Evaluation of environmental seismic intensities of all known historical and recent earthquakes felt in Zakynthos Island, Greece using the Environmental Seismic Intensity (ESI 2007) scale

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Evaluation of environmental seismic intensities of all known historical and recent earthquakes felt in Zakynthos Island, Greece using the Environmental Seismic Intensity (ESI 2007) scale Spyridon Mavroulis∗, Eirini-Spyridoula Stanota, Efthymios Lekkas Department of Dynamic Tectonic Applied Geology, Faculty of Geology and Geoenvironment, School of Sciences, National and Kapodistrian University of Athens, Panepistimiopolis, Athens, 15784, Greece

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ABSTRACT

Keywords: Earthquake environmental effects Central Ionian islands Neotectonics ESI-07 Seismic hazard

The complete and detailed knowledge of the historical earthquakes, the past earthquake environmental effects (EEE) and the respective seismic intensities has become significant in recent years due to the fact that among others it serves as a valuable tool for revealing and highlighting sites of significant earthquake-related hazards. Many efforts have been made to record the EEE of individual recent earthquakes and evaluate their seismic intensity based on the Environmental Seismic Intensity 2007 scale (ESI 2007) in Greece and around the world. But fewer studies have focused on the complete seismic history including historical and recent earthquakes of an area and the respective intensities based on the induced EEE. The Central Ionian Islands (Western Greece) and especially Zakynthos Island are considered appropriate for the development of this approach. The complete history of earthquakes with destructive impact on Zakynthos from 1513 to present is presented. Emphasis is given on EEE, while the respective ESI 2007 intensities are assigned. Based on the EEE's distribution on the affected fault blocks, it is concluded that eastern Zakynthos has been affected more often and severely by earthquakes. This selective distribution is attributed to the neotectonic setting of Zakynthos. The recording of the EEE over the past five centuries and the study of possible correlation with the seismotectonic structure of the affected area could be used as a basic guide for the reduction of the future seismic risk and the risk from EEE through effective land-use planning and preparedness in earthquake-prone areas.

1. Introduction Earthquake environmental effects (EEE) are the effects induced by an earthquake on the natural environment (Michetti et al., 2007; Audemard et al., 2015). The coseismic environmental effects considered more diagnostic for intensity evaluation can be classified into two main types: (a) primary effects, which are directly linked to the earthquake energy and (b) secondary effects, which are generally induced by the ground shaking (Michetti et al., 2007; Audemard et al., 2015). The use of the EEE not only provide useful data and information in order to understand the type and the basic parameters of an earthquake but also allows insight into the earthquake size and intensity evaluation through the application of the Environmental Seismic Intensity (ESI 2007) scale introduced by Michetti et al. (2007). Moreover, they are independent of peculiar cultural and local socio-economic conditions or different building practices through time. Thus, the EEE can be used for the evaluation of the seismic intensity not only of recent

∗

but also of historical and palaeo-earthquakes. Furthermore, they can be used for the comparison among future, recent, historical and palaeoearthquakes and between earthquakes generated in different tectonic environments. The complete and detailed study and knowledge of the historical earthquakes and the assessment of the seismic intensities based on the historical EEE has become more important in recent years. This is attributed to the fact that the EEE serve as a valuable tool for revealing and highlighting sites of significant earthquake-related hazards, where no macroseismic damage data are available, and for testing the susceptibility and the vulnerability of the affected areas to the same effects. As a result, the disaster preparedness and the land-use planning in this areas could be improved, aiming to overcome the changes that an earthquake induces on the natural environment of the affected area. The ESI 2007 scale has been already applied in historical earthquakes in various tectonic environments around the world. More specifically, it has been applied in individual historical earthquakes in

Corresponding author. E-mail address: [email protected] (S. Mavroulis).

https://doi.org/10.1016/j.quaint.2019.09.006 Received 8 May 2019; Received in revised form 6 September 2019; Accepted 11 September 2019 1040-6182/ © 2019 Elsevier Ltd and INQUA. All rights reserved.

Please cite this article as: Spyridon Mavroulis, Eirini-Spyridoula Stanota and Efthymios Lekkas, Quaternary International, https://doi.org/10.1016/j.quaint.2019.09.006

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order to enrich the existing databases of several countries including Italy (Comerci et al., 2015), Greece (Papanikolaou and Melaki, 2017; Papathanassiou et al., 2017), Spain (Sanz Pérez et al., 2016), Central Asia (Tatevossian, 2007), northwestern Kashmir Himalaya (Ahmad et al., 2014) and Mexico (Rodríguez-Pascua et al., 2017). Moreover, it has been applied in a set of chosen historical events in the same region in order to reassess the historical events in the same region and to contribute to the reduction of the risk from the EEE. Characteristic examples of the last approach are the southern Apennines in Italy (Esposito et al., 1987; Porfido et al., 2002, 2007; Serva et al., 2007, 2015; Nappi et al., 2017), the Betic Cordillera in SE Spain (Silva et al., 2017) and the Eastern Siberia (Radziminovich and Shchetnikov, 2013). Many efforts have been made to record the environmental effects of individual recent earthquakes and evaluate their seismic intensity based on the ESI 2007 scale in Greece (e.g. Papathanassiou et al., 2007; Fokaefs and Papadopoulos, 2007; Fountoulis and Mavroulis, 2013; Mavroulis et al., 2013; Lekkas and Mavroulis, 2015; Papanikolaou and Melaki, 2017; Lekkas et al., 2018) and around the world (e.g. Tatevossian, 2007; Lalinde and Sanchez, 2007; Tatevossian et al., 2009; Ota et al., 2009; Ali et al., 2009; Mosquera-Machado et al., 2009; Berzhinskii et al., 2010; Lekkas, 2010; Di Manna et al., 2012; Gosar, 2012; Silva et al., 2015; Porfido et al., 2015; Heddar et al., 2016; Sanchez and Maldonado, 2016; Porfido et al., 2016; Rodríguez-Pascua et al., 2017). But fewer studies have focused on the complete seismic history including all known historically reported and recent instrumentally recorded earthquakes of an area and the respective intensities based on the induced EEE (e.g. Mavroulis and Lekkas, 2018). The Central Ionian Islands and especially Zakynthos Island was considered appropriate for the development of this approach based on various sources listed below. Zakynthos along with Lefkada, Cephalonia and Ithaca (Fig. 1) constitute the Central Ionian Islands, which are characterized as one of the most seismically active parts of the Mediterranean region (Makropoulos and Burton, 1984; Papazachos and Papazachou, 1989, 1997, 2003; Lekkas, 1993, Lekkas et al., 1996-1997; Lagios et al., 2007). This study offers the complete catalogue of all known destructive historical and recent earthquakes covering a time period from 1513 to present, generated in the Central Ionian Islands and the offshore western Peloponnese with significant impact on Zakynthos Island. It reexamines the available literature on historical earthquakes and their EEE, including analysis and interpretation of historical archives and contemporary sources. This analysis aims to precisely and accurately describe the impact of these earthquakes on Zakynthos Island and to construct a meaningful and useful reference for understanding mainly their environmental impact. Moreover, this study presents a list of newly classified localities on the base of the recognized primary and secondary EEE along with the characteristics and the parameters of the EEE (type, size and distribution). Based on these qualitative and quantitative information, the ESI 2007 scale is applied to the historical and recent earthquakes of the catalogue and the respective environmental seismic intensities are assigned. Based on the results of the aforementioned approach, this study aims to present a more complete picture of the impact of each earthquake on the study area and to identify areas and zones most prone to the occurrence of ground effects and more susceptible to local geological instabilities. It also aims to interpret the intensity values and their distribution according to the seismotectonic regime.

1993, Lekkas et al., 1996-1997; Lagios et al., 2007). As in all regions and time periods where no instrumental recordings occurred, the pre-1800 seismicity record of Zakynthos is not well documented and therefore it could be considered as incomplete. However, there are historical and instrumentally recorded information and data proving that the Central Ionian Islands in general and Zakynthos Island in particular has been repeatedly stricken from large and destructive earthquakes from 1513 AD to present with significant impact to public health, the natural and built environment comprising earthquake environmental effects and damage to structures respectively (Lekkas, 1993; Lekkas et al., 1996–1997; Papazachos and Papazachou, 1989, 1997, 2003; Papadopoulos et al., 2010, 2014a, 2014b; Papadopoulos, 2016; Soloviev et al., 2000; AUTH, 2019). Based on the seismicity catalogues of the permanent regional seismological network operated by the Aristotle University of Thessaloniki (AUTH, 2019) covering the period from 550 BC to 2018, it is concluded that many earthquakes with magnitudes larger than 5.0 have been concentrated in four seismic zones around Zakynthos Island (Fig. 2). These zones include significant seismic sources with high productivity and frequent occurrence of destructive earthquakes during historical and recent times (Lekkas et al., 1996–1997; Papazachos and Papazachou, 1989, 1997, 2003) and they are the following: (a) The western offshore Cephalonia and Lefkada area, where the prevailing active tectonic structure is the CTFZ. The CTFZ is composed of two segments; the Lefkas segment (LS in Fig. 2) to the north located west of Lefkas Island and the Cephalonia segment (CS in Fig. 4) to the south located west of Cephalonia Island (Scordilis et al., 1985; Kiratzi and Langston, 1991; Louvari et al., 1999; Sachpazi et al., 2000; Kokinou et al., 2005, 2006). The LS extends with a length of about 40 km from the northwestern offshore part of Lefkas to the northern offshore part of Cephalonia Island (Underhill, 1988; IGME, 1989; Louvari et al., 1999) (Fig. 2). It strikes in a NESW direction, dips to the ESE and is characterized by a dextral strike-slip motion (Fig. 2) combined with a small thrust component involved in the movement. The CS occurs with a length of about 90 km close to the western offshore part of Cephalonia (Sachpazi et al., 2000; Kokinou et al., 2005). The southern segment of CTFZ is the zone with the highest seismicity in the western part of the Hellenic Arc and Trench system comprising earthquakes with magnitudes up to Mw 7.4 and high intensities (Papazachos and Papazachou, 1989, 1997, 2003; Lekkas, 1993; Lekkas et al., 1996-1997). The largest earthquakes for this zone are the February 4, 1867, Mw 7.4 Cephalonia earthquake (Fig. 2) along the CS with maximum intensity X assigned to Lixouri town located in the western part of Cephalonia Island (Paliki peninsula) and the August 14, 2003, Mw 6.2 Lefkada earthquake (Fig. 2) along the LS with maximum intensity VIII assigned to the western part of Lefkada Island (Papazachos and Papazachou, 1989, 1997, 2003; Stiros et al., 1994; Lekkas et al., 2018).

2. Seismotectonic setting of Zakynthos Island

(b) Within the Zakynthos channel and the respective structural basin between Zakynthos Island and Peloponnese. The Zakynthos channel reaches water depth of 520 m and contains the NW-SE trending offshore Zakynthos basin with geometry strongly related to the presence of reverse faults or thrusts and diapiric phenomena of Triassic evaporites (Sorel, 1976; Dermitzakis, 1978; Brooks and Ferentinos, 1984; Underhill, 1985, 1988).

From the geotectonic point of view, Zakynthos Island is located in the external part of the Hellenic Arc and in the east of the Hellenic Trench (Fig. 1), which represents the convergence boundary between the African and Eurasian plates, and is one of the most tectonically and seismically active parts of the Mediterranean region (Makropoulos and Burton, 1984; Papazachos and Papazachou, 1989, 1997, 2003; Lekkas,

Taking into account the tectonic interpretations of crustal velocity models run by Makris and Papoulia (2014), it is concluded that the Zakynthos basin and the Kyparissiakos Gulf located southwards are affected by the westward offshore extensions of major active onshore normal faults, mapped and studied by IGME (1989), Mariolakos et al. (1991), Lekkas et al. (1992, 2000), Fountoulis (1994), Mariolakos et al. 2

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Fig. 1. Map of the Hellenic Arc showing the location of Zakynthos Island at the northwesternmost part of the Hellenic Arc along with the prominent morphological features of the Hellenic Arc and the major morphoneotectonic features based on Mariolakos and Papanikolaou (1981, 1987) and the seismic risk zones of Greece. Zakynthos Island falls in the third seismic zone of Greece which is characterized by a ground acceleration coefficient of 0.36 g corresponding to the greatest seismic strength demand according to the Greek code for Seismic Resistant Structures.

(1998) and Papanikolaou et al. (2007) onshore western Peloponnese (Fig. 2). This zone is mainly characterized by moderate seismic events (Lekkas et al., 1990; Mariolakos et al., 1991, 1995; Papazachos and Papazachou, 2003; Roumelioti et al., 2004; Serpetsidaki et al., 2010). The largest recent earthquakes include: (i) the offshore March 28, 1955, M 5.7 Vartholomio earthquake with maximum intensities VIIMM and VII + EMS-98 assigned to Amaliada town (western Peloponnese) and VIMM and VEMS-98 maximum intensities assigned to Zakynthos city (Misailidis et al., 2014), (ii) the offshore October 16, 1988, Mw 5.6 Kyllini earthquake (e.g. Lekkas et al., 1990; Mariolakos et al., 1991, 1995) with maximum intensities VIIIMM and VIIIEMS-98 assigned to

Kyllini and Vartholomio in western Peloponnese (Papazachos and Papazachou, 2003; Misailidis et al., 2014) and VI + MM and VI + EMS-98 to Pantokratoras, Vanato, Mouzaki and Kalamaki settlements on Zakynthos Island (Misailidis et al., 2014; Papazachos and Papazachou, 2003) and (iii) the offshore December 2, 2002 Mw 5.6 Vartholomio earthquake (Roumelioti et al., 2004) with maximum intensity V+ (Serpetsidaki et al., 2010) (Fig. 2). (c) The offshore area between Zakynthos and Strofades Islands. Focal mechanisms of shallow earthquakes determined by waveform modelling provide strong evidence of strike-slip faulting in the area southeast of Zakynthos Island (Kirarzi and Louvari, 2003; 3

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Fig. 2. Significant historical and recent earthquakes and their epicenters that affected the geodynamic evolution of all Ionian Islands along with the distribution of earthquakes with magnitude M ≥ 4.5 from 550 BC to present based on earthquake catalogues provided by UOA (2019) and AUTH (2019). Onshore and offshore faults derived from active seismic observations of Makris and Papoulia (2014) and neotectonic mapping of Mariolakos et al. (1986), Fountoulis (1994), Papanikolaou et al. (2007), Mavroulis (2009), Vassilakis et al. (2011), Mavroulis et al. (2013) and Fountoulis et al. (2015).

Roumelioti et al., 2004). This evidence has been confirmed and strengthened by an active seismic experiment conducted by Makris and Papoulia (2014) and the SEAHELLARC working Group (2014). Crustal velocity models deduced from ENE-WSW seismic profiles revealed the presence of several N–S and ENE-WSW striking normal faults dissecting the Peloponnese continental margin in offshore southern Zakynthos as well as a major fault corresponding to the western limit of the Ionian geotectonic unit (Makris and Papoulia, 2014) (Fig. 2). The same velocity models also revealed a noticeable shift of the Ionian geotectonic unit and the Ionian thrust to the west (Makris and Papoulia, 2014). This shift could be attributed to the southwestward extension of the NE-SW striking dextral strike-slip Western Achaia Fault Zone (WAFZ) from Northwestern Peloponnese to offshore southern Zakynthos (Makris and Papoulia, 2014) (Fig. 2), which is a deformation zone arranged sub-parallel to the

Cephalonia Transform fault zone (CTFZ). The WAFZ is linked with an offshore pull apart basin NE of Strofades Island (Camera et al., 2014; Wardell et al., 2014) (Fig. 2). A specially confined deep seismicity comprising earthquakes with focal depths larger than 90 km and focal mechanisms indicating dextral strike slip deformation is observed in this transtensional basin (Papoulia et al., 2014; Camera et al., 2014). It is strongly related to fracture of the oceanic lithosphere due to differential bending of the subducted slab below a continental crust of laterally variable thickness (Papoulia et al., 2014). The onshore WAFZ is responsible for the generation of the 2008, June 8, Mw 6.4 Andravida (NW Peloponnese) strike-slip earthquake (Fig. 2). It is significant to note that the WAFZ has no direct surficial morphotectonic or geological evidence in onshore Western Peloponnese 4

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(Mavroulis et al., 2010, 2013), but its occurrence is deduced mainly by seismological evidence including the focal mechanism of the 2008 Andravida earthquake and the spatial distribution of its aftershocks (Konstantinou et al., 2009; Feng et al., 2010).

Upper Cretaceous - Eocene limestones composing the largest part of Vrachionas Mt and the broader area of Keri (Vrachionas-Keri limestones in Fig. 3). The Paxoi unit is also composed of two formations with various lithologies. The first formation occurs from Machairados settlement to Keri bay and consists of a Lower-Middle Miocene – Oligocene sequence (Lagopodo formation in Fig. 3) of marly limestones at the base, alternations of marls and limestones in the middle and diatomites, cohesive conglomerates, limestones and marls in its upper part. The second formation is the Middle-Upper Miocene Agios Sostis formation (Fig. 3). The post-alpine formations of this fault block comprise marly limestones and clayey marls of the Lower Pliocene Keri formation, eroded clayish-marly beds of the Middle-Upper Pliocene Kastro formation, calcitic sandstones, conglomerates and marls of the Pleistocene Gerakas formation, alluvial and elluvial formations, recent marsh deposits, coastal deposits and scree (Fig. 3). The Central Zakynthos fault block is bounded to the north by the VFZ and to the south by the active Kamaroti fault zone (KAFZ in Fig. 4). Kamaroti fault zone is a 6-km-long, almost E-W striking and southwards dipping normal tectonic structure with throw larger than 150 m. It mainly dissects formations of Paxoi unit and especially Vrachionas limestones and Lagopodo limestones and diatomites (Fig. 3)

(d) The area southwest of Zakynthos Island. This zone constitutes a downthrown block of the external Hellenides at the northern end of the Hellenic Trench and comprises tectonic structures like flat thrusts, strike-slip faults and normal faults, whose reactivations have resulted moderate seismicity (SEAHELLARC Working Group, 2014) (Fig. 2). More specifically, thrusting prevails over strike – slip or normal faulting (Kokinou et al., 2005, 2006), with the most important feature of this area being a 46-km-long NW-SE trending thrust system (SEAHELLARC Working Group, 2014). This system is responsible for the generation of the 1997 Mw 6.6 earthquake (Fig. 2) (SEAHELLARC Working Group, 2014) as well as for the October 26, 2018 Mw 6.8 offshore Zakynthos earthquake (Fig. 2). Thrust events are also observed along the Hellenic subduction zone located west and southwest of Zakynthos Island (Kiratzi and Louvari, 2003; Roumelioti et al., 2004). As regards the microseismicity of the greater Zakynthos area, two main seismic clusters are related to the NNW-SSE striking tectonic structures formed onshore Zakynthos Island and in the continental backstop to the west (Papoulia et al., 2014). Focal mechanisms indicate extension along with strike slip movements, and are related to the occurrence of extensional basins behind the NNW-SSE thrust fronts (Papoulia et al., 2014).

This fault block can be divided in two smaller blocks: (a) the western one comprising the Vrachionas Mt (main fault block IIa in Fig. 4) and (b) the eastern one from Vrachionas Mt to the eastern coastline of Zakynthos Island (main fault block IIb in Fig. 4). In the northwestern part of the Central Zakynthos fault block, where Vrachionas-Keri limestones and Lagopodo formations occur, many secondary faults dissect the large anticline of Vrachionas Mt (Figs. 3 and 4). They are active NE-SW striking faults with throw ranging from 50 to 100 m. In the southwestern part of Central Zakynthos fault block, many secondary NW-SE striking active oblique normal and normal faults with throw ranging from 50 to 150 m are arranged parallel to the coastline of the area and dissect Vrachionas limestones. The eastern coastal part of the Central Zakynthos fault block is composed of recent formations and has been affected by slight tectonic deformation expressed by the presence of recently activated faults in the Bochali – Gerakari area (Fig. 4). It is also characterized by the presence of NW-SE striking and NE-dipping secondary active normal faults with throw ranging from 50 to 100 m. They are observed in the broader Tragaki area (Tragaki faults – TRF in Fig. 4) and they are arranged parallel to the eastern coastline of Zakynthos Island. They dissected the Kastro and Gerakas formation and alluvial deposits.

3. Geological formations, faults and fault blocks of Zakynthos Island Zakynthos can be divided into fault blocks bounded by major active faults or fault zones (Figs. 3 and 4). They have different lithostratigraphic structure, they have suffered different tectonic deformation and consequently they have followed different tectonic and neotectonic evolution (Lekkas, 1993). The main neotectonic fault blocks are the following from the northern to the southern part of Zakynthos Island (Fig. 4):

• The Northern Zakynthos fault block (main fault block I in Fig. 4)

•

is located in the northern part of the island and is extended from the area north of Volimes and Katastari Cape (Figs. 3 and 4). It is composed of white chalky and easily weathered Upper Cretaceous Eocene limestones of Paxoi unit (Vrachionas – Keri limestones in Fig. 3), scree and eluvial deposits (Fig. 3). It is bounded to the south by the Volimes fault zone (VFZ in Figs. 3 and 4), a 10-km-long active fault zone with a semicircular form, NNW-SSE striking and SSW dipping in its western and NE-SW striking and SE dipping in its eastern part with activity during Holocene and throw larger than 100 m. It mainly dissects the Vrachionas – Keri limestones resulting in formation of scree. The VFZ bounds and dissects some residual occurrences of a Middle-Upper Miocene sequence composed of conglomerates, limestones, sandstones with bitumens, marls, sands and sandy-marly beds with gypsum (Agios Sostis formation in Fig. 3). Other secondary faults within this fault block (Northern Zakynthos Faults - NFZ in Fig. 4) bound the eluvial deposits. These secondary faults are classified as active and probably active. They are of normal character, WNW-ESE mean strike and throw of about 50 m. The Central Zakynthos fault block (main fault block II in Fig. 4) occupies the largest part of the island (Fig. 4). It is composed of the central part of Vrachionas Mt, the lowland Laganas – Alykes area as well as the higher Bochali – Gerakari area (Figs. 3 and 4). It is composed of Paxoi formations and post-alpine deposits. The Paxoi formations mainly comprise white, chalky and easily weathered

• The Keri bay fault block (main fault block III in Fig. 4) corresponds

to the morphologic depression of Keri bay – Logos (Fig. 4). It comprises mainly limestones of Vrachionas-Keri, Lagopodo and Agios Sostis formations, marly limestones and clayish marls of Keri as well as alluvial, eluvial, lagoonal, coastal deposits and scree. It is bounded to the north by the Kamaroti fault zone and to the south by the Keri bay fault zone (KEFZ in Figs. 3 and 4). The Keri fault zone is a 6-km-long, E-W striking and N-dipping normal active fault zone with throw larger than 150 m. It dissects Vrachionas limestones in its western part and marly limestones of Keri formation in its eastern part. The boundary between Vrachionas and Keri limestones is not always clear, mostly due to lithofacies similarities.

Secondary faults are also observed within Keri fault block (Keri faults - KEF in Figs. 3 and 4). They are of normal character with strike varying from E-W to WSW-ENE. They dissected Keri limestones and each of them has throw of about 50 m. The Keri bay fault block is a typical graben, whose formation and evolution have been strongly and impressively related to the presence of active faults repeatedly activated from Early Miocene to Holocene (Lekkas, 1993). The presence of hydrocarbons in Herodotus springs as 5

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Fig. 3. Neotectonic map of Zakynthos Island based on Lekkas (1993) illustrating alpine formations and post-alpine deposits, major active faults bounding the main fault blocks and secondary faults.

well as the gas release are attributed to the activation of these fault zones. These active structures represent the most significant neotectonic structures on Zakynthos Island.

• The Southern Zakynthos fault block (main fault block IV in Fig. 4)

•

occur in the southern part of the island. It is bounded to the north by the Keri fault zone (KEFZ in Figs. 3 and 4). It is mainly composed by the Vrachionas-Keri limestones along with eluvial deposits (Fig. 3). It is a graben dissected by a small number of NΕ-SW and ΝW-SΕ striking faults (Southern Zakynthos Faults – SZF in Fig. 4) with throw of about 50 m. Some of them seem to bound the eluvial

deposits and thus their activation probably occurred during Holocene (Lekkas, 1993). This suggestion can be strengthened by the natural petroleum gas release along the south and southeastern part of the island. The Skopos fault block (main fault block V in Fig. 4) occupies the Skopos peninsula. It is composed of Triassic evaporates, limestones and dolomites of Skopos Mt, the Dafni, Keri and Gerakas formations as well as scree and alluvial deposits (Fig. 3).

This neotectonic macrostructure is characterized by diapiric phenomena controlling its formation and evolution. Folds, fault-bend folds 6

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Fig. 4. Fault blocks of Zakynthos Island based on Lekkas (1993) along with major and secondary faults.

and steep slopes are attributed to these phenomena and are mainly and locally observed within the Dafni formation comprising basal conglomerates, sandstones, marls and siltstones in frequent alternations, as well as gypsum layers. This intense tectonic deformation fades significantly to the southeastern part of Skopos peninsula, where Kastro and Gerakas formation occur. Their beds are slightly inclined and are clearly separated from the formations in Skopos Mt by a major N–S striking fault. This area is also characterized by repeated palaeoenvironmental changes during Pleistocene (Dermitzakis et al., 1979). The western boundary of this fault block is the fault of Zakynthos city (ZCF in Figs. 3 and 4). This is an almost 7-km-long NNE-SSW striking and SSE dipping active normal scissor fault with throw of about 100 m. It juxtaposes Gerakas formation and Kastro formation that occurred on its footwall with alluvial deposits in its hangingwall. Except from the Zakynthos city fault, there are secondary N–S, NW-SE, NE-SW striking active normal faults within the Skopos fault block (Skopos faults – SKF in Fig. 4) dissecting the abovementioned alpine and postalpine formations. Each of them has throw ranging from 50 to 100 m.

earthquakes and their EEEs in Greece (e.g. Papazachos and Papazachou, 1989, 1997, 2003) or in the broader Zakynthos area (e.g. Lekkas et al., 1996-1997; Papadopoulos and Plessa, 2001; Papadopoulos and Fokaefs, 2005; Papadopoulos et al., 2010, 2014a, 2014b, 2014c; Misailidis et al., 2014; Papadopoulos, 2016). (c) Scientific literature referring to the impact of individual earthquakes in Zakynthos (e.g. Lekkas et al., 1996-1997). (d) Official field survey and reconnaissance reports (e.g. Lekkas and Mavroulis, 2018). (e) Official reports of applied scientific research projects (e.g. Lekkas, 1993). All this literature has been reviewed with emphasis given on seismic events which have induced EEE including primary (surface faulting and permanent ground dislocation of tectonic origin) and secondary effects (ground cracks, slope movements, liquefaction phenomena, hydrological anomalies, hydrocarbon seepages). Qualitative and quantitative information on EEE were collected and entered in a database specially designed and developed for the purpose of this study in Geographic Information Systems (GIS) environment. The database comprises information and data including coordinates of the EEE site, type and main category of the EEE and quantitative information such as surface rupture length and maximum surface displacement for primary effects, length, width and areal density for ground cracks, volume and mobilized material and total area affected for slope movements as well as tsunami height at shore and run-up as well as its effects on the local population, the natural and built environment among other parameters where available. This approach was developed in order to present a list of newly classified localities according to the ESI 2007 scale on the base of the EEEs induced in Zakynthos by each historical and recent earthquakes along with their dimensional characteristics and the respective environmental intensities (Tables 1 and 2).

4. Environmental effects and ESI 2007 intensities 4.1. Methodology For achieving the aim of this study, data and information on historical and recent earthquakes generated in the Central Ionian Sea and the western Peloponnese and on their impact on Zakynthos Island were obtained from the following sources: (a) Official earthquake catalogues from universities, seismological institutes and observatories (AUTH, 2019; UOA, 2019; NOA, 2019; EMSC, 2019). (b) Scientific literature containing catalogues or information of 7

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Table 1 List of sites with the EEE induced in Zakynthos Island by historical and recent earthquakes. Occurrence date of earthquake

Type of induced EEE

Area affected by EEE (affected fault block)

Description of EEE

ESI 2007 Intensity

1513, April 16

Primary effect – Fault reactivation

Ancient Psofida – Agios Elias (IIb: eastern part of Central Zakynthos fault block)

VIII-IX

Secondary effect – Slope movements

Ancient Psofida – Agios Elias (IIb: eastern part of Central Zakynthos fault block) In various sites. (IIb? eastern part of Central Zakynthos fault block) In various sites. No detailed location available. Agios Sostis Cape (IIb: southeastern part of Central Zakynthos fault block)

Fault reactivation on the hill of Kastro in Psofida – Agios Elias hill accompanied by possible generation of landslides and rockfalls that buried a part of the ancient city Landslides and rockfalls Landslides

VI-VII

Ground cracks

VI-VII

The Agios Sostis Cape was swept by the sea indicating tsunami generation Ground cracks observed in several places accompanying sulphureous gas emissions and subsequent flames coming out of ground cracks indicating combustion of natural gas Slope movements

VIII

Slope movements

VI-VII

The Agios Sostis Cape in the southeastern side of Zakynthos suffered collapse and sept away by the sea. Ground cracks

VI-VII

Small tsunami offshore Cephalonia island On October 3 coastal inundation occurred. The water was mixed with asphalt-pitch. The most violent agitation of the sea

V-VI

Ground cracks

VI-VII

Sulphureous gas emissions

VIII

Boiling of asphalt – Not included in the ESI 2007 criteria for assessing intensities Liquefaction phenomena

Intensity not assigned

Along the south bank of Agios Charalampos River (V: Skopos Mt fault block) Along the bank of Agios Charalambos River in Episkopiani district located in the southern part of Zakynthos city (V: Skopos Mt fault block) No detailed location available

Ground cracks with width of almost 4 cm accompanied by sulphureous gas emissions Sand boils, the most common form of liquefaction phenomena

VIII

Subsidence

VII

No detailed location available

Ground cracks

VI-VII

No detailed location available

Sulphureous gas emissions

VIII

No detailed location available

Roughness in the sea surface

V

No detailed location available

Local landslide or rockfalls

VI

1591, April 14 1622, May 5

1633, November 5

1636, 30 September

1791, November 2

1809, June 2

Secondary effect – Slope movements Secondary effect – Ground cracks Secondary effect – Anomalous waves/tsunamis Secondary effect – Hydrogeological anomalies

Keri or Alykes area (III: Keri Bay fault block or IIb? eastern part of Central Zakynthos fault block)

Secondary effect – Slope movements Secondary effect – Slope movements Secondary effect – Slope movements

Skopos Mt (V: Skopos fault block)

Secondary effect – Ground cracks Secondary effect – Anomalous waves/tsunamis Secondary effect – Anomalous waves/tsunamis and asphalt-pitch seepages Secondary effect – Anomalous waves/tsunamis Secondary effect – Ground cracks Secondary effect – Hydrogeological anomalies Secondary effect – Asphalt phenomena

In various sites of Zakynthos

Secondary effect – Liquefaction phenomena Secondary effect – Hydrogeological anomalies Secondary effect – Liquefaction phenomena

1820, December 29

1837, August 3

Secondary effect – Liquefaction phenomena Secondary effect – Ground cracks Secondary effect – Hydrogeological anomalies Secondary effect – Anomalous waves/tsunamis Secondary effect – Slope movements

Vrachionas Mt (IIa: Western part of Central Zakynthos fault block) Agios Sostis Cape (IIb: southeastern part of Central Zakynthos fault block)

Offshore Cephalonia Island Coastal mire/swamp zone of Keri, located in southern Zakynthos (III: Keri Bay fault block) Zakynthos Straits between Zakynthos Island and Peloponnese Zakynthos city area (V: Skopos Mt fault block) Zakynthos city area (V: Skopos Mt fault block) Keri swamp area (III: Keri Bay fault block) No detailed location available

VII

IX

VI-VII

VI-VII

VI-VII V-VI

VII

VII

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Table 1 (continued) Occurrence date of earthquake

Type of induced EEE

Area affected by EEE (affected fault block)

Description of EEE

1840, October 18/30

Secondary effect – surface deformation of secondary origin Secondary effect – Hydrogeological anomalies

No detailed location available

Ground oscillation

No detailed location available

Water effects in the surface were reported including rising of the water about 5 or 6 feet from the ground. Wells presented increased discharge.

VII

Increased flow of a river.

VIII

Some streams in the wider area overflowed their banks and flooded the adjacent areas. The groundwater level was uplifted about 1 m above earth's surface. Sulphureous gas emissions

VII

Some olive trees with height of 2–3 m touched the ground. A small island (Trenta Nove) was submerged due to sea level rise. Boiling of asphalt – Not included in the ESI 2007 criteria for assessing intensities Asphalt-pitch seepages – Not included in the ESI 2007 criteria for assessing intensities Liquefaction phenomena included ejection of sand/water mixture along ground cracks. Liquefaction phenomena included ejection of sand/water mixture along ground cracks. Liquefaction phenomena included ejection of sand/water mixture along ground cracks. Liquefaction phenomena included ejection of sand/water mixture along ground cracks. Rockfalls resulting in the destruction of a church and along the slopes of the castle. Rockfalls

VIII

Rockfalls and landslides

VI-VII

Rockfalls and landslides

VI-VII

Rockfalls and landslides along slopes

VI-VII

Rockfalls and landslides

VI-VII

Coseismic surface ruptures affected mainly embankments and secondarily stable deposits on gentler slopes. Rockslide

VIII-IX

Rockfalls

VI-VII

Moreover, many rockfalls and rockslides were generated along slopes. Sea withdrawal was reported in Zakynthos

VI-VII

Secondary effect – Hydrogeological anomalies Secondary effect – Hydrogeological anomalies Secondary effect – Hydrogeological anomalies

1893, January 19/31

A river in Zakynthos (Agios Charalambos River) (IIb: eastern part of Central Zakynthos fault block) No detailed location available

Secondary effect – Hydrogeological anomalies Secondary effect – Hydrogeological anomalies Secondary effect – Trees shaking Secondary effect – Anomalous waves/tsunamis Secondary effect – Asphalt phenomena

Avissos area close to Keri swamp (III: Keri bay fault block) Avissos area close to Keri swamp (III: Keri bay fault block) No detailed location available

Secondary effect – Asphalt phenomena

Keri swamp (III: Keri bay fault block)

Secondary effect – Liquefaction phenomena

Keri coastal area (III: Keri bay fault block)

Secondary effect – Liquefaction phenomena

Laganas Bay coastal area (IIb: Eastern part of the Central Zakynthos fault block)

Secondary effect – Liquefaction phenomena

Alykes Lagoon (IIb: Eastern part of the Central Zakynthos fault block)

Secondary effect – Liquefaction phenomena

Agios Charalambos River (V: Skopos Mt fault block)

Secondary effect – Slope movements

Kryoneri Cape (IIb: Eastern part of the Central Zakynthos fault block)

Secondary effect – Slope movements Secondary effect – Slope movements

Gerakas Beach (V: Skopos Mt fault block)

Secondary effect – Slope movements Secondary effect – Slope movements Secondary effect – Slope movements Primary effect – Coseismic surface ruptures 1893, April 5/17

No detailed location available

Secondary effect – Slope movements Secondary effect – Slope movements Secondary effect – Slope movements Secondary effect – Anomalous waves/tsumamis

Northwest of Kryoneri Cape (IIb: Eastern part of the Central Zakynthos fault block) Avissos area close to Keri swamp (III: Keri bay fault block)

Kryoneri – Bochali – Kokkinos Vrachos hill (IIb: Eastern part of the Central Zakynthos fault block) Various sites in Lithakia municipality located in the southwestern part of the island. (IIa: Western part of the Central Zakynthos fault block) Geraki Cape (V: Skopos Mt Fault block) Keri Cape (IV: Southern Zakynthos fault block) Schoinarion Cape (Korithi site) (Ι: Northern Zakynthos fault block) Along the slopes of Megalo Vouno at the western coast, close to the Agalas village (IIa: Western part of the Central Zakynthos fault block) Geraki Cape (V: Skopos Mt fault block) In the southeastern and southwestern part of the island No detailed location available No detailed location available

ESI 2007 Intensity

VII

VII VIII

VIII Intensity not assigned Intensity not assigned VII VII VII VII VI-VII VI-VII

VI-VII

V

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Table 1 (continued) Occurrence date of earthquake

Type of induced EEE

Area affected by EEE (affected fault block)

Description of EEE

ESI 2007 Intensity

1896, November 5

Secondary effect – Anomalous waves/tsumamis

Laganas Bay (IIb: Eastern part of the Central Zakynthos fault block)

VI-VII

1898, December 3

Secondary effect – Anomalous waves/tsumamis

No detailed location available

1899, January 22

Secondary effect – Anomalous waves/tsumamis

1912, January 24

Secondary effect – Ground cracks Secondary effect – Slope movements

IIb: Eastern part of the Central Zakynthos fault block III: Keri Bay fault block IV: Southern Zakynthos fault block V: Skopos Mt fault block On the cobblestone road of Zakynthos city (V: Skopos Mt fault block) Kryo Nero location of Kokkinos Vrachos area (IIb: Eastern part of the Central Zakynthos fault block) Bochali hill (IIb: Eastern part of the Central Zakynthos fault block) Zakynthos city (V: Skopos Mt fault block)

Dry torrent that streams from the villages Mouraki and Pissinontas and flows into the Laganas Bay filled in with sea water. It is not clear if this indicates small local flood due to the shock or inundation due to other cause. The sea rose 0.4 m and returned to its usual place between 10 a.m. and 3 p.m. Tsunami of about 20–40 cm in Zakynthos Island Ground cracks of approximately 20 m Rockfalls resulting in destruction of the road network

VI

Rockfalls generated along slopes

VI-VII

Ground cracks observed along the coastal road and in many sites within the city Ground cracks

VI-VII

Sulphureous gas emissions from ground cracks also noticeable. Subsidence was observed along the coastal road. Small ground cracks caused along the quay. Slope movements

VIII

A sea wave of about 0.5 m height was observed. Sea retreat in Zakynthos. Landslides and rockfalls along the steep coastal slopes and scarps Landslides and rockfalls along the steep coastal slopes and scarps Landslides and rockfalls along the steep coastal slopes and scarps Landslides and rockfalls along the steep coastal slopes and scarps A small tsunami wave generated and detected based on sea level changes (offset: 0.549 m) An increase in sea level of about half a meter (far field effect)

VI

Asphalt-pitch – water mixture observed few days after the generation of the main shock – Not included in the ESI 2007 criteria for assessing intensities

Intensity not assigned

1953, August 12

Secondary effect – Slope movements Secondary effect – Ground cracks

1959, November 15

1983, January 17 2018, October 26

Secondary effect – Ground cracks Secondary effect – Hydrological anomalies Secondary effect – Slope movement Secondary effect – Ground cracks Secondary effect – Slope movements Secondary effect – Anomalous waves/tsumamis

Kokkinos Vrachos area (IIb: Eastern part of the Central Zakynthos fault block) No detailed location available

Secondary effect – Slope movements Secondary effect – Slope movements Secondary effect – Slope movements Secondary effect – Slope movements Secondary effect – Anomalous waves/tsunamis

Navagio beach (IIa: Western part of the Central Zakynthos fault block) Mizithra beach (IV: Southern Zakynthos fault block) Panagoula (V: Skopos Mt fault block)

Secondary effect – Anomalous waves/tsunamis

Along the coast between Santa Maria di Leuca Cape and Otranto located in the eastern coast of the Salento peninsula (Italy) Marathias offshore area (IV: Southern Zakynthos fault block)

Secondary effect – Asphalt phenomena

Zakynthos city (V: Skopos Mt fault block) Zakynthos city (V: Skopos Mt fault block) No detailed location available. No detailed location available.

Kryoneri (V: Skopos Mt fault block) Offshore southwestern Zakynthos

4.2. Earthquakes, EEEs and ESI 2007 intensities

V-VI V-VI

VI-VII

VI-VII

VI-VII VI-VII VI-VII

VI-VII VII VI-VII VI VI Intensity not assigned

Ground cracks were induced by the earthquake along a NNW-SSE striking fault zone disrupting the hill of the fortress (Bochali hill) (Figs. 6 and 7a; Table 1). Based on the available description, “… the mountain of the fortress was cut from top to bottom and the hill of Agios Andreas was formed …” (Barbiani and Barbiani, 1864; Chiotis, 1886; Schreiner, 1975; Lekkas et al., 1996-1997; Papazachos and Papazachou 2003). This cracking constitutes a clear and significant evidence of the reactivation of the Zakynthos city fault zone and the formation of coseismic surface ruptures (Fig. 6; Table 1). Thus, this effect is considered as primary environmental effect induced by the 1513 earthquake and the assigned intensity is VIII-IXESI 2007 (Fig. 6;

In this paragraph we show the full records of earthquakes from 1513 to present, with magnitudes varying from 6.1 to 7.2 that have had a significant impact on the natural environment of Zakynthos Island (Fig. 5). 1513, April 16, 37.6° N, 20.8° E, h = n, M = 6.5 (Fig. 5). Very strong earthquake that caused damage in Zakynthos city, Psophida and Gialos (Fig. 6). Most of the buildings and the castle in Bochali hill (Fig. 7) were destroyed, causing many fatalities (Manousakas, 1967, Lekkas et al., 1996-1997; Papazachos and Papazachou, 2003). 10

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Table 2 Earthquakes that affected Zakynthos Island along with the induced EEE and the respective ESI 2007 intensities. FR: fault reactivation, SR: surface ruptures, HA: hydrological anomalies, AWT: anomalous waves/ tsunamis, GC: ground cracks, SM: slope movements, TREE: tree shaking, LIQ: liquefaction phenomena, H/S: hydrocarbon seepages, N/A: not assigned.

Table 1). Landslides and rockfalls (VIIESI 2007) were caused on Bochali hill (Fig. 6), which buried a part of the ancient Psophida city (Fig. 6; Table 1). The slope movements in this location are entirely justified by the morphological and geological-geotechnical conditions. More specifically, the morphological slope of the hill exceeds 100%, while it is composed of Kastro formation consisting of eroded clayish-marly beds, as well as the overlaying Gerakas formation consisting of calcitic sandstones. Due to landslides of the first formation, there is a loss of support of the overlaying rocky calcitic, sandy masses resulting in the occurrence of rockfalls. The main cause of landslide phenomena can possibly be attributed to the presence of the fault zone that cuts through the western part of the city and has simultaneously formed morphological discontinuities. The possible activation of the aforementioned fault zone, may have also caused loss of support of the entire post-alpine sequence of the hill. 1591, April 14 (Fig. 5). A strong earthquake occurred and was followed by three strong aftershocks. The castle in Bochali hill suffered damage, especially the tower and the bell tower (Konomos, 1970; Lekkas et al., 1996-1997). As regards the EEE, landslides were generated in various sites. Aside from the fact that the castle was damaged, there is no detailed geographic information about the location of these landslides. Taking into account this fact and the geological, geomorphological and geotechnical conditions and characteristics of the castle area, this is considered as the most suitable area for the generation of these landslides (Fig. 6). Moreover, no quantitative information such as the volume of the landslide material are also available. Thus, an intensity VIII-IXESI 2007 (Fig. 6; Table 1) is assigned to the generation of these slope movements. 1622, May 5, 37.7° N, 20.6° E, h = n, Μ = 6.0 (Fig. 5). The earthquake caused collapse of buildings resulting in fatalities and injuries and was followed by a large aftershock. Both of them lasted long (Barbiani and Barbiani, 1864; Katramis, 1880; Zois, 1893; Tsitselis,

1960; Mouyiaris, 1994; Lekkas et al., 1996-1997; Papazachos and Papazachou, 2003). The mainshock caused ground cracks (VI-VIIESI 2007) in various sites, while on the southern part of the island the Agios Sostis Cape was swept by the sea indicating the generation of a tsunami (Galanopoulos, 1960; Lekkas et al., 1996-1997) (VIIIESI 2007; Fig. 6; Table 1). 1633, November 5, 37.7° N, 20.8° E, h = n, M = 7.0 (Fig. 5). Α strong earthquake occurred in Zakynthos causing collapse of many residential buildings and consequently many fatalities. As far as the induced secondary EEE are concerned, ground cracks, slope failures and a small tsunami wave were generated (Galanopoulos, 1960; Lekkas et al., 1996-1997; Papadopoulos and Plessa, 2001; Papadopoulos et al., 2010) (Fig. 6; Table 1). More specifically, ground cracks were observed in several places and flames were coming out of them indicating that sulphureous gas emissions probably burnt along cracks. An IXESI 2007 intensity was assigned to these hydrological anomalies (Fig. 6; Table 1). Slope failures were generated probably along the high mountains of Zakynthos (Skopos Mt or Vrachionas Mt) (Figs. 6 and 7b; Table 1) corresponding to a VI-VIIESI 2007 intensity (Fig. 6; Table 1). A landslide happened also during the 1633 earthquake, when the Agios Sostis Cape sunk and the small homonymous island was created (Fig. 6; Table 1). The collapses/rock failures occurred in a part of the peninsula and in rocky sections of Keri marly limestones deposited unconformably on the marls of Agios Sostis formation probably eroded by sea waves. As a result of this landslide, the edge of the peninsula was cut off from the island. The intensity assigned to this site is VI-VII ESI 2007 (Fig. 6; Table 1). Papadopoulos (1994) supported that an active fault occurs between the Agios Sostis and the adjacent land and its activation resulted in cut-off. The occurrence of an active fault is not valid because (i) there is an uninterrupted continuity of the beds of Agios Sostis formation both offshore and onshore and (ii) the unconformity surface from both sides of the marine channel between 11

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Fig. 5. The epicenters of the studied earthquakes along with the major onshore and offshore faults of the Central Ionian Sea. Onshore faults from Lekkas (1993) and Lekkas et al. (2015), offshore faults from Makris and Papoulia (2014) and earthquake epicenters from UOA (2019), AUTH (2019) and Papazachos and Papazachou (1989, 1997, 2003).

Agios Sostis and Zakynthos Islands is not deformed by the occurrence and the activation of faults. 1636, 30 September, 38.1° N, 20.3° E, h = n, M = 7.2 (Fig. 5). This earthquake had significant impact on Cephalonia and Zakynthos islands and its aftershock sequence lasted until next spring (Sathas, 1867; Partsch, 1890; Zois, 1893; Romas, 1973; Mouyaris, 1994; Spyropoulos, 1997; Papazachos and Papazachou, 2003). Especially in Zakynthos, many residential buildings collapsed resulting in hundreds of fatalities. A fire that started from the powder keg of the castle spread quickly and aggravated damage and losses. In Strophades monastery the tower and parts of the walls collapsed. As far as the ΕΕΕ are concerned, only secondary effects were observed. Ground cracks were formed in various sites in Zakynthos, while the captain of a ship noticed a small tsunami offshore Cephalonia island (Lekkas et al., 1996-1997; Papazachos and Papazachou, 2003) (Table 1). The exact locations of these phenomena are not available, but VI-VIIESI 2007 and V-VIESI 2007 are assigned based on the guidelines of the ESI 2007 (Table 1). On October 2, weaker seismic events followed the mainshock of September 30, while on October 3 coastal inundation occurred. It is significant to note that the water was mixed with asphalt-pitch. The location of these phenomena is indirectly extracted. Taking into account that the coastal mire/swamp zone of Keri, located in southern Zakynthos, is famous for the asphalt-pitch seepages (Nikolaou, 1986,

2001; Avramidis et al., 2017) and the fact that similar phenomena were also observed after the 1791 and 1840 earthquakes (Lekkas et al., 19961997) and in Marathias offshore area after the 2018 Zakynthos earthquake (this study), it can be inferred that the asphalt-pitch – water mixture after the 1636 event was also observed in the Keri offshore area. The mechanism of the asphalt-pitch seepages are described in detail by Nikolaou (1986, 2001): the water infiltrates through fault and cracks from the Vrachionas Mt. limestones into deeper layers and exfiltrates through marginal normal faults of Keri swamp. The Keri swamp is separated from the open sea by a low relief sand barrier and it communicates with the open sea when the barrier is open (Fig. 7). Based on this data and after considering these phenomena as hydrological anomalies, an intensity VI-VIIESI 2007 is assigned to the phenomena observed in the Keri coastal area (Fig. 6; Table 1). 1791, November 2, 37.9° N, 21.0° E, h = n, M = 6.8 (Fig. 5). The western part of Zakynthos Island remained intact by this earthquake, while the eastern hilly areas suffered damage. The aftershock sequence lasted for more than 6 weeks, while the largest aftershock occurred 8 days after the generation of the mainshock. The mainshock claimed the life of 20 people, while 300 people were injured. The most violent agitation of the sea occurred in Zakynthos Straits (V-VIESI 2007) between Zakynthos and Peloponnese (Table 1), while due to the earthquake ground cracks (VI-VIIESI 2007), sulphureous gas emissions in Zakynthos city area (VIIIESI 2007) and boiling of asphalt 12

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in Keri swamp area (intensity not assigned) were also observed (SaintSauver, 1800; Mallet, 1858; Barbiani and Barbiani, 1864; Katramis, 1880; Zois, 1893; Chiotis, 1886; Barbiani, 1863; Kolyva, 1997; Lekkas et al., 1996-1997; Papazachos and Papazachou, 2003) (Figs. 6 and 7c; Table 1). Liquefaction phenomena were also observed without further detailed information on exact location and quantitative or qualitative characteristics (VIIESI 2007; Table 1). 1809, June 2 (Fig. 5). On June 2, an explosion of Etna volcano occurred. At the same time in Zakynthos Island, ground cracks with width of almost 4 cm were observed along the south bank of Agios

Charalampos River accompanied by sulphureous gas emissions and liquefaction phenomena (Chiotis, 1886; Zois, 1893; Barbiani and Barbiani, 1864; Lekkas et al., 1996-1997) (Fig. 6; Table 1). The site affected by liquefaction during this event is precisely determined along the bank of Agios Charalambos River in Episkopiani district located in the southern part of Zakynthos city (Fig. 6). Geological maps show that recent marsh and coastal deposits occur, while the aquifer is almost on the ground surface, due to its low elevation and its proximity to the sea and the Agios Charalambos River. Thus, the occurrence of the liquefaction in the form of sand boils, which is the most common type, is

Fig. 6. EEE and related assigned intensities for 16 historical and recent earthquakes that have partially or totally affected Zakynthos Island. It is concluded that anomalous waves/tsunamis are the most frequently reported EEE followed by slope movements, ground cracks, hydrological anomalies, liquefaction phenomena, and hydrocarbon seepages. Primary EEE including fault reactivation and coseismic surface ruptures have been also observed. The most susceptible areas to the generation of EEE are the Skopos Mt fault block, followed by the eastern part of the Central Zakynthos fault block, the Keri Bay fault block, the western part of the Central Zakynthos fault block, the Southern Zakynthos fault block and the Northern Zakynthos fault block. 13

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Fig. 6. (continued)

completely justified and confirmed by the existing geological - geotechnical conditions. The assigned intensities are VIIIESI 2007 for the observed hydrological anomalies and VIIESI 2007 for the liquefaction phenomena (Fig. 6; Table 1). 1820, December 29, 03:45, 37.8° N, 21.1° E, h = n, M = 6.9 (Fig. 5). The earthquake lasted for 60 s and a rotational movement was noticeable. Earthquake precursor phenomena included unusual explosive sounds reported by the local population 4 h before the main shock as well as luminous phenomena just before the main shock. It was a destructive event during which the recent structures of the city behaved better that the older ones. Structures along the coast suffered the most. 79 buildings in Zakynthos Island were totally destroyed, while 807 suffered significant damage resulting in 6 fatalities and 29 injured (Papazachos and Papazachou, 2003). Based on Lekkas et al. (1996-

1997), 500 buildings collapsed, more than 1000 were still standing but severely damaged, 4 fatalities and 39 injured people were reported. As regards the EEE, ground cracks (VI-VIIESI 2007), sulphureous gas emissions (VIIIESI 2007), roughness in the sea surface (VESI 2007) and subsidence (VIIESI 2007) (Table 1) were induced by the main shock as well as post-earthquake luminous phenomena and explosive sounds during the aftershock sequence (Chiotis, 1886; Lekkas et al., 19961997; Papazachos and Papazachou, 2003). On January 6, 1821, at 20:00, an earthquake occurred and lasted for 20 s and razed the still standing buildings to the ground. Damage were also observed in Elis (Lalas village) and Achaia (Patras) in western and Northwestern Peloponnese respectively (Perrey, 1850; Mallet, 1858; Mercati, 1811; Pouqueville, 1824; Barbiani and Barbiani, 1864; Papazachos and Papazachou, 1989, 1997, 2003). Both earthquakes were accompanied 14

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(Fig. 5). Earthquakes occurred on October 13, 21 and 30 October. The last one was the most destructive event. The earthquake was felt throughout the Ionian Islands, in Epirus located in northwestern Greece and in Messinia located in Southwestern Greece (Peloponnese) (Perrey, 1848; Mallet, 1858; Barbiani and Barbiani, 1864; Chiotis, 1886; Sieberg, 1932; Lekkas et al., 1996-1997; Papazachos and Papazachou, 2003). The mainshock was followed by 100 aftershocks during the first week. Earthquake precursor phenomena were observed and included luminous phenomena and unusual explosive sounds. 12 people were fatally injured and many others were only slightly injured. Secondary EEE included ground oscillation, hydrological anomalies, sea level rise or tectonic subsidence, trees shaking, boiling of asphalt, liquefaction phenomena and slope failures (Fig. 6; Table 1). Water effects in the surface were reported including rising of the underground water about 5 or 6 feet from the ground in the area of Avissos (VIIESI 2007) (Figs. 6 and 7c; Table 1). According to geological maps, an active fault zone disrupts the area and separates the Keri Bay fault block from the western part of the Central Zakynthos fault block. The detected changes in the spring of Avissos can be attributed to: (i) the activation of Keri fault zone resulting in easier escape routes of the underground water, (ii) the rotation of the fault blocks, and especially the Vrachionas fault block, resulting in the deformation of the underground aquifer and (iii) the reduction of porosity of loose formations due to the earthquake. Moreover, some wells presented increased discharge (VIIESI 2007), a river in Zakynthos (Agios Charalambos River) presented increased flow (VIIIESI 2007) and some streams in the wider area overflowed their banks and flooded the adjacent areas (VIIESI 2007) (Fig. 6; Table 1). A small island (Trenta Nove) located northwest of Kryoneri Cape was submerged due to sea level rise (VIIIESI 2007) (Fig. 6; Table 1). Some olive trees with height of 2–3 m touched the ground (VIIIESI 2007) (Table 1). Boiling of asphalt was observed in Avissos area. Slope failures comprised rockfalls in Kryoneri Cape, in Gerakas located in the southeastern part of Zakynthos (Fig. 7) resulting in the destruction of a church and along the slopes of the castle (VI-VIIESI 2007) (Fig. 6; Table 1). Liquefaction phenomena included ejection of sand/water mixture along ground cracks along beaches (VIIESI 2007) (Fig. 6; Table 1). According to Lekkas (1993), coastal sites in which such phenomena can occur are the Keri coastal area, Laganas Bay, Agios Charalambos River and Alykes Lagoon (Figs. 6 and 7d; Table 1). Recent marsh and coastal deposits occur in these locations. These formations are characterized by the proper lithological composition, geotechnical characteristics and hydrogeological conditions for the generation of liquefaction phenomena of the aforementioned type during an earthquake. 1893, January 19/31 (Fig. 5). On January 19 and 20, two earthquakes caused major damage (Lekkas et al., 1996-1997). The first earthquake caused damage to Zakynthos city, while the second affected mainly rural settlements. Except from damage, the earthquake claimed many lives. As regards the induced EEE, slope failures occurred. More specifically, landslides were observed in Bochali, Kokkinos Vrachos and Kryoneri areas in the eastern part of the Central Zakynthos fault block, landslides and rockfalls in various sites in Lithakia municipality located in the southwestern part of the island, rockfalls in Geraki Cape in the southeastern part of Skopos Mt fault block as well as landslides and rockfalls in the southern and western coastal slopes of Zakynthos Island (VI-VIIESI 2007) (Figs. 6 and 7e; Table 1). Moreover, during the 1893 earthquake, ground cracking was observed in the northern part of Zakynthos Island (Korithi site, close to Skinari Cape in the northern edge of Zakynthos Island) (Zois, 1893; Issel, 1893; Issel and Agamennone, 1894; Mondessus de Ballore, 1990; Lekkas et al., 1996-1997), where an E-W striking active fault runs through this area. This effect indicates the reactivation of this fault, which affected mainly embankments and secondarily unstable deposits on gentler slopes (Lekkas et al., 1996-1997) and thus, it is considered as primary environmental effect (VIII-IXESI 2007) (Fig. 6; Table 1). 1893, April 5, 37.68° N, 20.81° E, h = n, M = 6.5 (Fig. 5). This

Fig. 7. Views of characteristic sites affected more often and severely by environmental effects induced by the historical and recent earthquakes on Zakynthos Island. (a) Partial view of the Vrachionas Mt in the area of Skoulikado village where the Ionian Thrust is located. (b) Panoramic view of Zakynthos city from SE to NW, with Bochali hill composed of gray, bluish and brown clayey-marly beds with sandstone and sandy intercalations. This site is prone and vulnerable to the generation of earthquake-induced landslides and rockfalls. (c) Skopos Mt is located southeast of Zakynthos City. It is composed mainly of alpine formations of Ionian geotectonic unit and post-alpine deposits. The Skopos Mt fault block is prone to the generation of earthquake-induced slope movements. (d) Gerakas beach and Geraki Cape in the eastern end of the Skopos Mt fault block. (e, f) The impressive and famous Navagio beach in the northwestern part of the Central Zakynthos fault block. It was affected by rockfalls during September 2018 before the generation of the October 26, 2018 earthquake and during the mainshock as well. (g) The coastal area of Keri bay and the Keri swamp, which has suffered repeatedly by hydrological anomalies including sulphureous gas emissions, anomalous waves including tsunamis, hydrocarbon seepages and liquefaction. (h) The Keri and Laganas coastal area often affected by tsunamis and Agios Sostis Cape in the left part of the picture often affected by slope movements. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

by stormy weather with heavy rainfall and flash floods (Pouqueville, 1824; Papazachos and Papazachou, 2003; Papadopoulos and Plessa, 2001; Papadopoulos et al., 2010). 1837, August 3 (Fig. 5). An earthquake was felt at around 9 am in Zakynthos, Cephalonia (Schmidt, 1879) and Peloponnese (Mourikis, 1934) and induced a local landslide or rockfalls (Papadopoulos and Plessa, 2001) corresponding to VIESI 2007 intensity (Table 1). No further description for the location and the quantitative parameters are available for these phenomena. 1840, October 30, 09:29, 37.8° N, 20.8° E, h = n, M = 6.5 15

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earthquake is the largest of a seismic sequence that started in December of 1892 and continued until May 1893 (Mitsopoulos, 1893; Philipson, 1893; AOA, 1899). It was a destructive event, that not only aggravated the damage induced by the January 19/31, 1893 foreshock, but also caused further building effects. It was predominantly felt in Zakynthos and secondarily in Strophades Islands, the northwestern Peloponnese (Patras, Katakolon, Pyrgos, Kyllini) and the western mainland Greece (Messolonghi). The main shock caused 23 fatalities, and 100 injured resulted from heavy damage to Zakynthos city and its suburbs including Bochali in its northwestern part and to villages located in the southern and southwestern part of the island. Many of them were completely devastated (Keri, Gaitani, Lithakia, Episkopiana, Agalas). 2000 buildings were totally destroyed and 1700 suffered heavy structural damage from a building stock comprising 4500 buildings. Slope movements and anomalous waves/tsunamis were generated (Zois, 1893; Issel and Agamennone, 1894; Lekkas et al., 1996-1997). A rockslide occurred along the slopes of Megalo Vouno located at the western coast, close to the Agalas village (VI-VIIESI 2007) (Fig. 6; Table 1). Rockfalls were generated in Geraki Cape located in the southeastern part of Zakynthos Island (VI-VIIESI 2007) (Figs. 6 and 7f; Table 1). Moreover, many rockfalls and rockslides were generated along slopes in the southeastern and southwestern part of the island (VI-VIIESI 2007) (Table 1). Sea withdrawal was reported in Zakynthos (VESI 2007) (Table 1) (Papadopoulos et al., 2010). 1896, November 5 (Fig. 5). This is a moderate shock which occurred in the area of Zakynthos. According to AOA (1896) the dry torrent that streams from the villages Mouzaki and Pantokratoras and flows into the Laganas Bay located in the southern side of Zakynthos Island, filled in with sea water (VI-VIIESI 2007) (Fig. 6; Table 1). However, it is not clear if this indicates small local flood due to the shock or inundation due to other cause. Papadopoulos et al. (2010) also referred to sea-water inundation in Zakynthos and assigned intensity II in the Seiberg-Ambraseys 6-grade scale (Ambraseys, 1962) and II to Papadopoulos and Imamura 12-grade scale (Papadopoulos and Imamura, 2001). 1898, December 3, 37.42° N, 20.48° E (Fig. 5). This is another strong earthquake which occurred in Zakynthos. According to AOA (1898) the sea rose 0.4 m and returned to its usual place between 10 a.m. and 3 p.m. (V-VIESI 2007) (Table 1). Papadopoulos et al. (2001) also referred to the induced sea-wave and assigned intensity II in the Seiberg-Ambraseys 6-grade scale (Ambraseys, 1962) and II to Papadopoulos and Imamura 12-grade scale (Papadopoulos and Imamura, 2001). 1899, January 22, 09:56, 37.2° N, 21.6° E, h = n, M = 6.5 (Fig. 5). This very strong earthquake caused no fatalities, many injuries, severe damage and extensive EEE in towns and villages in the western part of Peloponnese (Galanopoulos, 1941, 1947, 1955). Slope movements included mainly landslides and rockfalls occurred in Messinia, hydrological anomalies in Messinia and Arcadia, liquefaction and slumping of geological material along the Ionian coast of Messinia and the onshore area of Messiniakos Gulf. The subsequent earthquake-induced tsunami was about 1 m high and resulting in inundation of Marathopolis coastal area, while in Zakynthos Island was about 20–40 cm (V-VIESI 2007) (Fig. 6, Table 1) (Mitsopoulos, 1900; Galanopoulos, 1941; Papazachos and Papazachou, 1989, 1997, 2003; Ambraseys and Jackson, 1990; Ambraseys, 2009). The tsunami was possibly triggered by submarine slumps, but no damage occurred to the submarine cables between Zakynthos and Western Peloponnese (Galanopoulos, 1941). Mavroulis and Lekkas (2018) assigned a tsunami intensity VI-VII to Marathopolis coastal area and VI to Zakynthos Island based on the ITIS-2012 12-grade scale. 1912, January 24, 16:22:51, 38.11° N, 20.67° E, h = n, M = 6.8 (Fig. 5). The earthquake partially devastated Cephalonia and Zakynthos. Villages in the northern part of Zakynthos were destroyed in their largest part (Papazachos and Papazachou, 2003). On the cobblestone road of Zakynthos city, ground cracks of approximately 20 m

were also observed (VIESI 2007) (Fig. 6; Table 1). 1953, August 12, 11:23:52 (local time), 38.3° N, 20.8° E, h = n, M = 7.2 (Fig. 5). The 1953 Cephalonia earthquake sequence comprised three strong and destructive earthquakes. The first one occurred on August 9 at 09:41 with magnitude M 6.4, the second one on August 11 at 05:32 with magnitude M 6.8 and the third one on August 12 at 11:23 with magnitude M 7.2. The largest earthquake was followed by many aftershocks, the strongest of which happened the same day as the main shock. These earthquakes were predominantly felt throughout Ionian Islands and completely destroyed their central part comprising Lefkada, Cephalonia, Ithaca and Zakynthos. The last one was also felt in southern Italy. The first event caused significant damage to Cephalonia and Ithaca, while Zakynthos was affected to a lesser extent. The second event also caused significant damage to Zakynthos, while the third event completely devastated the Central Ionian Islands. From a building stock of 33300 houses in total, 27659 were completely destroyed, 2780 were severely damaged, 2394 suffered less damage and only 467 remained untouched. The earthquakes claimed the life of 455 residents and 2412 got injured. Damage were also observed in Aetoloakarnania (western mainland Greece) and in Elis (Northwestern Peloponnese). The largest intensities (IX-X) were assigned to Argostoli, Lixouri, Valsamata, Asprogeraka, Havdata and Agia Efthimia on Cephalonia Island and in Volimes (IX+) and the Zakynthos city (IX) on Zakynthos Island. More specifically, Zakynthos city was totally destroyed by a postearthquake fire that lasted for days. As regards the EEE in Zakynthos Island, slope failures, ground cracks, and sulphureous gas emissions were reported (Fig. 6; Table 1) (e.g. BGINOA, 1953; Di Filippo and Marcelli, 1954; Galanopoulos, 1955; Stravolaimos, 1958; Lekkas et al., 1996-1997; Papazachos and Papazachou, 2003). Rockfalls were generated in Kryroneri location of Kokkinos Vrachos area (VI-VII ESI 2007) (Fig. 6; Table 1) resulting in destruction of the road network as well as along the slopes of Bochali hill (VI-VII ESI 2007) (Fig. 6; Table 1). Ground cracks were observed along the coastal road and in many sites within the city and in Kokkinos Vrachos area (VI-VII ESI 2007) (Fig. 6; Table 1). Sulphureous gas emissions from ground cracks were also noticeable (VIII ESI 2007) (Table 1). 1959, November 15, 17:08:43, 37.78° N, 20.53° E, h = n, M = 6.8 (Fig. 5). The earthquake caused little damage to Zakynthos Island and minor in Elis and Aetoloakarnania. Subsidence due to slope instability was observed along the coastal road (VI-VIIESI 2007) (Fig. 6; Table 1) and small ground cracks were caused along the quay of Zakynthos city (VI-VIIESI 2007) (Fig. 6; Table 1) (BGINOA, 1959). According to the data, the coastal road was constructed over coastal deposits and debris originated by collapsed buildings resulted from the devastating 1953 Cephalonia earthquakes. Earlier on that day (15:40) a foreshock (M 4.0) occurred, while the strongest aftershock occurred on December 1 (12:38, M 5.8) (Papazachos and Papazachou, 1989, 1997, 2003). 1983, January 17, 12:41:31, 38.1° N, 20.2° E, h = n, Mw = 7.0 (Fig. 5). It was a very strong earthquake, but only moderately damaging in Cephalonia Island. No damage was caused in Zakynthos, where local people reported that during the earthquake, a sea wave of about 0.5 m height was observed (VIESI 2007) (Table 1) (Eleftheriou and Mouyiaris, 1983; Papadopoulos et al., 2010). Papadopoulos et al. (2010) also referred to sea retreat in Zakynthos and assigned intensity II in the Seiberg-Ambraseys 6-grade scale (Ambraseys, 1962) and III to Papadopoulos and Imamura 12-grade scale (Papadopoulos and Imamura, 2001). 2018, October 26, 22:54:52, 37.52° N 20.55° E, h = 14 km, Mw = 6.8 (Fig. 5). The last earthquake was generated offshore southwestern Zakynthos and it was predominantly felt on Zakynthos and throughout the Ionian Islands, Peloponnese and the mainland Greece fortunately with no casualties or injuries reported. Despite the fact that it was a strong earthquake with significant recorded time-histories of acceleration, velocity and displacement (Karakostas et al., 2018), the new modern reinforced concrete buildings with infill and partition 16

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Table 3 The earthquakes of Zakynthos Island and the affected fault blocks along with the maximum assigned ESI 2007 intensities. I: Northern Zakynthos fault block, IIa: the westen part of the Central Zakynthos fault block, IIb: the eastern part of the Central Zakynthos fault block, III: Keri bay fault block, IV: Southern Zakynthos fault block, V: Skopos fault block, N/A: not assigned.

of the Sea Level Station Monitoring Facility (http://www.iocsealevelmonitoring.org/station.php?code=kata&period=0.5& endtime=2018-10-26T09:00). Based on several witnesses, an increase in sea level of about half a meter were observed along the coast between Santa Maria di Leuca Cape and Otranto located in the eastern coast of the Salento peninsula (Italy) (Table 1). Asphalt-pitch – water mixture was also observed in Marathias offshore area few days after the generation of the main shock, but intensity was not assigned (Fig. 8; Table 1).

Fig. 8. Representative photos from environmental effects induced by the 2018 Zakynthos earthquake. Landslides and rockfalls were observed (a, b, c, d) in the areas of Bochali – Kokkinos Vrachos – Kryoneri Cape, (e) along the northwestern part of Central Zakynthos fault block and (f) in Navagio beach. (g, h) Asphalt-pitch seepage was reported after the mainshock in the offshore area of Marathias located southeast of Keri swamp.

walls behaved very well during the earthquake and withstood safely and successfully the strong motions. Traditional buildings of Zakynthos Island suffered non-structural damage including detachment of plasters from the masonry load-bearing walls and cracks of the masonry walls. During the subsequent aftershocks, slight non-structural damage to infill walls of reinforced concrete buildings was also observed. The most significant damage was observed in the southern part of Zakynthos and more specifically in Kalamaki, Laganas, Lithakia Keri, Machairado and Lagopodo settlements located in the coastal area of Laganas bay. Severe damage comprising longitudinal cracks of the jetties parallel to the seashore as well as detachment, displacement and rotation of the quay seawalls were observed in Zakynthos port, but it remained operational. As regards the environmental effects induced by the 2018 Zakynthos earthquake, secondary earthquake environmental effects were observed including slope failures, a small tsunami and asphaltpitch seepages. Landslides and rockfalls were mainly generated in various parts of the island and more specifically along the steep coastal slopes and scarps in its northwestern part (e.g. Navagio beach, VIIESI 2007) (Figs. 6, 7g-7h, 8; Table 1), in its southwestern part (e.g. along Mizithres beach, VIIESI 2007) (Fig. 6; Table 1), in its central-eastern part (e.g. Panagoula and Kryoneri areas, VI-VIIESI 2007 and VIESI 2007 respectively) (Figs. 6 and 8; Table 1) (Lekkas and Mavroulis, 2018). A small tsunami wave was generated offshore southwestern Zakynthos and was detected based on sea level changes (offset: 0.549 m) (VIESI 2007) (Fig. 6; Table 1) that occurred after the earthquake and were recorded by the Katakolo (offshore central western Peloponnese) station

5. Discussion The aim of this study is providing a contribution to seismic hazard evaluation of Zakynthos Island through the recording of the EEE over the past five centuries and the study of possible correlation with the seismotectonic structure of the affected area. According to the results of geological mapping and to historical seismicity data of the Central Ionian Sea, major active structures have been activated repeatedly in the geological past of Zakynthos Island and they are still active during the recent period. These major structures are faults mainly in the eastern, northern and southern parts of the island (e.g. Zakynthos City, Volimes and Keri areas respectively). These fault systems caused several primary and secondary earthquake environmental effects comprising boiling of asphalt and sulphureous gas emissions and subsequent ignitions, landslides and rockfalls as well as cracking of formations. All these factors strongly affect the extent and severity of disaster-related impact. The complete seismic history of destructive historical and recent earthquakes presented herein comprises 20 significant earthquakes generated not only onshore Zakynthos Island but also offshore with great impact on population, nature, buildings and infrastructures of Zakynthos. The impact on the natural environment comprises mainly secondary effects including hydrological anomalies, anomalous waves/ 17

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Fig. 9. The most affected fault blocks on Zakynthos Island along with the areas with significant building damage and areas with frequent generation of EEE.

tsunamis, ground cracks, slope movements, tree shaking and liquefaction phenomena. Primary effects have also been reported after strong earthquakes in Zakynthos Island including reactivation of faults and coseismic surface ruptures (Tables 1 and 2). Primary effects in the Central Ionian Islands have been recently observed also on Cephalonia Island after the early 2014 Cephalonia earthquakes (Lekkas and Mavroulis, 2015, 2016). In Lefkada Island surface rupturing of tectonic origin has never been observed during strong earthquakes, while rare small-length cracks were considered as secondary phenomena (Rondoyanni et al., 2012; Lekkas et al., 2016, 2018). Based on the guidelines for applying ESI 2007 scale provided by Michetti et al. (2007) and updated by Audemard et al. (2015), the sulphureous gas emissions are considered as hydrological anomalies in the environment and correspond to intensities from VIIIESI 2007 to XIIESI 2007. Lekkas et al. (1996-1997) strongly related these emissions with liquefaction phenomena generated in recent deposits that contain many trapped plant remains and produce natural gas during the decomposition phase. Both processes during an earthquake have the potential to create gas escape routes such as cracks in the fractured formations or sand dikes in the liquefied deposits respectively and to release the trapped sulphureous gas through the upward flow. Based on the study

of the environmental effects induced by Zakynthos earthquakes, sulphureous gas emissions were observed during the 1633, 1791, 1809, 1820, 1840 and 1953 earthquakes in various sites along not only with hydrological anomalies but also with liquefaction phenomena. Thus, environmental seismic intensities for sulphureous gas emissions were assigned taking into account the accompanied phenomena observed in each site (Table 1). The occurrence of asphalt-pitch seepages and sulphureous gas emissions are associated with existing hydrocarbon deposits in the subsoil. These effects have been generated in the southern and the northeastern parts of the island, which are characterized by the presence of large fault blocks bounded by major fault zones, and are generally attributed to the fractured geological formations and the differential movement of the fault blocks bounded by fault zones, through which the hydrocarbon products flow upwards. According to the data presented, similar phenomena including boiling of asphalt, sulphureous gas emissions, asphalt-pitch seepages and sea water/oil mixture occurred in 1636, 1791, 1840 and 2018 earthquakes and they affected the Keri, Avissos and Marathias areas in the southern part as well as Alykes areas in the northeastern part of the island respectively. Based on the available data and information on the environmental 18

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effects induced by historical earthquakes, it becomes clear that related data are often limited to the earthquake occurrence date with no further detailed information on the exact location and quantitative characteristics (e.g. length, width, density, volume) of the induced environmental effects. However, by taking into account these descriptions and correlating them with the geological and tectonic setting of the affected areas, it is likely to gain valuable information about not only the generation of these effects but also about understanding of the neotectonic processes. For understanding this fact, some characteristic examples are mentioned. During the 1622 earthquake “… many ground cracks were created …” and during the 1633 earthquake “… many holes appeared in the ground …” with no detailed geographic information mentioned. As a result it is impossible to conclude whether it is a primary effect or a secondary one and to assign the corresponding seismic intensity. The 1791 and 1820 earthquakes also caused ground cracks with no determination of the exact location. The 1893, 1953 and 1959 earthquakes caused ground cracks in various sites with recent fills and embankments (e.g. along the roads constructed on the coastal areas of Zakynthos city). The formation of ground cracks in these areas is frequent during an earthquake and it is attributed to the compaction of loose materials or to pre-existing lateral instability. Thus, these ground cracks are considered as secondary phenomena attributed to the ground shaking and not as primary effects related to the surface expression of the seismogenic fault. In contrast, the following description “… the mountain of the fortress was cut from top to bottom and the hill of Agios Andreas was formed …” refers to ground cracking induced by the 1513 earthquake along a NNW-SSE striking fault zone disrupting the particular location and constitutes a clear and significant evidence of the reactivation of this fault zone and the formation of coseismic surface ruptures. Moreover, during the 1893 earthquake, ground cracking was observed in the northern part of Zakynthos Island (Korithi site, close to Schinarion Cape), where an E-W striking active fault disrupting the area. This effect indicates the reactivation of this fault.

has been affected more often and severely by earthquakes and their subsequent geodynamic phenomena that the other parts of the island (Fig. 8; Table 3). This selective distribution of EEE is attributed to the geological structure of the western part of Zakynthos, which is composed mainly of alpine formations and more specifically of Vrachionas and Keri limestones, while the eastern part of the island is composed mainly of post-alpine deposits with all these characteristics and properties that make them susceptible to the generation of these phenomena. (e) It is a true and indisputable fact that specific areas of Zakynthos Island are characterized by geological and geotechnical conditions ideal for the generation of earthquake environmental effects resulting in high susceptibility and vulnerability. The recording of the EEE over the past six centuries and the study of possible correlation between these effects and the geological and tectonic structure of the affected areas contribute to the proper and effective urban design and planning along with land use management. Moreover, this approach can contribute to the reduction of the vulnerability of island and mainland urban areas against earthquakes and the earthquake-induced effects on the natural environment and as a result to the reduction of the earthquake disaster risk and the subsequent risks of EEE. Zakynthos as well as the Central Ionian Islands and many more island and mainland regions can benefit from this study. It is not only of historical interest and important from an academic point of view, but it could constitute a basic guide for the future urban design and planning and the sustainable local development since all scientists and agencies competent to the prevention and management of natural disasters can informed and guided. For developing modern and novel risk mitigation strategies, it is fundamental to consider, study and evaluate the effect of active faults and seismic zones to the formation and evolution of the seismic active areas and to emphasize the role and the contribution of the EEE on the assessment of the national and regional seismic hazard.

6. Conclusions

Conflict of interest

The study of earthquake environmental effects (EEE) and the respective seismic intensities for the Zakynthos Island, has confirmed the importance of this approach for identifying the areas with high earthquake-related hazards. In detail, the most relevant results of this study are:

All authors have participated in (a) conception and design, or analysis and interpretation of the data; (b) drafting the article or revising it critically for important intellectual content; and (c) approval of the final version. This manuscript has not been submitted to, nor is under review at, another journal or other publishing venue. The authors have no affiliation with any organization with a direct or indirect financial interest in the subject matter discussed in the manuscript.

(a) Anomalous waves/tsunamis are the most frequently reported EEE in the 20 historical and recent earthquakes of this study (11 out of 20 events), followed by the slope movements (10 out of 20), ground cracks (7 out of 20), hydrological anomalies (6 out of 20), liquefaction phenomena (4 out of 20) and hydrocarbon seepages (4 out of 20) (Table 2). Primary effects are limited to fault reactivation (1 out of 20 events) and coseismic surface ruptures (1 out of 20) (Table 2). (b) The maximum assigned local environmental seismic intensities are VIII-IXESI-07 for the Northern Zakynthos fault block, VIIESI-07 for the western part of the Central Zakynthos fault block, VIII-IXESI-07 for the eastern part of the Central Zakynthos fault block, IXESI-07 for the Keri Bay fault block, VIIESI-07 for the Southern Zakynthos fault block and VIIIESI-07 for the Skopos Mt fault block (Tables 1 and 3). (c) The most susceptible areas to the generation of EEE are the Skopos Mt fault block, which has been affected by EEE during 11 earthquakes, followed by the eastern part of the Central Zakynthos fault block with EEE during 9 earthquakes, the Keri Bay fault block affected by EEE during 5 earthquakes, the western part of the Central Zakynthos fault block affected 4 times, the Southern Zakynthos fault block affected 3 times and the Northern Zakynthos fault block affected only once (Fig. 9; Table 3). (d) Based on this geographical distribution of the EEE and the affected fault blocks, it is concluded that the eastern half of Zakynthos Island

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