George Papathanassiou

Estimating slope failure potential in an earthquake prone area: a case study at Skolis Mountain, NW Peloponnesus, Greece

One of the most important issues in the evaluation of geological hazards is the delineation of landslide prone areas and the estimation of the slope failure potential. In this study, the GIS-based Newmark’s displacement method was applied for the first time in Greece, in order to compute the permanent displacement values and evaluate the slope failure potential in the area of the Skolis mountain in NW Peloponnesus, Greece. The resulting maps were validated using the distribution of secondary effects triggered by the 8 June 2008 earthquake. The study indicated that as the source areas of the rock falls were delineated by the Newmark displacement method, this may be a useful approach to predict earthquake-induced slope failures when the seismic potential of an area has been previously evaluated.RésuméUn des thèmes les plus importants dans l’évaluation des aléas géologiques est la délimitation des zones sujettes à glissements de terrain et l’estimation des risques de rupture de terrain. Dans cette étude, la méthode de Newmark, sur une base SIG, a été appliquée pour la première fois en Grèce, afin de calculer des valeurs de déplacement permanent et d’évaluer le risque de rupture de pente dans la région des montagnes de Skolis dans le nord-ouest du Péloponnèse (Grèce). Les cartes produites ont été validées à partir de la répartition des effets secondaires déclenchés par le séisme du 8 juin 2008. L’étude a montré que, la zone source de chutes de blocs étant délimitée par la méthode des déplacements de Newmark, cette approche peut être utile pour prévoir les glissements déclenchés par des séismes lorsque l’activité sismique d’une région a été au préalable évaluée.

A Rock Fall Analysis Study in Parnassos Area, Central Greece

The paper presents the kinematics of rock instability of a high promontory, where Tithorea village is situated, in the Central Greece peninsula. The instability phenomena pose a significant threat on the town situated immediately down slope. Rock fall episodes occurred in the past, as it has been noted by local authorities and published reports. A more recent rockfall is investigated in this paper, which caused considerable damages to two village houses. The detailed rock fall path was mapped obtaining more than 15 hit points and rolled on different scree and vegetation material till it stopped and crushed upon the house roofs. The predominant types of kinematic instability are of planar or wedge failure and toppling of large blocks. In order to investigate the existing stability conditions and decide upon the protection measures, rockfall analysis was carried out. Such an analysis can be compared in detail with the actual situation, since the geometrical data of the recent rockfall incident are very accurate. On the other hand, other parameters of rockfall hazard such as the run-out distance of the boulders was examined.

Towards the Evaluation of Landslide Hazard in the Mountainous Area of Evritania, Central Greece

The mountainous area of Evritania, in central Greece, is considered one of the most hazardous zones, in the country, in providing landslides. The main reason lies to the presence of flysch formations, which are highly tectonized. Characteristic cases of slope failures have been reported after a field survey. The basic aim of this study is to delineate zones prone to slope failure and to quantify the landslide hazard. For every case of slope failure basic parameters have been collected including the critical engineering geological characteristics for 11 flysch rock mass types, which induce a certain mechanism of failure. The information was imported into a GIS environment in order to compile an inventory and data facts map. In addition, the spatial distribution of slope failures was correlated to the geological map, the slope angle map and the aspect map of the area.

Evaluation of the Temporal Probability of Earthquake-Induced Landslides in the Island of Lefkada, Greece

In the island of Lefkada, earthquakes are considered as the main causal factor of slope instabilities according to the seismic catalogues. In this study, the temporal probability was estimated as a joint probability of the exceedance probability of earthquake threshold per decade and the probability of landslide occurrence once the threshold has been exceeded. Initially, landslide inventory maps were compiled for the instrumental period of seismicity in Greece, indicating that the occurrence of landslide phenomena is mainly concentrated within the western part of the island. For this period, 21 earthquakes were recorded with magnitudes M > 5.3 and 6 of M > 6.0, while five landslide events were defined. Thus, the mean recurrence interval per decade between successive threshold exceedance was estimated at 0.48 and 1.66 for earthquake magnitudes M = 5.3 and M = 6.0, respectively. Taking into account that the probability of occurrence of landslide after the threshold has been exceeded is 0.23 and 0.83 for M > 5.3 and M > 6.0, the estimated temporal probability of earthquake-induced landslides for different time periods, from 10 to 500 years, ranges from 0.2 to 0.23 for earthquake magnitudes M > 5.3 and from 0.37 to 0.83 for magnitude M > 6.0.

Liquefaction Susceptibility Map of the Broader Thessaloniki Urban Area

Τhe occurrence of liquefaction and the generation of liquefaction-induced deformations can result in severe damages to the manmade environment particularly in urban areas constructed in coastal areas. The first event close to an urban environment and studied in detail, was the 1906 San Francisco earthquake while the last decade, severe structural damages were induced due to soil liquefaction (CES 2010–2011, Great East Japan 2011, Emilia Romagna 2012 and Cephalonia 2014 earthquakes). In order to prevent the occurrence of soil liquefaction and to minimize its effects to the manmade environment, studies regarding the susceptibility of the geological units should initially take place, oriented to the assessment of the depositional environment. The goal of this study is to delineate susceptible to liquefaction geological units within the broader Thessaloniki urban area. In order to achieve this, information regarding the surficial distribution of geological units was taken into account in conjunction with the historical seismicity background of the area. The result obtained by this study is that the industrialized area, located to the western edge of the urban area, is constructed upon sediments classified as high to very high liquefaction susceptibility. The outcome of this study can be used by urban planners for the future extension of the city of Thessaloniki.

Multitemporal Landslide Mapping and Quantification of Mass Movement in Red Beach, Santorini Island Using Lidar and UAV Platform

Santorini Island constitutes one of most unique geological structures as it compromises a distinctive Miocene volcano. Rockfalls and landslides are widespread phenomena in Santorini due to orientation and steepness of the cliffs which are mainly formed as high elevated lava domes with loose material inside, extruded by sticky, slow-flowing dacite lava. Especially, landslide events in high vulnerable areas can cause significant environmental and socioeconomic impacts. Following a landslide hazard, comprehensive and reliable information on the geotechnical and geometric properties but also on the actual consequences of the phenomenon is mandatory. Today, the evolution of geoscience enables the use of innovative tools such as Unmanned Aerial Vehicles (UAVs) to address emergency response in disastrous situations. As a result, landslide identification and monitoring of large areas can be accomplished economically and timely. During the last decade, the UAV platform has become a useful and reliable research tool in emergency situations such as landslide monitoring. The current research performs a semi-automated method to evaluate and quantify site-specific landslide hazard in the area of Red Beach on the island of Santorini, Greece. Red Beach shows very high tourist activity during the whole year, so it is mandatory for safety reasons to establish guidance to eliminate landslide hazard in prone areas. Considering the availability of high spatial and spectral datasets, multitemporal change detection techniques were performed between two different datasets acquired in February and September of 2017 with a UAV platform. The methodology implemented in the current research revealed the accumulation area of failure, flow direction but also allowed the quantification of the mass movement in the area of interest. The outcome of the current research aims to highlight the usefulness of photogrammetry and UAV platforms to assess and mitigate the potential negative consequences of landslide hazard.

3D Modelling of the ancient underground quarries of the famous Parian marble in the Aegean Sea, Greece and assessment of their stability using LiDAR scanning

Laser scanning has proven useful in the stability assessment of underground openings. High accuracy points are used to generate 3D surface models to evaluate their stability by assessing rockmass structural features. A series of scans obtained from underground ancient quarries in Paros Island were processed. The structural analysis of the openings was complemented by field observations and measurements, serving as input parameters for numerical software used to evaluate the potential failure mechanisms and overall stability. Blocks with a higher risk of detaching are encountered mostly at tunnel portals where support is required to secure unstable blocks to preserve this geotope.

AN ENGINEERING GEOLOGICAL ASSESSMENT USING TERRESTRIAL LASER SCANNING FOR THE STABILIZATION OF A VOLCANIC SLOPE IN SANTORINI ISLAND

The present work presents an engineering geological assessment using terrestrial laser scanning for the study of stabilization of a rock slope in Santorini island that has suffered from several rock slide incidents. The use of LiDar laser scanning and focused surveying provided valuable data for the engineering geological zonation of the area and the rock block quantification for the stability analysis. In order to prevent failure and the accompanied consequences, slope stability analysis and various mitigation methods were examined based on the site’s geological and geotechnical conditions, cost, local architecture restrictions and constructability.

Simulation of Off-Fault Surface Effects from Historical Earthquakes: The Case of the City of Thessaloniki (Northern Greece)

Thessaloniki, the second largest city in Greece, is located at the northeastern shore of Thermaikos Gulf, a shallow water body and remnant of the Axios valley sedimentary system that has received massive sedimentation. The broader region is surrounded by neotectonic fault zones, several of which can be considered as active, based on their kinematic data compatibility with the active stress field and the frequent microseismicity. Part of the city is built on basement rocks of primarily Mesozoic age, but most of it is founded on the sedimentary filling of the Axios basin that has been active since Eocene. Historical records indicate that Thessaloniki, has suffered several earthquake disasters, both from sources close to the city, as well as from more distant sources (e.g. 1829 M7 Drama and 1904 M7 Kresna events). The event of 1759 (MMI IX; M6.5) is reported as the most destructive for Thessaloniki with a postulated epicenter at the Axios River estuary in Thermaikos Gulf. We combine the most updated tectonic and seismological data for the broader Thessaloniki area to postulate a scenario of a moderate-magnitude earthquake and model the expected strong shaking and off-fault coseismic surface effects, with emphasis on liquefaction phenomena that match the damage patterns described in historical archives. In particular, we delineate zones prone to liquefaction occurrence and evaluate the potential of the geological units. The latter is achieved based on data provided by in situ tests and the former based on information provided by geological maps regarding the age and depositional process of the sediments.