Lefkas Island Research Articles

Tectonic Inversion and Deformation Differences in the Transition from Ionian Basin to Apulian Platform: The Example from Ionian Islands, Greece

The studied areas (the Ionian Islands: Paxoi, Lefkas, Kefalonia, and Zakynthos), are situated at the western ends of the Ionian Basin in contact with the Apulian Platform and named as Apulian Platform Margins. The proposed model is based on fieldwork, previously published data, and balanced geologic cross-sections. Late Jurassic to Early Eocene NNW–SSE extension, followed by Middle Eocene to Middle Miocene (NNW–SSE compression, characterizes the Ionian basin). The space availability, the distance of the Ionian Thrust from the Kefalonia transform fault and the altitude between the Apulian Platform and the Ionian Basin that was produced during the extensional regime were the main factors for the produced structures due to inversion tectonics. In Zakynthos Island, the space availability (far from the Kefalonia Transform Fault), and the reactivation of normal bounding faults formed an open geometry anticline (Vrachionas anticline) and a foreland basin (Kalamaki thrust foreland basin). In Kefalonia Island, the space from the Kefalonia Transform Fault was limited, and the tectonic inversion formed anticline geometries (Aenos Mountain), nappes (within the Aenos Mountain) and small foreland basins (Argostoli gulf), all within the margins. In Lefkas Island, the lack of space, very close to the Kefalonia Transform Fault, led to the movement of the Ionian Basin over the margins, attempting to overthrust the Apulian Platform. Because the obstacle between the basin and the platform was very large, the moving part of the Ionian Basin strongly deformed producing nappes and anticlines in the external part of the Ionian Basin, and a very narrow foreland basin (Ionian Thrust foreland basin).

Characteristics of Recent Aftershocks Sequences (2014, 2015, 2018) Derived from New Seismological and Geodetic Data on the Ionian Islands, Greece

In 2014–2018, four strong earthquakes occurred in the Ionian Sea, Greece. After these events, a rich aftershock sequence followed. More analytically, according to the manual solutions of the National Observatory of Athens, the first event occurred on 26 January 2014 in Cephalonia Island with magnitude ML = 5.8, followed by another in the same region on 3 February 2014 with magnitude ML = 5.7. The third event occurred on 17 November 2015, ML = 6.0 in Lefkas Island and the last on 25 October 2018, ML = 6.6 in Zakynthos Island. The first three of these earthquakes caused moderate structural damages, mainly in houses and produced particular unrest to the local population. This work determines a seismic moment tensor for both large and intermediate magnitude earthquakes (M > 4.0). Geodetic data from permanent GPS stations were analyzed to investigate the displacement due to the earthquakes.

Middle Helladic Tombs at Nydri Plain, Lefkas Island. An Archaeological and Paleoanthropological Study

The prehistoric record of Lefkas and the smaller neighboring islands is fairly extensive. The oldest archaeological material dates back to the Middle Palaeolithic period. The Neolithic period is also attested by archaeological finds in five caves. Even though Early (EH) and Middle Helladic (MH) periods have been known since the 1920s due to Dőrpfeld’s excavations, the archaeological data from the Late Bronze Age are scarce. A small Mycenaean tholos tomb has been excavated at Agios Nikitas, while several LH tumuli have been unearthed at the neighboring Meganissi Island.

Cephalonia-lefkas Transform Fault Zone (CLTFZ) Complexity: Insights from 2015 Lefkas Earthquake Sequence

In order to define a better model for the Cephalonia-Lefkas Transform Fault Zone the sequence of 2015 Lefkas earthquake was examined. On 17 November 2015 (07:10 GMT) a major earthquake (Mw=6.4) occurred on the central-western part of Lefkas island. Several destructive events were located in the past in this fault zone, so an extensive seismotectonic study is feasible for that area. Manual analysis was performed using a custom velocity model that was determined for that purpose, applying the average travel-time residuals and location uncertainties errors minimization method. Several clusters belonging to the aftershock sequence were identified, whereas three are directly related to the causative fault, covering an area of about 25 km. The central one, which includes the mainshock, comprises of only a few aftershocks. The northern, within which the majority of aftershocks are located, lies in the central part of Lefkas island and the southern occurred close to the SW edge of the island. In addition, offshore clusters with distinct characteristics have been identified to the south, between Lefkas and Cephalonia islands. The temporal evolution of the aftershock sequence indicates that no migration was observed, given that after the occurrence of the mainshock the entire epicentral area was activated. Focal mechanisms of the Seismological Laboratory of the University of Athens showed dextral strike-slip faulting for both mainshock and major aftershocks of the sequence. Taking into account the spatial distribution of the aftershocks, supported by the tectonic and geomorphological settings of the region, a deformation pattern, consisting of the Cephalonia-Lefkas and Ithaca-Lefkas major fault zones which converge in the area of Vassiliki bay is proposed. The appearance of the southernmost clusters was interpreted by the positive Coulomb stress changes transfer due to major earthquake Mw=6.4.

On the Lefkas (Ionian Sea) November 17, 2015 Mw=6.5 Earthquake Macroseismic Effects

ABSTRACT We exploit macroseismic observations and instrumental data aiming at explaining the effects of the 17th November 2015 Mw=6.5 earthquake, occurred beneath the southwestern peninsula of Lefkas Island (Ionian Sea) causing two casualties, major geo-environmental and slight-to-moderate structural effects. The spatial distribution of the structural damage of the local building stock is well correlated with the deformation pattern deduced from satellite geodesy, it appears though considerably low with respect to the ground deformation. Comparison with the previous strong earthquake on 14.8.2003 with Mw=6.2 occurred about 20 km to the north, shows that structural damage was significantly lower during the recent quake and also manifests good behaviour of the local buildings. This is partly explained by the characteristics of the ground motion and primarily explained by the unique concepts applied to the non-engineered buildings of Lefkas to resist ground motions.

The 17 November 2015 Mw 6.4 Lefkas (Ionian Sea, Western Greece) Earthquake: Impact on Environment and Buildings

On Tuesday, November 17, 2015 at 07:10:07 (UTC) a strong earthquake struck Lefkas Island (Ionian Sea, Western Greece) with magnitude Mw 6.4, depth of about 6 km and epicenter located 20 km southwest of Lefkas town. It was felt in Lefkas Island and the surrounding region and caused the death of two people, the injury of eight others, many earthquake environmental effects (EEE) and damage to buildings and infrastructures. Secondary EEE were observed in western Lefkas and classified as ground cracks, slope movements and liquefaction phenomena. Primary effects directly linked to surface expression of seismogenic source were not detected in the field. The maximum intensity VIIIESI 2007 was assigned to large-volume slope movements along western coastal Lefkas. Damage to buildings was mainly observed in villages located in Dragano-Athani graben arranged almost parallel to the northern segment of the Cephalonia Transform Fault Zone (Lefkas segment). Among structures constructed with no seismic provisions, the stone masonry buildings and monumental structures suffered most damage, while the traditional buildings of the area with dual structural system performed relatively well and suffered minor damage. Reinforced-concrete buildings were affected not so much by the earthquake itself but by the generation of secondary EEE. The maximum seismic intensities VIIIEMS-98 were assigned to villages located in Dragano-Athani graben due to very heavy structural damage observed on masonry buildings mainly attributed to the combination of the recorded high PGA values, the poor antiseismic design and construction of buildings and the geological and tectonic structure of the affected area.

FIELD OBSERVATIONS OF THE 2015 (NOVEMBER 17, MW 6.4) LEFKAS (IONIAN SEA, WESTERN GREECE) EARTHQUAKE IMPACT ON NATURAL ENVIRONMENT AND BUILDING STOCK OF LEFKAS ISLAND

On Tuesday, November 17, 2015 at 07:10:07 (UTC) a strong earthquake struck Lefkas Island (Ionian Sea, Western Greece) with magnitude Mw 6.4, depth of about 7 km and epicenter located 20 km southwest of Lefkas town. The seismic activity in the region was essentially expected after the 2003 Lefkas earthquake and the 2014 Cephalonia earthquakes. Between these earthquake affected areas, the tectonic stresses were not released after the 1948 Lefkas earthquakes. Instead, they were intensifying and accumulating until the generation of the 2015 event. It was felt in Lefkas and the surrounding region and caused the death of 2 people, the injury of 8 others, earthquake environmental effects (EEE) and damage to buildings and infrastructure. Secondary EEE were observed in western Lefkas and included ground cracks, slope movements and liquefaction. Primary effects were not detected in the field. Buildings damage were mainly observed in villages of Dragano-Athani graben in southwestern Lefkas. Among structures constructed with no seismic provisions, the stone masonry buildings and monumental structures suffered most damage, while the traditional buildings with dual structural system performed relatively well. Reinforced-concrete buildings were affected not so much by the earthquake itself but by the generation of secondary effects.

CALCAREOUS NANNOFOSSIL BIOSTRATIGRAPHY OF LANGHIAN DEPOSITS IN LEFKAS (IONIAN ISLANDS)

Spanochorion composite section (NE Lefkas) and Aghios Vassilios section (W Lefkas) have been studied semiquantitatively for their calcareous nannofossil content. The calcareous nannofossil biostratigraphy, performed at the Paxos unit atypical flysch sediments of Aghios Vassilios section, permits their biostratigraphic correlation with the uppermost part of NN4 biozone and the lower part of NN5 biozones (Martini, 1971) or MNN4b and MNN5a (Fornaciari and Rio, 1996), which chronostratigraphically point to Early Langhian age and range between 15.974 and 14.53 Ma. Τhe most interesting biostratigraphic result is the detection of the Sphenolithus heteromorphus Paracme End at the base of Aghios Vassilios section, a biohorizon that marks the basal Langhian deposits in the Mediterranean region. The mollasic sediments of Spanochorion section are more or less of the same age, ranging between 15.5 and 14.53 Ma. The biostratigraphic results of the present study reinforce the hypothesis that the molassic sediments of NE Lefkas Island behind the Ionian thrust were isochronously deposited with Paxos atypical flysch sequences deposited in front of Ionian thrust, at least until Early Langhian times.

Contingent Valuation Method (CVM) to Measure the External Cost From the Pollution of the Lagoon of Lefkas Island in Greece

Contingent Valuation Method (CVM) to Measure the External Cost From the Pollution of the Lagoon of Lefkas Island in Greece

Seismic damage scenarios in Lefkas old town (W. Greece)

Damage scenarios were developed for the Lefkada old town (LOT). LOT is the historical center of the capital of Lefkas Island, part of the Ionian Islands, one of the most seismically prone areas within the Mediterranean region that suffered numerous devastating earthquakes in the past. The most recent strong earthquake with Mw = 6.2 occurred on 14 August 2003 at a distance of about 13 km from LOT. A peak ground acceleration (PGA) equal to 0.42 g was recorded in LOT, one of the highest values ever recorded in Greece. Nevertheless, the 2003 event produced limited damage to the buildings of LOT, comprising traditional construction practices of architectural significance and a high seismic behaviour. Towards the development of damage scenarios the following tasks were performed: (a) buildings inventory and vulnerability indices determination using EMS-98 along with behaviour modification scores, (b) subsoil response functions and 1D visco-elastic models calculation using microtremors and (c) stochastic PGA simulation using site amplification deduced from the subsoil response functions. Two scenarios were developed considering the sources of the 2003 M6.2 earthquake and a future M7 event, located at the same fault. The discrete damage probability was resolved by formulating a beta distribution of an average damage grade related to the vulnerability index and the simulated PGA through empirical equations. The deduced models are comparable with the observed 2003 damage distribution, hence they are likely useful for preparing future emergency plans. In the aftermath, although further investigation is needed to explain outliers, the implementation of the followed methodology into an automated procedure for near real time shake and damage maps generation in case of a seismic crisis is highly recommended. The work herein, with proper adaptations, is potentially fairly applicable for other towns and regions in Europe.

Morphotectonic characteristics of Lefkas Island during the Quaternary (Ionian Sea, Greece)

This paper focuses on the study of the Quaternary morphotectonic evolution of the Lefkas Island using morphological and tectonic data. Based on our mainly morphological studies, two principal tectonic structures have been distinguished: (i) the Nydri-Vasiliki Fault Zone (NVF) and (ii) the Ionian Zone overthrust onto the Paxos Zone, which both divide the island into three morphotectonic units of Karya (KMTU), Maradochori (MMTU) and Athani (AMTU). Paleogeographically, it has been suggested that the broader Lefkas area was dominated by a NE–SW to ENE–WSW compressional regime since the early Pliocene that continuous in our days, resulting in the uplift of the southwestern part of the island and the submergence of the northeastern part of the island. This deformation is characterized by principal fault structures of a N110°–130° strike. These structures, combined with a fault system of N40°–60° strike representing an older Miocene fault zone, control the main morphological characteristics such as watersheds and streams and are responsible for the KMTU formation producing a northeastward plunging monoclinal en-echelon structure. During the Early Pleistocene, a shift of the compressional regime towards the east was responsible for the formation of a N70°–90° strike fault system. These faults had a morphotectonic effect throughout the island, controlling the watersheds and stream evolution. The continuous compressional regime recorded in our days results in the formation of fault structures, striking N20°–40° and N170°–190° that bound almost exclusively the coastal area of the island.

Comparison of Site-Effects Estimation Methods Using the Lefkas, Greece, 2003 Earthquake Aftershocks

Abstract This study aims at the comparison of different site-effect estimation techniques. We used aftershock records of the 14 August 2003 ( M w 6.2) earthquake that occurred in the western part of Greece next to Lefkas Island. A data set of 15 events recorded at five stations was selected for the period of 20 August 2003–30 September 2003. The moment magnitudes range between 3.3 and 4.2, and the hypocentral distances vary from 5 to 34 km. The site effects are determined using the standard spectral ratio (SSR) method, the horizontal-to-vertical spectral ratio for earthquake data method (the so-called receiver function [RF] method), and an inversion method for simultaneous source, path, and site-effects estimation. These methods are applied to both P and S waves, using exactly the same time windows in order to compute the spectra. Finally, using geotechnical information available for the city of Lefkas, the experimental site transfer functions at each site are compared to theoretical one-dimensional 1D transfer functions computed using Kennett’s reflectivity coefficient method. We show that the SSR method and the inversion method give similar site transfer functions. However, the experimental SSR fits even better with the spectral ratios of the inversion results, which indicates that our reference site is affected by a small site effect. Moreover, the RF method results fit well with the horizontal-to-vertical ratios coming from the inversion results, indicating in this case a nonnegligible amplification on the vertical component. This is the reason why the RF gives only the frequency of amplification but not the amplitude. These conclusions hold for both P and S waves. Finally, the site effects computed using the whole record or a window including only S waves are similar, while the P waves results, although showing some similar features with those for S waves, give different transfer functions for the sites investigated.

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.

Seismic response of traditional buildings of Lefkas Island, Greece

This paper deals with the seismic response of the traditional buildings of Lefkas (also called ‘Lefkada’) Island in western Greece. These structures, with two up to four storeys, are met mainly in the old town of Lefkas, the capital of the island. The special structural characteristic of the traditional buildings of Lefkas (that is the issue of this article) is the dual load-carrying system used on ground floor level to handle vertical loads and seismic actions. The first main load-carrying system consists of single-storey stone masonry walls, while the second main load-carrying system consists of multistorey wooden frames at the inner perimeter of the ground-floor masonry walls. Therefore, the load-bearing system of the upper storeys, over the ground floor, consists of a wooden 3D frame with diagonal trusses that are infilled by walls of bricks and lime mortar. There is, also, a tiled wooden roof. During the earthquake of August 14th, 2003 ( M = 6.2 ) , partial collapse of the masonry walls took place in some cases at the low storey and some cracks around the openings were observed. In the upper floors, the load-bearing wooden frames did not suffer damage, but the brick infill cracked and out-of-plane falls were observed. In this paper, the analytical investigation of the seismic behaviour of such structures is developed, the identification of the main way of vibration of these systems is achieved, and both the explanation of the observed damages and the interpretation of the reasons for the development of these damages are performed. Finally, the results of a representative numerical model of a typical traditional building—from a series of examined models—with dual bearing system are presented and discussed, using as base excitation the two horizontal recorded components of the main earthquake ground motion that took place on August 14th, 2003, at Lefkas town, Greece.

Origin of barite in Tertiary marine sedimentary rocks from Lefkas Island, Greece

Siliceous and calcareous biogenic rocks host pervasive, although volumetrically minor, authigenic barite on Lefkas Island, Greece. The barite composes up to 10 percent of the carbonate-free fraction of samples. In decreasing order of abundance, authigenic barite occurs as infilled foraminifera and radiolarian tests, disseminated grains and aggregates in porcelanite and siliceous limestone, replaced siliceous and calcareous biogenic tests, and thin laminae and lenses. The laminae form anastomosing networks that were probably originally rich in organic matter. Barite abundance shows a good correlation with the abundance of skeletal biogenic components. Biogenic silica diagenesis produced opal-CT and clinoptilolite, which compose the cement of porcelanite, replaced radiolarians, diatoms, and echinoderms and filled in some foraminifera tests. Subsequent silica diagenesis produced chalcedony at deeper stratigraphic levels, which also filled in pore space and calcareous microfossils in siliceous limestones and formed chert beds. Textural evidence shows that barite mineralization postdated opal-CT formation and preceded chalcedony formation. Barite precipitated in microenvironments including microfossil chambers, fecal pellets, and areas rich in organic matter. The source of the barium was probably from the decomposition of organic matter derived from plankton and bacteria and from the diagenesis of silica and carbonate tests. The source of the sulfate ion was seawater; possibly additional sulfate was produced and utilized in microenvironments from degradation of organic matter.

Paleomagnetic results from the Pindos, Paxos, and Ionian zones of Greece

Maastrichtian to Miocene sediments were sampled at 12 sites in central Greece (Meso-Hellenic basin, Pindos zone and Western Thessaly unit) and four sites on Lefkas island (Paxos and Ionian zones). The samples were subjected in the laboratory to thermal and alternating field (AF) cleaning. As a result, characteristic remanent magnetizations (ChRMs) were obtained for 11 sites. In comparing these paleomagnetic directions with others already obtained for the external Hellenides, the following points are noteworthy. 1. (1)|In post-Alpine times a slight clockwise rotation seems to be common to the zones of the external Hellenides in Western Greece; 2. (2)|independent movement of the Pindos flysch and the Ionian Paleogene sediments is plausible as the sampled sites from the former exhibit large counter-clockwise declination rotation with respect to the latter; 3. (3)|the net rotation of the Mesozoic sediments of both Pindos and Ionian zones is similar. These observations may indicate that the two zones rotated independently as ‘blocks’ before the Miocene, for the detachment of the Paleogene from the Mesozoic sediments is opposed on geological grounds. Thus the similar declinations observed for the Mesozoic sediments of the Pindos and Ionian zones must be regarded as coincidence.