Geomorphological Analysis of Xilokastro Fault, Central Gulf of Corinth, Greece

The Gulf of Corinth is a rapidly opening area with high seismicity associated with extensive building collapses, destruction of cities, and even the deaths of inhabitants. Rapid residential development, especially in the southern part of the Gulf of Corinth, and the construction of crucial technical infrastructures necessitate understanding the activity across crustal-scale faults that host devastating earthquakes. The evolution of landforms affected by fault action is a dominant issue in geological science. In the present study, was selected the 20 km long Xilokastro pure normal fault. In this fault, we apply eight geomorphological indices in footwall catchments that drain perpendicular to its trace. In total, more than 5000 measurements were made in 102 catchments. The determination of geomorphological indices requires the construction of morphological profiles either perpendicular to the faults or perpendicular to the main tributaries of the drainage basins under consideration through the use of the geographical information systems (ArcGIS platform). Τhe application of these indices along catchments draining the Xilokastro fault scarp show high active tectonics. Its high activity is evidenced by the high values of the length-slope index near the fault trace, the low values of the width to height ratio index, the strong asymmetry of the drainage basins, especially in the overlapping zones between its segments, and the elongated shape of the drainage basins. This study supports the idea that the application of a single morphometric index is unable to reflect the distribution of active tectonics across faults, which makes inevitable the systematic comparison of a series of tectonic morphometric indices from which a new combined index emerges (Iat). The Iat classifies the Xilokastro fault in the high degree of activity at a rate of 75% of its length.

The 373 B.C. Helike (Gulf of Corinth, Greece) Earthquake and Tsunami, Revisited

Ancient authors report the destruction and drowning in 373 B.C. in the Gulf of Corinth (Greece) of Helike (Helice and Eliki), an important, nearly coastal town, and of Boura (Bura, Buris, Bouris, and Voura), another town in the hinterland, as a punishment by the ancient God Poseidon because of a serious crime committed in his shrine. This narrative has been regarded as a description of a true event, though with some exaggerations, and the 373 B.C. event is included in earthquake and tsunami catalogs. In the first part of this article, it is shown that (1) local natural hazards exclude the possibility (risk) of total loss of the ancient “polis” (town state) of ancient Helike because of its vulnerability due to its geography. (2) Systematic geoarchaeological studies confirm this prediction because they reveal essentially undisturbed archaeological layers predating and postdating 373 B.C., with no signs of a tsunami. (3) Archaeological excavations have recently brought to light, among other findings, remains of the harbor of Boura and of the shrine of Poseidon at Helike, as well as coins issued by Helike several decades after its alleged loss. This evidence permits a reconsideration of ancient texts related to the loss of Helike in a supervised learning-type approach. It was found that genuine ancient Greek texts do not mention any catastrophe of Helike, but rather that the legend of its total loss appeared several centuries later in Roman times, in local legends, rumors, and forged or manipulated ancient texts (e.g., by pseudo-Aristotle). The ancient legend became important because it explained the collapse of the town state of Helike and it fit ancient religious ideas in a tectonically active region because of the rapid burial (“disappearance”) of ancient Greek remains under sediments in a young delta and because of the prominent location of Helike in the seafaring route between Rome and the eastern Mediterranean. For earthquakes before our era, historical and archaeological data have attracted interpretations… attributing to earthquakes… the demise of flourishing city-states. …The reason for the revival of catastrophe hypotheses is perhaps that they are easy to explain. They are too simple, too obvious and too coincidental and chiefly because they have become fashionable in recent years. (Ambraseys, 2006)

Holocene environmental evolution and climate variability inferred from multi-proxy coastal sediment records of southern Greece

This thesis presents a multi-proxy reconstruction from 5 different coastal wetlands of southern Greece spanning in the Holocene period and an in-depth review and application of non-destructive systems (CT scanning, X-ray Fluorescence) in paleoenvironmental research. During this thesis, the acquired dataset used consisted of a) X-ray Fluorescence scanning (XRF), b) Computed Tomography (CT) scanning, c) Stable isotopes δ18O and δ13C, d) micropaleontology (foraminifera, ostracods, diatoms, pollen), e) mineralogical analysis, and f) standard sedimentological techniques (grain size, magnetic susceptibility (MS), Total Organic Carbon (TOC), carbonates content). The chronological framework for the sediment cores was established through 14C radiocarbon dating. The wetlands studied are Aliki salt pond (NE Gulf of Corinth), Klisova lagoon (SW Greece), lake Vouliagmeni (E Gulf of Corinth), Agoulinitsa marsh field (W Peloponnese) and Pappas lagoon (NW Peloponnese). The sites form an E-W transect of southern Greece, an area with high climatic and environmental spatial variability, whereas human occupancy on all areas is recorded from antiquity. Non-destructive, high-resolution techniques used in this thesis have been established as standard in the last decades and have been a great asset in geosciences. Computed Tomography (CT) was conducted in lake Vouliagmeni, Agoulinitsa marsh and Pappas lagoon cores. In contrast, XRF was performed on all core sections except for Pappas lagoon core, where the high assemblage of bivalve shells could lead to bias of Ca. The fundamental parameter behind CT analysis is the Hounsfield units that reflect relative density variations in the sediment. Correlation between HU values and heavy elements like Zr has been detected in all cores, whereas distinct sedimentological facies were recorded according to HU variations. In Agoulinitsa and Pappas lagoon, microstructural characteristics like shells/sediment ratio and root remnants were examined through 3D sections, in which HU boundaries were set accordingly. Lake Vouliagmeni sediment core was characterized by extreme stratigraphic variations with lamination structures alternating with homogenous deposits and event layers. Laminae thickness and boundaries were recorded through 3D rendered volumes with HU boundaries. Micro CT scanning and thin section analysis was also performed to cross-check possible variations. Event sedimentation layers were distinguished through the combined use of CT scanning, XRF and MS, with layers responding to increased HU, MS and Mn values. Statistical assessment of elemental distribution and HU revealed 3 different clusters. Cluster A responded to sedimentation during enhanced evaporation in the area, Cluster B emulated sedimentation during increased chemical weathering in the catchment, and Cluster C reflected the homogenous deposits. Aliki salt pond is located in a highly tectonic region, at the northeast part of the Gulf of Corinth in Greece. Beachrock deposits that form a barrier between the salt pond and the marine environment play an essential role in the evolution of the area. The chronological framework was set at ca. 3100 cal BP by four 14C radiocarbon dates and the established evolutionary model indicate four different changes taking place during this period in the study area. From around 3100 to 1600 cal BP, a transition from a closed to an open lagoonal environment, was identified, interrupted by a terrestrial fluvial deposit at ca. 2500 cal BP. A shift toward a closed lagoonal system at around 1600 cal BP and the establishment of a salt pond environment seem to correlate with tectonic activity. The study provides important information about the evolution of the coastal landscape in such an active tectonic region and points the interaction between regional human activity and climatic changes during the late-Holocene period.Klisova lagoon is located in the eastern part of Messolonghi-Etoliko wetland, the biggest lagoonal complex of Greece and an area of great environmental interest. For the last 4700 cal BP, the freshwater influx, the progradation of the Evinos river delta and related geomorphological changes control the environmental conditions (e.g. depth and salinity) in the lagoon system. Considering the centennial temporal resolution of our analyses, small offsets of c.a. 50 years due to the lack of regional reservoir correction do not impact the reported radiocarbon ages considerably. Prior to 4000 cal BP, a relatively shallow water depth, significant terrestrial/freshwater input and increased weathering in the lagoon area are inferred. Elemental proxies and increased dinoflagellate and foraminifera abundances, which indicate marine conditions with prominent freshwater influxes, point to the gradual deepening of the lagoon recorded at the drilling site up to 2000 cal BP. The marine and freshwater conditions equilibrium sets at 1300 cal BP, and the lagoonal system seems to reach its present state. Maxima of anthropogenic pollen indicators during the Mycenaean (3200 cal BP), Hellenistic (2200 cal BP) and Late Byzantine (800 cal BP) periods suggest intervals of increased anthropogenic activities in the study area. Lake Vouliagmeni is subjected to intense climatically and tectonic forces, causing stratigraphic variations, with laminated sediments frequently interrupted by homogenous and event sedimentation deposits. Lamination couplets consist of aragonite layers alternating with detrital and organic residues and form during periods of seawater intrusion and stratification of the lake water. The discontinuous occurrence of laminated deposits excludes a varve based chronology from being established but still highlights the susceptibility of the lake to record environmental and climatically driven changes. Our synthesis model for regional climatic reconstruction and local environmental changes derives from δ18O and δ13C data from the laminated and homogenous sediments studied separately depending on the dominant carbonate mineral. This is further strengthened by high-resolution geochemical proxies, diatom and sedimentological data. Regional climatic trajectories from key sites and possible links to the lake Vouliagmeni record are explored in response to atmospheric circulation patterns variations. Phases of overall humid conditions are recorded by the increased inflow of siliciclastic material in the lake and negative δ18Obulk values. In contrast, periods of marine intrusion and enhanced evaporation are recorded by aragonite precipitation, increased δ18OAr values and laminations. The driving mechanism behind laminae formation seems to be marine intrusions, leading to pycnocline stabilization and increased evaporation of lake surface waters during summer months. Climatic oscillations recorded during the Holocene, apart from their duration, do not exceed events of the last millennia.

Virtual field trips as a tool for indirect geomorphological experience: a case study from the southeastern part of the Gulf of Corinth, Greece

Field trips are an essential part for geoscience students, as the field is intrinsic for understanding what they are taught in the classroom. Yet, distance learning has never been more necessary than today. Despite their significance in the students' education, field trips cannot be performed under the present conditions with the COVID-19 pandemic. Educators are called to find, use and evolve various tools in order to offer students quality education, with an effort to eliminate the drawbacks of the lack of physical contact and “live” field work. Virtual field trips are one such tool through which one can virtually see any place on the globe by means of a computer, tablet, or even mobile phone, without physically visiting it. In this paper, we present the results of a virtual field trip developed for students following the Geomorphology course of the Faculty of Geology and Geoenvironment, National and Kapodistrian University of Athens; it can, however, be used from other universities with similar courses not only in Greece but also in other countries as well. The purpose of this study is the evaluation of virtual field trips, both as an alternative to and/or substitute for in situ field work and as a means of preparation for live field trips, by taking into consideration the students' views through an anonymous questionnaire. Our findings suggest that virtual field trips are useful for geoscience students, and they provide a good alternative during restriction periods, and although they can under no circumstances substitute real field trips, they can be a valuable additional tool when preparing for a live field trip.

Estimation of Earthquake Early Warning Parameters for Eastern Gulf of Corinth and Western Attica Region (Greece). First Results

The main goal of an Earthquake Early Warning System (EEWS) is to alert before the arrival of damaging waves using the first seismic arrival as a proxy, thus becoming an important operational tool for real-time seismic risk management on a short timescale. EEWSs are based on the use of scaling relations between parameters measured on the initial portion of the seismic signal after the arrival of the first wave. To explore the plausibility of EEWSs around the Eastern Gulf of Corinth and Western Attica, amplitude and frequency-based parameters, such as peak displacement (Pd), the integral of squared velocity (IV 2) and the characteristic period (τc), were analyzed. All parameters were estimated directly from the initial 3 s, 4 s, and 5 s signal windows (tw) after the P arrival. While further study is required on the behavior of the proxy quantities, we propose that the IV 2 parameter and the peak amplitudes of the first seconds of the P waves present significant stability and introduce the possibility of a future on-site EEWS for areas affected by earthquakes located in the Eastern Gulf of Corinth and Western Attica. Parameters related to regional-based EEWS need to be further evaluated.

Spatiotemporal Properties of Seismicity and Variations of Shear-Wave Splitting Parameters in the Western Gulf of Corinth (Greece)

The Western Gulf of Corinth (WGoC) exhibits significant seismicity patterns, combining intense microseismic background activity with both seismic swarms and short-lived aftershock sequences. Herein, we present a catalogue of ~9000 events, derived by manual analysis and double-difference relocation, for the seismicity of the WGoC during 2013–2014. The high spatial resolution of the hypocentral distribution permitted the delineation of the activated structures and their relation to major mapped faults on the surface. The spatiotemporal analysis of seismicity revealed a 32-km-long earthquake migration pattern, related to pore-pressure diffusion, triggering moderate mainshock-aftershock sequences, as fluids propagated eastwards in the course of ~15 months. The anisotropic properties of the upper crust were examined through automatic shear-wave splitting (SWS) analysis, with over 2000 SWS measurements at local stations. An average fast shear-wave polarization direction of N98.8° E ± 2.8° was determined, consistent with the direction of the maximum horizontal regional stress. Temporal variations of normalized time-delays between fast and slow shear-waves imply alterations in the level of stress or microcrack fluid saturation during the long-lasting pore-pressure diffusion episode, particularly before major events. The present study provides novel insights regarding seismicity patterns, active fault structures, anisotropic properties of the upper crust and triggering mechanisms of seismicity in the WGoC.

Ancient Thebes

The principal city of Boiotia, Thebes exerted influence and at times control over the great expanse of Central Greece, from the South Euboean Gulf at east to the Gulf of Corinth at west. Lying north of the massif of Parnes (and its most famous spur, Cithaeron), Thebes bestrides the western reaches of a low mountain range running east toward Tanagra and governs access to the flatlands along the Asopus river to the south, to the plains stretching north and east toward Helicon and the Copais (the Teneric plain), and to the level expanses extending west toward the sea south of the Messapion-Ptoon line (the Aonian plain). Thebes itself sits on a dense cluster of hills. One such hill, the Cadmea, is the age-old acropolis. The river Dirce runs just west of the Cadmea. Two rivers lie east: the Strophia (or Chrysoroas), which runs immediately next to the Cadmea, and, further east, the Ismenos. Thebes has a grand mythic history. Founded by the Phoenician Cadmus (in one tradition) while in search of his sister, Europa, the city is the birthplace of two sons of Zeus, Dionysus and Heracles, and an imposing mortal line which includes Oedipus. Impressive Bronze Age remains have long lent intrigue to these traditions. Thebes had regional and extra-regional aspirations by the 6th century, with mythic, epigraphic, and historical references indicating rivalry with neighboring Boiotian communities as well as Athens and Thessaly. Famous for medizing during the Persian Wars, Thebes likely acted within a Boiotian collective by the middle of the 5th century. Thebans joined the Peloponnesian cause in the Peloponnesian War but thereafter came into running conflict with Sparta. The city expelled an imposed Spartan garrison in 379, and the leaders Epaminondas and Pelopidas brought forth a period of expansive Theban hegemony after Leuctra (371). Following the shared defeat at Chaeronea in 338—where Thebes’ renowned Sacred Band came to ruin—the city endured a Macedonian garrison. Destroyed by Alexander in 335 for rebellion, Thebes was rebuilt in the time of Cassander (316). The city functioned as a member of a Boiotian collective subsequently, but Sulla stripped its territory in 86 for Thebes’ backing of Mithridates. Thebes sank to relative insignificance thereafter and did not rise to prominence again until Byzantine times. A prosperous international city after Justinian and into the Middle Ages, Thebes’ importance receded under Ottoman domination.

Source Process-Related Delays in Earthquake Early Warning for Example Cases in Greece

ABSTRACT We explore a hypothetical zero-latency earthquake early warning (EEW) system in Greece, aiming to provide alerts before warning thresholds of the intensity of ground motion are exceeded. Within the seismotectonic context of Greece, both shallow- and intermediate-depth earthquakes (along the Hellenic subduction zone) are plausible and, thus, examined. Using regionally applicable attenuation relations, we combine and adjust the methodologies of Minson et al. (2018) and Hoshiba (2020) to examine what are the minimum magnitudes required to invoke the warning thresholds at the user site. With simple modeling, we examine how fast an alert can be issued and what is the available warning time when taking into account delays due to finite-fault rupture propagation, alongside other delays. These computations are merged with delays introduced due to the present-day configuration of the Greek national monitoring network (varying spatial density of permanent monitoring stations). This approach serves as a tool to assess the feasibility of an EEW system at specific sites and to redesign the national permanent monitoring network to serve such a system more effectively (we provide results for four sites.). Warning times for on-land crustal earthquakes are found to be shorter, whereas for intermediate-depth earthquakes in Greece an EEW system is feasible (provides warning times of several tens of seconds at large cities, e.g., on Crete Island) even with the current configuration of the national monitoring network, which is quite sparse in the southern part of the country. The current network configuration also provides sufficient early warning (e.g., of the order of 10 s for a warning threshold of 0.05g) at the center of Athens from earthquakes of the eastern Gulf of Corinth—a zone posing elevated hazard in the broader area of the Greek capital. Several additional assumptions and factors affecting the operability of an EEW system in Greece (i.e., source process complexity and uncertainty in attenuation laws) are also discussed.