Early post-breakup kinematic adjustments of continental-oceanic transform fault zones; Cape Range, Coromandal and Romanche transform margin case study

A comparison of transform margins that started their evolution as continental transforms shows differences in their tectonic style, which can be attributed to the variable kinematic adjustments they underwent during the post-breakup continental-oceanic stage of their development. Three end-member examples are presented in detail. The Cape Range transform fault zone (Western Australia) retained its strike-slip character during its entire continental-oceanic stage, as documented by the transform-perpendicular system of spreading-related magnetic stripe anomalies. The Coromandal transform fault zone (Eastern India) adjusted its kinematics to a transtensional one during its continental-oceanic stage, as indicated by the transform-oblique system of magnetic stripe anomalies and extensional component of movement indicated by a narrow zone of crustal thinning. The Romanche transform fault zone (Equatorial Africa) adjusted its kinematics to transpressional, as documented by the changing geometries of magnetic stripe anomalies and transpressional folding during its continental-oceanic development stage. Based on the recognition of the aforementioned adjustments, we suggest a new categorization of transforms into (1) those that experience transpressional adjustment, (2) those that experience transtensional adjustment and (3) those that do not experience any adjustment during their continental-oceanic development stage.Supplementary material at https://doi.org/10.6084/m9.figshare.c.5762388

ANALISIS KARAKTERISTIK LAPISAN SEDIMEN BERDASARKAN DATA MIKROTREMOR DI AREA RUMAH SAKIT PENDIDIKAN UNILA

Provinsi Lampung memiliki kondisi geografis kompleks, dilewati jalur bukit Bukit barisan Barisan dan terletak diantara dua lempeng yaitu Indo-Australia dan Eurasia. Provinsi Lampung juga merupakan wilayah yang dekat dengan Samudera Hindia dan terletak pada zona Sesar Semangko (Sumatra Transform Fault Zone) yang terbentang dari Aceh sampai Teluk Semangka Lampung. Dengan demikian penataan wilayah dan pembangunan infrastruktur yang berbasis mitigasi gempabumi merupakan hal yang sangat penting dilakukan. Tujuan penelitian ini adalah mengetahui karakteristik lapisan batuan sedimen pada area pembangunan Rumah Sakit Pendidikan Universitas Lampung (RSP Unila) sebagai bagian dari tahapan mitigasi bencana gempabumi. Penelitian ini dilakukan dengan menggunakan metode mikrotremor dan analisis geologi permukaan. Data pengukuran metode mikrotremor dianalisis dengan menggunakan metode analisis Horizontal Vertical Spectrum Ratio (HVSR) untuk mendapakan nilai amplifikasi (A0) dan frekuensi dominan (f0).Hasil dari analisis HVSR diperoleh nilai frekuensi dominan (f0) berkisar 0.65–1.2 Hz, amplifikasi (A0) berkisar 3.33–5.62, dan nilai periode dominan (T0) 0.6–1.5 sekon. Nilai A0, f0, T0 diinterpreasikan dengan mengacu data literatur dan geologi permukaan yang relevan. Hasilnya menunjukkan bahwa daerah penelitian memiliki jenis lapisan sedimen yang rentan terhadap peristiwa gempabumi.

Τhe February-March 2019 Seismic Swarm Offshore North Lefkada Island, Greece: Microseismicity Analysis and Geodynamic Implications

A quite energetic seismic excitation consisting of one main and three additional distinctive earthquake clusters that occurred in the transition area between the Kefalonia Transform Fault Zone (KTFZ) and the continental collision between the Adriatic and Aegean microplates is thoroughly studied after the high-precision aftershocks’ relocation. The activated fault segments are in an area where historical and instrumental data have never claimed the occurrence of a catastrophic (M ≥ 6.0) earthquake. The relocated seismicity initially defines an activated structure extending from the northern segment of the Lefkada branch of KTFZ with the same NNE–SSW orientation and dextral strike slip faulting, and then keeping the same sense of motion, its strike becomes NE–SW and its dip direction NW. This provides unprecedented information on the link between the KTFZ and the collision front and sheds more light on the regional geodynamics. The earthquake catalog, which was especially compiled for this study, starts one year before the occurrence of the Mw5.4 main shock, and adequately provides the proper data source for investigating the temporal variation in the b value, which might be used for discriminating foreshock and aftershock behavior.

Τhe February – March 2019 Seismic Swarm Offshore North Lefkada Island, Greece: Microseismicity Analysis and Geodynamic Implications

A quite energetic seismic excitation consisting of one main and additional three distinctive earthquake clusters that occurred in the transition area between the Kefalonia Transform Fault Zone (KTFZ) and the continental collision between Adriatic and Aegean microplates, is thoroughly studied after high–precision aftershocks’ relocation. The activated fault segments are in an area where historical and instrumental data have never claimed the occurrence of a catastrophic (M>6.0) earthquake. The relocated seismicity initially defines an activated structure extending from the northern segment of the Lefkada branch of KTFZ with the same NNE–SSW orientation and dextral strike-slip faulting and then keeping the same sense of motion its strike becomes NE–SW and its dip direction NW. This provides unprecedented information on the link between the KTFZ and the Collision front and sheds more light on the regional geodynamics. The earthquake catalog, which is specially compiled for this study, starts one year before the occurrence of the Mw5.4 mainshock and adequately provides the proper data source for investigating the temporal variation of the b–value, which might be used for discriminating foreshock and aftershock behavior.

Seismicity characterization of oceanic earthquakes in the Mexican territory

We analyzed the seismicity of oceanic earthquakes in the Pacific oceanic regime of Mexico. We used data from the earthquake catalogues of the Mexican National Service (SSN) and the International Seismological Centre (ISC) from 1967 to 2017. Events were classified into two different categories: intraplate oceanic (INT) and transform fault zone and mid-ocean ridges (TF-MOR) events, respectively. For each category, we determined statistical characteristics such as magnitude frequency distributions, the aftershocks decay rate, the nonextensivity parameters, and the regional stress field. We obtained b values of 1.17 and 0.82 for the INT and TF-MOR events, respectively. TF-MOR events also exhibit local b-value variations in the range of 0.72–1.30. TF-MOR events follow a tapered Gutenberg–Richter distribution. We also obtained a p value of 0.67 for the 1 May 1997 (Mw=6.9) earthquake. By analyzing the nonextensivity parameters, we obtained similar q values in the range of 1.39–1.60 for both types of earthquakes. On the other hand, the parameter a showed a clear differentiation, being higher for TF-MOR events than for INT events. An important implication is that more energy is released for TF-MOR events than for INT events. Stress orientations are in agreement with geodynamical models for transform fault zone and mid-ocean ridge zones. In the case of intraplate seismicity, stresses are mostly related to a normal fault regime.

A seismological and remote sensing approach of geodynamic phenomena

Οι γεωδαιτικές μετρήσεις μπορούν να συνεισφέρουν στη μελέτη πολλαπλών φαινομένων. Η τεχνολογική εξέλιξη και οι δορυφορικές αποστολές ραντάρ, προσέφεραν καινούργιους τρόπους μελέτης της επιφανειακής παραμόρφωσης. Στην παρούσα διατριβή, γίνεται χρήση αυτών των δεδομένων ραντάρ, για την παρακολούθηση της γήινης επιφανειακής παραμόρφωσης, μέσω της ανάλυσης τους με τη χρήση των τεχνικών συμβολομετρίας ραντάρ συνθετικού ανοίγματος (InSAR). Μελετώνται δυο τύποι επιφανειακής παραμόρφωσης: ανθρωπογενής και τεκτονική. Όσον αφορά την πρώτη κατηγορία η εστίαση τίθεται στη βόρεια Ελλάδα και ειδικότερα στη Θεσσαλονίκη (τη δεύτερη μεγαλύτερη πόλη της Ελλάδας) και τα περίχωρά της. Η περιοχή ήταν γνωστό ότι ήταν υπό καθεστώς παραμόρφωσης, και σε αυτή τη διατριβή παρουσιάζονται νέα, αποτελέσματα των οποίων τα χαρακτηριστικά αναλύονται λεπτομερώς. Επιπρόσθετα, οι κύριοι μηχανισμοί που προκαλούν το φαινόμενο προτείνονται με βάση και την αξιοποίηση εξωτερικών δεδομένων. Οι περιοχές ενδιαφέροντος είναι το Καλοχώρι, η λεκάνη του Ανθεμούντα και η ευρύτερη περιοχή του Ωραιοκάστρου. Τα αποτελέσματα δείχνουν ότι υπάρχουν σημαντικά σήματα παραμόρφωσης, τα οποία κατά περιπτώσεις μπορεί να προκαλέσουν σοβαρές αστοχίες σε σπίτια, κτίρια και υποδομές υψηλής σημασίας. Αξιολόγηση των in-situ δεδομένων, μαζί με τα Τηλεπισκοπικά, δείχνουν ότι η δραστηριότητα των υδροφορέων είναι η κύρια αιτία της επιφανειακής παραμόρφωσης και ότι η δραστηριότητα του υπόγειου νερού καθορίζει τις επιφανειακές ανυψωτικές και υποχωρητικές τάσεις, των υπό μελέτη περιοχών. Ως γενική πρόταση, η ανάγκη για διαχείριση των υπόγειων υδάτων, θεωρείται κρίσιμη προτεραιότητα για την ευρύτερη περιοχή της Θεσσαλονίκης, για την αποφυγή μελλοντικών κινδύνων. Στη δεύτερη ενότητα της παρούσας διατριβής, γίνεται μελέτη σήματος τεκτονικής παραμόρφωσης αρχικά για την περίπτωση του σεισμού Ιράν-Ιράκ 2017 με σκοπό τον προσδιορισμό της πηγής του συμβάντος. Οι άγονες περιβαλλοντικές συνθήκες της περιοχής μελέτης, συνέβαλαν σε ένα σήμα παραμόρφωσης υψηλής ποιότητας, το οποίο χρησιμοποιήθηκε για την αντιστροφή των γεωδαιτικών δεδομένων, με σκοπό τον υπολογισμό των παραμέτρων του ρήγματος, που σχετίζεται με την παραμόρφωση της ενεργού τεκτονικής των πλακών που παρατηρείται στην περιοχή. Μια άλλη μελέτη που παρουσιάζεται εδώ, επικεντρώθηκε στην περιοχή του Ιονίου, το πιο ενεργό τεκτονικό τμήμα της ανατολικής Μεσογείου. Με την αξιοποίηση δεδομένων ραντάρ πολλαπλών ζωνών συχνοτήτων από διαφορετικές γεωμετρίες λήψης και με βάση τα αποτελέσματα των προηγούμενων μελετών της ακολουθίας της Λευκάδας του 2015, προτείνεται μια νέα γεωμετρία ρηξιγενών τεμαχών για τη ζώνη Κεφαλονιάς-Λευκάδας (Cephalonia-Lefkada Transform fault Zone -CTF). Στην παρούσα διατριβή, γίνεται επίσης εστίαση στις νέες δυνατότητες που προσφέρουν οι τελευταίες τεχνολογικές εξελίξεις των δορυφόρων ραντάρ και στη συμβολή που θα μπορούσαν να προσφέρουν στην κοινότητα των επιστημών της γης. Μελετώντας την ακολουθία σμηνοσεισμών της χερσονήσου της Μπίγκα του 2017 και χρησιμοποιώντας μια συγκεκριμένη στρατηγική μοντελοποίησης, αποδεικνύεται ότι υπάρχουν νέες δυνατότητες σήμερα για να εξετάσουμε τις ακολουθίες σμηνοσεισμών. Οι τελευταίες είναι ελάχιστά μελετημένες απο τη σκοπιά της δορυφορικής συμβολομετρίας ραντάρ, σε σύγκριση με τις ακολουθίες που συνδέονται με σεισμούς μεγάλου μεγέθους.

Magma-rich transform margins: example from the Limpopo transform fault zone between Mozambique and Antarctica

<p>Transform continental margins known across the Earth represent 31% of passive margins. Resulting from first-order plate tectonic processes, transform margins record a diachronous evolution mainly defined by three successive stages, including intra-continental transform faulting, active and passive transform margin. Due to their high complexity and a lack of large hydrocarbon discoveries (i.e. not a target for oil industry), they have only been sparsely studied, especially when compared with other margin types (i.e. divergent or convergent).</p><p>                  We present the structure and evolution of the NS-trending Limpopo Transform Fault Zone (LTFZ), corresponding to the main fracture zone from western part of the Africa-Antarctica Corridor (AAC). Here, we combine published and unpublished dataset (seismic reflection profiles, wells, multibeam bathymetry, gravity, magnetic data) in order to propose an interpretation of the LTFZ structure and adjoining segments and their evolution through time, from rifting to spreading.</p><p>The LTFZ is composed of two main segments: the East Limpopo segment and the Astrid conjugate one and the North and South Natal segment including the Dana-Galathea Plateau (Mozambique side) and the Maud rise/east of Grunehogna craton (Antarctica margin). The LTFZ offsets the segments of divergent conjugate margins (Southern Natal-off Grunehogna craton in the west and Beira High Angoche-Riiser Larsen Sea in the east) since 155 Ma (chron M25). We focus on the evolution of the transform fault zone from its initiation at chron M25 up to chron M0 (~126 Ma, Barremian). Oceanic spreading onset at chron M25 in the south of Beira High segment and Dana-Galathea Plateau triggered the uplift and erosion of the proximal parts of the margin and the formation of several seaward dipping reflectors wedges. Plate kinematic implies an NNW-SSE opening of the LTFZ. The oblique component of opening promotes the setting up of several volcanic wedges. These wedges rejuvenate southward trough time, which is consistent with the sliding of Antarctica with respect to Africa and thus confirm the diachronous evolution of the transform fault zone.</p>

On the Segmentation of the Cephalonia–Lefkada Transform Fault Zone (Greece) from an InSAR Multi-Mode Dataset of the Lefkada 2015 Sequence

We use Interferometric Synthetic Aperture Radar (InSAR) to study the Cephalonia–Lefkada Transform Fault Zone (CTF) in the Ionian Sea. The CTF separates continental subduction to the north from oceanic subduction to the south, along the Hellenic Subduction Zone. We exploit a rich multi-modal radar dataset of the most recent major earthquake in the region, the 17 November 2015 Mw 6.4 event, and present new surface displacement results that offer additional constraints on the fault segmentation of the area. Based on this dataset, and by exploiting available information of earthquake relocation, we propose a new rupture process for the 2015 sequence, complementary to those published already. Our modelling includes an additional southern fault segment, oblique to the segment related with the mainshock, which indicates that the CTF structure is more complex than previously believed.

Cephalonia-lefkas Transform Fault Zone (CLTFZ) Complexity: Insights from 2015 Lefkas Earthquake Sequence

In order to define a better model for the Cephalonia-Lefkas Transform Fault Zone the sequence of 2015 Lefkas earthquake was examined. On 17 November 2015 (07:10 GMT) a major earthquake (Mw=6.4) occurred on the central-western part of Lefkas island. Several destructive events were located in the past in this fault zone, so an extensive seismotectonic study is feasible for that area. Manual analysis was performed using a custom velocity model that was determined for that purpose, applying the average travel-time residuals and location uncertainties errors minimization method. Several clusters belonging to the aftershock sequence were identified, whereas three are directly related to the causative fault, covering an area of about 25 km. The central one, which includes the mainshock, comprises of only a few aftershocks. The northern, within which the majority of aftershocks are located, lies in the central part of Lefkas island and the southern occurred close to the SW edge of the island. In addition, offshore clusters with distinct characteristics have been identified to the south, between Lefkas and Cephalonia islands. The temporal evolution of the aftershock sequence indicates that no migration was observed, given that after the occurrence of the mainshock the entire epicentral area was activated. Focal mechanisms of the Seismological Laboratory of the University of Athens showed dextral strike-slip faulting for both mainshock and major aftershocks of the sequence. Taking into account the spatial distribution of the aftershocks, supported by the tectonic and geomorphological settings of the region, a deformation pattern, consisting of the Cephalonia-Lefkas and Ithaca-Lefkas major fault zones which converge in the area of Vassiliki bay is proposed. The appearance of the southernmost clusters was interpreted by the positive Coulomb stress changes transfer due to major earthquake Mw=6.4.