Paliki Peninsula Research Articles

Active shortening and aseismic slip along the Cephalonia Plate Boundary (Paliki Peninsula, Greece): Evidence from InSAR and GNSS data

We present a comprehensive analysis of geodetic data, including InSAR and GNSS, to assess the interseismic deformation of the Paliki peninsula in western Cephalonia, Greece. The region is prone to frequent earthquakes, due to its proximity to the Cephalonia Transform Fault (CTF), a 140 km long dextral strike-slip fault (striking NNE-SSW) that accommodates the relative motion between the Apulian and Aegean lithospheric plates. Our analysis covers the period from 2016 to 2022 and leverages LiCSBAS, an open-source package, for InSAR time series analysis with the N-SBAS method. The results of the InSAR analysis demonstrate deformation rates between 2 and 5 mm/yr in the line-of-sight (LOS) direction of the satellite. We constructed E-W velocity profiles that provided a velocity gradient terminating against the outcropping trace of the seismic fault of the 3 February 2014 earthquake (M6.1) near Atheras. This geodetic evidence indicates a ∼ 1 mm/yr minimum aseismic slip (fault creep) motion along the February 2014 seismic fault, during 2016–2022. Positive LOS values in both satellite imaging geometries show that the coastal town of Lixouri experiences uplift of 1 mm/yr. East-West velocity cross-sections across western Cephalonia including Gulf of Argostoli reveal several velocity discontinuities, possibly bounded by active faults and/or landslides. The E-W shortening rate between Lixouri and Argostoli areas amounts to 1.5 mm/yr corresponding to −187 ns/yr (nanostrain/yr) of tectonic strain. Our results suggest a complex deformation pattern on the Paliki peninsula with strain accumulation along strike-slip and reverse-slip faults. We also inverted GNSS velocities from Italy and Greece, across the Cephalonia segment of CTF (assuming elastic half-space) and obtained a locking depth of 13 km, and a slip rate of 17.3 ± 0.8 mm/yr.

Quantative calcareous nannofossil biostratigraphy of Bay Akrotiri section (Cefallinia island, W. Greece). Tracing the gephyrocapsid size-trend in an early Pleistocene terrigenous sequence

Bay Akrotiri section represents a thick terrigenous sequence laying along the southern part of Paliki peninsula (southwest Cefallinia). The data of the calcareous nannofossil quantitative analysis indicate that the studied sediments are ranging in terms of age between 1.95 Ma and 1.238 Ma approximately. The provided distribution patterns clearly depict an apparent evolutionary lineage close to the Plio/Pleistocene boundary, from small Gephyrocapsa placoliths to larger forms of gephyrocapsids.

The Mw6.5 17 November 2015 Lefkada (Greece) Earthquake: Structural Interpretation by Means of the Aftershock Analysis

The 2015 Mw6.5 Lefkada main shock occurred at the south western part of Lefkada Island (Greece), less than 2 years after the occurrence of a doublet along the western part of the nearby Kefalonia Island, Paliki peninsula (on 25/01/2014, with Mw6.1 and 03/02/2014 with Mw6.0) and 12 years after the 2003 Mw6.2 main shock that struck the northwestern part of Lefkada Island. The four failed dextral strike slip fault segments belong to the Kefalonia transform fault zone (KTFZ), the major active boundary that bounds from the west the area of central Ionian Islands, namely Lefkada and Kefalonia. It is associated with several known historical earthquakes and is considered the most hazardous area in the Greek territory. The KTFZ fault segments are characterized by high slip rates (of the order of tens of millimeters per year), with maximum earthquake magnitudes up to 6.7 for Lefkada and 7.2 for Kefalonia fault zone, respectively. The double difference location technique was employed for relocating the aftershocks revealing a seismogenic layer extending from 3 to 16 km depth and multiple activation on well-defined fault planes, with strikes that differ than the main rupture and dips either to east or to west. This implies that strain energy was not solely released on a main fault only, but on secondary and adjacent fault segments as well. The reliable definition of their geometry forms the basis for the structural interpretation of the local fault network. The aftershock spatial distribution indicates three main clusters of the seismic activity, along with activation of smaller faults to an extent of more than 50 km. A northeasterly striking cluster is observed to the north of the main shock epicenter, with a remarkable aftershock density. The central cluster is less dense than the previous one with an epicentral alignment in full accordance with the strike provided by the main shock centroid moment tensor solution, and is considered as the main rupture with a length of 17 km. The third cluster, encompassing a large number of aftershocks, is located in the offshore area between Lefkada and Kefalonia Islands with a NE–SW epicentral alignment, alike the first cluster. The northeast–southwest striking secondary faults positioned obliquely and in continuation of the main fault segment, reveal that the KTFZ is being deformed in a complex tectonic setting. The presence of faults with this geometry implies strain partitioning and sheds light to new components necessary to be taken into account in the seismic hazard assessment. Stress transfer models of the M ≥6.0 main shocks were investigated and the calculated static stress changes may well explain their sequential occurrence. Static stress changes due to the 2015 coseismic slip were also calculated with the main objective of exploring the aftershock occurrence pattern and it was found as the driving mechanism that triggered the vast majority of the off-fault aftershocks.

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, Mw6.1, Mw5.9) and 2015 (Lefkas, Mw6.4) 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.

GEOMORPHIC EVOLUTION OF WESTERN (PALIKI) KEPHALONIA ISLAND (GREECE) DURING THE QUATERNARY

Kephalonia Island is located in the Ionian Sea (western Greece). The active subduction zone of the African lithosphere submerging beneath the Eurasian plate is placed just west of the island. The evolution of the island is depended mostly on the geodynamic processes derived from this vigorous margin. The geomorphic evolution of the western part of the island (Paliki peninsula) during the Quaternary was studied, by carrying out detailed field geomorphological mapping focusing on both coastal and fluvial landforms, utilizing aerial photos and satellite image interpretation with the use of GIS technology. The coastal morphology of Paliki includes beachrocks, aeolianites, notches and small fan deltas which were all studied and mapped in detail. The drainage systems of the peninsula comprise an older one on carbonate formations in the northwest and a younger Quaternary one in the south and southeast. Eight marine terraces found primarily on the Pliocene marine formations range in elevation from 2 m to 440 m are tilted to the south. In the Late Pleistocene some of the main drainage networks flowed towards the newly-formed gulf of Argostoli to the east.

Evaluation of the macroseismic intensities triggered by the January/February 2014 Cephalonia, (Greece) earthquakes based on ESI-07 scale and their comparison to 1867 historical event

On January 26th and February 3rd, 2014, two earthquakes occurred onshore the island of Cephalonia inducing severe environmental effects. The main types of earthquake-induced failures were liquefaction and slope failures and were mainly widespread at the western and central part of the island at Paliki peninsula. Although the fact that the generated strong motions by the second event were the highest ever recorded in Greece, medium severity structural damages were induced. Few days after the occurrence of the events, two post-earthquake field surveys took place in order to document the generated environmental effects. By taking into account these field observations, it was decided to evaluate the macroseismic intensity of the February 3rd, 2014 earthquake by applying the ESI-07 scale. As an outcome, the epicentral intensity of Feb. 3rd, 2014 was evaluated as VII ESI-07 by taking into account the total affected area. Furthermore, as it is pointed out in primary contemporary sources and seismic catalogues, similar environmental failures such as liquefaction phenomena at coastal areas and large scale rockfalls were triggered by historical earthquakes at the island of Cephalonia. The most accurately described effects are the ones triggered by the 1867 earthquake. Therefore, it was decided to assess the macroseismic intensities of this historical event based on the ESI-07 scale and compare it with the relevant ones of Feb 3rd, 2014 event. As an outcome, it is concluded that similar intensities were assessed for both events, VIII ESI-07 for 1867 event and VII ESI-07 for the February 2014 one, despite the fact that the relevant earthquake magnitudes strongly differ. Outlining the results of this study, it is reconfirmed the importance of the recording and documentation of earthquake environmental effects in order to maintain the consistency with the historical earthquake catalogues. Moreover, a re-evaluation of historical earthquake magnitudes at the area of Cephalonia island could be realized in order to accurately update the seismic hazard.

Fault zones ruptured during the early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquakes (January 26 and February 3, Mw 6.0) based on the associated co-seismic surface ruptures

The early 2014 Cephalonia Island (Ionian Sea, Western Greece) earthquake sequence comprised two main shocks with almost the same magnitude (moment magnitude (Mw) 6.0) occurring successively within a short time (January 26 and February 3) and space (Paliki peninsula in Western Cephalonia) interval. Each earthquake was induced by the rupture of a different pre-existing onshore active fault zone and produced different co-seismic surface rupture zones. Co-seismic surface rupture structures were predominantly strike-slip-related structures including V-shaped conjugate surface ruptures, dextral and sinistral strike-slip surface ruptures, restraining and releasing bends, Riedel structures (R, R′, P, T), small-scale bookshelf faulting, and flower structures. An extensional component was present across surface rupture zones resulting in ground openings (sinkholes), small-scale grabens, and co-seismic dip-slip (normal) displacements. A compressional component was also present across surface rupture zones resulting in co-seismic dip-slip (reverse) displacements. From the comparison of our field geological observations with already published surface deformation measurements by DInSAR Interferometry, it is concluded that there is a strong correlation among the surface rupture zones, the ruptured active fault zones, and the detected displacement discontinuities in Paliki peninsula.

Rupture process of the 2014 Cephalonia, Greece, earthquake doublet (Mw6) as inferred from regional and local seismic data

We study the 26 January and 3 February, 2014 (~Mw6) events in Cephalonia, combining weak and strong motion waveforms from regional and local stations. The hypocenter of the January 26 event is located at the southernmost tip of the Paliki Peninsula, at a depth of ~15km. The centroid moment tensor (CMT) solution indicates rupture along a N20°E dextral strike-slip fault, dipping to the east. The hypocenter of the February 3 event is 10km NNE of the first, at shallower depth (~5km). The CMT solution of this event is highly uncertain. The kinematic slip model for the January 26 event indicates that the rupture was mainly confined to shallow depths, and it propagated upwards and towards NE. The major slip patches, when projected to the surface, cover the western part of the Paliki Peninsula and include the areas where surface ruptures were observed. Our preferred slip model for the event of February 3 is based on a published two-segment fault model. Although this is our preferred slip model, it is worth noting, that the single segment inversion provided a similar slip pattern. The rupture propagated predominantly southwards along both segments. The main slip episode on both segments occurred almost simultaneously. Total duration of the rupture propagation did not exceed 9 and 6s, respectively. The 2014 Cephalonia doublet did not rupture the Cephalonia Transform Fault (CTF). The diffuse pattern of the aftershocks implies the activation of a network of faults on-shore the Paliki Peninsula, in accordance with the local stress field derived from aftershocks. The 2014 sequence has implications for the seismic hazard assessment: active faults in western Cephalonia exist on-shore; some have gentle dip angles; the strike-slip motions can be combined with thrust components; and the segmented ruptures may introduce time delays that increase the duration of strong ground shaking.

Earthquake environmental effects and ESI 2007 seismic intensities of the early 2014 Cephalonia (Ionian Sea, western Greece) earthquakes (January 26 and February 3, Mw 6.0)

The early 2014 Cephalonia (Ionian Sea, western Greece) earthquake sequence comprised two main shocks with almost the same magnitude (Mw 6.0) occurring successively in short time (January 26 and February 3) and space (western Cephalonia, Paliki peninsula). Two different almost parallel NE–SW striking and SE dipping at a steep-angle dextral strike-slip fault zones with small reverse component corresponded to the two events and were activated onland in Paliki. Many earthquake environmental effects (EEE) were induced by both earthquakes in Paliki offering the possibility to apply the ESI 2007 scale. They are classified into primary and secondary effects. Primary effects comprise uplift, subsidence and surface ruptures. Secondary effects include ground cracks, slope movements, liquefaction and hydrological anomalies. The VIIIESI2007 intensities are assigned to sites of maximum uplift in the central–eastern part of Paliki. The VIIESI2007 intensities are assigned to sites with maximum subsidence, surface ruptures and large-volume slope movements in the eastern coastal zone of Paliki, the northern part of Paliki and the northern part of Argostoli peninsula, respectively. The VIESI2007 intensities are assigned to sites with ground cracks and slope movements mainly in the northern part of Paliki and the western part of Aenos Mt, respectively. The lowest VESI2007 intensities are observed in the southern part of Paliki associated with small-volume slope movements. From the comparison of all data, it is concluded that there is a strong correlation between the active faults, the displacement discontinuities detected from already published interferometric analysis and the spatial distribution of the EEE.

Fault modelling of the early-2014 ~ M6 Earthquakes in Cephalonia Island (W. Greece) based on GPS measurements

Modelling of ground deformation was performed based on GPS measurements that were collected in the western part of Cephalonia (Paliki Peninsula) which had experienced two moderate magnitude earthquakes in early 2014. Both events of magnitudes Mw=6.1 (Jan. 26) and Mw=5.9 (Feb. 3) had the same right-lateral character of motion with a small thrust component, but different focal depth of 16km and 5km, respectively. The GPS results show large amplitudes of displacement in both horizontal (6–40cm) and vertical (8–15cm) components in the vicinity of Paliki for the period 2010–2014, which is attributed primarily to those earthquakes. Various scenarios of activated faults were considered to model the observed ground deformation that was associated with those events. The most plausible case that accounts for a realistic and best-fit approximation of the measured displacement vectors has a solution of two activated faults of SSW–NNE trending direction that are separated by a small offset. Consistent with the lack of surface exposure of the seismogenic faults, the best-fit model assumes two easterly dipping fault planes at burial depths of 0.8km and 0.5km and at angles of 72° and 65°, respectively. The GPS deduced models clearly resolved the plane ambiguities associated with the focal mechanism solutions of the two major events. A small “normal-fault” component along the upper part of the fault planes should be considered to further improve the best-fit approximation of the observed vertical deformation. It was also found that a non-negligible deformational component from aftershocks is inherent in the observed data mainly from Paliki.

The February 2014 Cephalonia Earthquake (Greece): 3D Deformation Field and Source Modeling from Multiple SAR Techniques

Online Material: Tables of moment tensor elements; SAR image pairs and coseismic displacement gradients; figures of InSAR data, model parameter tradeoffs, and observed and modeled displacements. On 26 January 2014 at 13:55 UTC, an M w 6.0 earthquake struck the island of Cephalonia, Greece, followed five hours later by an M w 5.3 aftershock and by an M w 5.9 event on 3 February 2014 at 03:08 UTC (National Observatory of Athens, Institute of Geodynamics [NOA‐GI]). The epicenter of the M w 6.0 event was relocated 2 km east of the town of Lixouri, and that of the M w 5.9 event at the tip of the Gulf of Argostoli, in the northern part of the Paliki peninsula (Fig. 1; Karastathis et al. , 2014; Papadopoulos et al. , 2014). Extensive structural damage and widespread environmental effects were induced throughout the Paliki peninsula and along the eastern coast of the Gulf of Argostoli (Valkaniotis et al. , 2014). Quays, sidewalks, and piers were damaged in the waterfront areas of the towns of Lixouri and Argostoli, the island capital, and liquefactions, road failures, rock falls, and small landslides were observed. Most of the latter effects took place in the aftermath of the 26 January 2014 event and were reactivated one week later by the 3 February earthquake. Figure 1. Location map showing the tectonic context of the Aegean Sea and the main faults on the island of Cephalonia. Colored rectangles and magenta triangles indicate the Synthetic Aperture Radar (SAR) frame coverage and the continuous Global Positioning System (cGPS) stations, respectively. Yellow stars indicate the relocated epicenters of the three largest events of the 2014 sequence (Karastathis et al. , 2014). Diverging and converging arrows in the top‐right map indicate approximate horizontal projections of the P axis (compression) and T axis (extension), respectively, based on figure 5b in …

Field observations of geological effects triggered by the January–February 2014 Cephalonia (Ionian Sea, Greece) earthquakes

On Jan. 26 and Feb 3, 2014 Cephalonia Island, Ionian Sea, Greece, was struck by two strong, shallow earthquakes (NOA local magnitudes ML5.8 and ML5.7, respectively). The earthquakes ruptured two sub-parallel, strike-slip faults, with right-lateral kinematics. During both earthquakes, ground-shaking phenomena such as liquefaction, road failures, rock falls, small/medium size landslides and stonewall failures were widespread all over the western part of the island. No primary fault surface ruptures were observed, however one patch of the rupture of the second event may have nearly reached the surface, because of the abundance of surface cracks with cm-size offsets mapped in the field in north Paliki Peninsula. Liquefaction phenomena were mainly triggered in reclaimed lands in the waterfront areas of Argostoli and Lixouri inducing structural damages to quays, sidewalks and piers. We also present evidence for liquefaction-induced ground disruption in the recently constructed marina of Argostoli, which is situated at the opposite (eastern) shore to the town.

Locating Ithaca: Continuing the search for Odysseus' island kingdom

Many assume that Ithaki, one of the Ionian Islands, is the site of ancient Ithaca, home of Odysseus, the Greek hero of the Trojan War. However, some experts claim that the Paliki Peninsula, on nearby Kefalonia, conforms more closely to descriptions of Odysseus' homeland and, crucially, was once itself an island.

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.

Biostratigraphical and paleoenvironmental determination of a marine Plio/Pleistocene outcrop in Cefallinia Island (Greece)

Lofos section is located in the southern part of Paliki peninsula in southwest Cefallinia island. It consists of 15-20 m of homogenous bluish marls with macrofossils and coarse grained alternations, followed by almost 10 m of sandy marls. The data from the quantitative biostratigraphic analysis based on calcareous nannofossils of the studied material allow the correlation of the lower part of Lofos section with MNN19a biozone which chronostratigraphically points to an uppermost Pliocene. At about 16 m from the base of the section, it is possible to recognize the MNN19a/MNN19b biozones boundary which points to the Pliocene/Pleistocene boundary. The rest of the section can be assigned to Pleistocene. Finally, a paleoenvironmental reconstruction and determination of the paleoenvironmental parameters is attempted based on the quantitative, paleoecological analysis of the benthic foraminifera. The paleoecological data indicate that the lower part of the studied sequence have been deposited in waters with lowered oxygen levels. Stagnant conditions became less severe in the course of deposition.