TECHNIQUES OF ROCKFALL INVENTORY IN EARTHQUAKE PRONE ROCK SLOPES

Στις ορεινές περιοχές ένας κίνδυνος που ενσκήπτει είναι οι απότομες βραχώδεις κλιτύες, οι οποίες σε μεγάλο βαθμό ελέγχονται από πτώσεις βράχων. Αν και οι πτώσεις βράχων χαρακτηρίζονται από μικρούς όγκους ολισθαίνουσας μάζας πετρωμάτων σε σύγκριση με τους υπόλοιπους τύπους κατολισθήσεων, μπορούν να προκαλέσουν σημαντικές βλάβες σε κτήρια, υποδομές εξαιτίας της ξαφνικής και της πολύ γρήγορης κίνησής τους. Βασική παράμετρος στην κατανόηση της διεργασίας που οδηγεί στην εκδήλωση πτώσεων βράχων αποτελεί η μελέτη των κυριότερων αιτιών που επηρεάζουν την ευστάθεια της κλιτύος. Η εξέλιξη της βραχώδους κλιτύος μπορεί να συσχετίζεται με πολλά γενεσιουργά αίτια όπως τεκτονικά, γεωμορφολογικά, σεισμικά, κλιματολογικά ή ανθρωπογενή. Η παρούσα εργασία παραθέτει σε σύνοψη μεθοδολογίες που σχετίζονται με το φαινόμενο της πτώσης βράχων. Επιπλέον χρησιμοποιεί παραδείγματα από την περιοχή της Βόρειας Πελοποννήσου ώστε να καταστεί σαφής η χρησιμότητα των μεθόδων ανάλυσης του φαινομένου στην ελληνική επικράτεια. Συλλέγοντας δεδομένα και παράγοντας θεματικούς χάρτες και συνδυάζοντας την υπαίθρια παρατήρηση με την τηλεπισκόπηση, είναι δυνατή η απόδοση βέλτιστων αποτελεσμάτων τα οποία μπορούν να ενταχθούν σε τεχνικές σκιαγράφησης της επικινδυνότητας και τη λήψη προστατευτικών μέτρων.A relevant hazard in mountainous regions is the steep rock slopes concentrating rock falls. Although rock falls are characterized by smaller rock volumes compared to other landslide types, can also provoke severe damage to buildings, infrastructures and human life due to their sudden and highly fast movement. The key to understand the processes that result in rock fall onset is an integrated study of the major causing parameters that affect slope stability. A rock slope may be subjected to many forms of triggering factors including tectonic, geomorphic, seismic, climatic or even human induced damages. This contribution provides an overview of the previous and current research related to rock falls and uses case studies of North Peloponnese in order to prove the usefulness of these methods in the Greek territory. Collecting data and production of thematic maps by means of field and remote sensing investigations can yield far more updated results incorporated in hazard assessment techniques and protection measures.

UNMANNED AERIAL VEHICLES FOR GEOLOGICAL APPLICATIONS

Οι τεχνικές της Τηλεπισκόπησης και της Φωτογραμμετρίας ανέκαθεν χρησιμοποιούνταν σε γεωλογικές εφαρμογές. Η πρόοδος της τεχνολογίας των μη Επανδρωμένων Αέριων Οχημάτων (UAV), σε συνδυασμό με τη συνεχόμενη μείωση του κόστους απόκτησης των και την αυξημένη διαθεσιμότητα φωτογραμμετρικού λογισμικού, καθιστούν ολοένα και πιο δημοφιλή την επιλογή τους για μικρής έκτασης και μεγάλης κλίμακας χαρτογραφήσεις. Με τη χρήση των UAVs σε χαρτογραφήσεις, ξεπερνιούνται τα προβλήματα του αυξημένου κόστους, της κατανάλωσης χρόνου και των πιθανών δυσκολιών στην προσβασιμότητα –λόγω απότομου αναγλύφου. Σε αυτή τη μελέτη, ένα εξακόπτερο UAV το οποίο φέρει δύο κάμερες, χρησιμοποιείται γα την παρακολούθηση δύο πολύπλοκων –από πλευράς τοπογραφίας- περιοχών στη Δυτική Ελλάδα. Ένα λατομείο ασβεστολίθου και μία κατολίσθηση επί αμμοαργιλωδών ιζηματογενών πετρωμάτων. Και οι δύο περιοχές χαρτογραφήθηκαν με τη βοήθεια τοπογραφικών οργάνων, όπως ταχύμετρα και γεωδαιτικά GPS, όπως επίσης και με το προαναφερθέν UAV. Τρισδιάστατα μοντέλα δημιουργήθηκαν και για τις δύο περιοχές 1663 με τη χρήση ειδικού φωτογραμμετρικού λογισμικού. Για τη δημιουργία των τρισδιάστατων μοντέλων, πολλαπλοί στόχοι τοποθετήθηκαν στο έδαφος και μετρήθηκαν με τη βοήθεια διαφορικού GPS για να χρησιμοποιηθούν ως σημεία εδαφικού ελέγχου. Οι στόχοι αυτοί με γνωστές συντεταγμένες μπορούσαν εύκολα να εντοπιστούν στις υψηλής ανάλυσης αεροφωτογραφίες, και να χρησιμοποιηθούν στην φωτογραμμετρική διαδικασία για να διατηρηθεί χαμηλό το σφάλμα RMS, κατά τη διάρκεια της δημιουργίας Ψηφιακού Μοντέλου Επιφανείας και ορθοεικόνων. Επιπρόσθετα, εξετάστηκε το κατά πόσο οι όποιες παραμορφώσεις προκαλούνται από τον ευρυγώνιο φακό των καμερών επηρεάζουν τη συνολική γεωμετρική ακρίβεια των μοντέλων. Τέλος, τα τρισδιάστατα μοντέλα συγκρίθηκαν με τις τοπογραφικές μετρήσεις και τα αποτελέσματα παρουσιάζονται σε αυτή τη μελέτη.Remote Sensing and photogrammetric techniques have always been used in geological applications. Current advancements in the technology behind Unmanned Aerial Vehicles (UAVs), in accordance with the consecutive increase in affordability of such devices and the availability of photogrammetric software, makes their use for large or small scale land mapping more and more popular. With the UAVs being used for mapping, the problems of increased costs, time consumption and the possible accessibility problems -due to steep terrain-, are all solved at once. In this study, a custom-made UAV with 2 cameras onboard, is used to monitor two complex –regarding their topography- regions in Western Greece. One open pit limestone mine and a landslide occurring on sandy-clayous sediments. Both regions were mapped using surveying instruments like tachymeters and geodetic GPS, as well as using the aforementioned UAV system. 3D models of both regions were created using off-the-shelf photogrammetric software. For the creation of the 3D models, multiple targets were placed on the ground, to indicate GCPs with precisely known coordinates that could be identified in the high-resolution air photos, in order to maintain low Root Mean Square Error, while creating the DSMs and Orthophotos. In addition, the fish-eye effect caused by the cameras’ wide-angle lens was taken into consideration, regarding whether or not it affects the models’ overall geometric accuracy. Finally, the 3D models were compared to the survey measurements and the results are presented in this paper.

PATTERNS OF DUCTILE DEFORMATION IN ATTICO-CYCLADIC MASSIF

The area of Lavrion constitutes the westernmost part of the Attico-Cycladic massif where the allochthonous Cycladic Greenschist-Blueschist unit overthrusts the para-authochthonous Basal unit. The tectonic contact of these units forms a crustal scale thrust zone which is the continuation of the Evia thrust. Our research was focused on quartz-rich schists of the overlying allochthonous unit. Combination of microstructural, finite strain data and quartz and calcite c-axis fabrics analysis was used to characterize the kinematics of rock flow within the thrust zone. The latter was formed under conditions of progressive exhumation and decompression of the high-pressure schists of the AtticoCycladic massif. A dominant top-to-the-ENE sense of shearing along the thrust zone is inferred by several shear sense criteria. The analysis of several specimens collected from various structural depths manifest that the deformation close to the thrust zone occurred under approximately plane strain conditions and was characterized by an Rxz strain ratio which fluctuates between 3 and 6.5

PALAEOSEISMOLOGICAL INVESTIGATION OF THE GYRTONI FAULT (THESSALY, CENTRAL GREECE)

Two paleoseismological trenches were excavated across the Gyrtoni Fault in NE Thessaly and studied in order to understand the recent seismotectonic behavior of this structure. Twenty five fluvial-colluvial sediment and pottery samples from both the upthrown and the downthrown fault blocks were investigated. Optically Stimulated Luminescence (OSL) dating has been applied to date both sedimentary depositspalaeosoils and pottery fragments. Paleoseismological analysis of the two trenches indicates evidence of three surface faulting events in the time span between 1.42 ±0.06 ka and 5.59 ± 0.13 ka. The observed vertical displacement per event of ~0.50 m corresponds to an Mw 6.5 ± 0.1 earthquake. An average fault slip rate of 0.41 ± 0.01 mm/yr and an average recurrence of 1.39 ± 0.14 ka for earthquakes were estimated. The results documented the activity of the fault and since the return period from the most recent event (minimum age 1.42 ± 0.06 ka) has expired, the possibility for reactivation of this active structure in the near future should be included in Seismic Hazard Assessment

Structural and seismological segmentation of the Gulf of Corinth fault system: implications for models of fault growth

The positions and dimensions of fault segments within the Gulf of Corinth fault system have been identified by analysing spatial variation in fault displacements and fault kinematics. Growth of these fault segments is assessed by comparing their geometry and kinematics with the geometry and kinematics of the three sets of earthquake surface ruptures that are known to have affected the area in the last -200 years. Areas along the Gulf of Corinth fault system exhibiting low fault displacement (tens of metres) are identified as persistent segment boundaries which separate the fault system into a number of fault segments characterised by displacements which achieve maxima of -3 km. Fault-slip directions defined by lineations on fault planes vary systematicalIy with fault displacement, showing a converging pattem towards the hanging-walls of the fault segments: the fault-slip directions change by -90° across persistent segment boundaries. It is unclear where fault segments end and persistent segment boundaries begin, but if the persistent segment boundaries are considered to be -10-15 km across, the intervening fault segments achieve lengths of 30-35 km. In contrast known surface ruptures during the last -200 years, including those for the 1995 Egion earthquakes, have all been < 15 km in Length. these so-caIled earthquake segmens are, therefore, considerably shorter than the fault segmentss that hosted the earthquakes. Also, the positions of earthquake segments have varied relative to the positions of the fault segments during successive earthquakes. It appears, therefore, that a Modified Overlap Model is more appropriate than the Characteristic Earthquake Model to describe the seismological behaviour of fault segments around the Gulf of Corinth through a number of earthquake cycles. A pattern of coseismic slip vectors converging towards the hanging-wall has been measured for the surface ruptures to 1995 Egion earthquakes; a similar pattern was noted for the 1981 Alkyonides earthquake ruptures. Repetition of such ruptures in different positions along fault segments, in accordance with a Modified Overlap Model, will produce systematic variations in the scatter of fault-slip directions, with fault displacement. Thus, scatter in the orientations of lineations on fault planes may contain information concerning the lengths and positions of numerous pre-historic earthquake segments; information which may be used to constrain both the palaeoseismology and the future seismjcity in areas of active extension

Structural and seismological segmentation of the Gulf of Corinth fault system: implications for models of fault growth

The positions and dimensions of fault segments within the Gulf of Corinth fault system have been identified by analysing spatial variation in fault displacements and fault kinematics. Growth of these fault segments is assessed by comparing their geometry and kinematics with the geometry and kinematics of the three sets of earthquake surface ruptures that are known to have affected the area in the last -200 years. Areas along the Gulf of Corinth fault system exhibiting low fault displacement (tens of metres) are identified as persistent segment boundaries which separate the fault system into a number of fault segments characterised by displacements which achieve maxima of -3 km. Fault-slip directions defined by lineations on fault planes vary systematicalIy with fault displacement, showing a converging pattem towards the hanging-walls of the fault segments: the fault-slip directions change by -90° across persistent segment boundaries. It is unclear where fault segments end and persistent segment boundaries begin, but if the persistent segment boundaries are considered to be -10-15 km across, the intervening fault segments achieve lengths of 30-35 km. In contrast known surface ruptures during the last -200 years, including those for the 1995 Egion earthquakes, have all been < 15 km in Length. these so-caIled earthquake segmens are, therefore, considerably shorter than the fault segmentss that hosted the earthquakes. Also, the positions of earthquake segments have varied relative to the positions of the fault segments during successive earthquakes. It appears, therefore, that a Modified Overlap Model is more appropriate than the Characteristic Earthquake Model to describe the seismological behaviour of fault segments around the Gulf of Corinth through a number of earthquake cycles. A pattern of coseismic slip vectors converging towards the hanging-wall has been measured for the surface ruptures to 1995 Egion earthquakes; a similar pattern was noted for the 1981 Alkyonides earthquake ruptures. Repetition of such ruptures in different positions along fault segments, in accordance with a Modified Overlap Model, will produce systematic variations in the scatter of fault-slip directions, with fault displacement. Thus, scatter in the orientations of lineations on fault planes may contain information concerning the lengths and positions of numerous pre-historic earthquake segments; information which may be used to constrain both the palaeoseismology and the future seismjcity in areas of active extension