Summary:
Earthquake environmental effects are natural phenomena triggered by an earthquake. They can be used as a useful and diagnostic tool for the assessment of the environmental seismic intensities. They are classified into primary and secondary effects. Primary effects are directly related to seismic energy and include surface expressions of the seismic source comprising surface faulting, surface ruptures and permanent surface deformation of tectonic origin (uplift or subsidence, tilting). Secondary earthquake environmental effects are attributed to the ground motion and are classified into the following 8 categories: (1) hydrogeological anomalies, (2) anomalous waves / tsunamis, (3) ground cracks, (4) slope failures, (5) effects on vegetation, (6) liquefaction phenomena, (7) dust clouds and (8) jumping stones.
The recording, analysis and understanding of the earthquake environmental effects provides basic and essential data and important information not only for understanding the type and the basic parameters of an earthquake, but also for estimating the magnitude of the ground motion and above all the seismic intensity in the earthquake-affected area. Moreover, they are independent of the cultural and local socio-economic conditions and of the various construction techniques and building practices adopted from time to time. Therefore, they can be used to assess seismic intensity of not only recent but also of historical earthquakes, to compare intensities of past earthquakes with intensities of recent earthquakes and to compare intensities of earthquakes in different tectonic environments.
The creation of the ESI (Environmental Seismic Intensity) scale 2007 (Michetti et al., 2007) was based on this advantage of the accompanying geodynamic phenomena. This is a 12-degree scale with the basic structure of existing macroseismic 12- degree scales. It takes into account all the seismic effects on the environment. By applying this scale, the following can be assessed: (a) the epicentral intensity based on the total surface rupture length and the maximum displacement induced by the earthquake as well as on the spatial distribution of the secondary earthquake environmental effects and (b) the local intensities, based on the qualitative and quantitative properties of the primary and the secondary effects induced in several affected localities.
The advantages of this scale are the following: (a) the classification, quantification and measurement of various known geological, geomorphological, hydrological and biological indicators for each degree of intensity, (b) the analysis and parametrization of the primary and secondary earthquake environmental effects of previous earthquakes, including historical and palaeo- earthquakes, (c) a comparison not only between earthquakes generated in the same area, but at different times, but also between earthquakes that have occurred in different tectonic environments, (d) the assessment of seismic intensities in sparsely populated areas, resulting in greater spatial distribution and higher spatial analysis of seismic intensities, (e) independent assessment of seismic intensities in areas where structural building damage and generally building damage diagnostic data lose their diagnostic capacity or are saturated, (f) the use of the total spatial distribution of secondary environmental effects as an independent tool for accurately estimating the focal intensity when primary effects are not induced, (g) its application either alone or in combination with other traditional macroseismic scales, such as the European Macroseismic Scale 1998, which has the effect of filling the gaps and disadvantages of the traditional scales, (h) the extension of the time period and the enrichment of the earthquake catalogues in many areas worldwide based on the parameterization of the past earthquake environmental effects and (i) the use of earthquake geology and palaeoseismology to extend the time window of historical seismicity to a few tens, hundreds or even thousands of years.
All these characteristics and properties of the ESI 2007 scale and its use contribute to a full and detailed knowledge of historical earthquakes, their effects and intensities. They reveal sub-areas with significant risks associated with the occurrence of significant environmental effects, they test the susceptibility and the vulnerability of the affected area to the occurrence of these geodynamic phenomena and improve preparedness and land use planning aiming to successful and effective management of these effects and their future mitigation.
This approach is applied in this study. Western Greece was chosen as the study area and especially the islands in the central part of the Ionian Sea (Lefkada, Cephalonia, Ithaca and Zakynthos) and the southwestern Peloponnese including mainly the prefecture of Messinia and Laconia where necessary. These areas have been characterized as the most tectonically and seismically active areas in Greece and one of the most active in the Eastern Mediterranean and worldwide. They have been repeatedly affected both during the recent period of instrumental recordings and during the historical years by large-scale destructive earthquakes with significant effects on the local population, the natural environment, the buildings and infrastructure. The most recent episodes in the geodynamic evolution of these areas are the earthquakes of Kalamata in September 1986, Cephalonia in early 2014, Lefkada in 2017 and Zakynthos in 2018 with significant effects on the natural and built environment of Western Greece. Their generation is attributed to the rupture of main tectonic structures, which significantly affect the geodynamic evolution of western Greece, such as the Cephalonia transform fault, the fault zone along the eastern margin of the Lower Messinia basin, among other equally important faults, characterized by high potential to cause significant environmental impact, in addition to strong ground motion.
For these areas, the following approach was applied:
• Detailed description of the geological and neotectonic structure.
• Detailed and complete recording and review of historically and recently recorded seismicity with all known historical and instrumental earthquakes from all available sources.
• Detailed and complete recording of the earthquake impact with special emphasis on earthquake environmental effects from all available sources.
• Field research in earthquake-affected areas shortly after the occurrence of recent earthquakes and recording of their effects.
• Classification of the observed and recorded earthquake environmental effects into primary and secondary based on their available qualitative characteristics.
• Application of the Environmental Seismic Intensity (ESI) 2007 scale for the estimation of local seismic intensities of all known historical and recent earthquakes with impact on the research area and estimation of the maximum environmental seismic intensity based on their available quantities characteristics.
• Identification of the most frequently observed earthquake environmental effects and correlation of them with various factors that affect their occurrence.
• Spatial distribution of intensity in the fault blocks of the study areas and extraction of fault blocks most affected by the earthquake environmental effects as well as the most susceptible areas within these fault blocks.
• Reassessment of all available already published seismic intensities and isoseismal maps produced by the application of traditional macroseismic scales, based mainly on damage to the built environment of the affected area.
• Comparison of the macroseismic intensities of the traditional macroseismic scales with the earthquake seismic intensities in order to reveal similarities and differences and to precisely record the impact of the earthquake on the affected area.
• Application of methodologies for the assessment of landslide and liquefaction susceptibility, and susceptibility to tsunami effects in order to reveal vulnerable and susceptible zones.
• Compilation of inventory maps for earthquake environmental effects and their correlation with the aforementioned susceptibility maps in order to determine whether these phenomena occur randomly or are controlled and determined by specific factors and distributed in susceptible zones with characteristics favorable for the triggering of these phenomena.
• Correlation of primary, where existing, and secondary seismic environmental effects with the active tectonic structures of the study area. For this correlation, the study and description of the geological, tectonic and neotectonic structure of the research area was deemed necessary. Particular emphasis was given on the active tectonic structures and the fault blocks, as they derived not only from the relevant literature review but also through field work, which was carried out from time to time during the conduction of the study and covered all earthquake-affected areas including the horsts and grabens in the southwestern Peloponnese and the islands in the central part of the Ionian Sea (Lefkas, Cephalonia, Ithaca and Zakynthos).
• Analysis and correlation of primary and secondary earthquake environmental effects with seismological parameters comprising magnitude and intensity.
• Compilation of a database for earthquakes and their triggered environmental effects.
The complete seismic history of destructive historical and recent earthquakes in the islands located at the central part of the Ionian Sea and the southwestern Peloponnese includes 52 major earthquakes, which have occurred not only onshore but also offshore with significant effects on the local population, the natural environment, buildings and infrastructure. Secondary effects on the natural environment include hydrological anomalies, tsunamis, ground cracks, slope failures, liquefaction phenomena, vegetation effects, jumping stones and hydrocarbon related phenomena. Dust clouds were not observed in any of the studied seismic events. Primary environmental effects have also been reported after strong earthquakes including reactivation of faults and coseismic surface ruptures as well as permanent surface deformation.
For the time covered by the compiled inventories, the following are observed:
• the Lefkada inventory extends from 1630 to 2015
• the Cephalonia inventory extends from 1636 to 2014
• the Ithaca inventory extends from 1915 to 1953
• the Zakynthos inventory extends from 1513 to 2018
• the southwestern Peloponnese inventory extends from 1842 to 2011
Historically recorded earthquakes dating back to before 1900 have triggered primary and secondary environmental effects at 157 sites. Instrumental earthquakes from 1900 to present have caused primary and secondary environmental effects at 260 sites. In particular, primary and secondary environmental effects have been recorded:
• • in Lefkada in 33 sites from historical earthquakes and in 96 sites from recent ones,
• • in Kefalonia in 29 sites from historical earthquakes and in 88 sites from recent ones,
• • in Ithaca in 8 sites from recent,
• • in Zakynthos in 53 sites from historical earthquakes and in 11 sites from recent ones,
• • in the Peloponnese in 42 sites from historical earthquakes and in 57 sites from recent ones.
The most common secondary earthquake environmental effects in the studied Ionian Islands and the southern Peloponnese are:
• slope failures in 39 seismic events,
• the ground cracks in 32 events,
• tsunami in 31 events,
• the liquefaction phenomena in 23 events,
• the hydrological anomalies in 18 events,
• hydrocarbon related phenomena in 5 events,
• jumping stones in 1 event and
• effects on vegetation in 1 event.
Primary earthquake environmental effects are limited to:
• surface deformation in 6 earthquakes and
• surface seismic ruptures in 5 earthquakes.
From the correlation of number of earthquakes with earthquake magnitude it is concluded that:
• Most seismic events that have affected the studied areas have a magnitude ranging from 6.6 to 7.0. Subsequently:
• 17 earthquakes of magnitude ranging from 6.1 to 6.5,
• 6 earthquakes with magnitude from 5.1 to 5.5,
• 5 earthquakes with magnitude from 5.6 to 6.0,
• 5 earthquakes with magnitude from 7.1 to 7.5 and
• 2 earthquakes with magnitude from 4.5 to 5.0.
• The magnitude in 11 earthquakes could not be determined.
The correlation of earthquakes and environmental seismic intensities for the studied areas shows the following:
• in Lefkada, most earthquakes have triggered effects with maximum environmental seismic intensity VI-VIIESI 2007 and VIIIESI 2007.
• •in Cephalonia, most earthquakes have triggered effects with maximum environmental seismic intensity VIIIESI 2007.
• in Ithaca, all earthquakes have triggered effects with maximum environmental seismic intensity ΙΧESI 2007.
• in Zakynthos, most earthquakes have triggered effects with maximum environmental seismic intensity VIIIESI 2007.
• in the southwestern Peloponnese, most earthquakes have triggered effects with maximum environmental seismic intensity VIIESI 2007.
The most susceptible areas of Lefkada for triggering of earthquake environmental effects are:
• the fault block of Lefkada city,
• the western coastal and northeastern coastal part of the Tsoukalades – Katouna fault block,
• the fault block of Agios Nikitas,
• the northern part of the Drymonas fault block,
• the central-northern part (slopes of Mega Mt), the eastern part (slopes of Skaroi Mt) and the southern part (area south of Elati Mt) of the Mount Mega – Skaroi Mt fault block,
• the central and western part of the Lefkata peninsula and
• the eastern part of the Vlychos - Poros fault block, where the Vlychos and Poros peninsulas and the Vlychos bay are formed.
The most susceptible areas in Cephalonia for triggering of earthquake environmental effects are:
• in the fault block of the Argostoli peninsula: the coastal area of Argostoli city, the coastal areas of Argostoli gulf and the southeastern shores of the homonymous peninsula,
• in the fault block of the Paliki peninsula: the coastal area of the Lixouri city, the coastal area from Lixouri to Agios Dimitrios village and from Agios Dimitrios to the Livadi coastal swamp, as well as the western, southern and northern coast of Paliki and the southeast part of Paliki peninsula,
• in the fault block of Ainos and eastern Cephalonia: the western slopes of the mountain, the coastal areas of the bays of Agia Efimia, Sami and Poros, the central-eastern and southeastern shores of the fault block and the southeastern slopes of the mountain,
• in the fault block of the Erisos peninsula: the wider Assos area,
• in the fault block of southeastern Kefalonia: the wider Skala area,
• in the transitional zone from Ainos Mt to Erissos peninsula: the area of Myrtos,
• in the transitional zone from Ainos Mt to Paliki peninsula: the area of Zola.
The most susceptible areas of Ithaca for triggering of earthquake environmental effects are:
• the northern rupture and
• the southern rupture.
The most susceptible areas of Zakynthos for triggering of earthquake environmental effects are:
• the fault block of Skopos Mt,
• the eastern part of Central Zakynthos fault block,
• the fault block of the Keri bay,
• the western part of Central Zakynthos fault block,
• the fault block of South Zakynthos
• the fault block of North Zakynthos.
The most susceptible areas of the Southwestern Peloponnese for the occurrence of seismic environmental effects are:
• the plain and coastal part, where the main branch of Pamisos River and its estuary are respectively formed,
• the eastern margin of the Lower Messinia basin, where the seismic fault zone responsible for the 1986, 2004 and 2011 earthquakes occurs,
• the coastal areas of the Messinian Gulf, especially in Kalamata, Petalidi and Koroni areas,
• the northern part of Taygetos, where gorges and steep slopes are formed in alpine formations,
• the margins of the Dimiova – Perivolakia graben with its significant marginal fault zones,
• the transitional zone from the Pyrgos – Christianoi morphotectonic graben to the Kyparissia Mts,
• the western coastal part of Messinia and
• the southeastern part of the Pylia peninsula.
Keywords:
Neotectonics, Active tectonics, Active Faults, Seismic Faults, Natural Disasters, Earthquakes, historical earthquakes, instrumental earthquakes, Earthquake environmental effects, earthquake magnitude, environmental seismic intensity, landslide susceptibility, liquefaction susceptibility, surface deformation, slope movements, rockfalls, tsunami, ground cracks, GIS, geoinformation, earthquake disaster database
