1 Introduction

The Cephalonia Island earthquake sequence early in 2014 comprised two main shocks with almost the same magnitude occurring successively within a short time and space interval; therefore, one of which cannot be considered as a foreshock or an aftershock of the other. The first earthquake struck on January 26 at 13:55:42 GMT (15:55:42 local time) and was assessed as moment magnitude (Mw) 6.0 (National Observatory of Athens (NOA)) or Mw 6.1 (University of Athens (UOA), Harvard (HARV), Istituto Nazionale di Geofisica e Vulcanologia (INGV), Aristotle University of Thessaloniki (AUTH), Helmholtz Centre Potsdam (GFZ)) (Fig. 1). It was felt predominantly on Cephalonia and throughout the Ionian Islands, the western continental Greece, and the Peloponnese. It was also felt in the southern part of Italy and in Albania. Fortunately, no fatalities or serious injuries were reported. The western part of Cephalonia and particularly Paliki peninsula suffered most damage in masonry and reinforced concrete buildings, stonewalls, road network infrastructures, port facilities, and monumental structures including monasteries, churches, and cemeteries (Lekkas et al. 2014a, b; Papadopoulos et al. 2014). Atheras village was evacuated in order to avoid rockfall risk during the aftershock activity. The rest part of the island was only slightly affected. It is significant to note that no building underwent total collapse due to the first earthquake. Based on macroseismic field surveys and the European Macroseismic Scale (EMS) applied by Papadopoulos et al. (2014), the maximum EMS intensities of the first event reached VI–VII and were assigned to Paliki peninsula, the western part of Aenos Mt., and the northwestern part of Argostoli peninsula.

Fig. 1
figure 1

Geological map of Cephalonia based on Lekkas (1996) and Lekkas et al. (2001). Field observations including primary earthquake environmental effects (surface ruptures) induced by the early 2014 Cephalonia earthquakes, seismological data (epicenters and focal mechanisms), and rose diagrams displaying azimuth of the mapped surface ruptures in Paliki peninsula are also shown. NAFZ North Anatolian fault zone, CTFZ (CS) Cephalonia transform fault zone (Cephalonia segment), AEF Agia Efimia fault, KAF Kontogourata-Agon fault, AFZ Aenos fault zone, PF Paliokastro fault, IO Ionian overthrust

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The epicenter of the January 26 earthquake was located in the southeastern part of Paliki peninsula, 7 km W of Argostoli (Fig. 1). Based on data provided by national and international seismological institutes (UOA, HARV, INGV, AUTH, GFZ), the main shock is consistently located at depths of 10–17 km, and the focal mechanisms demonstrate a NNE-SSW trending dextral strike-slip seismic fault zone that dips southeastwards at a steep angle with a small reverse component (Fig. 1).

Eight days after the first earthquake, on February 3, at 03:08:44 GMT (05:08:44 local time), a second earthquake hit the island and was assessed Mw 5.9 (UOA, NOA) or Mw 6.0 (HARV, AUTH, GFZ) (Fig. 1). It was felt in the same areas as the earlier event was. No fatalities or serious injuries were also reported. The second earthquake struck the same area and increased the damage induced by the first event in Paliki peninsula (Lekkas et al. 2014a, b). Based on macroseismic field surveys and the EMS applied by Papadopoulos et al. (2014), the maximum EMS intensities of the second event were bigger than that of the first event reaching VII–IX and were assigned to the eastern part of Paliki peninsula.

The peak ground acceleration (PGA) of the second main event recorded at the Chavriata site (Fig. 1), in the southern part of Paliki peninsula, was 0.77 g, which is larger than that recorded during the first event (0.56 g) (Karakostas et al. 2014a, b). It is significant to note that 0.77 g is the largest PGA value recorded in Greece since the early 1970s, when the first accelerographs were installed at large cities of Greece. A slightly smaller PGA value of the second main event was measured as 0.68 g at the Lixouri site (Fig. 1), in the eastern coastal part of the Paliki peninsula (Karakostas et al. 2014b).

The epicenter of February 3 event was located in the northwestern part of Paliki peninsula, 17 km NW of Argostoli (Fig. 1). The provided focal mechanism by national and international seismological institutes (UOA, HARV, INGV, AUTH, GFZ) demonstrated depths around 10 km and a NE-SW trending dextral strike-slip fault dipping southeastwards at a steep angle (Fig. 1).

The majority of buildings constructed to strict earthquake standards after the catastrophic 1953 Cephalonia earthquake sequence showed good performance during both earthquakes since none of them collapsed and no resident was killed or seriously injured. This result takes on even greater importance when considering that the early 2014 Cephalonia earthquakes presented the largest PGA values ever recorded in Greece, and all building structures in Cephalonia are designed on the basis of a PGA equal to 0.36 g, which is the highest possible value for the behavior factor according to the Greek seismic code.

Many earthquake environmental effects were induced by both earthquakes in the western part of Cephalonia (Valkaniotis et al. 2014; Benekos et al. 2015; Lekkas and Mavroulis 2015). Primary effects included tectonic uplift, subsidence, and surface ruptures (Benekos et al. 2015; Lekkas and Mavroulis 2015), and secondary effects included ground cracks, slope movements, liquefaction phenomena, and hydrological anomalies (Valkaniotis et al. 2014; Benekos et al. 2015; Lekkas and Mavroulis 2015). Based on the ESI 2007 scale applied to the early 2014 Cephalonia earthquakes by Lekkas and Mavroulis (2015), the higher ESI 2007 intensities were assigned to sites with permanent ground dislocations induced by tectonic uplift and subsidence, surface ruptures, and large-volume slope movements in Paliki peninsula, the western part of Aenos Mt., and the northwestern part of Argostoli peninsula, while the maximum VIIIESI 2007 intensity was assigned to sites of maximum uplift in the central-eastern part of Paliki peninsula.

This paper focuses on the co-seismic surface rupture structures and the displacement distributions induced by the early 2014 Cephalonia earthquakes based on direct field observations recorded during our field reconnaissances in the island right after each earthquake (first reconnaissance after the January 26 earthquake and second reconnaissance after the February 3 earthquake). We also investigate the relationship between the surface ruptures and the pre-existing geological structures, which provide important field geological data for further assessment of kinematics of rupturing process during this earthquake sequence.

2 Geology

Cephalonia comprises (a) formations that belong to Paxoi (Pre-Apulian) and Ionian geotectonic units and (b) Plio-Quaternary sediments that lie on the basement formed by Paxoi and Ionian units (Renz 1955; Aubouin 1959; Aubouin and Dercourt 1962; Georgiadou-Dikaioulia 1967; BP Co 1971; BP Co et al. 1985; Stavropoulos 1988; Underhill 1989; Lekkas 1996; Lekkas et al. 2001; Fig. 1). The Paxoi unit forms the major part of Cephalonia and comprises mainly Triassic-Middle Miocene neritic and locally pelagic carbonates and the Middle Miocene-Early Pliocene clay-clastic sequence of flysch type consisting of alternations of marls, clays, and mudstones (Lekkas 1996; Lekkas et al. 2001; Fig. 1). The Ionian unit is the allochthonous tectonic nappe occurring along the eastern part of the island (Fig. 1). It consists of Triassic evaporitic sequence of gypsum beds and limestone breccia and Jurassic-Cretaceous thick-bedded limestones, red nodular limestones, and slates without the flysch at the top of the sequence (Lekkas 1996; Lekkas et al. 2001; Fig. 1) as known from mainland Greece. The western boundary of the Ionian unit is defined by the Ionian overthrust (IO in Fig. 1), which is the most external Hellenide structure. The formations of Paxoi and Ionian units are dissected by major thrust faults (Underhill 1989) (Fig. 1).

The Plio-Quaternary sediments comprise the Pliocene-Calabrian sequence and the Middle-Late Pleistocene-Holocene formations (Lekkas 1996; Lekkas et al. 2001; Fig. 1). The Pliocene-Calabrian sequence is a marine sequence observed in an uplifted coastal zone with width ranging from 2 to 10 km, particularly in the largest part of Argostoli peninsula and the eastern and southern part of Paliki peninsula (Fig. 1). It is developed transgressively over the Paxoi formations with thickness ranging from 200 to 500 m. The lower part of the sequence is composed of oligomictic conglomerates and breccia, limestones, and massive calcarenites of Early Pliocene age. The intermediate part consists of yellowish marls with siltstones, intercalations of sandstones, sands, and conglomerates of Middle-Late Pliocene age. The upper part contains marls with intercalations of sands and coarse regressive calcarenites (BP Co 1971; Lekkas 1996; Lekkas et al. 2001).

The Middle-Late Pleistocene-Holocene formations occur in many locations, mainly in Argostoli peninsula (Fig. 1). They are continental formations and classified into slope breccia, Middle Pleistocene marine calcarenites, Mindel-Riss decalcified clays, Riss scree cones, and Holocene deposits (Lekkas 1996; Lekkas et al. 2001). Recent pieces of formation are scattered in many sites. Red terrestrial clayey sands and coastal conglomerates as well as red calcareous crusts occur in Minia area and WSW of Skala area (Fig. 1). Scree and torrential fans cover the Pliocene formations along the western foot of Aenos Mt. Thin paleosols cover many limestone slopes. Loose fine- and coarse-grained alluvial formations including sands, gravels, and clays are observed in the narrow beaches and the wide valleys of the island.

3 Tectonic setting and seismicity

3.1 Tectonic and neotectonic structures

Cephalonia is located at the northwesternmost part of the Hellenic Arc developed only a few tens of kilometers east of the Hellenic Trench in the Ionian Sea, representing the active plate boundary between the European and African plates, which has a complex history of oceanic and continental subduction (Lekkas 1996; Lekkas et al. 2001; Royden and Papanikolaou 2011; Fig. 1). The tectonic setting of Cephalonia is the result of successive compressional episodes interrupted by periods during which phenomena associated with extensional regime occurred. The main tectonic structures developed during the Alpine deformation phase are strongly related to the overthrusting of the Ionian nappe on the Paxoi nappe and are distinguished to the following: (a) NW-SE and NE-SW striking reverse faults and broad open folds with axes with the same orientation in Paxoi formations. These reverse faults have been considered as pre-existing syn-sedimentary tectonic structures defining several paleogeographic units of the Pre-Apulian platform during the sedimentation phase. Some of these faults [e.g., Agia Efimia fault (AEF in Fig. 1) in the southern part of Kalon Mt. and Aenos fault zone (AFZ in Fig. 1) in the western part of Aenos Mt.] present not only dip-slip but also strike-slip motion; (b) thrusts, ramp anticlines, and duplexes with NW-SE trending axes; and (c) NE-SW and E-W trending listric faults in the Ionian unit. Axial planes and thrusts are parallel to the IO (Fig. 1).

The neotectonic structures comprise faults bounding the neotectonic macrostructures of Cephalonia. These faults have throws ranging from a few to tens of meters and present not only dip-slip but also strike-slip motion. They are classified into reverse, strike-slip, and normal faults.

Five major sets of faults are recognizable based on their geometry. Primary faults strike NW-SE, NNE-SSW, and NNW-SSE while secondary strike NE-SW and E-W (Fig. 1). The first set comprises the NW-SE striking Aenos and Agia Efimia faults (Fig. 1). The second set includes the NNE-SSW striking Kontogourata-Agon and Paliokastro faults (Fig. 1). The third set consists of NNW-SSE striking faults formed in Argostoli peninsula during Pliocene-Pleistocene and some faults in Paliki peninsula (Fig. 1). The last two sets include reactivated pre-existing oblique-normal faults within the Ionian nappe including the Atros fault in Poros area as well as the faults in Fragata, Minia, and Arginia area (Fig. 1). Primary faults are pre-existing syn-sedimentary normal faults formed during Cretaceous-Eocene that have been reactivated and continued as reverse faults since the Eocene-Miocene.

Four sets of faults are recognizable based on their kinematics: (a) pure reverse faults in the Argostoli peninsula cutting Middle-Late Pleistocene formations, (b) oblique reverse faults and thrusts in the western boundary of Aenos Mt. (Aenos fault zone) and in the southern boundary of Erissos peninsula (Agia Efimia fault), (c) strike-slip faults with horizontal striations in Argostoli peninsula, the southeastern part of Cephalonia, and Paliki peninsula, and (d) normal faults in the Arginia graben and within the Ionian nappe (Fig. 1).

3.2 Neotectonic macrostructures

Based on lithology, similar structural features and common evolution during the Late Quaternary, Cephalonia can be divided into the following four major neotectonic macrostructures (Lekkas 1996; Lekkas et al. 2001): (a) the Aenos Mt., (b) the Erissos peninsula, (c) the Paliki peninsula, and (d) the Argostoli peninsula.

The Aenos Mt. is located in the central and eastern part of the island (Fig. 1). It is bounded to the SW by the AFZ (Fig. 1), to the NW by the Kontogourata-Agon fault (KAF in Fig. 1), to the ΝE by the AEF (Fig. 1), and to the SE by the Paliokastro fault (PF in Fig. 1). Aenos Mt. is characterized by a considerable uplift (>1,500 m) and remarkable incision since the Early Pliocene (Sorel 1976; Lekkas 1996). The region around Aenos Mt. is dominated by a major west-verging, asymmetric, NW-SE to N-S trending anticline. Its axis can be traced for more than 20 km from Myrtos in the north, where numerous meso-scale open folds represent its surface trace, to Markopoulo in the southeast.

The Erissos peninsula is located in the northern part of the island (Fig. 1). It is bounded to the SW by the Agia Efimia fault and is also characterized by uplift and erosion. Since Pleistocene and after the deactivation of Agia Efimia fault, the Erissos peninsula presents the same evolution with the Aenos Mt.

The Paliki peninsula is located in the western part of the island (Fig. 1). It is bounded to the west by the Cephalonia segment (CS in Fig. 1). The evolution of this macrostructure is defined by this active fault throughout the Plio-Pleistocene. This definition is expressed through strong uplift movements (<1,000 m) which are smaller than the Aenos Mt. ones (Sorel 1976; Lekkas 1996). Complex halotectonic dynamics control local vertical tectonic movements (Lagios et al. 2007; Hollenstein et al. 2008) playing an important role for the evolution of the local relative sea level.

The Argostoli peninsula is located southwest and south of the Aenos Mt. (Fig. 1). It is bounded to the east and north by the Aenos fault zone (Fig. 1). Possible increased compression in the Argostoli peninsula during Pleistocene resulted in shortening and development of back-thrust faults exclusively in this macrostructure.

3.3 Neotectonic evolution

Based on the geometry and kinematics of the neotectonic macrostructures of Cephalonia, four main stages can be recognized in the tectonic evolution of the island:

  1. 1.

    During Early Pliocene, the emplacement of the Ionian nappe on Paxoi unit took place under a compressional tectonic regime. This regime resulted in the emergence of the island, the generation of folds, and the reactivation of major pre-existing syn-sedimentary faults. Pre-existing normal faults were converted into reverse faults during this stage (e.g., Aenos, Argostoli, Kontogourata-Agon, Agia Efimia faults).

  2. 2.

    During Late Pliocene-Early Pleistocene, the compression prevailed in the lower crustal parts, while major tectonic structures were activated as normal faults in the upper crustal parts. Aenos and Agia Dinati Mt. horsts and the grabens on either side of them were also created.

  3. 3.

    During Early Pleistocene, structural elements related to compressional tectonic regime were once again developed. Reverse faults were mainly activated in Argostoli peninsula, in the southeastern part of Cephalonia, and in Paliki peninsula. New folds with NNW-SSE trending axes were generated. During Early-Middle Pleistocene, the marine terrace in Minia area was uplifted. The compressional tectonic regime had a WSW-ENE trend.

  4. 4.

    Normal or oblique-normal slip reactivation of pre-existing structures took place following the compression of the previous stage.

3.4 Historical seismicity

Historical seismicity over the last 500 years is well documented for Cephalonia. Historical seismic data indicate that the island has been repeatedly struck by seismic events with Mw up to 7.2 and seismic intensity up to X+ (Papazachos and Papazachou 1989, 1997, 2003). It is not the first time that sequences of two or three earthquakes with the same or similar magnitude in or close to Cephalonia occurred successively within relatively short time and space intervals. Analogous earthquake sequences were also generated in 1767 (July 11 and 22), in 1912 (January 24, 25, 26, and February 10), in 1953 (August 9, 11, and 12), and in 1983 (January 17 and 19 and March 23) (Papazachos and Papazachou 1989, 1997, 2003; Lekkas et al. 1997).

4 Co-seismic surface rupture structures associated with the early 2014 Cephalonia earthquakes

4.1 Surface ruptures during the first earthquake (January 26)

4.1.1 Zola area

The Thinia valley is formed in the transition zone between the northwestern part of Aenos Mt. located eastwards and the northeastern part of Paliki peninsula located westwards (Fig. 1). Its length is 6 km, its width is up to 2 km, and its elevation in the central part is around 180 m. It is bounded on either side by steep hill ranges, rising up to almost 1 km on its eastern flank. It is mainly composed of the Middle Miocene-Early Pliocene clay-clastic sequence of Paxoi unit and the Middle Pleistocene interglacial deposits, which occur only along its eastern margin. In this margin, the Triassic-Middle Miocene limestones of Paxoi unit thrust over the aforementioned clay-clastic sequence. This reverse fault is the NE-SW striking Kontogourata-Agon fault, which also presents strike-slip sense and is entirely covered by slope breccia. Moreover, parallel active faults bound the western and eastern margins of the Thinia valley resulting in the uplift of the carbonate formations on both sides of the valley and the respective subsidence of the clay-clastic sequence.

The Zola area is located in the northwestern margin of the Thinia valley and precisely on the road leading from Zola village to Agia Kiriaki bay (Fig. 1). The surface ruptures were mapped during our field reconnaissance in this area right after the first earthquake. They were observed in two sites on the road asphalt surface constructed on the Paxoi clay-clastic sequence. It is significant to note that these surface ruptures were observed close to a parallel N25° E striking fault, which is mapped and characterized as active by Lekkas (1996).

In the first site located southwards, V-shaped conjugate surface ruptures were observed. The first one strikes N50°E and the second one strikes N10°W (Fig. 2a, b). The NE-striking surface rupture was 2-m long and 1-cm wide. The SE block moved towards NE, while the NW block moved towards SW (Fig. 2a). The sinistral strike-slip offset revealed by the movements of the blocks on either side of the surface rupture was measured at 1.4 cm. The N-striking surface rupture was 6-m long and 1-cm wide and caused the failure of an adjacent perimeter stone wall. The eastern block moved southwards, while the western block moved northwards (Fig. 2a). The dextral strike-slip offset revealed by the movements of the blocks on either side of the rupture was measured at 0.5 cm. The road asphalt surface was destroyed at the intersection point of the V-shaped conjugate surface ruptures due to their opposite sense of shear (Fig. 2a).

Fig. 2
figure 2

a V-shaped conjugate surface ruptures observed in the first site of Zola area. The yellow arrows indicate the failure of an adjacent perimeter stone wall and the destruction of the road asphalt surface at the intersection point of the V-shaped conjugate surface ruptures due to their opposite sense of shear. b Sinistral strike-slip offset in the Zola first site. c Representative photograph of surface ruptures observed on road asphalt surface in the second site of Zola area. They caused (d) damage (pointed by the yellow arrow) on an adjacent concrete entrance ramp onto a field. e Structure involving small-scale bookshelf faulting in the second site of Zola area. f The tectonic model of bookshelf faulting involves faulting, fault-bound blocks formation, and anticlockwise rotation of blocks in a “bookshelf” manner

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In the second site located northwards, surface ruptures were observed on the road asphalt surface constructed on the Paxoi clay-clastic sequence (Fig. 2c), and they caused damage on an adjacent concrete entrance ramp onto a field (Fig. 2c, d). They strike N20°E and were 20-m long, 3–5-cm wide, and 0.5–1-m deep. The eastern block was uplifted and moved northwards, while the western block was subsided and moved southwards (Fig. 2c), indicating a sinistral strike-slip motion. The co-seismic sinistral strike-slip offset was approximately 7 cm.

A structure involving strike-slip deformation was detected in Zola’s second site (Fig. 2e). It was 1-m long and 20-cm wide. It consists of blocks, which were formed between two major left-lateral surface ruptures and separated by right-lateral block-bounding deformation bands almost transverse to the main shear direction. The deformation bands were originally oriented according to the basic Riedel shear structure (Fig. 2f) and later rotated anticlockwise during the same seismic event. As the deformation bands divided this block into smaller ones, it is possible that the blocks rotated in a domino style. This mechanism is consistent with the tectonic model of bookshelf faulting (Fig. 2f), which involves faulting, fault-bound blocks formation, and anticlockwise rotation of blocks in a “bookshelf” manner (Mandl 1987). Bookshelf faulting can be observed in various tectonic environments and at various observation scales. Its most important manifestations are cross-faults between normal faults or parallel wrench faults, the extension of deltaic slope deposits, and all forms of tilted block tectonics (Mandl 1987 and references therein).

4.1.2 The broader area of Livadi swamp

Surface ruptures were induced by the first earthquake in the area northeast of Livadi swamp (Fig. 1). They were formed in an area comprising Triassic-Middle Miocene limestones of Paxoi unit. They were located along the central part of an almost 2-km-long active fault striking NE-SW, dipping 40–55° to the NW, and cutting across the aforementioned limestones (Fig. 3a). They caused extensive damage on the road asphalt surface and considerable failure of the stone retaining walls along the road leading to the Livadi swamp.

Fig. 3
figure 3

All co-seismic surface rupture structures presented in this figure were induced by the first earthquake (January 26) and are observed along the surface rupture zone located northeast of the Livadi swamp area (a–f) and close to the eastern end of Livadi swamp (g). a A surface rupture zone is observed in the northern part of a pre-existing NE-SW striking active fault. The active fault cuts across the Triassic-Middle Miocene limestones of Paxoi unit forming thick scree on the hillside. b Destruction of a concrete retaining wall transverse to the dextral strike-slip surface rupture. c Co-seismic dip-slip (normal) surface ruptures in the same site. The maximum vertical offset was measured at 30 cm. d Destruction of the road asphalt surface. Restraining and releasing bends due to minor changes in fault trend were observed along the surface rupture zone. Black arrows indicate uplift and subsidence. e The co-existence of normal component was indicated by small grabens (sinkholes) observed along the same surface rupture zone. f A dextral strike-slip surface rupture cut large boulders. g N20°E striking major surface ruptures accompanied by N40°W striking minor surface ruptures

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These surface ruptures generally strike N20°–30°E. Their total length was 300 m, their width varies from 1 cm to 1.5 m, and their depth from few centimeters to 1 m. The geometry and the kinematics of the observed surface ruptures and the associated structures indicate predominantly a dextral strike-slip motion (Fig. 3b). The dextral offset ranged from 10 to 20 cm.

Apart from the strike-slip motion, dip-slip motion was also observed along these surface ruptures (Fig. 3c). The NW block subsided, while the SE one was uplifted. The maximum vertical offset detected in the field was 30 cm (Fig. 3c). The subsidence of the NW block was detected not only by direct field observations but also by surface deformation measurements from DInSAR analysis performed by Benekos et al. (2015). Based on their deformation map, this area presented a subsidence close to −7 cm.

Restraining and releasing bends caused by minor changes in fault trend were observed in the same site (Fig. 3d). The length of these structures was about 5 m, and topographic subsidence (or uplift) ranged from 10 to 30 cm.

In the northern part of this zone, a ground opening (sinkhole) was observed along a segment of this zone (Fig. 3e). It was 4-m long and its maximum diameter at the surface was measured at 1.5 m, stretching downwards into the scree for 1 m (Fig. 3e). It could result from the shaking during the first earthquake and the collapse of the surface layer revealing that extension (black arrows in Fig. 3e) and dextral strike- slip (red arrows in Fig. 3e) operated contemporaneously in the same site. In the southern tip of this zone, a surface rupture struck NNE-SSW, dipped 70° to the ENE, presented vertical offset of 30 cm, and cut large boulders of recent scree (Fig. 3f).

Surface ruptures induced by the first earthquake occurred also in the area located close to the eastern end of Livadi swamp (Fig. 3g). These ruptures were observed on the road asphalt surface with a length of about 7 m. They extended to the adjacent limestone slope, which was partially covered by scree. The major surface ruptures strike N20°E, and they were accompanied by minor ruptures striking N40°W (Fig. 3g). The major surface ruptures are considered to be synthetic Riedel shears (R), while the minor ones are considered to be antithetic Riedel shears (R′) (Fig. 3g).

Surface ruptures were also found in the coastal area of Livadi bay comprising recent alluvial deposits. Along the eastern and southern margins of the swamp, NW-SE, N-S, and NE-SW striking active faults juxtapose the alluvial formations of the coastal area against the Triassic-Middle Miocene limestones of the Paxoi unit (Fig. 1). Along the western margin of the swamp, NW-SE striking faults juxtapose the alluvial formations against the Pliocene-Calabrian marine sequence (Fig. 1), which is developed transgressively over Paxoi formations. A surface rupture induced by the first earthquake was detected on the asphalt surface of the road leading from Argostoli to Livadi, particularly in the site of the old Livadi rural prison located close to the coastline (Fig. 1). It strikes N50°W and has length of about 10 m. It could be considered as an antithetic Riedel shear (R′) within the principal dextral strike-slip surface rupture zone. It is significant to note that the masonry prison buildings located in the same site were completely devastated by the first earthquake and had to be demolished.

4.1.3 The area north of Atheras village

Surface ruptures were induced by the first earthquake and observed north of Atheras village (Fig. 1), precisely on the road from Atheras village to Atheras bay. This area is located in the northwestern part of Paliki peninsula. The surface ruptures occurred in an area composed of the Triassic-Middle Miocene limestones of Paxoi unit (Fig. 1). They strike N30°–50°E. Their total length was 100 m and their width 3 cm. The co-seismic dextral offset was 2 cm. The surface ruptures were accompanied by flower structures and landslides along the slopes (Fig. 4a, b). Due to the fact that the first earthquake was generated by a dextral strike-slip fault zone with a small reverse component, the splay faults tend to be convex upward with gentle dips at the surface (Fig. 4b). They form a positive (or palm tree) flower structure, which appears as an uplifted, commonly antiformal area (a rhomb horst or push-up) (Fig. 4b).

Fig. 4
figure 4

Surface ruptures induced by the first earthquake were observed north of Atheras village. They were accompanied by flower structures and landslides along the slopes (a). The flower structure is characterized as positive due to the small reverse component observed in the field resulting in the uplift of the northwestern block and the subsidence of the southeastern block (b)

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4.2 Surface ruptures during the second earthquake (February 3)

4.2.1 The area south of Atheras village

Surface ruptures were induced by the second earthquake south of Atheras village, in the northwestern part of Paliki peninsula (Fig. 1). Eight surface ruptures were arranged across a 600-m-long part of the road that is constructed on an area comprising mainly of the Triassic-Middle Miocene limestones of Paxoi unit (Fig. 1). They are distinguished into surface ruptures with (a) strike-slip and (b) dip-slip motion (reverse displacement) (Fig. 5a–c). Their length did not exceed 10 m. The first strikes N20°–50°E and presented width ranging from 1 to 10 cm and depth ranging from 1 to 20 cm. The last also strikes N20°–55°E.

Fig. 5
figure 5

a–c Representative photographs of surface ruptures induced by the second earthquake and observed south of Atheras village. d Co-seismic surface rupture structures observed in Petanoi area. e Surface ruptures in Petanoi area were observed along a pre-existing NE-SW striking active fault juxtaposing the Triassic-Middle Miocene limestones of Paxoi unit against the Middle Miocene-Early Pliocene clay-clastic sequence of the same unit. f, g Representative photographs of co-seismic surface rupture structures observed in Petanoi area

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Where the strike-slip motion prevailed, the NW block moved towards SW and the SE block moved towards NE revealing sinistral strike-slip motion. The sinistral offset ranged from 1 to 2.4 cm. Where the dip-slip motion prevailed, the NW block was uplifted, while the SE block was subsided. The vertical offset ranged from 1 to 3 cm.

The structure and orientation of these surface ruptures correspond to the upper surficial part of a flower structure developed in the area. The displacements along the ruptures revealed the presence of a small reverse component (Fig. 5c). Thus, the flower structure is characterized as positive, which appears as an uplifted area (Fig. 5c).

4.2.2 Petanoi area

Petanoi area is located in the western part of Paliki (Fig. 1). Surface ruptures induced by the second earthquake occurred in two sites. NW striking surface ruptures were located along a fault juxtaposing the Triassic-Middle Miocene limestones of the Paxoi unit against the Middle Miocene-Early Pliocene clay-clastic sequence of the same unit (Fig. 5e). Their length was 5 m and their width 1 cm. The NE block moved towards NW and the SW block moved towards NE indicating sinistral strike-slip motion (Fig. 5d, f).

A co-seismic surface rupture was observed in the second site in a rural road constructed on the Middle Miocene-Early Pliocene clay-clastic sequence of the Paxoi unit. It strikes N20°E and presented dextral strike-slip motion (Fig. 5g). The respective zone consisted mainly of R-bands, synthetic to the sense of shear across the zone, antithetic R′-bands, secondary synthetic P shears, and tensional cracks (T fractures) (Fig. 5g). Restraining and releasing bends caused by minor changes in fault trend were also observed in the same site (Fig. 5g). Topographic subsidence (or uplift) was 10 cm.

5 Fault zones ruptured during the early 2014 Cephalonia earthquakes

Based on direct field observations immediately after each earthquake including geometry, slip distribution, and kinematics of the surface ruptures induced by the early 2014 Cephalonia earthquakes, it is revealed that each earthquake was induced by the rupture of a different pre-existing active fault zone in the western part of Cephalonia.

The January 26 earthquake was induced by the rupture of a fault zone located in the eastern and northwestern part of Paliki peninsula (Fig. 6a). The eastern part of this fault zone is about 18-km long starting from the southeastern tip of Paliki peninsula (Cape Agios Georgios) and extending to the northeastern part of the same macrostructure (Agia Kiriaki Bay) (Fig. 6a). It can be divided into three segments based on morphological characteristics, geometry of structures, and the spatial distribution of surface ruptures.

Fig. 6
figure 6

Geological maps presenting the strong correlation among the (a) geological field data (locations of ruptured active fault zones and surface rupture zones) and (b) the displacement discontinuities derived from already published surface deformation measurements (Benekos et al. 2015). CTFZ (CS) Cephalonia transform fault zone (Cephalonia segment)

Full size image

The first segment starts from the Cape Agios Georgios in the southeastern tip of Paliki and extends northwards to Agios Dimitrios area (Fig. 6a). This segment consists of a N-S striking active fault with significant strike-slip component that defines the eastern straight coastline of Paliki and juxtaposes the Pliocene-Calabrian marine sequence against the submarine formations of the Livadi bay and the recent alluvial deposits in Agios Dimitrios area (Lekkas 1996; Lekkas et al. 2001; Fig. 6a). Despite the considerable structural damage and earthquake environmental effects induced by the first earthquake in Lixouri-Agios Dimitrios area (Lekkas et al. 2014a, b; Papadopoulos et al. 2014; Lekkas and Mavroulis 2015), no distinct surface rupture occurs in this first 10-km-long segment. This fact can be attributed to the unconsolidated or loose character of the geological formations in the area, which consists of Pliocene-Calabrian marine and Holocene alluvial and coastal formations.

The second segment starts north of Agios Dimitrios area and extends to Livadi swamp (Fig. 6a). In this segment, the aforementioned active fault zone is most likely to extend deeper below the Livadi bay located northwards as well as along the almost N-S trending morphological escarpment that follows approximately the contact between the Pliocene-Calabrian marine sequence and the recent deposits in the broader area of the Livadi village located westwards (Fig. 6a). Despite the fact that villages located close to this morphological escarpment suffered extensive structural damage from the early 2014 Cephalonia earthquakes, no distinct surface rupture also occurs in this segment.

The third segment starts from the Livadi swamp and extends to the Agia Kiriaki bay (Fig. 6a). In this segment, the aforementioned active fault zone is again superficially observed northeastwards, extending from the Livadi swamp to the Zola area in the northwestern margin of Thinia valley. It consists of NE-SW striking strike-slip faults disrupting the Triassic-Middle Miocene limestones of Paxoi unit or juxtaposing the Middle Miocene-Early Pliocene clay-clastic sequence of the same unit against the aforementioned limestones (Fig. 6a). The strike-slip-related structures were observed in this segment and especially in Zola and Livadi broader areas. They are V-shaped conjugate surface ruptures, dextral and sinistral surface ruptures, restraining and releasing bends, Riedel structures, and small-scale bookshelf faults. An extensional component across this surface rupture zone was derived from ground openings (sinkholes), small-scale grabens, and co-seismic dip-slip displacements. The addition of the extensional component across this zone produced transtension during the first seismic event. Transtension resulted in clockwise rotation of the extension axis in the dextral shear (broader area of the Livadi swamp) and anticlockwise rotation in the sinistral shear zone (Zola area).

Co-seismic displacements were measured in the third segment by examining offset surface markers, such as roads and mountain paths that crossed the co-seismic surface rupture zone. The displacements were measured in Zola area and in the Livadi swamp broader area. In Zola area, the co-seismic dextral offset was 0.5 cm, while the sinistral offset ranged from 1.4 to 7 cm. In the Livadi broader area, the co-seismic dextral offset ranged from 10 to 20 cm, while the maximum vertical offset detected in the same area was 30 cm.

The northwestern part of the fault zone ruptured during the January 26 earthquake is a 4-km-long N-S striking fault zone which starts south of Atheras village and extends to Atheras bay area (Fig. 6a). It comprises faults cutting the Triassic-Middle Miocene limestones of Paxoi unit. The co-seismic surface rupture structures were strike-slip surface structures and flower structures revealing also the co-existence of reverse component. The co-seismic displacements were measured north of the Atheras village. The co-seismic dextral offset was 2 cm.

The February 3 earthquake ruptured a fault zone located in the western part of Paliki peninsula (Fig. 6a). The western fault zone strikes NNE-SSW and is divided into two segments. The first 12-km-long segment starts from the area located southwest of the Chavriata village and extends northwards to Agia Thekli area (Fig. 6a). It is significant to note that the PGA recorded close to this segment (at the Chavriata site) is of 0.77 g (Karakostas et al. 2014a, b). This segment consists of NNE-SSW and NE-SW striking active faults that form two fault blocks, the western and the eastern. The western fault block comprises Triassic-Middle Miocene limestones and Middle Miocene-Early Pliocene clay-clastic sequence of Paxoi unit. Recent formations including Holocene and Middle Pleistocene deposits are also observed in this fault block. The eastern fault block comprises Middle Miocene-Early Pliocene clay-clastic sequence of Paxoi unit, Pliocene-Calabrian sequence, and Middle Pleistocene deposits.

The strike-slip-related structures were also observed in this segment and especially in Petanoi area. They are strike-slip surface ruptures and Riedel structures including R, R′, and P shears as well as T fractures. Restraining and releasing bends were also observed.

The second segment starts from the area north of Agia Thekli and extends northwards to the Cape Kartergaki. This block is exclusively built of Triassic-Middle Miocene limestones. NE-SW and E-W striking active faults disrupt these limestones and present significant dextral strike-slip motion (Lekkas 1996; Lekkas et al. 2001).

The strike-slip-related structures in this segment were observed south of Atheras area. They are strike-slip surface ruptures, Riedel structures, and flower structures. A compressional component across the surface rupture zone was derived from the aforementioned structures. The addition of the compressional component across this zone produces transpression during the second seismic event.

Co-seismic displacements were measured in the second segment of the western active fault zone by examining offset surface markers, such as roads that crossed the co-seismic surface rupture zone. South of Atheras, the sinistral offset ranged from 1 to 2.4 cm, while the vertical offset ranged from 1 to 3 cm.

6 Discussion–conclusions

The January 26, 2014 Cephalonia earthquake was induced by the rupture of a 18-km-long fault zone developed in the eastern part of Paliki peninsula and a 4-km-long fault zone developed in the western part of the same neotectonic macrostructure. The February 3, 2014 Cephalonia earthquake was induced by the rupture of a 20-km-long fault zone developed in the western part of Paliki peninsula.

The first earthquake produced a 6-km-long surface rupture zone in the northeastern part of Paliki peninsula and a 4-km-long surface rupture zone in the northwestern part of the same neotectonic macrostructure, while the second earthquake produced a surface rupture zone in the northwestern part of Paliki peninsula.

By performing the DInSAR analysis, Benekos et al. (2015) succeeded in measuring and mapping the surface deformation of Cephalonia during the period from January 28 to February 8. Based on these surface deformation measurements, it is detected that the ground in the central part of Paliki peninsula was uplifted by 12 cm, while its eastern coastal part was subsided by 7 cm (Fig. 6b). More precisely, maximum displacement towards the Satellite’s Line of Sight (LOS) of +12 cm was observed in different zones at the central part of Paliki, while displacements away from the satellite were measured along the 600-m-wide eastern coastal zone of Paliki with maximum displacement of −7 cm at Agios Dimitrios area (Fig. 6b). As for the eastern part of Cephalonia is concerned, a displacement of −2.5 cm (LOS) in Argostoli area was also detected, while the rest of the island displayed minimum surface deformation.

Taking into account the interferometric product of Benekos et al. (2015), we identified distinct displacement discontinuities in the eastern, northern, and central part of Paliki (Fig. 6b). In the eastern coastal part of Paliki peninsula, two displacement discontinuities can be detected. The easternmost displacement discontinuity with a length of about 9 km extends from the southeastern tip of Paliki, close to the Cape Agios Georgios, to Agios Dimitrios village located north of Lixouri (Fig. 6b). It juxtaposes the easternmost coastal part of maximum subsidence [−7 cm (LOS) in Agios Dimitrios area and −6 cm (LOS) in Lixouri area] against the adjacent zone with displacement close to 0 cm (Fig. 6b). It coincides with the first segment of the eastern part of the fault zone ruptured during the first earthquake and with the surface rupture zone in the northeastern part of Paliki peninsula (Fig. 6a).

This displacement discontinuity is most likely to extend deeper below the Livadi bay located northwards, reappears in the area of Livadi swamp, and is observed on land along NE-SW striking strike-slip faults disrupting the Triassic-Middle Miocene limestones of Paxoi unit until Cape Kartergaki and Zola area in the northern part of Paliki (Fig. 6b).

Another displacement discontinuity is detected to the west of the previous one. It extends with a length of 9 km from the southeastern Paliki to the area north of Livadi village (Fig. 6b). It juxtaposes the abovementioned zone with displacements close to 0 cm against the central Paliki characterized by maximum uplift (Fig. 6b).

Some discontinuities are also observed in the central and the northwestern part of Paliki (Fig. 6b). Indicatively for this region, we can refer to the displacement discontinuity with a length of 11 km extending from the central part of Paliki east of Petanoi bay to Atheras area. It juxtaposes the central Paliki of maximum uplift (+12 cm) against the adjacent western zone with displacement close to 0 cm (Fig. 6b). This displacement discontinuity coincides with the northern part of the fault zone ruptured during the second earthquake as well as with the surface rupture zone in Petanoi and southern Atheras areas (Fig. 6a).

From the comparison of the field geological observations with the aforementioned DInSAR analysis results, it is concluded that there is a strong correlation among the ruptured active fault zones, the surface rupture zones, and the detected displacement discontinuities in Paliki peninsula. Conclusively, there is an imperative need to compare field observations and surface deformation measurements from DInSAR immediately after strong earthquakes in order to further assess the kinematics of their rupturing process.

It is more evident now than ever that Cephalonia is exposed to significant seismic hazard not only due to the proximity to the offshore segment of the Cephalonia Transform Fault Zone but also due to the existence of several active fault zones and faults occurred on land. These active faults are capable of generating moderate to strong earthquakes releasing parts of the accumulated strain energy and causing extensive primary and secondary earthquake environmental effects and severe damage on the building environment. Thus, these faults pose additional significant seismic hazard to Cephalonia and the surrounding region.