the Ionian Islands and ending to the southeast at the south of Rhodes. In the Ionian Islands region lies the transition zone (shaded rectangle) between the Hellenic subduction to the south and the continental collision in the north linked by the Cephalonia transform fault (Modified after Sachpazi et al. 2000) ...... 7 Figure 5: Map of the seismicity in western Greece for ML>3.5 for the last 55 years (1964-2019) based on the NOAGI catalogue. The color scale represent the number of events...... 9 Figure 6: The Cephalonia Transform Fault (dashed thick line) with the Cephalonia segment of the fault marked as C and the Lefkada segment as L. The focal mechanisms of strong events are also shown. (Bathymetry contours from the topographic map 1: 500,000 from GYS) (After Louvari et al., 1999)...... 12 Figure 7: Fault plane solutions of the stronger earthquakes (M > 5) that occurred in the central Ionian Islands during the last five decades (1970-2020). The focal mechanism data are reported in the Appendix (Annex I). Black quadrants indicate compressional parts of the focal sphere...... 14 Figure 8: Neotectonic map of Lefkas Island based on Lekkas et al. (1999, 2001) and Rondoyanni et al. (2012)...... 16 Figure 9: Sketch Geological Map of Cephalonia Island by Lagios et al. (2007)...... 18 Figure 10: Nadir view of GNSS antenna at station VLSM (Cephalonia). Date of photo: 22 May 2014...... 20 Figure 11: Map of central Ionian Sea showing GNSS stations and earthquake epicentres of 26/01/2014, 03/02/2014, 17/11/2015 and 25/10/18 events. Red solid circles indicate earthquake epicentres (NKUA solutions). Solid triangles indicate locations of GNSS stations...... 22 Figure 12: Data processing to calculate relative displacement in three components (DE, DN, DUP), in mm. 23 Figure 13: Diagrams showing the time series of AGRI (Agrinion, Western Greece) and PONT station (Lefkada Island, Ionian Sea, Greece). Co seismic offsets caused by November 17, 2015 event, are observed. Top-panel: East-West component, Middle-panel: North-South component, Bottom-panel: Up-Down component. Time series of all stations are shown in Appendix II...... 24 Figure 14: Offsets calculation process...... 25 Figure 15: Co-seismic offsets observed in AGRI station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color...... 28 Figure 16: Co-seismic offsets observed in KARA station after the seismic event of Lefkada (2015). The least square trendline is in green color...... 29 Figure 17: Co-seismic offsets observed in KTCH station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color ...... 30 Figure 18: Co-seismic offsets observed in PAT0 station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color...... 31 Figure 19: Co-seismic offsets observed in PATR station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color...... 32 Figure 20: Co-seismic offsets observed in PONT station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015;right panel). The least square trendline is in green color...... 33 Figure 21: Co-seismic offsets observed in RLSO station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015;right panel). The least square trendline is in green color...... 34 Figure 22: Co-seismic offsets observed in SISS station after the seismic event of Zakynthos (2018). The least square trendline is in green color...... 35 Figure 23: Co-seismic offsets observed in SKAL station after the seismic event of Lefkada (2015). The least square trendline is in green color...... 36 Figure 24: Co-seismic offsets observed in SPAN station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color...... 37 Figure 25: Co-seismic offsets observed in VLSM station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color...... 38 Figure 26: Co-seismic offsets observed in VLSM station after the seismic events of Zakynthos (2018). The least square trendline is in green color...... 39 Figure 27: Co-seismic offsets observed in ZAKY station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color...... 40 Figure 28: Co-seismic offsets observed in ZAKY station after the seismic events of Zakynthos (2018). The least square trendline is in green color...... 41 Figure 29: Map showing seismic offset for each station due to 26/1/2014 earthquake, as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse...... 43 Figure 30: Map showing seismic offset for VLSM station as well as the corresponding error, in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse...... 44 Figure 31: Map showing seismic offset for each station (AGRI, PAT0, PATR, PONT, RLSO SPAN and ZAKY) as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse. Station VLSM Is plotted in Fig. 30 ...... 45 Figure 32: Map showing seismic offset for each station as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse. Stations SPAN and PONT are plotted in Fig. 33 ...... 46 Figure 33: Map showing seismic offset for PONT and SPAN stations as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse...... 47 Figure 34: Map showing seismic offset for PONT and SPAN stations, in the vertical direction. Offsets are shown as red arrows...... 48 Figure 35: Map showing seismic offset for each station as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse...... 49 Figure 36: Bar charts showing seismic offsets per seismic event for all studied GNSS stations. Top panel: 26 Jan. 2014 event, middle panel: 3 Feb. 2014 event, bottom panel: 17 Nov. 2015 event...... 50 Figure 37: Data processing to calculate baseline change rate between two stations, in mm / year...... 52 Figure 38: Map of Ionian Islands showing of pre- seismic baseline change rates (scale -8 mm/yr to +6 mm/yr) for several pairs of GNSS stations. The baseline rate changes have been measured before the Cephalonia (2014), Lefkada (2015) and Zakynthos (2018) earthquakes. The seismic epicentres are after NKUA. Green triangles show the GNSS stations used in this study ...... 53 Figure 39: Graph showing baseline rate changes between stations PONT-VLSM for the period 2009-2017. 55 Figure 40: Graph showing baseline rate changes between stations PONT-VLSM for the period 1/1/2009- 25/1/2014 ...... 55 Figure 41: Graph showing baseline rate changes between stations PONT-VLSM for the period 1/7/2014- 16/11/2015 ...... 56 Figure 42: Diagram showing baseline changes between VLSM-KARA stations for the period 01/07/14- 16/11/15...... 57 Figure 43: Diagram showing baseline changes between SPAN-PONT stations for the period 2009 -2017. Star indicates earthquake occurrence...... 59 Figure 44: Diagram showing baseline changes between SPAN-PONT stations for the pre-seismic period 2009 -2015, showing shortening of -2.48 mm/yr. A seasonal signal is also present in the data (see fluctuations), however, the trend is clear...... 60 Figure 45 : Diagram showing baseline changes between PONT-SPAN stations for the post-seismic period 2015 -2016 the first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots...... 60 Figure 46: Diagram showing baseline changes between PONT-SPAN stations for the post-seismic period 2016 -2017...... 61 Figure 47: Diagram showing baseline changes between stations PONT-VLSM stations for the period 2014 - 2015...... 62 Figure 48: Diagram showing baseline changes between stations PONT-VLSM for the period 18/11/15- 06/01/18, the first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots...... 62 Figure 49: Diagram showing baseline changes between PONT-KARA stations for the periods 2014 -2016. .. 63 Figure 50: Diagram showing baseline changes between PONT-KARA stations for the pre-seismic period 2014 -2015...... 64 Figure 51: Diagram showing baseline changes between PONT-KARA stations for the post-seismic period 2015 -2018. The first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots...... 64 Figure 52: Diagram showing baseline changes between stations PONT-AGRI for the period 2013 -2016. .... 65 Figure 53: Diagram showing baseline changes between stations PONT-AGRI for the pre-seismic period 2013 -2015...... 66 Figure 54: Diagram showing baseline changes between stations PONT-AGRI for the post-seismic period 2015 -2018. The first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots...... 66 Figure 55: Diagram showing baseline changes between stations ZAKY-VLSM for the period 2013 -2019. .... 69 Figure 56: Diagram showing baseline changes between stations ZAKY-VLSM for the pre-seismic period 2016 -2018...... 69 Figure 57: Diagram showing baseline changes between stations ZAKY-VLSM for the post-seismic period 2018 -2019. The first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots...... 70

List of tables

Table 1: List of the strong earthquakes and their focal plane solutions that occurred in the study area during the last four decades. Shaded cells indicate events studied in this thesis...... 10 Table 2: List of GNSS stations in Ionian Islands, Greece, used in this study. For a map view see Fig. 11...... 21 Table 3: List of seismic offsets determined in this study (in mm) for Cephalonia 2014 and Lefkada 2015 events. AE, AN, AUP indicate offsets measured on East-West, North-South and Up-Down components, respectively...... 26 Table 4: List of seismic offsets determined in this study (in mm) for Zakynthos 2018 event. AE, AN, AUP indicate offsets measured on East-West, North-South and Up-Down components, respectively...... 27 Table 5: List of the hypocentre-to-station distance, calculated using the Pythagoras theorem and published depths from the NOA catalogue...... 42 Table 6: Baseline rate change for stations VLSM-PONT, VLSM-KARA before and after the two 2014 seismic events. N is number of common days...... 54 Table 7: Baseline rate change for stations VLSM-PONT, VLSM-KARA and VLSM-SKAL before and after the two 2014 seismic events. N is number of common days...... 58 Table 8: Baseline rate change for stations and ZAKY-VLSM before and after the 2018 Zakynthos seismic event. N is number of common days...... 68

CHAPTER 1 INTRODUCTION

1. CHAPTER 1: INTRODUCTION

In the Central Ionian Islands region, lies the transition zone between the active Hellenic subduction to the south and the continental collision zone between the Apulian microplate and Eurasia to the north (McClusky et al. 2000; Shaw and Jackson, 2010; Howell et al. 2017). The kinematic transition is characterized by horizontal motion along a major tectonic structure, called the Cephalonia Transform Fault. The sense of motion is right-lateral (dextral) because of the northward motion of Africa-Apulia and the southward motion of Eurasia (Hatzfeld et al. 1995; Hollenstein et al. 2006; 2008). The Zakynthos Island and the offshore area west of it, is dominated by thrust faulting with a minor or considerable strike– slip component (Ganas and Parsons, 2009; Haddad et al. 2020).

In this work we present results on the co-seismic deformation of central Ionian islands as measured from the GNSS networks in the area, following the large shallow earthquakes of 2014 (Cephalonia), 2015 (Lefkada) and 2018 (Zakynthos). In addition, we present preliminary results of baseline changes between GNSS stations in this area and discuss the implication for crustal deformation.

The earthquakes occurred in the western part of Cephalonia (Paliki Peninsula and bay of Argostoli) on January 26, 2014 13:55 UTC (Mw= 6.1) and February 3, 2014 03:08 UTC (Mw= 5.9) and ruptured two, 10+ km long, strike-slip faults that generated environmental effects and ground deformation (Valkaniotis et al., 2014; Sakkas and Lagios, 2017).

The November 17th 2015 07:10 UTC (Mw= 6.4) earthquake which was located to the southwestern part of the Lefkada island north of Cephalonia, along a N20±5°E, east-dipping strike-slip fault with right-lateral sense of slip (Ganas et. al., 2016; Avallone et al. 2017).

The earthquake offshore Zakynthos island on October 25, 2018 22:54 UTC (Mw= 6.6). The event occurred ~36 km to the SW of Zakynthos and ~40 km to the NW of the Strofades Island (Romano et al., 2019; Cirella et al. 2020; Ganas et al., 2020).

1 CHAPTER 1 INTRODUCTION

1.1 Tectonic Setting

The tectonic setting of Greece, as a part of the East Mediterranean region, is dominated by the long-term convergence between the African and the Eurasian tectonic plates, which has currently reached the final stages of collisional orogeny (see Fig. 1). All over this area, the oceanic lithospheric domains (the “neo-Tethys”) originally present between the Eurasian and African-Arabian plates have been subducted and partially obducted (in East Anatolia region), except for the Ionian basin in the SW (i.e. at the WHA) and the Levantine basin (Herodotus basin) in the SE of the East Mediterranean ridge (Fig. 1). A recent publication by Granot (2016) supports the idea that the Levantine oceanic crust is of Paleozoic age.

Figure 1: Tectonic map of the Eastern Mediterranean (after Jolivet et al. 2013). Thick black lines are main plate boundary faults with ticks/arrows indicating relative sense of movement.

The geodynamic evolution of this area of the Mediterranean, which includes the orogenic system of the Hellenides, has been a subject of discussion and several different models have been proposed for its interpretation. At first, the Hellenides were proposed as a typical area for the theory of geosynclines (Aubouin, 1965). Later on, with the development of plate tectonics theory in the 1960-1970s, new tectonic models were proposed, which were focused on the identification of the ophiolite zones representing the lost oceanic

2 CHAPTER 1 INTRODUCTION basins of Tethys. However, this simple Atlantic-type paleogeographic organization could not explain, the occurrence of more than one ophiolite belt within the pelagic sediments, alternating with shallow water carbonate platforms (Aubouin, 1976; Aubouin et al., 1977; Dercourt, 1970, 1972; Dewey et al., 1973; Le Pichon and Angelier, 1979). For this reason, several tectonic and paleogeographic models have been developed, including different geodynamic settings and several oceanic basins associated with different ophiolite belts (Biju-Duval et al., 1977; Dercourt et al., 1985; Dewey and Şengör, 1979; Robertson, 2002; Robertson and Dixon, 1984; Robertson et al., 1991; Smith and Rassios, 2003; Stampfli and Borel, 2004). The outcrops of continental crust (Pre-Cambrian and/or Paleozoic age) in between the ophiolite suture zones of the Hellenides, covered by shallow-water carbonate platforms of Mesozoic–early Cenozoic age, were thought to represent microcontinents (Papanikolaou 1984, 1986; Robertson and Dixon, 1984; Sengor, 1979, 1984, 1989; Sengör et al., 1984, 1988).

According to Papanikolaou (Papanikolaou, 1989, 1997, 2009, 2011; Papanikolaou et al., 2004), the geodynamic evolution of the Hellenides, from the late Triassic (230 Myr ago) until present, includes the successive drifting, towards North, of several continental fragments (tectonostratigraphic terranes), which where detached of the passive African margin through rifting in the early Mesozoic. As the drifting (autonomous motion of the terranes) went on, new oceanic basins opened consecutively (mainly in the Jurassic), in- between the terranes, southwards to Africa and pre-existing oceanic basins started to close northwards to Europe (from Jurassic until today). The oceanic closure was accompanied by northward subduction beneath Eurasia and collision of the tectonostratigraphic terranes to the European active margin. This resulted in the successive accretion of the latter, from early Jurassic until late Miocene.

Jolivet and Brun (2010) proposed the hypothesis that a single oceanic (African) northward subduction has been active throughout most of the Mesozoic and the entire Cenozoic, and they presented evidence that the geological record (mainly on the Cyclades) is compatible with this hypothesis. They suggested that a) slab-roll back and b) delamination of lower- crust and upper mantle were the primary mechanisms leading to the exhumation of metamorphic rocks and the subsequent formation of extensional metamorphic domes in the Aegean back-arc region.

3 CHAPTER 1 INTRODUCTION

1.2 Hellenic Arch-Trench System

(Greek orogen), which includes a number of features such as deep troughs on the East- Mediterranean Sea floor, the subducted slab, upper plate faults etc. (Fig. 1). The main feature is the active subduction zone, along which the current remnants with oceanic lithosphere (mainly the Ionian basin) of the African plate are being subducted, under the continental crust of the Eurasian plate forming an amphitheatric shape (Fig. 2; Ganas and Parsons, 2009). It was believed that subduction occurs since Neogene (Miocene–Pliocene) and it is just before the final collisional stage with Africa (Finetti et al., 1990; Mascle and Chaumillon, 1998), which is estimated to occur in about 8 million years (Papanikolaou, 2011).

A breakthrough occurred when seismic tomographic models showed an oceanic slab much longer than expected (1500 km) and thus suggested a much longer history of subduction and extension with a single slab (Spakman et al., 1988), a hypothesis that has been widely developed from then onwards (Wortel and Spakman, 2000; Faccenna et al., 2003; Jolivet and Brun, 2010).

The Hellenic Arc-Trench system is in its current state since the late Miocene. It has a total length of about 1200 km from approximately 37.5°N, 20.0°E offshore the island of Zakynthos to 36.0°N, 29.0°E offshore the island of Rhodes (Ganas and Parsons, 2009). The Arc starts with a NNW-SSE orientation from the Ionian Islands and continental Greece (WHA), bending to E-W from Kythera to Crete and ends with a NE-SW direction east of Dodecanese island complex up to Turkey. It is interesting to note that for two decades, since the 1970s (at least) the front of the subduction was misplaced by many workers as being located in the Hellenic trench (Fig. 4) instead of south of the Mediterranean Ridge, that was not yet fully interpreted as an accretionary wedge (Jolivet et al. 2013).

4 CHAPTER 1 INTRODUCTION

Figure 2: 3-D visualization of the Hellenic subduction beneath Eurasia (after Ganas and Parsons, 2009).

In its current geometry (Fig. 2 and 3) it includes:

• The Hellenic trench which is most clearly developed in the western part of the Hellenic Arc-Trench system, mainly in the Ionian domain, splitting into the Pliny and Strabo trenches, south of Crete. Τhe depth of the trench is approximately 5 km. The deepest point along the trench (also deepest point in the Mediterranean Sea), is in the Ionian Sea south-west of Pylos and has a maximum depth of 5267 m. It is noted that the term trench does not refer to oceanic trench like Mariana’s or Sumatra etc. (see comment above), rather it denotes several bathymetric features of the upper (Eurasian) plate (such as the Matapan trench; Mascle and Le Quellec, 1980), that are characterized with linear geometry.

• The non-volcanic (island) arc which is characterized by a raised topography and includes the land masses of Peloponnese, Crete and Dodecanese.

• The back arc in the Northern Cretan basin which is caused by N-S extension in the Aegean area, mostly developing in Miocene and Pliocene

5 CHAPTER 1 INTRODUCTION

• The active volcanic arc in the south Aegean Sea which extends for 450 km along- arc, from Methana on the north-eastern coast of the Peloponnese in the west to the island of Nisyros in the east.

Figure 3: N-S compiled lithospheric-scale cross-section of the Aegean region from the Rhodope massif to the African passive margin across Greece and the Cyclades (after Jolivet and Brun 2010). This figure highlights the most significant parts of the present-day Hellenic Arc-Trench system, such as the accretionary complex, the back-arc extension, the volcanic arc (Santorini) and others.

The eastern end of the subduction zone is located south of Rhodes and terminates before the Antalya region (Turkey; Howell et al. 2017) because of the slab tear (Wortel and Spakman, 2000). This part of the arc displays distributed deformation in the overriding upper-plate, including a mixture of strike-slip and splay-thrust faulting (Shaw et.al, 2010). In the west, the subduction zone terminates in a zone of sub-parallel NE–SW strike-slip faults following the submarine Cephalonia valley west of the island chain from Lefkada to Cephalonia, of which the most prominent is the Cephalonia Transform Fault Zone (CTF; Louvari et al., 1999; Ganas and Parsons, 2009; Sachpazi et al. 2000; Ganas et al. 2016; Fig. 4).

6 CHAPTER 1 INTRODUCTION

Figure 4: Schematic location map of the Africa-Europe convergence in the Greek region. The map shows the Hellenic Arc-Trench system starting to the northwest in the Ionian Islands and ending to the southeast at the south of Rhodes. In the Ionian Islands region lies the transition zone (shaded rectangle) between the Hellenic subduction to the south and the continental collision in the north linked by the Cephalonia transform fault (Modified after Sachpazi et al. 2000)

South of the Hellenic Arc-Trench system, from southwest Peloponnesus to south of Crete and Rhodes, the seafloor is characterized by a large, arc-shaped, sedimentary wedge (Huguen et al., 2006), more than 1500 km long and 200–250 km wide, called the Mediterranean Ridge (Fig. 1, Fig. 4; Heezen and Ewing, 1963; Emery et al., 1966). It is an accretionary prism, which results from the northward subduction of the African plate beneath Europe (Fig. 4; Olivet et al., 1982; Le Pichon et al., 1995; Dewey and Sengör, 1979; Kreemer and Chamot-Rooke, 2004; Reillinger et al., 1997; McClusky et al., 2000). It stretches from the Ionian to the Levantine Basins and consists of a thick pile (up to 12 km) of off-scrapped and stacked sediments (De Voogd et al., 1992) deposited since the Mesozoic. It also contains, in its upper sedimentary cover, thick Upper Miocene/Messinian

7 CHAPTER 1 INTRODUCTION

evaporitic sequences (locally up to 2 km; Finetti, 1976; Chaumillon, 1995; Chaumillon et al., 1996; Le Meur, 1997).

1.3 Seismicity of the Central Ionian Sea

The central Ionian Islands (Lefkada, Cephalonia, Ithaki and Zakynthos; Fig. 4) region is the most active zone of shallow seismicity in the broader Hellenic region.

Historical information and instrumental data reveal that the occurrence frequency for the stronger (M>6.5) events in the study area is almost constant during the last four centuries with one such shock per decade (Papadimitriou & Papazachos, 1985).

The maximum observed earthquake magnitude in Cephalonia equals to 7.4 and in Lefkada to 6.7 (Papazachos & Papazachou, 2003). Moderate magnitude events are also very frequent, often located onshore, constituting an additional threat from the seismic hazard view point. Earthquake locations (Papazachos et al. 2005; Papadimitriou et al. 2006; Haddad et al. 2020) demonstrate that the seismicity is well confined in a narrow zone running along the western coast of Cephalonia Island and in the sea area southwestwards, which is bounded by a steep bathymetric slop (Fig. 6). Earthquakes are more widely distributed to the south and west, offshore Zakynthos Island (Fig. 5; Ganas et al. 2020).

The NOA seismicity data of the instrumental period (1964-2019; M>3.5; Fig. 5) highlight the concentration of epicentres near the islands of Cephalonia, Lefkada and Zakynthos, respectively. The map in Fig. 5 shows the frequency of earthquakes per 100 km2 over the last 55 years. This image of seismicity may not be entirely characteristic of the 20th century (i.e. the 1953 earthquakes are missing) but it is in agreement with the location of the main tectonic lines of this area (see Fig. 4), i.e. the CTF and the west Hellenic Arc which is part of the Hellenic subduction zone.

8 CHAPTER 1 INTRODUCTION

Figure 5: Map of the seismicity in western Greece for ML>3.5 for the last 55 years (1964-2019) based on the NOAGI catalogue. The color scale represent the number of events.

Information of the strong earthquakes and their plane solutions that occurred in the study area during the last four decades is given in Table 1. The list of events starts in 1976 with a thrust event west of Zakynthos (Global Centroid Moment tensor magnitude M=6.4). On Jan. 17, 1983 a strong strike-slip event (M=6.8) occurred SW from Cephalonia followed by a strong aftershock (23 March; M=6.2). On Nov. 18, 1997 a strong earthquake occurred near Strofades islands about 50 km south of Zakynthos (M=6.6). On Aug. 14, 2003 a magnitude M=6.2 strike-slip event occurred in northwestern Lefkada. In 2014 and 2015 three events occurred in Cephalonia (26 January 2014; M=6.1 and 3 February 2014; M=5.9)

9 CHAPTER 1 INTRODUCTION and south Lefkada (17 November 2015; M=6.4;). Finally, on Oct. 25, 2018 a strong earthquake (M=6.6) occurred southwest of Zakynthos. In this study we analyze the GNSS data of the 2014 and 2015 and 2018 events only.

Table 1: List of the strong earthquakes and their focal plane solutions that occurred in the study area during the last four decades. Shaded cells indicate events studied in this thesis. Focal Mechanism Ref Depth Origin Epicenter Mw Nodal Plane 1 Nodal Plane 2 (km)

Time Lat. Long. Strike Dip Rake Strike Dip Rake Year Date (GMT) (˚N) (˚E) (°) (°) (°) (°) (°) (°)

1976 May 11 16:59:48.20 37.56 20.35 15.0 6.4 339 14 110 139 77 85 1

1983 Jan. 17 12:41:30.14 38.07 20.25 14.0 6.8 34 14 153 151 84 78 2

1983 March.23 23:51:05.50 38.23 20.29 12.6 6.2 27 59 175 120 86 32 3

1997 Nov. 18 13:07:38.33 37.48 20.69 10.0 6.6 8 31 162 113 81 60 3

2003 Aug. 14 05:14:54.20 38.82 20.60 12.0 6.2 18 59 -174 285 85 -31 2

2014 Jan. 26 13:55:44.29 38.21 20.47 16.0 6.1 30 70 169 124 80 20 3

2014 Feb. 3 03:08:44.57 38.28 20.41 5.0 5.9 294 78 35 196 56 166 3

2015 Nov. 17 07:10:07.24 38.68 20.59 14.0 6.4 22 72 161 118 72 19 3

2018 Oct. 25 22:54:51.46 37.36 20.50 20.0 6.6 119 84 66 16 25 166 3 1 = epicentre: Makropoulos et al 2012/magnitude & depth: GCMT catalog (focal mechanism), 2= epicentre & depth from www.geophysics.geol.uoa.gr / magnitude and focal mechanism from GCMT 3 = epicentre, depth, magnitude & focal mechanism retrieved from the NKUA seismic catalogue (www.geophysics.geol.uoa.gr)

10 CHAPTER 1 INTRODUCTION

1.4 The Cephalonia Transform Fault (CTF)

In the central Ionian Islands region, lies the transition zone between the active Hellenic subduction to the south and the continental collision zone to the north (McKenzie, 1978; Le Pichon et al., 1995; Papazachos and Kiratzi, 1996; Shaw and Jackson, 2010). The kinematic transition is characterized by horizontal motion along a major tectonic structure, called the Cephalonia Transform Fault (CTF; Fig. 4). The sense of motion is right-lateral (dextral) because of the northward motion of Africa-Apulia and the southward motion of Eurasia at a rate of about 25 mm/yr (firstly measured by Kahle et al. 1995). The first suggestions of strike-slip faulting in the area of the Ionian Islands has been based on bathymetric data (Finetti and Morreli, 1973; Stride et al., 1977; Finetti, 1982) and geological mapping on land (British Petroleum, 1971; Mercier et al., 1976; Cushing, 1985). The first evidence for strike-slip motion in the area based on seismological data, was presented by

Skordilis et al. (1985). They used fault plane solutions for the January 17, 1983 (Ms = 7.0) main shock and its largest aftershock (March 23, 1983, Ms = 6.2), as well as the distribution of the foci of this seismic sequence to support the idea of the existence, west of Cephalonia and Zakynthos, of a strike-slip dextral fault with a thrust component which strikes in an about NE-SW direction dipping to SE. This idea has been further confirmed by fault plane solutions (Anderson and Jackson, 1987; Papazachos et al., 1991), on waveform modeling (Papadimitriou, 1988; Kiratzi and Langston, 1991; Papadimitriou, 1993), by microseismicity studies (Hatzfeld et al., 1995; Haddad et al. 2020) and geodetic measurements (Kahle et al., 1993, 1995, 1996; Caporali et al., 2011; Hollenstein et al., 2006, 2008; Jenny et al., 2004; Nocquet, 2012).

These studies, as well as microseismicity studies (Kassaras et al., 1994; Makropoulos et al., 1996) supported by geological information (Cushing, 1985; Sorel, 1989; Underhill, 1989; IGME, Seismotectonic Map of Greece 1:500.000, 1989) have pointed out that the strike- slip motion continues further north in the western part of Lefkada Island. To the west of the Cephalonia island there is an area with a deep bathymetric trough striking at N20◦E with water depths of more than 3 km (Sachpazi et al., 2000; EERI Special Earthquake Report, 2003), possibly marking the surface outcrop of the right-lateral CTF. North of the

11 CHAPTER 1 INTRODUCTION

Cephalonia Island both the seismicity and the bathymetry change to an NNE direction (Stiros et al. 1994).

Figure 6: The Cephalonia Transform Fault (dashed thick line) with the Cephalonia segment of the fault marked as C and the Lefkada segment as L. The focal mechanisms of strong events are also shown. (Bathymetry contours from the topographic map 1: 500,000 from GYS) (After Louvari et al., 1999).

According to Louvari et al., (1999), the CTFZ consist of two segments. The Cephalonia segment which is the main one and its northeastward prolongation, the Lefkada segment. The Cephalonia segment exhibits strike-slip motion with a thrust component, strikes in a NE direction and dips to the SE (typical focal mechanism with strike=38˚, dip=63˚, rake=172). It has a length of ≈90 km, its width is ≈20 km and its slip rate equal to 2.7 cm/yr (Papazachos et al., 1994; Papazachos and Kiratzi 1996; Kahle et al., 1995, 1996). These values indicate that the maximum earthquake magnitude for the Cephalonia segment is equal to 7.4, based on a scaling relation by Papazachos and Papazachou (1997). The Lefkada branch starts from the northern part of Cephalonia Island and continues alongside the

12 CHAPTER 1 INTRODUCTION western coast of Lefkada Island. It also exhibits strike-slip motion with a thrust component, strikes in an NNE–SSW direction and dips to the ESE (typical focal mechanism with strike=14˚, dip=65˚, rake=167˚) has a length of 40 km. The strike-slip kinematics of the two segments of the CTF is clearly seen in the focal mechanisms of shallow earthquakes in the central Ionian Sea (Fig. 6).

Recent analyses of CTF segmentation using space geodesy (Ilieva et al. 2016; Ganas et al. 2016; Avallone et al. 2017) have shown that the Lefkada segment is further segmented in two segments that ruptured in 2003 (north) and 2015 (south), respectively. The segmentation of the Cephalonia segment is less clear due to the absence of geodetic data. The new information we have from the 2014 earthquakes (Ganas et al. 2015; Karastathis et al. 2015) is that Paliki peninsula is separated from the rest of Cephalonia by a strike-slip zone with dextral-slip kinematics (Sakkas and Lagios 2015). Kokinou et al., (2006) who combined sea bottom topography and fault plane solutions suggested the prolongation of the CTFZ into the Ionian Abyssal Plain towards the direction between Cephalonia and Zakynthos Islands at depth greater than 15 km. This southern part of the CTFZ exhibits a strike–slip motion with a thrust component according to fault plane solutions of moderate and strong earthquakes (Fig. 7). The Zakynthos Island and the offshore area west of it, is dominated by thrust faulting with a minor or considerable strike–slip component being present in some of these fault plane solutions (Fig. 7; Cirella et al. 2020; Ganas et al. 2020). The transition from thrusting to strike–slip faulting is taking place north from about 37.8˚ (Kokinou et al., 2006).

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Figure 7: Fault plane solutions of the stronger earthquakes (M > 5) that occurred in the central Ionian Islands during the last five decades (1970-2020). The focal mechanism data are reported in the Appendix (Annex I). Black quadrants indicate compressional parts of the focal sphere.

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1.5 Geological Settings

The central Ionian Islands comprises the western part of the fold-belt of the External Hellenides. Central Ionian Islands mainly consist of Alpine Mesozoic and Cenozoic sedimentary rocks of the Paxos (or pre-Apulian) and the overthrusted Ionian zone, covered by Post-Alpine formations. Paxoi unit is the most external unit of the Hellenides originated from the western external part of the huge External Carbonate Platform of the Hellenides. This unit is considered to be relatively autochthonous one due to small horizontal displacement resulted from the alpine deformation phase. It constitutes a neritic carbonate platform from Jurassic to Miocene (Bornovas, 1964; BP 1971; Rondoyanni, 1997; Lekkas et al. 1999, 2001). The oldest alpine formations of the zone are Lower Jurassic dolomites and Middle Jurassic cherts and bituminous shales (Bornovas, 1964; BP Co, 1971). Marly formations and turbiditic limestones were formed during Early Miocene (Aquitanian). Moreover, clays and marls were deposited in Burdigalian and in Middle Miocene and it is therefore generally accepted that Paxoi unit lacks the typical flysch sedimentation of the other geotectonic units of the External Hellenides (Papanikolaou, 2015).

The Ionian unit is composed of the sequence of the Jurassic and Middle Cretaceous cherts and schists, the Upper Triassic-Upper Cretaceous carbonate sequence and the Oligocene- Lower Miocene flysch sediments (Bornovas, 1964; IGRS-IFP, 1966). The Ionian zone is dominated by compressional tectonics. The western boundary of the zone is defined by a major west-directed thrust, the Ionian thrust (Aubouin, 1959; Jacobshagen, 1986; Fig. 2), which is marked by evaporate intrusion (Karakitsios and Rigakis, 2007) and is generally considered to represent the most external structure of the Hellenides.

1.5.1 Lefkada Island

Lefkada Island consists of i) alpine formations that belong to Ionian and Paxoi geotectonic units, which cover the largest part of the island, ii) molassic formations and (c) recent deposits that lie unconformably on the previous formations (Renz, 1955; Bornovas, 1964; BP Co, 1971; Lekkas et al. 1999, 2001; Rondoyanni, 1997; Triantafyllou, 2010; Rondoyanni et al. 2012; Fig. 8).

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Figure 8: Neotectonic map of Lefkas Island based on Lekkas et al. (1999, 2001) and Rondoyanni et al. (2012).

The largest part of Lefkada comprises formations which belong to the Ionian unit, while Paxoi unit formations are observed only on the southwestern part of the island and more specifically on Lefkata peninsula (Fig. 8). The Ionian unit is considered to be the paraautochthonous unit of Lefkada. Paxoi formations are found in the central- and south- western part of Lefkas and extends eastwards under the Ionian formations (BP Co, 1971; Fig. 8). The molassic formations comprise mostly marine Aquitanian-Tortonian (Bornovas, 1964; Lekkas et al. 2001) marls, bioclastic limestones, conglomerates and sandstones that are unconformably overlying the deformed Ionian formations and few outcrops of Ionian flysch turbidites (Cushing, 1985; Rondoyanni, 1997; Triantafyllou, 2010). The post-alpine formations of Lefkada are Quaternary deposits lagoonal deposits, alluvial deposits, scree, talus cones, terra rossa and coastal deposits) unconformably overlying its alpine and molassic formations (Lekkas et al. 2018).

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The current tectonic setting of Lefkada is compressional and it is established on the island since Miocene-Quaternary times (Cushing, 1985; Mercier et al. 1987; Sorel et al. 1988; Lekkas et al. 1999, 2001) It is expressed by a large number of faults, mainly normal or strike- slip faults with a sinistral or dextral sense of shear (Fig. 8) which affect both the alpine formations of the Ionian and Paxoi geotectonic and the unconformably overlying recent deposits and they are classified into active, probably active and inactive structures (Lekkas et al. 1999, 2001). These faults are classified into active, probably active and inactive structures (Lekkas et al. 1999, 2001).

1.5.2 Cephalonia Island

The Cephalonia Island mainly consists of Alpine Mesozoic and Cenozoic sedimentary rocks belonging to the Paxoi zone and the overthrusted Ionian zone of the external Hellenides (Fig. 9; Aubouin and Dercourt, 1962; Lekkas et al., 2001).

The major part of the island is formed by the Paxoi zone. The stratigraphy contains a thick series of Mesozoic to Paleogene carbonates that are overlain by a series of folded Oligocene to earliest Tortonian deep marine marls and interbedded turbiditic calcareous sandstones (Mercier et al., 1972). Uncomformably overlying these, mainly in the SSW part of the island, are younger Plio-Quaternary sediments. Along the southern coast of the island, a series of Messinian clays alternating with mass-transported conglomerate and sand beds emerges beneath a thick series of Messinian evaporites and Lower Pliocene Turbi facies sediments and marls. A series of gently dipping Upper Pliocene silts, sands and calcarenites are also developed in the area (Underhill, 1989).

The region around Aenos Mt. (Fig. 9) is dominated by a major asymmetric NW–SE to N–S trending anticline. At the northern tip of this anticline, a NE-dipping Cretaceous to Miocene succession emerges, which was overthrusted by cleaved Cretaceous carbonates of Aenos Mt. (Mercier et al., 1972). Further to the east, a zone of intense brecciation occurs bounding these carbonates. Shortening is evident in the western areas of Cephalonia where a series of approximately N–S trending folds and at least one major thrust fault can be traced (Underhill, 1989). Mesozoic carbonates and sheared evaporites are found onto Miocene marls (Underhill, 1989).

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Figure 9: Sketch Geological Map of Cephalonia Island by Lagios et al. (2007).

The Ionian zone is dominated by compressional tectonics. The boundary of the zone is defined by the Ionian thrust, which is generally considered to represent the most external structure of the Hellenides. The thrust is well exposed in Cephalonia where a distinct scarp has formed with Mesozoic carbonates of the hanging wall lying next to eroded Miocene marls. In southern Cephalonia, a subhorizontal thrust brings evaporites onto Miocene marls (Underhill, 1989).

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2. CHAPTER 2: DATA PROCESSING AND ANALYSIS

2.1 GNSS Theory

The GNSS technique relies on precise measurements of electromagnetic waves between a GNSS antenna and a constellation of satellites with precisely known orbits. Geodetic analysis of the dual-frequency GNSS data results in reliable and unsaturated measurements of ground motion displacement (e.g. Melgar et al., 2015). GNSS directly measures displacements in an absolute global reference frame (WGS84), but rigorous post- processing of the data is necessary to measure offsets at the sub-millimetric level. Thus, the space observations are useful for extracting ground displacements following moderate to large events (5≤M≤9.2) as it has been demonstrated in many cases (see Melgar et al., 2015 and Ruhl et al. 2018, for a summary of recent literature).

Accordingly, GNSS technology is widely used in earthquake source studies where it is usually inverted on its own, or jointly with other geophysical data sets (e.g. InSAR, seismology), to define the kinematic source process of M6+ events (e.g. Avallone et al., 2017; Cirella et al. 2020). High-rate GNSS has also been employed in studies of long-period ground motions, and in structural monitoring (e.g. Moschas and Stiros, 2011). The latest application of GNSS is the incorporation of the geodetic component in earthquake early warning systems (e.g. Murray et al., 2018). A review of the evolution, uses, and algorithms behind GNSS can be found in Bock and Melgar (2016).

2.2 GNSS networks in Western Greece - Station Features

In this study we used geodetic data from fifteen (15) stations of two Greek GNSS networks located on central Ionian Islands, northwestern Peloponnese and southwestern Central Greece (Table 2, Fig. 11). The stations were in operation during the 2014-2018 period when four (4) strong and shallow seismic events occurred in the central Ionian islands, Greece

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(see Table 1 for chronology of events). The stations are equipped with dual-frequency geodetic receivers and belong to NOANET (Ganas et al., 2008; 2013), HxGN SmartNet Greece, EUREF and NKUA networks (Sakkas and Lagios, 2015, 2017).

In Greece NOA operates a national GNSS network, NOANET (Ganas et al., 2008; 2011; 2012; 2013) under an open data policy. Thus, it is possible to access the data (raw GPS observations) after a seismic event. The GNSS data are collected in a centralized repository and they are accessible via the NOA GSAC tool (Argyrakis et al., 2016) or the local GLASS node of NOA http://194.177.194.250:8080/glasswebui/#/site. All stations are built on small houses, directly on bedrock or up-to 2 story buildings so as to avoid tilting or other effects due to building weight response.

Figure 10: Nadir view of GNSS antenna at station VLSM (Cephalonia). Date of photo: 22 May 2014.

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Τhe following Table (2) shows the key features of the GNSS stations, which are i) the location (longitude and latitude), ii) the start and end date which indicate the period for which we have acquired GNSS data for each station (i.e. not the date at which a station started and ended operating), iii) the technical features of each station that is mainly the geodetic equipment (antenna and receiver types), and iv) the network to which every station belongs. An example of a geodetic station is shown in Fig. 10 (VLSM station on NOANET). A map of the stations is shown in Fig. 11.

Table 2: List of GNSS stations in Ionian Islands, Greece, used in this study. For a map view see Fig. 11. Site name Location Start Date End Date Network Lon Lat AGRI 21.4107 38.6266 2/12/2013 30/4/2018 HxGN ARGO 20.4941 38.1716 1/5/2016 30/4/2018 HxGN KARA 20.5843 38.1307 6/5/2014 17/9/2016 NKUA KIPO 20.3499 38.2058 15/6/2013 24/6/2016 NOANET KTCH 21.2486 38.4142 20/11/2013 24/4/2018 PPGNET PAT0 21.7884 38.2863 1/1/2013 30/4/2018 EUREF PATR 21.7346 38.2435 1/1/2014 30/4/2018 HxGN PLAT 21.7812 38.3701 20/3/2014 25/11/2015 NOANET PONT 20.5868 38.6216 1/1/2009 5/1/2018 NOANET RLSO 21.4664 38.0585 1/8/2006 27/2/2017 NOANET SISS 20.6610 38.1036 19/3/2014 12/3/2018 NKUA SKAL 20.7953 38.0772 4/3/2015 1/10/2017 NKUA SPAN 20.6753 38.7839 1/1/2009 28/4/2018 NOA VLSM 20.5902 38.1795 1/1/2009 30/4/2018 NOA ZAKY 20.8866 37.7818 2/12/2013 31/3/2019 HxGN

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Figure 11: Map of central Ionian Sea showing GNSS stations, earthquake epicentres and focal mechanisms of 26/01/2014, 03/02/2014, 17/11/2015 and 25/10/18 events. Red solid stars indicate earthquake epicentres (NKUA solutions). Yellow solid triangles indicate locations of GNSS stations.

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2.3 Data Processing

To study the ground deformation due to the strong seismic events we used daily observations from the GPS satellite constellation sampled at 30-s intervals. The daily data are provided by the network operators as RINEX version 2.11 files. The method of study is as follows (see data flow in Fig. 12):

1. The GNSS data are processed using the Bernese 5.2 (Dach et al. 2015) software following the double-difference method

2. Daily position calculations in IGb08 Cartesian coordinates (X, Y, Z) were obtained for each station.

3. The positions were then converted to EGSA 1987 (Greek projection system)

4. Finally, in order to calculate relative displacement in all three components, DE, DN,

DUP, in mm we normalized the daily position data by subtracting from the value of each day the average of all days (Fig. 12).

Figure 12: Data processing to calculate relative displacement in three components (DE, DN, DUP), in mm.

The resulting displacement values are in mm and are plotted in diagrams showing the position time series of each station (Fig 13 and ANNEX II). In several stations we observe an abrupt change in the time series trendlines caused by the strong seismic events. This indicates a rapid change in the position of a station (horizontally and/or vertically), after the occurrence of the event.

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Figure 13: Diagrams showing the time series of AGRI (Agrinion, Western Greece) and PONT station (Lefkada Island, Ionian Sea, Greece). Co seismic offsets caused by November 17, 2015 event, are observed. Top-panel: East-West component, Middle-panel: North-South component, Bottom-panel: Up-Down component. Time series of all stations are shown in Appendix II.

In some cases, such as that of the PONT station (south Lefkada) we observe a change of several cm in the position of the station in all three components as a result of the November 17th, 2015, Lefkada event. In relation to that we notice that the position of AGRI station (located in Agrinio, Western Greece), changes by a few mm especially in East-West and Up- Down direction. The difference between the two stations clearly represents the effect of the epicentral distance on the observed ground deformation. PONT station that is located closer to the epicenter of the 2015 earthquake (hypocenter distance 12.5 km, Tab. 4) shows an order of magnitude larger deformation than the AGRI station where is further away (hypocenter distance 71.42 km).

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2.4 Seismic Offsets

The procedure followed for calculating station offsets after Cephalonia earthquakes (1st 26/1/2014 13:55 UTC - 2nd 3/2/2014 03:08 UTC), after Lefkada earthquake (17/11/2015 07:10 UTC) and after Zakynthos earthquake (25/10/2018 22:54 UTC) is as follows:

• Initially we calculated the average of the normalized values (DE, DN, DUp,) for 8 days before and 8 days after each earthquake (excluding the day of the event).

• Then, we subtracted the pre-earthquake average from the post-earthquake and the result is the offset caused by each seismic event in each station, in three components. The positive values indicate northward motion in the North-South component, eastward motion in the East-West component and an upward motion in the Up-Down component. On the contrary, negative values indicate southward motion in the North-South component, westward motion in the East-West component and downward motion in the Up-Down direction (Fig. 14).

Figure 14: Offsets calculation process.

In the case of the Cephalonia earthquakes, due to the temporal proximity of the events (7 days difference between them) we only used the position data we have for the days between the two events to avoid the overlapping of these data. For some stations there are no or not enough data to calculate the offsets.

25 CHAPTER 2 DATA PROCESSING AND ANALYSIS

We also found the standard deviation (SD) of the values (DE, DN, DUp,) for 8 days before

(SDB) and 8 days after (SDA) each earthquake (excluding the day of the event). Then we computed the offset error (E) using the quadratic error formula:

2 2 퐸 = √푆퐷퐵 + 푆퐷퐴

The following table (Tab. 3) shows the offset and the corresponding error of each station after Cephalonia earthquakes (1st 26/1/2014 13:55 UTC - 2nd 3/2/2014 03:08 UTC), and after Lefkada earthquake (17/11/2015 07:10 UTC).

Table 3: List of seismic offsets determined in this study (in mm) for Cephalonia 2014 and Lefkada 2015 events. AE, AN, AUP indicate offsets measured on East-West, North-South and Up-Down components, respectively. EVENTS 26-01-2014 03-02-2014 17-11-2015 STATION OFFSETS NAME (mm)

AE AN AUP AE AN AUP AE AN AUP

AGRI -0.56±0.9 0.26±0.9 2.92±4.5 0.01±1.0 -0.55±1.3 -1.98±5.8 -6.88±2.4 -0.46±2.0 4.21±5.2 ARGO No Data KARA No Data 1.18±1.6 -13.32±1.3 -1.61±6.7 KIPO No Data KTCH -2.66±1.6 -0.09±1.6 0.82±3.5 No Data -2.24±0.9 -0.62±2.0 4.53±6.5 PAT0 -0.52±1.7 1.44±2.3 0.98±4.5 -0.25±1.8 -2.20±2.1 3.13±4.07 0.71±2.2 0.34±1.4 0.88±1.4 PATR -0.73±2.0 0.45±1.2 2.84±5.6 1.25±1.9 -0.62±1.1 -3.80±5.25 -0.94±1.0 -0.74±1.0 3.05±6.2 PLAT No Data -0.76±1.8 -0.44±1.3 2.25±3.4 PONT -2.69±2.7 -0.81±2.2 4.75±6.8 -0.90±2.2 -2.04±1.9 -2.47±6.69 -217.99±2.1 -369.54±2.8 -57.64±3.7 RLSO -0.46±1.4 0.37±1.2 6.10±6.2 0.27±1.4 -0.90±1.2 -4.47±5.25 0.95±1.4 -0.80±1.1 -1.67±2.4 SISS No Data SKAL No Data 4.40±1.7 -9.39±1.7 1.08±9.1 SPAN 1.11±2.8 0.33±3.5 3.44±8.3 -0.45±3.1 -1.29±3.9 -4.45±8.92 -76.75±2.7 -56.03±2.6 -4.14±5.1 VLSM -12.58±2.8 -4.44±2.4 -3.77±2.6 -13.04±2.6 -8.02±1.6 -4.25±6.17 4.28±1.4 -16.91±1.4 1.26±10.2 ZAKY 3.59±2.3 1.20±2.1 6.98±7.1 0.36±1.5 -1.66±1.1 -4.22±6.50 2.65±3.9 -5.24±3.3 -1.55±3.7

26 CHAPTER 2 DATA PROCESSING AND ANALYSIS

Preliminary results of this part of the thesis were published in Partheniou et al. (2019). In addition, we present the seismic offsets of three GNSS stations due to the 25 October 2018 Zakynthos event (M6.6; Table 4 and Fig. 22, 26 and 28). The processed stations are SISS, VLSM on Cephalonia Island and ZAKY on Zakynthos Island.

Table 4: List of seismic offsets determined in this study (in mm) for Zakynthos 2018 event. AE, AN, AUP indicate offsets measured on East-West, North-South and Up-Down components, respectively.

EVENTS

25/10/2018 SITE NAME OFFSETS (mm)

AE AN AUP

SISS -4.45±4.72 -2.26±2.90 -4.52±7.83

VLSM -0.07±3.42 -1.51±3.59 -7.90±7.15

ZAKY -30.85±1.46 -38.64±1.91 -4.51±3.44

The offsets caused by all the studied events are also presented graphically in Figures 15-28.

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AGRI

Figure 15: Co-seismic offsets observed in AGRI station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color.

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KARA

Figure 16: Co-seismic offsets observed in KARA station after the seismic event of Lefkada (2015). The least square trendline is in green color.

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KTCH

Figure 17: Co-seismic offsets observed in KTCH station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color

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PAT0

Figure 18: Co-seismic offsets observed in PAT0 station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color.

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PATR

Figure 19: Co-seismic offsets observed in PATR station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color.

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PONT

Figure 20: Co-seismic offsets observed in PONT station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015;right panel). The least square trendline is in green color.

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RLSO

Figure 21: Co-seismic offsets observed in RLSO station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015;right panel). The least square trendline is in green color.

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SISS

25/10/18

25/10/18

25/10/18

Figure 22: Co-seismic offsets observed in SISS station after the seismic event of Zakynthos (2018). The least square trendline is in green color.

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SKAL

Figure 23: Co-seismic offsets observed in SKAL station after the seismic event of Lefkada (2015). The least square trendline is in green color.

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SPAN

Figure 24: Co-seismic offsets observed in SPAN station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color.

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VLSM

Figure 25: Co-seismic offsets observed in VLSM station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color.

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25/10/18

25/10/18

25/10/18

Figure 26: Co-seismic offsets observed in VLSM station after the seismic events of Zakynthos (2018). The least square trendline is in green color.

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ZAKY

Figure 27: Co-seismic offsets observed in ZAKY station after the seismic events of Cephalonia (2014; left panel) and Lefkada (2015; right panel). The least square trendline is in green color.

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ZAKY

25/10/18

25/10/18

25/10/18

Figure 28: Co-seismic offsets observed in ZAKY station after the seismic events of Zakynthos (2018). The least square trendline is in green color.

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In Table 5 we give the distance between each station and the hypocenters of the four seismic events because as it is observed in most cases there is a positive correlation between the proximity of a station to the hypocenter of an event and the magnitude of the offset caused by this event to the station.

Table 5: List of the hypocentre-to-station distance, calculated using the Pythagoras theorem and published depths from the NOA catalogue.

EVENTS STATION 26/1/2014 3/2/2014 17/11/2015 25/10/2018 NAME DISTANCE TO HYPOCENTER (km)

AGRI 90.48 97.61 71.95 - ARGO 17.17 12.98 57.90 - KARA 24.75 19.78 62.56 - KIPO 22.49 8.26 57.74 - KTCH 68.20 76.39 64.70 - PAT0 111.29 121.37 112.77 - PATR 106.46 116.66 110.48 - PLAT 111.61 121.35 108.94 - PONT 47.69 44.69 15.04 - RLSO 85.35 95.77 102.59 - SISS 23.55 28.47 64.74 83.78 SKAL 32.34 39.85 69.50 - SPAN 65.94 64.21 19.98 - VLSM 17.44 19.05 56.32 89.84 ZAKY 60.06 67.48 102.85 52.63

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2.4.1 Offsets caused by the January 26 2014, M6.1 Cephalonia event

The only station in which we observe seismic offset caused by the January 26 2014, Cephalonia event, is VLSM station which is located only 17 km away from the hypocenter (tab. 5). In this station we calculated offsets of a few mm and low errors in all components (Tab. 3, Fig. 29). This is probably due to the proximity of the station to the hypocenter of the earthquake. The station moved SSW (12.58±2.82 mm to the west, 4.44±2.44 mm to the south and 3.77±2.68 mm downwards. ZAKY station also shows offset off a few mm in the horizontal component but with considerable error. In all the other studied stations (AGRI, KTCH, PAT0, PATR, PONT, RLSO and SPAN), the calculated offsets are of low value with high errors in all components. For this reason it is considered that this earthquake did not cause displacement in these stations.

Figure 29: Map showing seismic offset for each station due to 26/1/2014 earthquake, as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse.

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2.4.2 Offsets caused by the February 3 2014, M5.9 Cephalonia event

The only station in which we observe seismic offset caused by the February 03 2014, Cephalonia event, is VLSM station, which is located 19 km away from the hypocenter (Tab 5). In this station we calculated offsets of a few mm and low errors in the horizontal component (Tab. 3, Fig. 30). This is probably due to the proximity of the station to the hypocenter of the earthquake, as in the case of January 26 2014 Cephalonia earthquake. The station moved SSW (13.04±2.69 mm to the west and 8.02±1.68 mm to the south).

Figure 30: Map showing seismic offset for VLSM station as well as the corresponding error, in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse.

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In vertical component the calculated offset shows high error so the result isn’t considered reliable.

In all the other studied stations (AGRI, PAT0, PATR, PONT, RLSO SPAN and ZAKY), the calculated offsets are of low value with high errors in all components (Tab. 3, Fig. 31). For this reason it is considered that this earthquake did not cause displacement in these stations.

Figure 31: Map showing seismic offset for each station (AGRI, PAT0, PATR, PONT, RLSO SPAN and ZAKY) as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse. Station VLSM Is plotted in Fig. 30

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2.4.3 Offsets caused by the November 17, 2015 M6.4 Lefkada event

The seismic offsets observed after the November 17 2015, Lefkada event, show values of tens of cm with low error values in stations located on Lefkada Island (SPAN, PONT) and of a few mm with low error values in stations located on Cephalonia Island (KARA, SKAL, VLSM; Tab. 3, Fig. 32-34).

Figure 32: Map showing seismic offset for each station as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse. Stations SPAN and PONT are plotted in Fig. 33

46 CHAPTER 2 DATA PROCESSING AND ANALYSIS

The highest offset values with the lowest error, for the two horizontal components are observed in PONT and SPAN stations (Fig. 33), which are located close to the hypocenter of the earthquake (20km and 15km respectively; Tab. 5). PONT station moved SW in the horizontal direction (217.99±2.18 mm to the west and 369.54±2.86 mm to the south, Tab. 3, Fig 33). Vertically the station moved downwards (57.64±3.79mm, Tab. 3, Fig 34).

Figure 33: Map showing seismic offset for PONT and SPAN stations as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse.

47 CHAPTER 2 DATA PROCESSING AND ANALYSIS

SPAN station which is also close to the hypocenter of the earthquake, moved SSW after the earthquake (76.75±2.71 mm to the west and 56.03±2.67 mm to the south Tab. 3, Fig. 33). This station exhibits downward motion (4.14±5.19 mm, Tab. 3, Fig. 34), but with a significant error.

We notice that this earthquake caused significant deformation even in stations located relatively far from its hypocenter. This is mainly due to its large magnitude.

Figure 34: Map showing seismic offset for PONT and SPAN stations, in the vertical direction. Offsets are shown as red arrows.

48 CHAPTER 2 DATA PROCESSING AND ANALYSIS

2.4.4 Offsets caused by the October 25, 2018 M6.6 Zakynthos event

In the case of October 25, 2018 Zakynthos event, the highest offset values with the lowest error, are observed in ZAKY station. It is located 50 km away from the hypocenter (Tab 5), but caused deformation to the station probably due to it high magnitude. The station moved SW in the horizontal direction (30.58±1.68 mm to the west and 38.54±1.93 mm to the south, Tab. 4, Fig 35). The station moved slightly downwards (4.67±3.39mm, Tab. 4). The other two stations (SISS and VLSM) which are located far from the epicentre of the earthquake, show no deformation in horizontal as well as in vertical direction.

Figure 35: Map showing seismic offset for each station as well as the corresponding error (60% confidence level of the error ellipse), in the horizontal direction. Offsets are shown as red arrows while their corresponding error is by a red ellipse.

49 CHAPTER 2 DATA PROCESSING AND ANALYSIS

2.5 Diagrams showing the position change in three components (East, North, Up,) for each station

The diagrams below show a comparison of seismic offsets for each GNSS station, in bar chart form. The largest offsets were calculated for the 2015 event.

Figure 36: Bar charts showing seismic offsets per seismic event for all studied GNSS stations. Top panel: 26 Jan. 2014 event, middle panel: 3 Feb. 2014 event, bottom panel: 17 Nov. 2015 event.

50 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

3. CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

In this chapter we studied the horizontal distance change or baseline change between several pairs of GNSS stations (e.g. SPAN-PONT in Lefkada, Fig. 43-46), located in the broader study area and its variation with time (i.e. the rate of change) since the installation of these stations. The main purpose is to determine how the baseline rate change between stations is affected by the deformation preceding the four major seismic events that occurred in the study area during the period 2014-2018. Earlier studies (e.g. Ganas et al. 2013; Chousianitis et al. 2016) have reported notable changes in baseline lengths for pairs of Lefkada stations.

For this reason, we studied the baseline change between two stations that are both located close to an earthquake epicenter (e.g. SPAN-PONT, Lefkada 17/11/2015 event, Fig. 43-46) as well as between a station that is located close to an epicenter and another station that is far from it (PONT-AGRI, Lefkada 17/11/2015, event Fig. 52-54). We also divided the study periods into pre-seismic and post-seismic for all seismic events and we calculated and studied the baseline change rate between two stations for each time period separately.

3.1 Method of processing

We calculated the horizontal distance between two stations or the baseline (e.g. SPAN- PONT, Fig. 43-46). The GNSS data used to calculate the baselines are the daily positions of each station (E, N, Up; in EGSA 1987 coordinates). For the calculations we took into account only the days for which both stations have recorded data (common days). The total number of common days included in the study period (N) for each pair of stations is given.

Then, we normalized the daily horizontal distance between two stations by subtracting the average of all values in the timeseries from the value of each day. The results for each pair of stations are presented in graphs relative to time.

51 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Finally, we calculated the baseline rate change between two stations, in mm / year by calculating the slope of the trendline (linear regression) from the graph of the daily distance between two stations relative to time (see Fig. 37 for a flow chart of the method).

Trendline function

풚 = 풂풙 + 풃

where, x is the independent variable; y is the dependent variable; a is the slope (gradient) of the line; b is a constant, equal to the value of y when x = 0.

∑(풙 − 풙)(풚 − 풚) 풂 = ∑(풙 − 풙)ퟐ

To estimate the reliability of the results we used the coefficient of determination R2, which is a statistical measure of how close the data are to the fitted regression line. R-squared is always between 0 and 1. In general the higher the R-squared, the better the model fits the data.

Preliminary results of this part of the thesis were presented in Partheniou et al. (2020; in Greek).

Figure 37: Data processing to calculate baseline change rate between two stations, in mm / year.

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Figure 38: Map of Ionian Islands showing of pre- seismic baseline change rates (scale -8 mm/yr to +6 mm/yr) for several pairs of GNSS stations. The baseline rate changes have been measured before the Cephalonia (2014), Lefkada (2015) and Zakynthos (2018) earthquakes. The seismic epicentres are after NKUA. Green triangles show the GNSS stations used in this study

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3.2 Cephalonia events (26/01/2014 and 03/02/2014)

In the case of the 2014 Cephalonia earthquakes, due to the temporal proximity of the events (7 days difference between them) we study them as one event. We calculated the baseline rate change between two pairs of GNSS stations (VLSM-PONT and VLSM-KARA; Fig. 38; Tab. 6). Stations VLSM and KARA are located on Cephalonia Island while PONT station is located on Lefkada Island (see Fig. 38).

Table 6: Baseline rate change for stations VLSM-PONT, VLSM-KARA before and after the two 2014 seismic events. N is number of common days.

EVENTS 26/01/2014 and 03/02/2014

2D 2D Station Station Pre-Seismic Baseline Post-Seismic Baseline R2 N R2 N A B Period Rate Period Rate (mm/yr) (mm/yr) 1/7/2014- VLSM PONT 1/1/09-25/1/14 2.92 0.79 1815 6.39 0.54 444 16/11/2015 1/7/2014- VLSM KARA No Data No Data No Data No Data -1.46 0.10 615 16/11/2015

3.2.1 Baseline VLSM-PONT

We calculated the baseline rate change between VLSM and PONT stations for a pre-seismic period from 01/01/2009 to 25/01/2014 (total number of common days N= 1815). We notice that during the pre-seismic period the horizontal distance rate change between the stations is positive (Tab. 6). This means that the distance between Cephalonia and South Lefkada increases with a rate of 2.92 mm/yr (Fig. 40).

Post-seismically, we skipped several months in order to avoid the post-seismic adjustment of the crust after the two seismic events and we calculated the baseline change rate from 01/07/2014 to 16/11/2015 (total number of common days N= 444). We observe that the baseline between these stations continues to increase after the occurrence of the two earthquakes (Fig. 41) with a higher rate (6.39 mm/yr; Tab.6). The results for this pair of

54 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES stations are considered to be quite reliable taking into account the high R2 for both pre- and post-seismic study period (Tab. 6).

Figure 39: Graph showing baseline rate changes between stations PONT-VLSM for the period 2009- 2017.

Figure 40: Graph showing baseline rate changes between stations PONT-VLSM for the period 1/1/2009-25/1/2014

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Figure 41: Graph showing baseline rate changes between stations PONT-VLSM for the period 1/7/2014-16/11/2015

3.2.2 Baseline VLSM-KARA

For the station pair VLSM-KARA (Fig. 38) we don’t have data for the time period before the seismic events of 2014 because station KARA was installed on May 2014. Post seismically we calculated the baseline change rate from 01/07/2014 to 16/11/2015 (total number of days N= 615). During this period the baseline between stations VLSM-KARA shortens (Fig. 42), with a rate of -1.46 mm/yr (Tab. 6). For this rate, the coefficient of determination is very low (R2=0.1). The distance between those two stations is 5.1 km and the relatively short time interval (one year and a half) may be responsible for this. In addition, lack of deformation is mainly the reason for this almost flat line of negligible rate changes.

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Figure 42: Diagram showing baseline changes between VLSM-KARA stations for the period 01/07/14-16/11/15.

57 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

3.3 Lefkada event (17/11/2015)

In the case of 2015 M6.4 Lefkada earthquake, we calculated baseline change rates between five pairs of GNSS stations (PONT-SPAN, PONT-VLSM, PONT-KARA, PONT-AGRI and PONT- KTCH). Stations PONT and SPAN are located on north and south Lefkada respectively, stations VLSM and KARA are located on Cephalonia island and stations AGRI- KTCH are located in Akarnania (Fig. 38).

Table 7: Baseline rate change for stations VLSM-PONT, VLSM-KARA and VLSM-SKAL before and after the two 2014 seismic events. N is number of common days. EVENT 17/11/2015

2D 2D Pre- Post- Baseline Baseline Station A Station B Seismic R2 N Seismic R2 N Rate Rate Period Period (mm/yr) (mm/yr) 18/11/15- 162.20 0.47 15 2/12/15 1/1/09- 3/12/15- PONT - SPAN -2.48 0.74 2408 18.80 0.69 146 16/11/15 30/4/16 1/5/16- -1.35 0.07 380 19/12/17 18/11/15- -326.35 0.85 15 4/4/14- 2/12/15 PONT - VLSM 5.99 0.61 531 16/11/15 3/12/15- -2.95 0.30 583 5/1/2018 18/11/15- -336.20 0.88 15 6/5/14- 2/12/15 PONT - KARA 5.48 0.52 488 16/11/15 3/12/15- -13.36 0.52 137 17/9/16 18/11/15- 77.12 0.12 15 2/12/13- 2/12/15 PONT - AGRI -8.69 0.79 547 16/11/15 3/12/15- -0.77 0.02 486 17/9/16

58 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

3.3.1 Baseline SPAN - PONT

The horizontal distance between PONT-SPAN stations (Fig. 38), shortens (fig. 44) with a rate of 2.48 mm/yr (Tab.7) during the pre-seismic period from 01/01/2009 to 16/11/2015 (total number of common days N=2408; Tab 7). We suggest that this baseline shortening between GNSS stations located in north and south Lefkada is due to pre-seismic deformation and it is related to fault “locking” before the earthquake rupture. Post seismically we notice that the baseline between PONT-SPAN stations increases sharply in the first fifteen days after the earthquake (Fig.45) with a high rate of 162.20 mm/yr. The distance between PONT-SPAN continues to increase (Fig.45) with a lower rate of 18.8 mm/yr (Tab.7) during the post seismic period from 3/12/2015 to 30/4/2016 (total number of days N=146; Tab.7). For the next period (01/05/2016 to 19/12/2017, total number of common days N= 380; Fig. 46) as we observe from the 2D Distance/time diagram, the baseline change rate between PONT-SPAN tends to become normalized.

Figure 43: Diagram showing baseline changes between SPAN-PONT stations for the period 2009 - 2017. Star indicates earthquake occurrence.

59 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Figure 44: Diagram showing baseline changes between SPAN-PONT stations for the pre-seismic period 2009 -2015, showing shortening of -2.48 mm/yr. A seasonal signal is also present in the data (see fluctuations), however, the trend is clear.

Figure 45 : Diagram showing baseline changes between PONT-SPAN stations for the post-seismic period 2015 -2016 the first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots.

60 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Figure 46: Diagram showing baseline changes between PONT-SPAN stations for the post-seismic period 2016 -2017.

3.3.2 Baseline PONT-VLSM

The distance between stations PONT-VLSM (north Lefkada and south Cephalonia respectively) increases (Fig. 47) with a rate of 5.99 mm/yr during the pre- seismic period, from 04/04/14 to 16/11/15 (total number of common days N=531; Tab. 7). Post seismically we observe a contrasting behavior: the distance between PONT-VLSM shortens with a high rate of -326.35 mm/yr for the first 15 days after the earthquake (Fig. 48). For the next period from 03/12/15 to 05/01/18 (total number of common days N= 583; Tab. 7), the distance between PONT-VLSM keeps shortening with a lower rate of -2.95 mm/yr (Tab. 7 and Fig. 48).

61 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Figure 47: Diagram showing baseline changes between stations PONT-VLSM stations for the period 2014 -2015.

Figure 48: Diagram showing baseline changes between stations PONT-VLSM for the period 18/11/15-06/01/18, the first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots.

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3.3.3 Baseline PONT-KARA

We observe a similar behavior in the baseline change between stations PONT- KARA (north Lefkada and south Cephalonia respectively; Fig. 38), with the baseline between these two stations to increase with a rate of 5.48 mm/yr during the pre- seismic period (Fig. 50), from 06/05/14 to 16/11/15 (total number of common days N=488; Tab.7). Post seismically the distance between PONT-KARA shortens with a high rate of -336.20 mm/yr for the first 15 days after the earthquake (Tab. 7 and Fig. 51). For the next period from 03/12/15 to 17/9/17 (total number of days N= 137), the distance between PONT-KARA keeps shortening with a lower rate of -13.36 mm/yr.

Figure 49: Diagram showing baseline changes between PONT-KARA stations for the periods 2014 - 2016.

63 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Figure 50: Diagram showing baseline changes between PONT-KARA stations for the pre-seismic period 2014 -2015.

Figure 51: Diagram showing baseline changes between PONT-KARA stations for the post-seismic period 2015 -2018. The first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots.

64 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

3.3.4 Baseline PONT-AGRI

The baseline between stations PONT-AGRI (Fig. 38) is oriented nearly E-W and it shortens with a rate of -8.69 mm/yr during the pre-seismic period (Fig. 53), from 02/12/13 to 16/11/15 (total number of common days N= 551; Tab.7). This observation points to a steady motion of station AGRI towards west (relative to PONT) therefore resulting in baseline shortening. Post-seismically the distance between PONT-AGRI increases abruptly with a relatively high rate of 77.12 mm/yr for the first 15 days after the earthquake (Tab. 7 and Fig. 54). For the next period from 03/12/15 to 17/9/17 (total number of common days N= 486; Fig. 54), the distance between PONT-AGRI tends to shorten with a lower rate of - 13.36 mm/yr going back to a normal state.

Figure 52: Diagram showing baseline changes between stations PONT-AGRI for the period 2013 - 2016.

65 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Figure 53: Diagram showing baseline changes between stations PONT-AGRI for the pre-seismic period 2013 -2015.

Figure 54: Diagram showing baseline changes between stations PONT-AGRI for the post-seismic period 2015 -2018. The first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots.

66 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

During the pre-seismic period of the Lefkada 17/11/2015 M6.4 earthquake, the baseline distance between PONT-VLSM and PONT-KARA (Lefkada-Cephalonia) increases with almost the same rate (5-6 mm/yr; Tab. 7) with a 0.52 < R2 < 0.61. However, during the same period the distance between PONT-AGRI and PONT-KTCH (Lefkada-Akarnania) shortens (Tab. 7) with a rate of -8.69 mm/yr (R2=0.79) and -3.32 mm/yr (but with a R2=0.16). During the post-seismic period we observe an abrupt change in baseline rate in the first few days after the event for all pairs of stations. The post-seismic change is marked by a non-linear behavior of the deformation in contrast to the linear behavior before the event.

67 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

3.4 Zakynthos M6.6 event 25/10/2018

In the case of 2018 Zakynthos M6.6 earthquake we calculated baseline change rates between ZAKY station, located on Zakynthos Island and VLSM station located on south Cephalonia (Tab. 8 and Fig. 38). We notice that during the pre-seismic period from 01/06/16 to 24/10/18 (total number of common days N= 403; Tab.8) the two stations move away from each other with a very low rate of 0.62 mm/yr (Fig. 56). The rate increases sharply for the first few days after the earthquake (43 mm/yr) and return to a pre-seismic value of 0.14 mm/yr for the rest period from 10/12/18 to 31/12/19 (total number of common days N= 767; Fig. 57 and Tab.8). It is noted that station ZAKY is located 54 km from the earthquake epicentre (Tab.4), i.e. it is located much further from the seismic fault than the previous cases. The low values of R2 also indicate that there is practically no deformation to be detected between these two pairs of stations.

Table 8: Baseline rate change for stations and ZAKY-VLSM before and after the 2018 Zakynthos seismic event. N is number of common days.

EVENT 25/10/2018

2D 2D Baseline Baseline Station Station Pre-Seismic Post-Seismic Change R2 N Change R2 N A B Period Period Rate Rate (mm/yr) (mm/yr) 26/10/18- 43.8 0.07 15 1/6/2016- 09/11/18 ZAKY VLSM 0.62 0.07 403 24/10/18 10/11/18- 0.14 0 767 31/12/19

68 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Figure 55: Diagram showing baseline changes between stations ZAKY-VLSM for the period 2013 - 2019.

Figure 56: Diagram showing baseline changes between stations ZAKY-VLSM for the pre-seismic period 2016 -2018.

69 CHAPTER 3: STUDY OF THE BASELINE RATE CHANGES

Figure 57: Diagram showing baseline changes between stations ZAKY-VLSM for the post-seismic period 2018 -2019. The first fifteen days after the seismic event are shown with orange dots, while the rest with blue dots.

70 CHAPTER 4: DISCUSSION AND CONCLUSIONS

4. CHAPTER 4: DISCUSSION AND CONCLUSIONS

In this study we used geodetic data from fifteen (15) permanent GNSS stations, to measure and interpret the co- and post-seismic crustal deformation due to four (4) strong seismic events in the central Ionian Islands, during the period 2007-2018. The earthquakes occurred on 26 January 2014 and 3 February 2014, on Cephalonia Island, on 17 November 2015, on Lefkada Island and offshore Zakynthos Island on 25 October 2018.

We used daily data, provided by the network administrators in the form of RINEX files (version 2.11) which were processed using Bernese 5.2 software (Dach et al. 2015) following the double-difference method. Daily position observations were obtained for each station in IGb08 Cartesian coordinates (X, Y, Z). The positions were then converted into EGSA 1987 system (Greek projection system). We used these data to a) observe the time series for changes due to seismic motions (offsets) and b) monitor changes in the horizontal distance (baselines) between several stations, in relation to time

Our conclusions are:

• The only station in which we observe seismic offset caused by the January 26 2014, Cephalonia event, is VLSM station (Tab. 3, Fig. 29), which is located only 17 km away from the hypocenter (Tab.5). The station moved 13.34mm SSW and 3.77±2.68 mm downwards. ZAKY station also shows offset off a few mm in the East-West direction (3.59±2.3 mm) but with considerable error.

• VLSM is also in the case of February 03 2014, Cephalonia event the only station in which we observe seismic offset (Tab. 3, Fig. 30). It is located 19 km away from the hypocenter (Tab. 5).The station moved 15.31 mm to the SSW. In vertical component the calculated offset shows high error so the result isn’t considered reliable.

• The seismic offsets with the highest values and the lowest errors in the case of November 17 2015, Lefkada event, are observed in stations PONT and SPAN on Lefkada Island which are located 15km and 20km away from the hypocenter, respectively (Tab. 5). PONT station moved 429. 04 mm to the SW in the horizontal direction (Tab. 3, Fig 33). Vertically the station moved downwards (57.64±3.79mm,

71 CHAPTER 4: DISCUSSION AND CONCLUSIONS

Tab. 3, Fig 34). SPAN, moved 95.03 mm to the SSW after the earthquake (Tab. 3, Fig. 33). This station exhibits downward motion but with a significant error. Offsets of a few mm with low error values are also observed in stations located on Cephalonia Island (KARA, SKAL, VLSM; Tab. 3, Fig. 32-34), although they are located more than 50 km away from the hypocenter.

• In the case of October 25, 2018 Zakynthos event, the highest offset values with the lowest error, are observed in ZAKY station which is located 50 km away from the hypocentre (Tab 5). The station moved 49.20mm to the SW in the horizontal direction (Tab. 4, Fig 35). The station moved slightly downwards (4.67±3.39mm, Tab. 4). The other two stations (SISS and VLSM) which are located far from the epicentre of the earthquake, show no deformation in horizontal as well as in vertical direction.

• We notice that the proximity of the stations to the hypocenter of the earthquakes is correlated to the value of the observed offset. The highest offset values are observed in most cases in the stations closest to each seismic event.

• We also notice that the magnitude of the earthquake is also positive correlated to the value of the observed offset. The highest offsets are observed after the 2015 November event (M6.5). Earthquakes with high magnitudes can cause deformation even in stations located quite far from their hypocenters (e.g Lefkada and Zakynthos earthquakes)

• We measured baseline shortening (negative baseline change rate) between some stations and baseline lengthening (positive baseline change rate) between other stations by a few mm/yr.

• Studying the baseline rate change between stations VLSM (located on Cephalonia) and PONT (located on south Lefkada), before and after Cephalonia, 2014 earthquakes, we observe that in both periods the distance between them increases by a few mm/yr.

• Studying the baseline rate change, before the Lefkada, 2015 earthquake, we notice that the horizontal distance between PONT-VLSM and PONT-KARA (Lefkada- Cephalonia) increases with almost the same rate (5-6 mm/yr; Tab. 7). On the contrary, during the same period the distance between PONT-AGRI and PONT-KTCH (Lefkada-Akarnania) shortens (note that the results for these pairs are questionable because of the low R2 value).

72 CHAPTER 4: DISCUSSION AND CONCLUSIONS

• Studying the baseline rate change, before and after Zakynthos, 2018 earthquake, we calculate very low baseline rate change values with very low R2, which indicates that there is practically no deformation to be detected between these two pairs of stations. This is to be expected taking into consideration that both studied stations are located much further from the seismic fault than the ones in the previous cases.

• The baseline between stations SPAN-PONT (Lefkada stations belonging to NOA) shortens at a constant rate of -2.48 mm/yr before the occurrence of the 2015 Lefkada event. We interpret this behavior as due to pre-seismic deformation, related to fault “locking” before the earthquake rupture.

• We also note that a few days (about 15) after the seismic events of Lefkada and Zakynthos, the baseline rate between all studied pairs of stations, changes abruptly before going back to lower values. This is a non-linear behavior characterizing the first days of the post-seismic period.

• The post-seismic change of the baseline rate is marked by a non-linear behavior of the deformation in contrast to the linear behavior before the earthquake nucleation (pre-seismic period).

Future work

One of the future goals of this research can be the detection of the onset of pre-seismic deformation (i.e. negative change in baseline rate between two GNSS stations) in longer time series. It would also be very useful to compare the results of this study, with those in similar cases in other areas e.g. the Aegean region.

73 REFERENCES

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85 ANNEX I

ANNEX I: List of focal mechanisms plotted in Fig. 7

Timestamp Long Lat Dep Strike Dip Rake Mag Agency 17/9/1972 20.340 38.280 8.00 39.0 61.0 -173.0 5.6 Baker et al., 1997 24/11/1972 20.430 39.390 9.00 243.0 46.0 37.0 5.2 Muço, 1994 4/11/1973 20.541 38.866 13.10 158.0 41.0 101.0 5.6 ETHZ 18/1/1976 20.510 38.810 5.00 226.0 47.0 -20.0 5.7 Main and Burton, 1990 11/5/1976 20.350 37.560 13.00 323.0 13.0 90.0 5.8 Baker et al., 1997 12/6/1976 20.600 37.500 8.00 297.0 20.0 90.0 5.8 Papazachos et al., 1991 6/11/1979 20.320 39.560 26.00 314.6 70.0 -18.1 5.4 Muço, 1994 11/11/1979 20.300 39.520 27.00 50.0 58.0 176.0 5.2 Muço, 1994 24/6/1981 20.100 37.870 20.00 27.0 60.0 171.0 5.2 Benetatos et al., 2004 28/6/1981 20.060 37.810 14.00 15.0 76.0 -179.5 5.7 Louvari et al., 1999 17/1/1983 20.200 38.100 9.00 40.0 45.0 168.0 7.0 Papazachos et al., 1991 19/1/1983 20.250 38.110 17.00 57.0 35.0 -168.0 5.6 CMT 31/1/1983 20.390 38.180 12.00 41.0 82.0 -177.0 5.6 Louvari et al., 1999 21/2/1983 20.130 37.860 24.00 75.0 42.0 -134.0 5.2 Benetatos et al., 2004 23/3/1983 20.300 38.200 7.00 31.0 69.0 174.0 6.3 Louvari et al., 1999 24/3/1983 20.290 38.100 18.00 62.0 70.0 172.0 5.5 Louvari et al., 1999 14/5/1983 20.330 38.440 13.00 126.9 77.0 4.1 5.5 Louvari et al., 1999 18/11/1984 20.540 39.730 22.00 293.0 56.0 -27.0 5.1 Muço, 1994 7/9/1985 21.200 37.500 29.00 24.0 57.0 168.0 5.3 Kiratzi and Louvari, 2003 17/12/1986 20.880 39.790 20.00 276.0 23.0 36.0 5.0 Muço, 1994 27/2/1987 20.360 38.420 13.00 26.0 61.0 168.0 5.8 Louvari et al., 1999 18/5/1988 20.420 38.360 23.00 45.0 70.0 163.0 5.3 Louvari et al., 1999 16/10/1988 20.920 37.930 25.00 301.0 76.0 -3.0 5.7 CMT 20/8/1989 21.140 37.260 16.00 193.0 74.0 -174.0 5.7 Kiratzi and Louvari, 2003 24/8/1989 20.140 37.940 16.00 36.0 46.0 142.0 5.3 Louvari et al., 1999 16/6/1990 20.540 39.160 7.00 352.0 33.0 105.0 5.5 Louvari et al.,2001 26/6/1991 21.040 38.340 22.00 354.1 41.4 -72.3 5.1 Kiratzi and Louvari, 2003 23/1/1992 20.570 38.400 9.00 345.0 19.0 68.0 5.6 Louvari et al., 1999 23/1/1992 20.280 38.390 10.00 21.0 72.0 172.0 5.0 Pondrelli et al., 1999 13/6/1993 20.490 39.280 9.00 325.0 30.0 106.0 5.3 Louvari et al., 2001 1/2/1994 19.860 37.760 19.00 24.0 39.2 114.9 5.6 Louvari et al., 1999 25/2/1994 20.540 38.760 9.00 22.0 58.0 168.0 5.5 Louvari et al., 1999 16/4/1994 20.630 37.360 22.00 304.0 14.0 90.0 5.5 Kiratzi and Louvari, 2003 29/11/1994 20.550 38.760 21.00 185.0 89.5 -179.5 5.0 CMT 1/2/1996 20.050 37.770 20.00 24.0 39.2 114.9 5.6 Louvari et al., 1999 10/1/1998 20.860 37.290 17.00 69.7 25.3 -43.2 5.3 Louvari, 2000 1/5/1998 20.750 37.620 13.00 143.7 52.9 49.0 5.1 Louvari, 2000 16/7/1998 20.520 38.910 18.00 287.0 55.0 -17.0 5.2 USGS/NEIC 6/10/1998 20.980 37.130 9.00 184.3 45.0 136.7 5.2 Kiratzi and Louvari, 2003 8/10/1998 20.380 37.720 10.00 266.0 4.0 33.0 5.1 USGS/NEIC 7/6/1999 20.830 37.180 5.00 326.0 8.0 75.0 5.0 INGV

86 ANNEX I

26/5/2000 20.580 39.110 8.00 85.0 69.0 -1.0 5.4 CMT 28/7/2002 20.775 37.970 7.30 34.0 51.0 180.0 5.1 USGS/NEIC 2/12/2002 21.151 37.809 12.50 36.0 56.0 -160.0 5.5 CMT 9/12/2002 20.010 37.880 10.00 255.0 28.0 -20.0 5.1 CMT 14/8/2003 20.711 38.887 0.10 18.0 60.0 -168.0 6.2 NKUA 14/8/2003 20.657 38.772 2.50 4.0 43.0 136.0 5.3 Konstantinou et al., 2010 31/1/2005 20.110 37.410 12.00 350.0 24.0 124.0 5.4 NKUA 18/10/2005 20.857 37.573 21.00 8.0 15.0 123.0 5.7 CMT 4/4/2006 20.944 37.610 14.80 13.0 34.0 127.0 5.2 NKUA 11/4/2006 20.933 37.692 12.90 18.0 37.0 120.0 5.6 USGS/NEIC 11/4/2006 20.939 37.655 16.40 19.0 39.0 139.0 5.2 NKUA 12/4/2006 20.952 37.627 16.10 36.0 41.0 152.0 5.3 NKUA 19/4/2006 20.931 37.676 16.90 339.0 40.0 98.0 5.0 NKUA 25/3/2007 20.436 38.368 12.40 128.0 82.0 0.0 5.5 NOA 29/6/2007 20.277 39.264 2.80 317.0 34.0 62.0 5.4 NKUA 29/6/2007 20.253 39.305 0.00 306.0 20.0 65.0 5.2 ETHZ 27/10/2007 21.251 37.699 10.00 297.0 70.0 -10.0 5.0 CMT 30/7/2008 20.216 38.082 8.10 127.2 81.0 1.0 5.0 NKUA 16/2/2009 20.771 37.097 12.00 88.0 61.0 8.0 5.4 NKUA 3/11/2009 20.310 37.420 18.00 280.0 10.0 35.0 5.7 NKUA 11/11/2009 20.200 37.400 4.00 291.0 44.0 88.0 5.3 AUTH 22/8/2010 20.200 37.400 10.00 316.0 12.0 81.0 5.3 UPSL 19/7/2011 20.000 37.300 13.00 320.0 31.0 83.0 5.0 UPSL 17/2/2013 20.740 37.330 6.00 270.0 38.0 36.0 5.1 AUTH 26/1/2014 20.467 38.213 16.00 30.0 70.0 169.0 6.1 NKUA 26/1/2014 20.419 38.235 15.00 18.0 65.0 162.0 5.2 NKUA 3/2/2014 20.388 38.269 5.00 35.0 62.0 175.0 5.9 NKUA 24/10/2014 21.140 38.910 10.00 313.0 52.0 17.0 5.1 NOA 8/11/2014 20.460 38.150 14.00 77.0 75.0 9.0 5.1 AUTH 17/11/2015 20.596 38.666 10.00 293.8 69.0 2.1 6.4 NOA 17/11/2015 20.557 38.652 8.00 119.0 57.0 27.0 5.0 NOA 18/11/2015 20.592 38.844 10.00 103.6 75.2 -9.3 5.0 NOA 29/3/2016 20.006 37.358 18.00 297.0 9.0 72.0 5.2 NKUA 15/10/2016 20.687 39.789 20.00 346.0 43.0 160.0 5.3 NKUA 25/10/2018 20.496 37.360 20.00 119.0 84.0 66.0 6.6 NKUA 25/10/2018 20.718 37.135 22.00 6.0 21.0 176.0 5.1 NKUA 26/10/2018 20.635 37.449 16.00 131.0 87.0 74.0 5.1 NKUA 30/10/2018 20.484 37.516 17.00 24.0 60.0 162.0 5.5 NKUA 19/11/2018 20.663 37.228 16.00 14.0 18.0 170.0 5.3 NKUA 5/2/2019 20.638 38.956 11.00 267.0 83.0 6.0 5.3 NKUA 19/1/2020 20.742 38.184 14.00 157.0 83.0 90.0 5.0 NKUA

87 ANNEX II

ANNEX II: Time Series Plots (E, N, Up) of GNSS stations in the Ionian Sea

AGRI

88 ANNEX II

ARGO

89 ANNEX II

KARA

90 ANNEX II

KTCH

91 ANNEX II

PAT0

92 ANNEX II

PATR

93 ANNEX II

PLAT

94 ANNEX II

PONT

95 ANNEX II

RLSO

96 ANNEX II

SISS

97 ANNEX II

SKAL

98 ANNEX II

SPAN

99 ANNEX II

VLSM

100 ANNEX II

ZAKY

101 ANNEX II

102