Η Κεφαλονιά στις σεισμικές δονήσεις του χρόνου - Ένα νησί στη ζώνη των ρηγμάτων

Greece is the most seismically active country in Europe, with approximately 50% of the continent’s total seismic energy released within Greek territory. The Greek seismic arc, starting from Kefalonia, runs through the southern Ionian Sea, passes west of the Peloponnese, south of Crete, and ends east of Rhodes. This seismic activity is primarily due to the collision of the Aegean microplate with the African plate, which subducts beneath the former. Most earthquakes are shallow, while intermediate-depth earthquakes are mainly found in the back-arc.

The Ionian region, particularly Kefalonia, is one of the world’s most seismically active areas and the most seismically prone in Greece. The geological structure of Kefalonia and Ithaca islands combines formations of the Paxos and Ionian units, with the latter prevailing geologically in Ithaca. The rocks of Kefalonia include Alpine and post-Alpine formations, with tectonic pressures having created successive phases of compression and tension (Lekkas et al., 2001). The high seismicity in the Kefalonia Transform Zone is attributed to the continuous movement of the Anatolian plate, the African plate moving northward, and Apulia moving northeast. These tectonic movements create strong geological pressures, which are released through frequent tremors.

Unfortunately, destructive earthquakes have struck this area over the years, resulting in the loss of many human lives and significant impacts at all levels. Consequently, the Greek Seismic Code mandates that all building structures in this area be designed to withstand a maximum ground acceleration (PGA) of 0.36 g, the highest earthquake resistance requirement in Greece.

In general, it is worth mentioning that studies on the island's seismic activity, such as those conducted by the Geodynamic Institute and international seismological organizations, have significantly contributed to improving earthquake prediction and understanding techniques. Data from Kefalonia's seismic recordings are used in international scientific research, making the island a natural laboratory for studying seismology.

The Strongest Earthquakes Over the Years

Historically, significant earthquakes occurred in 1469, 1636, 1767, 1867, and 1953, while more recent significant earthquakes were recorded in 1983 and 2014. The most destructive earthquakes in this area were those of 1867 and 1953, with the latter being the most catastrophic of all.

More specifically, in 1469, a 7.2-magnitude earthquake in Lixouri left over 500 victims. In 1743, a 7-magnitude earthquake caused extensive damage in the Ionian region. In 1767, a 7.2-magnitude earthquake caused serious losses in Lixouri, with dozens of casualties and the destruction of many buildings. The area’s water supplies were also contaminated, further straining the local community, while nearly all churches were destroyed, followed by aftershocks lasting 40 days.

The area continued to be struck by major tremors in the 19th century, notably the 1867 earthquake (7.4 magnitude), which destroyed thousands of houses, churches, and monuments in Paliki and Argostoli, leaving behind a significant number of victims. Another major earthquake occurred in 1912, centered in Asprogerakas, causing extensive damage in the region of Eleios-Pronnoi.

However, the most catastrophic earthquake in Kefalonia, with a magnitude of 7.2, was recorded in 1953 and destroyed 91% of the island's buildings, causing the death of 455 people. The Ionian earthquakes of 1953 were a sequence of three main tremors of increasing intensity that devastated Kefalonia and Ithaca. The first quake occurred on August 9, causing destruction in Ithaca and the town of Sami in Kefalonia. The second earthquake occurred on August 11, centered northwest of Zakynthos, was even stronger, and destroyed Argostoli, Lixouri, Agia Efimia, and Valsamata in Kefalonia. The third and most destructive earthquake occurred on August 12, with a magnitude of 7.2 according to McKenzie (1972) and a Modified Mercalli Intensity of IX-X, classifying it as "Extremely Destructive." This earthquake nearly completely destroyed Kefalonia, Ithaca, and Zakynthos, with its epicenter southeast of Kefalonia and a reverse fault mechanism.

Finally, in 2014, two strong earthquakes measuring 5.8 and 5.7 struck western Kefalonia, causing landslides and rockfalls but, fortunately, no casualties. These earthquakes were triggered by faults in the Kefalonia Transform Zone and were centered near Lixouri and nearby villages.

Technology and Seismological Observation in Kefalonia

Earthquakes are natural geological phenomena resulting from the sudden release of energy within the Earth’s crust, creating seismic waves that cause ground shaking. Tremors vary in magnitude, from imperceptible to destructive. In Greece, the systematic collection of macroseismic data began in 1858 by the Geodynamic Institute of the National Observatory of Athens (GI-NOA), using a network of observers and recording on the Rossi-Forel macroseismic scale (RF). Since 1950, measurements have been made on the Modified Mercalli scale, and since 1994, they have also been published electronically. Kefalonia has an extensive network of seismographs, recording tremors in real-time, even microearthquakes of 1-2 magnitude. This technology allows scientists to study the sensitivity of the subsurface to tectonic movements. The Geodynamic Institute has installed GNSS stations, such as VLSM and KIPO, to monitor tremors with great precision. Two GNSS stations operate in Kefalonia, in Valsamata and Kipouria, recording GPS observations at one-second intervals. These stations, operational since 2006 and 2010, respectively, contribute to monitoring tectonic shifts in the region.

After the 2014 earthquakes, scientific institutions from Greece and France, in collaboration with local organizations, created the Argostoli Earthquake Observatory (ARGONET). This network includes borehole and free-field seismographs. Additionally, modern accelerometers and temporary networks have been installed to study ground-structure interaction. New technologies, such as fiber optics, are also being tested for recording seismic data.

Indeed, assessing an evolving seismic action is of critical importance for civil protection. Everyone wants to know whether the activity represents background seismicity, a swarm, foreshocks, a mainshock, or aftershocks. Distinguishing these in real-time is challenging.

Recent research shows that the Ionian region has increased fluidity in the Earth's upper crust, facilitating the propagation of seismic waves and the intensity of surface earthquakes. These shifts largely explain the frequent and strong tremors that strike the island.

Anti-Seismic Technology in Kefalonia

The intense and continuous seismic activity in Kefalonia led to the development of one of the most advanced anti-seismic systems in Europe. According to the guidelines of the European Organization for Seismic Resistance, buildings, both private and public, are constructed to withstand tremors of over 8 on the Richter scale. Strict regulations, based on research by the School of Civil Engineering of the National Technical University of Athens, ensure resilience, as buildings equipped with dampers, braces, and flexible foundations absorb the tremors without collapsing. Studies show that these measures have reduced material damage by up to 70% and injuries by 60% compared to earlier earthquakes.

Conclusion

Kefalonia, with its intense seismic history, experiences significant economic impacts from earthquakes, as tourism is often affected after destructive events. Additionally, ongoing infrastructure projects and upgrades to strengthen anti-seismic structures raise costs, although these investments have proven effective.

Moreover, Kefalonia has emerged as a reference point for the development of prediction and crisis management programs, collaborating with international scientific organizations to implement advanced technologies to recognize strong aftershocks and prepare residents in real-time.

According to testimonies and records of those affected on the island, earthquakes have become a strong unifying force for the Kefalonian community. Local associations, societies, and citizens from all over the country provided help, with Kefalonians of the diaspora offering support and showing solidarity. However, the large-scale destruction, especially in 1953, forced many residents to migrate in search of safety and better living conditions. Today, Kefalonia remains a model of resilience and scientific innovation, continuing to invest in earthquake protection and the safety of its community.