Evaluation of ground motion characteristics,effects of local geology and liquefaction susceptibility: the case of Itea,Corinth Gulf (Greece)

In the present paper we analyze the effect of local geology on ground motion by means of numerical calculations (numerical models) using total (TS) and effective stress (ES) methods. These numerical calculations have been applied to the site of Itea, Corinth Gulf, which was chosen based on liquefaction susceptibility criteria and field inspection. Data from seismic refraction experiments and cone penetration test N-values as well as selected records of ground motion in nearby areas were used to construct the input file for the numerical model. By means of␣dynamic analysis such characteristics of ground motion as acceleration time histories, response spectra, and amplification function were evaluated. A one-dimensional soil amplification effect was clearly shown. Liquefaction probability at the Itea site was predicted based on the safety factor and the calculation of the induced settlement at the test site. Results of the TS and ES modeling lead us to conclude: (1) the presence of soft soil at Itea caused significant amplification (almost 2.5-fold higher magnitude) of the underlying bedrock motion and, therefore, can contribute to damage; (2) the area of Itea is highly susceptible to liquefaction due to presence of silty sand deposits at depths between 2.48 m (the position of the water table) and 12 m that demonstrate the rapid growth of the excess pore water pressure (EPWP) ratio with an increase in peak ground acceleration values.     

The 2014 Kefalonia Doublet (<Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript>6.1 and <Emphasis Type="Italic">M</Emphasis><Subscript>W</Subscript>6.0), Central Ionian Islands,Greece: Seismotectonic Implications along the Kefalonia Transform Fault Zone

The 2014 Kefalonia earthquake sequence started on 26 January with the first main shock (M_W6.1) and aftershock activity extending over 35 km, much longer than expected from the causative fault segment. The second main shock (M_W6.0) occurred on 3 February on an adjacent fault segment, where the aftershock distribution was remarkably sparse, evidently encouraged by stress transfer of the first main shock. The aftershocks from the regional catalog were relocated using a 7-layer velocity model and station residuals, and their distribution evidenced two adjacent fault segments striking almost N-S and dipping to the east, in full agreement with the centroid moment tensor solutions, constituting segments of the Kefalonia Transform Fault (KTF). The KTF is bounded to the north by oblique parallel smaller fault segments, linking KTF with its northward continuation, the Lefkada Fault.     

Markov based transition probability geostatistics in groundwater applications: assumptions and limitations

Markov based transition probability geostatistics (MTPG) for categorical variables, as implemented by the methodological framework introduced by Carle and Fogg (Math Geol 29(7):891–918, 1997) and extended thereafter, have been extensively applied for the three-dimensional (3D) statistical representation of hydrofacies in real-world aquifers, and the conditional simulation of 3D lithologies for groundwater flow and transport simulations. While conceptually simple and easy to implement, conditional simulation using the MTPG approach is not limitation free. However, to the best of our knowledge, there is no study that raises such concerns in the light of theoretical arguments and numerical findings. That said, the purpose of this study is twofold: (1) present a brief and coherent overview of the basic theory, fundamental assumptions, and limitations of the MTPG methodological framework, and (2) assess its capabilities on the basis of a simple two-dimensional test-case, using large ensembles of stochastic realizations. Contrary to real-world 3D aquifers, where the actual geology is unknown, and the quality of the simulations can be assessed solely on the basis of semi-quantitative arguments using properly selected sets of stochastic realizations, test-cases allow for direct quantitative assessments based on the application of statistical measures to large ensembles of synthetic realizations. Our analysis and obtained results show that stochastic modeling of actual geologies using the MTPG approach of Carle and Fogg (1997), is characterized by simplifying assumptions and theoretical limitations, with the simulated random fields exhibiting statistical structures that strongly depend on the problem under consideration and the modeling assumptions made, leading to increased epistemic uncertainties in the obtained results.     

Estimation of seawater retreat timescales in homogeneous and confined coastal aquifers based on dimensional analysis

Reliable estimation of the timescales of seawater retreat (SWR) in coastal aquifers has significant implications for sustainable groundwater management in coastal areas. Due to the complexity of coastal dynamics, analytical estimates of SWR timescales are limited in number, and numerical models are mainly utilized. Although numerical models allow for accurate analysis of the coastal systems, they concern particular aquifer cases, thus their results are not generally applicable. Here, we perform a dimensional analysis for the depth-integrated freshwater/seawater continuity equations and identify the dimensionless parameters that affect SWR in confined, homogeneous and flux-controlled aquifers. Based on numerical simulations, we gain insight into how these parameters affect SWR, and produce type curves which can be used as a simple tool for estimating SWR timescales in idealized coastal systems, independent of the hydraulic and geometric characteristics of the aquifer. The reliability of our estimates in real-world applications is also discussed.     

Space–time analysis, faulting and triggering of the 2010 earthquake doublet in western Corinth Gulf

Two moderate magnitude earthquakes (M5.5 and M5.4) occurred in January 2010 with their epicenters at a distance of about 5?km between them, in the western part of the Corinth Gulf. The recordings of the regional seismological network, which is dense locally, were used for the location of the two main events and aftershocks, which are concentrated in three clusters beneath the northern coasts of the Gulf. The first two clusters accompany each one of the two stronger earthquakes, whereas the third cluster comprises only low magnitude aftershocks, located westward of the two stronger events. Seismic excitation started in January 18, 2010, with the M?=?5.5 earthquake in the area occupied by the central cluster. Seismicity immediately jumped to the east with numerous aftershocks and the M?=?5.4 earthquake which occurred four days later (January 22, 2010). Cross sections normal to the long axis of each cluster show ruptures on north dipping faults at depths of 7?C11?km. Focal mechanisms of the stronger events of the sequence support the results obtained from the spatial distribution of the aftershocks that three different fault segments activated in this excitation. The slip vectors of all the events have an NNW?CSSE to NNE?CSSW orientation almost parallel to the direction of extension along the Corinth Gulf. Calculation of the Coulomb stress changes supports an interaction between the different clusters, with the major activity being coincided with the area of positive induced stress changes after the first earthquake.     

Rupture model of the great AD 365 Crete earthquake in the southwestern part of the Hellenic Arc

An M8.3 earthquake struck the southwestern part of the Hellenic Arc, near the Island of Crete, in AD 365, generating a tsunami that affected almost the entire eastern Mediterranean region. Taking into account that the time history of seismicity in this region is fairly complete for such earthquakes in the historical catalog, which can be dated as back as the 5th century B.C., there is no indication that this segment of plate boundary has been fully ruptured again. The seismic hazard associated with this part of the Hellenic Arc necessitates the evaluation of the rupture characteristics of this great event. The constraint of the faulting geometry was initially achieved by using information from seismicity, and the focal mechanisms of earthquakes that occurred during the instrumental period. A rupture model for this great earthquake is constructed by assuming an elastic medium and calculating the theoretical surface displacements for various fault models that are matched with the observed surface deformation gleaned from historical reports. The resulted fault model concerns thrust faulting with a rupture length of 160 km and a seismic moment of 5.7 × 10²⁸ dyn·cm, an average slip of 8.9 m and a corresponding moment magnitude equal to 8.4, in excellent agreement with the macroseismic estimation. The absence of such events recurrence is an indication of the lack of complete seismic coupling that is common in subduction zones, which is in accordance with the back arc spreading of the Aegean microplate and with previous results showing low coupling for extensional strain of the upper plate.     

Ground fissures in the area of Mavropigi Village (N. Greece): Seismotectonics or mining activity?

In the beginning of July 2010, a ground fissure was observed in the field near the village of Mavropigi (Northern Greece) and specifically in its NW side. Later on (early September), a second ground fissure was perceived, close and almost parallel to the first one and very close to the limits of the lignite exploitation mine (by the Public Power Corporation, PPC). It was observed that the village of Mavropigi slides away slowly towards the PPC lignite mine. Geological, seismological, as well as geotechnical survey in the field indicated that the phenomenon is related to the coal mining exploitation in the near vicinity of the village rather than to any seismotectonic activity in the surrounding area.     

Assessment of groundwater vulnerability to pollution: a combination of GIS, fuzzy logic and decision making techniques

The assessment of groundwater vulnerability to pollution aims at highlighting areas at a high risk of being polluted. This study presents a methodology, to estimate the risk of an aquifer to be polluted from concentrated and/or dispersed sources, which applies an overlay and index method involving several parameters. The parameters are categorized into three factor groups: factor group 1 includes parameters relevant to the internal aquifer system’s properties, thus determining the intrinsic aquifer vulnerability to pollution; factor group 2 comprises parameters relevant to the external stresses to the system, such as human activities and rainfall effects; factor group 3 incorporates specific geological settings, such as the presence of geothermal fields or salt intrusion zones, into the computation process. Geographical information systems have been used for data acquisition and processing, coupled with a multicriteria evaluation technique enhanced with fuzzy factor standardization. Moreover, besides assigning weights to factors, a second set of weights, i.e., order weights, has been applied to factors on a pixel by pixel basis, thus allowing control of the level of risk in the vulnerability determination and the enhancement of local site characteristics. Individual analysis of each factor group resulted in three intermediate groundwater vulnerability to pollution maps, which were combined in order to produce the final composite groundwater vulnerability map for the study area. The method has been applied in the region of Eastern Macedonia and Thrace (Northern Greece), an area of approximately 14,000 km². The methodology has been tested and calibrated against the measured nitrate concentration in wells, in the northwest part of the study area, providing results related to the aggregation and weighting procedure.