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Introduction – The Historical Event
Helike city, the capital of the twelve city-state alliance “the Achaean Dodecapolis” in the northern Peloponnesos, Greece, was suddenly devastated by a violent earthquake during a winter night in 372/373 BC. Many ancient writers such as Diodoros (1st c BC), Strabo (1stc BC–1stc AD) and Pausanias (2ndc AD) have described the destruction and disappearance of Helike.
Strabo reported that (i) «… and Heraclides stated that the submersion took place at night in his time and although the city was twelve stades (2km) from the sea, the whole district together with the city was hidden from sight…” and (ii) “… and Heratosthenes says that he saw the place and the ferryman said that there was a bronze Poseidon monument holding a hippocampus in his hand in a straight standing up position, which was dangerous for those who were fishing with nets.
Pausanias reported that: (i) “…the sea flooded much of their land and encircled the whole town all around and more over the sea covered the grove of Poseidon that only the tops of the trees remained visible. Then a sudden shock sent by God and with the earthquake the sea invaded and the wave sunk Helike and everyman in it…” (ii)...the traces of the town could still be seen, though damaged by salt water...”
Helike was situated between the Selinountas and Kerynitis rivers. (Fig.7.1) The two rivers have built an alluvial fan delta which has developed on a tilted block fault bounded to the north and south by active normal faults, which dip to the north (Fig 7.1)
The fault mapping all over the Gulf of Corinth shows that none of the onshore-offshore active faults length exceed the 26km, consequently the max. expected earthquake magnitude cannot exceeds the 6.7R whilst the max. expected fault displacement along a fault cannot exceeds the 0.8m. The coastal and submarine delta fans as well as the slope along the Gulf of Corinth are susceptible to sediment failure induced by earthquakes but non- earthquake induced sediment failures have also been observed. Published seafloor data from the Gulf of Corinth indicate the existence of noumerous submarine landslides. However foe a few only landslide the source area, the size and the volume are known. For the latter landslides the expected tsunami maximum wave height is 4.04m at the source and max. wave length 6.27km (Stefatos et al 2006)
A type of sediment failure that mainly affects the coastal zone along the Gulf of Corinth are the translational slides.
The translational slides occur at a low angle detachment plane (<1o), which lies between 5 and 6m below surface. At the head of a typical translational slide along the slides crown rotational cracks or a trench is formed due to the tensional forces, which act on the sliding mass. The sliding is attributed to liquefaction of subface sediment caused by cyclic loading during an earthquake.
Proposed scenarios regarding the destruction and disappearance of Helike underwater.
Taking into consideration: (i) the description of events that destroyed Helike as written by historians and geographers and (ii) the geological setting of the wider area surrounding Helike, it can be concluded that the causative factor for the Helikes destruction was undoubtedly an earthquake. What is not clear are the factors that caused the Helike to sink underwater and disappear out of sight.
The following proposed scenarios on the permanent submergence of Helike, based on all the aforementioned are:
The first scenario could not have been responsible for the submergence of the coastal zone where Helike was built, as it could not have caused enough subsidence for the permanent submergence of Helike. The max. expected displacement of the hangingwall block of the Helike fault cannot exceed the 0.6m.
The second scenario could not have been responsible for the submergence of Helike as it could not have caused sufficient subsidence in the coastal zone for Helike to have been permanently submerged.
The third scenario does not seem to have been responsible for the submergence of Helike as the expected wave height of a tsunami activated either by fault displacement or submarine land slide would not have been high enough to invade the coastal plain a 2km inland and cover Helike, even temporarily, by water.
The fourth scenario seems to be the most probable for causing the submergence of Helike. In this case a translational slide in the coastal zone could produce a land subsidence over the trench area along the slide’s crown between 6 and 7m (Fig. 7.2). This subsidenced is considered enough for the water to rush in the trench and cover permanently Helike.
Fig. 7.1: Schematic 3-D illustration showing the geological setting on which Helike (AH) was built. HF=Helike Fault, AF=Aegion Fault.
Fig. 7.2: Schematic 3-D illustration showing the fourth scenario, in which probable caused the submergence of Helike.