Interpretations of Reservoir Induced Seismicity may not always be valid : the case of seismicity during the impoundment of the Kremasta dam (Greece, 1965-1966)

The ‘Kremasta seismic sequence’ in western Greece is one of the most commonly cited examples of Reservoir Induced Seismicity (RIS). Here, we show that this ‘sequence’ is a result of normal tectonic activity and that only some small, unrelated microseismic events are reservoir induced. Shortly after the beginning of the impoundment of the Kremasta Dam in 1965, the then newly established seismic monitoring network in Greece recorded two Ms ≥ 6.0 events and numerous small shocks spread over a 120 km wide region. These were interpreted as a single seismic sequence (namely the Kremasta seismic sequence), and assumed to be reservoir induced. We revisit the epicenter locations of these events and interpret them in the framework of the regional tectonic context and the local hydrogeology. Placing these events into the local context shows that they represent an amalgamation of separate, ordinary (tectonic) seismic sequences. Further, the regional rocks are highly fragmented by small faults and the spatial distribution of seismic events is not consistent with a model of stress transfer from reservoir loading. In addition, it is not likely that events at such long (> 20-30 km) distances from the reservoir could be induced by an initial reservoir load head of 30 m. Whilst the larger magnitude events are tectonic, after impoundment local residents reported an unusual frequency of small microseismic events felt only within 10 km of the dam. We provide evidence that these are a result of the collapse of numerous shallow karstic cavities adjacent and beneath the reservoir due to increased water load (locally 100-150 m depth). This study has significant implications for interpretation of seismic triggering mechanisms in other regions: earthquake occurrence within the proximity of reservoirs during and after impoundment time cannot be assumed to be RIS unless supported by seismological, geological and hydrogeological evidence

Archaeological evidence of a destructive earthquake in Patras, Greece

Oriented collapse of columns, large-scale destruction debris and temporary abandonment of the area deduced from an archaeological excavation provide evidence for a major (intensity IX) earthquake in Patras, Greece. This, and possibly a cluster of other earthquakes, can be derived from archaeological data. These earthquakes are not included in the historical seismicity catalogues, but can be used to put constraints to the seismic risk of this city. Patras was affected by a cluster of poorly documented earthquakes between 1714 and 1806. The city seems to be exposed to risks of progressive reactivation of a major strike-slip fault. A magnitude 6.4 earthquake in 2008 has been related to it. This fault has also been associated with a total of four events in the last 20 years, a situation reminiscent of the seismic hazard at the western edge of the North Anatolian Fault

Tunneling and other engineering works in volcanic environments: Sousaki and Thessaly

Η εργασία αυτή έχει βασιστεί στις επιπτώσεις του Ηφαιστείου Σουσακίου Κορινθίας σε μία σήραγγα, και εξετάζει πιθανές επιπτώσεις της Τεταρτογενούς ηφαιστειότητας της Θεσσαλίας σε μεγάλα κατασκευαστικά έργα. Το ηφαίστειο Σουσακίου είχε σημαντική ηφαιστειακή δράση στο παρελθόν, αλλά η πρόσφατη δράση του περιορίζεται σε γεωθερμικά φαινόμενα. Μια σήραγγα για τη νέα Σιδηροδρομική Γραμμή Υψηλών Ταχυτήτων Αθηνών-Κορίνθου διανοίχθηκε διάμεσου της σολφατάρας του ηφαιστείου, σε περιοχή με πλήθος ρήγματα και φυσικά έγκοιλα. Κατά την εκσκαφή σημειώθηκε άνοδος της θερμοκρασίας και γεωθερμικά αέρια εισχώρησαν στη σήραγγα προκαλώντας προβλήματα στην συνέχιση του έργου. Αμεσα πραγματοποιήθηκαν επιτόπου έρευνες του υπεδάφους και αποφασίστηκε η λήψη έκτακτων μέτρων για την προστασία του έργου. Η περίπτωση της σήραγγας αυτής έκανε σαφές ότι και κατά την κατασκευή μεγάλων τεχνικών έργων και σε ό2λες περιοχές μπορεί να παρουσιαστούν προβλήματα συνδεόμενα με την ηφαιστειότητα στο μέλλον. Μία τέτοια περιοχή είναι οι Μικροθήβες και το Αχίλλειο, Μαγνησίας, όπου κατασκευάζονται σήραγγες για το νέο εθνικό οδικό και σιδηροδρομικό δίκτυο. Για την αντιμετώπιση αυτών των πιθανών προβλημάτων και την ελαχιστοποίηση των ηφαιστειακών κινδύνων κρίνονται απαραίτητες η βελτιστοποίηση της χάραξης των δικτύων μεταφορών σε συνδυασμό με την υιοθέτηση ειδικών τεχνικών κατασκευής και μέτρων ασφαλείαςThis study is inspired by the impacts on a tunnel of the Sousaki volcano, in the vicinity of Corinth and examines possible impacts of the Quaternary volcanism on major engineering works in Thessaly. The Sousaki volcano, at the NW edge of the Aegean Volcanic Arc has been associated with important volcanic activity in the past, but its current activity is confined to géothermie phenomena. A tunnel for the new Athens-Corinth High Speed Rail was excavated through the solfatara of the volcano, an area characterized by numerous faults and physical cavities. High temperatures and geothermal gases released in the underground opening through the faults caused disturbance to the tunnel construction, need for supplementary investigations and adoption of special measures to maintain tunnel stability. Experience from the tunnel at Sousaki indicates that similar risks may be faced in future major engineering works in other regions of Greece. Such an example is the area of Microthives and Achillio, Magnesia, Thessaly. Tunnels for the new highway and railway networks constructed or planned through at least two volcanic domes and other main engineering works may also face volcano-associated effects. Optimization of the network routes in combination with special construction techniques and safety measures need to be followed for minimization of such volcanic risks

Measuring sub-mm structural displacements using QDaedalus: a digital clip-on measuring system developed for total stations

The monitoring of rigid structures of modal frequencies greater than 5 Hz and sub-mm displacement is mainly based so far on relative quantities from accelerometers, strain gauges etc. Additionally geodetic techniques such as GPS and Robotic Total Stations (RTS) are constrained by their low accuracy (few mm) and their low sampling rates. In this study the application of QDaedalus is presented, which constitutes a measuring system developed at the Geodesy and Geodynamics Lab, ETH Zurich and consists of a small CCD camera and Total Station, for the monitoring of the oscillations of a rigid structure. In collaboration with the Institute of Structural Engineering of ETH Zurich and EMPA, the QDaedalus system was used for monitoring of the sub-mm displacement of a rigid prototype beam and the estimation of its modal frequencies up to 30 Hz. The results of the QDaedalus data analysis were compared to those of accelerometers and proved to hold sufficient accuracy and suitably supplementing the existing monitoring techniques

Seismic site characterization of the Kastelli (Kissamos) Basin in northwest Crete (Greece): Assessments using ambient noise recordings

Crete is actively seismic and site response studies are needed for estimating local site conditions subjected to seismic activity. In order to collect basic data, we performed ambient noise recordings to estimate the site response of the surface and near subsurface structure of the small-scale Kastelli Basin in northwest Crete. The spatial horizontal to vertical spectral ratios (HVSR) resonance pattern of the investigated sites in the centre of the Basin consists of either one or two peaks divided into low to high frequency range in different sites as follows: (a) in some sites only one amplified peak at low frequencies (0.6–1.2 Hz), (b) in other sites only one amplified peak at medium frequencies (2.9–8.5 Hz) and (c) in yet other sites two amplified peaks in the low to high frequency range (0.6–15.5 Hz). The investigated sites are amplified in the frequency range 0.6–15.5 Hz, while the amplitude reaches to a factor of 4 in the spectral ratios. The one HVSR amplified peak at low frequencies is related to locally soft or thick Quaternary deposits. Microtremors were measured in the coastal northwest part of the Basin in a well—lithified Cretaceous limestone site characterized by fractures and faults striking predominantly in a sector NNE to NNW. Sites of one amplified peak at medium frequencies are extended from coastal northwest to southwest delineating a structure striking to NNW. The two amplified peaks are attributed to shallow subsurface heterogeneities/irregularities, locally induced by fault zones and to the overlying Quaternary deposits. Spatial HVSR variations in the frequency and HVSR shape delineate four structures striking NNE, NNW and in a sector NW to WNW, crosscutting the dense populated Basin suggesting that microtremors could be a valuable tool for providing a first approximation of fault zone delineation at least for the Kastelli-Kissamos Basin. The Basin is classified into the X soil category of the Greek Seismic Code 2000.This work was implemented through the project entitled “Interdisciplinary Multi-Scale Research of Earth-quake Physics and Seismotectonics at the Front of the Hellenic Arc (IMPACT-ARC)” in the framework of action “ARCHIMEDES III—Support of Research Teams at TEI of Crete” (MIS380353) of the Operational Program “Education and Lifelong Learning” and is co-financed by the European Union (European Social Fund) and Greek national fund

Nothing Lasts Forever: Environmental Discourses on the Collapse of Past Societies

The study of the collapse of past societies raises many questions for the theory and practice of archaeology. Interest in collapse extends as well into the natural sciences and environmental and sustainability policy. Despite a range of approaches to collapse, the predominant paradigm is environmental collapse, which I argue obscures recognition of the dynamic role of social processes that lie at the heart of human communities. These environmental discourses, together with confusion over terminology and the concepts of collapse, have created widespread aporia about collapse and resulted in the creation of mixed messages about complex historical and social processes

Late Holocene relative sea level changes in SW Crete: evidence of an unusual earthquake cycle

Coastal challenges ill West Crete ill the last 4000 years can be described as a series of 11 relatively small (25 cm on the average) land subsidences alternating with short (150-250 year long) relatively still stands of the sea level. At 1500 B.P. an up to 9 m episodic relative land uplift and tilting of this part of the island occurred, but since then no significant coastal changes have been identified. There is strong evidence that these Late Holocene coastal changes are not a product of fluctuations of sea level, but reflect palaeoseismic events. The sequence of the latter is at variance with models of seismic deformation deduced from a wide range of observations in different tectonic environments, including coastal uplifts near major trenches: according to these models, strain buildup and release through earthquakes is described as a cyclic and rather uniform process, the earthquake cycle. In this process, the permanent seismic deformation accumulates after each earthquake to produce geological features, while the long-term deformation rate is approximately equal to the short term one. Obviously this is not the case with West Crete. The unusual pattern of seismic deformation in this island has been observed in other cases as well, but its explanation is not easy. The juxtaposition of different earthquake cycles, variations in the source and rate of stress or internal deformation of the uplifted hanging wall of a thrust in the pre-seismic period are some possible explanations for this unusual pattern of earthquake cycle in Greece