Ground deformation of Nisyros Volcano (Greece) for the period 1995–2002: Results from DInSAR and DGPS observations

Differential GPS (DGPS) and Differential Interferometric Synthetic Aperture Radar (DInSAR) analyses were applied to the Kos-Yali-Nisyros Volcanic Field (SE Hellenic Volcanic Arc) to quantify the ground deformation of Nisyros Volcano. After intense seismic activity in 1996, a GPS network was installed in June 1997 and re-occupied annually up to 2002. A general uplift ranging from 14 to 140 mm was determined at all stations of the network. The corresponding horizontal displacements ranged from 13 to 53 mm. The displacement vectors indicate that the island is undergoing extension towards the East, West and South. A two-source “Mogi” model combined with assumed motion along the Mandraki Fault was constructed to fit the observed deformation. The best-fit model assumes sources at a depth of 5500 m NW of the centre of the island and at 6500 m offshore ESE of Yali Island. DInSAR analysis using four pairs of images taken between May 1995 and September 2000 suggests that deformation was occurring during 1995 before the start of the seismic crisis. An amplitude of at least 56 mm along the slant range appeared for the period 1996 through 1999. This deformation is consistent with the two-source model invoked in DGPS modelling. Surface evidence of ground deformation is expressed in the contemporaneous reactivation of the Mandraki Fault. In addition, a 600 m long N-S trending irregular rupture in the caldera floor was formed between 2001 and 2002. This rupture is interpreted as the release of surface stress in the consolidated epiclastic and hydrothermal sediments of the caldera floor.     

Testing satellite and ground thermal imaging of low-temperature fumarolic fields: The dormant Nisyros Volcano (Greece)

The Nisyros Volcano (Greece) was monitored by satellite and ground thermal imaging during the period 2000–2002. Three night-scheduled Landsat-7 ETM⁺ thermal (band 6) images of Nisyros Island were processed to obtain land surface temperature. Ground temperature data were also collected during one of the satellite overpasses. Processed results involving orthorectification and 3-D atmospheric correction clearly show the existence of a thermal anomaly inside the Nisyros Caldera. This anomaly is associated mainly with the largest hydrothermal craters and has land surface temperatures 5–10 °C warmer than its surroundings. The ground temperature generally increased by about 4 °C inside the main crater over the period 2000–2002. Ground thermal images of the hydrothermal Stephanos Crater were also collected in 2002 using a portable thermal infrared camera. These images were calibrated to ground temperature data and orthorectified. A difference of about 0–2 °C was observed between the ground thermal images and the ground temperature data. The overall study demonstrates that satellite remote sensing of low-temperature fumarolic fields within calderas can provide a reliable long-term monitoring tool of dormant volcanoes that have the potential to reactivate. Similarly, a portable thermo-imager can easily be deployed for real-time monitoring using telemetric data transfer. The operational costs for both systems are relatively low for an early warning system.     

An algorithm developed in Matlab for the automatic selection of cut-off frequencies,in the correction of strong motion data

Strong motion recordings are the key in many earthquake engineering applications and are also fundamental for seismic design. The present study focuses on the automated correction of accelerograms, analog and digital. The main feature of the proposed algorithm is the automatic selection for the cut-off frequencies based on a minimum spectral value in a predefined frequency bandwidth, instead of the typical signal-to-noise approach. The algorithm follows the basic steps of the correction procedure (instrument correction, baseline correction and appropriate filtering). Besides the corrected time histories, Peak Ground Acceleration, Peak Ground Velocity, Peak Ground Displacement values and the corrected Fourier Spectra are also calculated as well as the response spectra. The algorithm is written in Matlab environment, is fast enough and can be used for batch processing or in real-time applications. In addition, the possibility to also perform a signal-to-noise ratio is added as well as to perform causal or acausal filtering. The algorithm has been tested in six significant earthquakes (Kozani-Grevena 1995, Aigio 1995, Athens 1999, Lefkada 2003 and Kefalonia 2014) of the Greek territory with analog and digital accelerograms.     

Geoelectric investigation of the Hellenic subduction zone using long period magnetotelluric data

The strongest evidence up to date for a subduction zone in the Hellenic region is a clearly identified Wadati-Benioff zone below the central Aegean Sea, to a maximum depth of 180 km. Alternative seismic tomography models suggest that subduction process continues deeper than the Wadati-Benioff zone to a maximum depth of at least 600 km. So far the lack of deep electrical studies in the region impeded scientists from imposing other control factors than seismic to the proposed models for the Hellenic Subduction Zone (HSZ). A Long Period Magnetotelluric (LMT) study was carried out in the southern part of the Greek mainland to study the deep electrical characteristics of the HSZ and examine whether prominent modelled features correlate with structures identified by the seismic methods. The study comprised collection, processing and modelling of magnetotelluric (MT) data in the period range 100–10000 s from ten sites located along a 250 km NE–SW trending profile. The dimensionality of the data was examined at a pre-modelling stage and it was found that they do not exhibit three-dimensional (3-D) features. The latter enabled to construct both one-dimensional (1-D) and two-dimensional (2-D) models. The proposed geoelectric model for HSZ was based on 2-D modelling, since it had better maximum depth resolution of about 400 km, and revealed structures not detected by 1-D modelling attempts. The model structure which was related to the African and Euro-Asian lithosphere is relatively resistive (> 800 Ω-m) and has an average thickness of 150–170 km. Although the bottom of the lithosphere is adequately resolved, the Wadati-Benioff zone that delineates the top of the subducting lithospheric slab is not identified by any electrical feature. The modelled structure associated with the subducting part of the African lithosphere penetrates a relatively conductive (< 200 Ω-m) asthenosphere with a dip angle of 42°. Intermediate electrical resistivities (200–800 Ω-m) are attributed to the ascending melting part of the lithosphere below the region of the Hellenic Volcanic Arc (HVA) and to a dipping zone below the south-western part of the profile, at 170–220 km depths.     

Ground deformation effects from the ~M6 earthquakes (2014–2015) on Cephalonia–Ithaca Islands (Western Greece) deduced by GPS observations

The implications of the earthquakes that took place in the central Ionian Islands in 2014 (Cephalonia, M _w6.1, M _w5.9) and 2015 (Lefkas, M _w6.4) are described based on repeat measurements of the local GPS networks in Cephalonia and Ithaca, and the available continuous GPS stations in the broader area. The Lefkas earthquake occurred on a branch of the Cephalonia Transform Fault, affecting Cephalonia with SE displacements gradually decreasing from north (~100 mm) to south (~10 mm). This earthquake revealed a near N–S dislocation boundary separating Paliki Peninsula in western Cephalonia from the rest of the island, as well as another NW–SE trending fault that separates kinematically the northern and southern parts of Paliki. Strain field calculations during the interseismic period (2014–2015) indicate compression between Ithaca and Cephalonia, while extension appears during the following co-seismic period (2015–2016) including the 2015 Lefkas earthquake. Additional tectonically active zones with differential kinematic characteristics were also identified locally.     

Cryptosporidium and Giardia in Natural,Drinking, and Recreational Water of Northwestern Greece

An investigation about distribution of Giardia cysts and Cryptosporidium oocysts in natural, drinking, and recreational water in Northwestern Greece was performed. Five rivers (Aoos, Arachthos, Kalamas, Louros, and Voidomatis) and one lake (Pamvotis Ioannina Lake) in Northwestern Greece were investigated during a 10‐month period. Drinking and recreational water (swimming pools) from the area were also examined. Samples were collected from prefixed sampling stations and processed following a modification of standard methods for the microbiological examination of water, as suggested by the APHA/AWWA/WEF. Both Giardia cysts and Cryptosporidium oocysts were isolated from Pamvotis Ioannina Lake (15 positive/27 examined samples). Significantly lower numbers of Cryptosporidium oocysts were detected in Arachthos River (1/5), Voidomatis River (1/5), drinking water (1/7), and pool water samples (1/9). No Giardia cysts were detected, neither in river water, nor in drinking, and pool water samples. The results clearly show that, with the exception of Pamvotis Ioannina Lake, where contamination of high level was observed, natural water sources of the investigated area have low pollution, resulting in low contamination with parasites.     

Crustal deformation in the Central Ionian Islands (Greece): Results from DGPS and DInSAR analyses (1995–2006)

Ground deformation studies based on Differential GPS (DGPS) and Differential Interferometric SAR (DInSAR) analyses have been conducted in the seismically active area of the Central Ionian Islands. Local GPS networks were installed in Cephallonia (2001) and Zakynthos (2005). The Cephallonian network has been remeasured five times and Zakynthos' once as of July 2006. The studies have yielded detailed information regarding both local and regional deformations that are occurring in the area.For Lefkas Island, DInSAR analysis (March to September 2003) revealed 56 mm of uplift in the central and western parts and is attributed to the August 2003 earthquake (M_w = 6.3) that occurred offshore to the west. Synthetic DInSAR modelling of the magnitude and extent of deformation is consistent with the seismologically deduced parameters for the ruptured segment along the Lefkas Transform Fault. Subsidence (< 28 mm) along the northern part of the island is attributed to local conditions unrelated to the earthquake. For Zakynthos Island, large-magnitude earthquakes that occurred offshore to the south in October 2005 and April 2006 most likely contributed to the observed deformation as deduced from DGPS measurements for an encompassing period (August 2005 to July 2006). The largest amount of horizontal deformation occurred in the south, where its western part moved in a W–NW direction, while the eastern part moved towards the NE, with magnitudes ranging from 15 to 26 mm. The southern part of the island uplifted a maximum of 65 mm whereas the north subsided from 12 to 28 mm.For Cephallonia Island, DInSAR analysis (1995 to 1998) indicated ground deformation up to 28 mm located in small sections of the island. Further interferometric analysis for the period 2003 to 2004, encompassing the occurrence of the Lefkas earthquake in August 2003, indicated 28 mm of uplift in the northern part, while during the next two years (2004 to 2005), further uplift of at least 56 mm had taken place at the western and northern part of the island.DGPS measurements for the period 2001 to 2006 revealed a clockwise rotation of the island with respect to a centrally located station on Aenos Mt. The horizontal component of deformation generally ranged from 6 to 34 mm, with the largest values at the western and northern parts of the island. Considering the vertical deformation, two periods are distinguished. The first one (2001 to 2003) is consistent with anticipated motions associated with the main geological and tectonic features of the island. The second one (2003 to 2006) has been tentatively attributed to dilatancy in which relatively small uplift (20–40 mm) occurred along the southern and southeastern parts of the island, while larger magnitudes (> 50 mm) happened at the western part (Paliki Peninsula). These large magnitudes of uplift over an extended area (> 50 km), in conjunction with an accelerated Benioff strain determined from the analysis of the seismicity in the broader region, are consistent with dilatancy. This effect commenced some time after 2003 and is probably centered in the area between Zakynthos and Cephallonia. If this interpretation is correct, it may foreshadow the occurrence of a very strong earthquake(s) sometime during 2007 to 2008 in the above designated region.     

Assessment of the Water and Habitat Quality of a Mediterranean River (Kalamas,Epirus, Hellas), in Accordance with the EU Water Framework Directive

In the present study, the water quality of Kalamas river (NW Greece) was evaluated using physicochemical and hydromorphological parameters and benthic macroinvertebrates. Statistical analyses (Cluster and FUZZY analyses) were performed and two biotic scores (BMWP' and HS) were used in order to classify the sites according to water quality. Kalamas river appeared to have excellent tomoderate water quality at all sampling sites except one (close to the delta area) which was ”fairly or significantly polluted”. During the low flow season water quality appeared poorer than during the high flow season. The ecological parameters (hydromorphological, chemical, and biological) used for this integrated approach are the ones proposed by the New Water Directive 2000/60 EC for an efficient surveying monitoring of running waters.     

Long term and seasonal ground deformation monitoring of Larissa Plain (Central Greece) by persistent scattering interferometry

The land subsidence which occurs at the Larissa Basin (Thessaly Plain, Central Greece) is due to various causes including aquifer system compaction. Deformation maps of high spatial resolution deduced by the Persistent Scattering Interferometry (PSI) technique (using radar scenes from ERS and ENVISAT satellites) for the period 1992–2006 were produced to study the spatial and temporal ground deformation. A developed GIS database (including geological, tectonic, morphological, hydrological, meteorological and watertable variation from wells in the area) offered the possibility of studying in detail the intense subsidence. The PSI based average deformation image clearly shows that subsidence generally takes place inside the Larissa Plain ranging from 5–250 mm. The largest amplitude rates (?25 mm/yr) are observed around the urban area of Larissa City (especially at Gianouli and Nikea villages), while the Larissa City center appears to be relatively stable with a tendency to subside. The rest of the plain regions seem to subside at moderate rates (about 5–10 mm/yr). The surrounding mountainous area is stable, or has slightly been uplifted with respect to the NE located reference point. It was found that there is a correlation between the seasonal water-table variation (deduced from wells data), the seasonal water demand for irrigation associated with specific types of cultivation (cotton fields), the monthly rainfall, and the observed subsidence rate in the rural regions of the Thessaly Plain.