Thomas Hasiotis

A pockmark field in the Patras Gulf (Greece) and its activation during the 14/7/93 seismic event

Abstract During a recent oceanographical-geophysical survey carried out in the southeastern part of the Gulf of Patras in Western Greece for the construction of an outfall, an active pockmark field was found. The pockmark field was formed in soft layered Holocene silts. The pockmarks are associated with acoustic anomalies attributed to gas-charged sediments. The pockmarks vary in size and shape from 25 to 250 m in diameter and from 0.5 to 15 m in depth and are among the largest and deepest observed in the world. On July 14th, 1993, during the survey, a major earthquake of magnitude 5.4 on the Richter scale occurred in the area. During the 24 hour period prior to the earthquake the bottom water temperature anomalously increased on three occasions, whilst for a few days after the earthquake it was noted that the majority of the pockmarks were venting gas bublles. It is considered that the three abrupt sea-water temperature increases were probably the result of upward migrating high-temperature gas bubbles in the water column. It is further suggested that the earthquake was the triggering mechanism and that the gas expulsion was caused by the reduction in the pore volume in the sediments resulting from changes in the stress regime prior to the earthquake. Therefore, it can be suggested that in seismic areas adjacent to pockmark fields, earthquake prediction may be achieved by monitoring the water temperature and/or the rate of gas venting in the pockmark field. Our analysis indicates that the pockmark field in the Patras Gulf has formed slowly during the Holocene by continuous gas venting, which is periodically being interrupted by short-duration events of enhanced gas seepage triggered by earthquakes.

Gas-charged sediments in the Aegean and Ionian Seas, Greece

Abstract During the last fifteen years, marine seismic surveys in the Aegean and Ionian Seas have revealed numerous acoustic anomalies; i.e. acoustic turbid zones, gas pockets, gas plumes, enhanced reflectors, columnar disturbances, wipe outs and meso- to micro-morphological features such as pockmarks, domes, mud volcanoes and elongated depressions. These are attributed to the presence of gas in sediment interstices. The gas-charged sediments are found in Pleistocene and present-day fjord-like environments, Pleistocene and present-day deltaic environments, lakes and open sea environments. The gas found in the Quaternary fjord-like and deltaic depositional environments is assumed to be of biogenic origin. The gas found in the pre-Quaternary open sea environments is associated with faulting and salt doming and may, therefore, be of thermogenic origin. Although the sediment grain size is favourable for the formation of pockmarks, the lack of them is probably due to the high seismicity which characterises these regions.

Earthquake-induced coastal sediment instabilities in the western Gulf of Corinth, Greece

Abstract On June 15 1995, a locally 6.2-R destructive earthquake occurred in the western Gulf of Corinth and caused extensive onshore and offshore sediment instabilities. A detailed offshore geophysical survey along the Tolofonas/Eratini coastline, soon after the earthquake, revealed the existence of two major sediment failures affecting the surficial 8 and 2 m of the recent sedimentary cover. The sizes of the two sediment failures are approximately 200 000 and 50 000 m 2 . The geotechnical study has shown that (1) the coastal sediments are stable under gravitational stresses and cyclic loading stresses induced by the 6.2-R earthquake and (2) the sediment failures were initiated due to liquefaction of subsurface layers. The failed sediments very quickly disintegrated and transformed to mass flows. The deformation of the failed sediment was caused by loss of sediment strength due to development of high pore water pressure induced by the earthquake. The study suggests that the alluvial fan deposits along the coastal zone of the western Corinth Gulf are susceptible to liquefaction by any earthquake event with a magnitude greater than 6 R, depending upon the proximity of the site to the earthquake epicenter. Earthquakes of magnitude 6 R are expected to occur every 22.7 years and therefore the recurrence interval for sediment failure events is about the same.

Assembling ecological pieces to reconstruct the conservation puzzle of the aegean sea

The effective conservation of marine biodiversity through an integrated ecosystem-based management approach requires a sound knowledge of the spatial distribution of habitats and species. Although costly in terms of time and resources, acquiring such information is essential for the development of rigorous management plans and the meaningful prioritization of conservation actions. Located in the northeastern part of the Mediterranean, the Aegean Sea represents a stronghold for marine biodiversity. However, conservation efforts are hampered by the apparent lack of spatial information regarding marine habitats and species. This work is the first to address this knowledge gap by assembling, updating, and mapping information on the distribution of key ecological components. A range of data sources and methodological approaches was utilized to compile and complement the available data on 68 ecological features of conservation interest (58 animal species, six habitat categories, and four other vulnerable ecological features). A standardized data evaluation procedure was applied, based on five semi-quantitative data quality indicators in the form of a pedigree matrix. This approach assessed the sufficiency of the datasets and allowed the identification of the main sources of uncertainty, highlighting aspects that require further investigation. The overall dataset was found to be sufficient in terms of reliability and spatiotemporal relevance. However, it lacked in completeness, showing that there are still large areas of the Aegean that remain understudied, while further research is needed to elucidate the distribution patterns and conservation status of several ecological features; especially the less charismatic ones and those found in waters deeper than 40 m. Moreover, existing conservation measures appear to be inadequate to safeguard biodiversity. Only 2.3% of the study area corresponds to designated areas for conservation, while 41 of the ecological features are underrepresented in these areas. Considering the high geomorphological complexity and transnational character of the Aegean Sea, this study does not offer a complete account of the multifaceted diversity of this ecoregion. Instead, it represents a significant starting point and a solid basis for the development of systematic conservation plans that will allow the effective protection of biodiversity within an adaptive management framework.

Assessment of island beach erosion due to sea level rise: the case of the Aegean archipelago (Eastern Mediterranean)

The present contribution constitutes the first comprehensive attempt to (a) record the spatial characteristics of the beaches of the Aegean archipelago (Greece), a critical resource for both the local and national economy, and (b) provide a rapid assessment of the impacts of the longterm and episodic sea level rise (SLR) under different scenarios. Spatial information and other attributes (e.g., presence of coastal protection works and backshore development) of the beaches of the 58 largest islands of the archipelago were obtained on the basis of remote-sensed images available on the web. Ranges of SLR-induced beach retreats under different morphological, sedimentological and hydrodynamic forcing, and SLR scenarios were estimated using suitable ensembles of cross-shore (1-D) morphodynamic models. These ranges, combined with empirically derived estimations of wave runup induced flooding, were then compared with the recorded maximum beach widths to provide ranges of retreat/erosion and flooding at the archipelago scale. The spatial information shows that the Aegean “pocket” beaches may be particularly vulnerable to mean sea level rise (MSLR) and episodic SLRs due to (i) their narrow widths (about 59 % of the beaches have maximum widths < 20 m), (ii) their limited terrestrial sediment supply, (iii) the substantial coastal development and (iv) the limited existing coastal protection. Modeling results indeed project severe impacts under mean and episodic SLRs, which by 2100 could be devastating. For example, under MSLR of 0.5 m – representative concentration pathway (RCP) 4.5 of the Fifth Assessment Report (AR5) of the Intergovernmental Panel on Climate change (IPCC) – a storm-induced sea level rise of 0.6 m is projected to result in a complete erosion of between 31 and 88 % of all beaches (29–87 % of beaches are currently fronting coastal infrastructure and assets), at least temporarily. Our results suggest a very considerable risk which will require significant effort, financial resources and policies/regulation in order to protect/maintain the critical economic resource of the Aegean archipelago. Published by Copernicus Publications on behalf of the European Geosciences Union. 450 I. N. Monioudi et al.: Assessment of island beach erosion due to sea level rise

Submarine Mass Movements On An Active Fault System In The Central Gulf Of Corinth

A very high-resolution shallow-seismic survey along the central part of the faultbounded Corinth Gulf southern margin (offshore Xylocastro town) revealed that three morphological zones characterize the area: the shelf, the slope and the basin. Three E-W trending, right stepping basin bounding faults define the basin-slope contact, producing a step like configuration along the base of slope. Steep scarps, caused by mass failures, sculpt the fault plane surfaces, which act as part of the slope. The shelf and the slope are dissected by submarine canyons and numerous minor channels. The largest canyons are located immediately off the river mouths, run perpendicular to the slope and are linear. Seismic profiles across the canyons suggest that some of them are fault controlled. The head and the walls of the canyons are affected by mass failures. It is estimated that about 1.1 km of mass failed sediments have been removed from the canyons and transported downslope to the basin floor.

Application of computational intelligence tools for the analysis of marine geotechnical properties in the head of Zakynthos canyon, Greece

This paper uses a computational approach to provide insight into the relationships among marine geotechnical properties that characterize the recent sedimentary cover at the head of Zakynthos Canyon in western Greece. Self-organizing maps (SOM) and generic interaction matrix (GIM) theory were used to investigate the tendency of the data to cluster and to examine the sediment property relationships. This analysis has also focused on the assessment of the dominance and interaction intensity between the related parameters following GIM theory definition. The principal results refer to the identification of clusters in the original multivariate data set. SOM-based analysis distinguished five clusters, with similar geotechnical characteristics, which led to the separation of the surficial (~80cm) unconsolidated sediments from the deeper normally consolidated sediments and depicted better relations between the geotechnical properties within each cluster. The combination of SOM with GIM theory also demonstrates the dominance of fine-grained sediments (especially silts) and their associated Atterberg limits. The strongest interaction intensity is observed between silt and water content, whereas the undrained shear strength of the surficial deposits appears to be least interactive. The application of computational intelligence methods in the study of marine geotechnical properties allows insight into the relationships between the various geotechnical parameters and provides a promising tool for knowledge extraction in marine geo-environments.

Modeling beach realignment using a neuro-fuzzy network optimized by a novel backtracking search algorithm

Beach realignment is caused by sediment movement across and along the beach due to the ever-changing incident waves. Its driving mechanisms are highly nonlinear and cannot be easily resolved by numerical process-response modeling which may also involve high computational costs. In this contribution, an alternative approach is developed to cope with the nonlinearity of beach realignment, which utilizes a neuro-fuzzy neural network optimized by a novel backtracking search algorithm. The network comprises multiple layers operating in sequence and establishes input–output relationships in terms of first-order fuzzy rules. The proposed backtracking search algorithm improves its performance by effectively modifying the existing mutation and crossover operations of the standard algorithm. A novel experimental setup was deployed in a touristic beach of Santorini island, Greece, to collect high frequency morphological and hydrodynamic information and generate a data set described by few representative input variables. Three additional networks were designed using the same data set. Standard criteria and nonparametric statistical analysis showed that the proposed approach outperforms all other tested methods in the case of modeling beach realignment, achieving also a significant improvement over previous efforts.

High Frequency Sediment Failures In A Submarine Volcanic Environment: The Santorini (Thera) Basin In The Aegean Sea

Seismic data have been used to evaluate the extent, characteristics and importance of the sediment failures in the Santorini basin. The failures are small but abundant, occupying almost half of the areal extent of the study area, and modify the relief of the basin and the surrounding slopes. The fact that surficial mass flow deposits are the source area of younger sediment failures is evident of the high intensity and frequency of the sediment instabilities. The major factors which are responsible for the observed instabilities are seismic activity and seismicity related to modern volcanic activity, steep slopes and the open sediment structure due to the specific texture of the volcanic material. Sediment failures are believed to compose a big part of the deeper sedimentary column.