Kolaiti E.1, Mourtzas, N.2
1GAIAERGON LtD, Athens, Greece, University of Peloponnese, Kalamata, Greece
2 GAIAERGON LtD, Athens, Greece
In this paper we attempt to approach the subsidence rate of the Rio-Antirio strait, based on data derived from the RSL change on the coast of Rio during the last 517 years. The underwater survey revealed four sea level indicators: the coastal construction -probably a beacon-of the Ottoman period, the base of the Ottoman fortification wall, the base of the fortification wall of the Venetian period, and the tidal notch formed on the blocks of the base of the Venetian wall. The fortress foundation should to have constructed in dry conditions and therefore, the current submerged position reflects the minimum RSL change from the period of construction to the present day.
Furthemore, the current submerged position of the Ottoman coastline defined by the position of the coastal construction reflects the maximum change in the sea-land relationship from the period of construction to the present day.
The Rio-Antirio strait lies in the western part of the Corinth Gulf, one of the most seismically active zones in Europe. The seismicity is not random in the area but formed distinctive clusters, indicating an E-W striking seismic zone. One main cluster is in the southern part along the Rio-Antirio strait with increased seismicity, which is probably correlated to the fluids presence that it seems to play an important role to the triggering of tectonic earthquakes (Karagianni et al., 2013).
The ENE trending Rio-Antirio tectonic graben, between the grabens of Patras Gulf and the West Corinth Gulf, is a transfer zone bounded by SW-NE faults, subject to a continuous regional N-S trending extension (Doutsos et al., 1988; Papanikolaou et al., 1997).
Data processing of the Rio-Antirio GPS Network revealed that the southern part of the Patras Gulf (Rio) seems to be extending towards the SSW direction with amplitudes of 8.7 - 13 mm/yr.
Concerning the vertical component, an uplift was generally observed with rate about 3 mm/yr in the northern part (Antirio), while in the southern part the uplift was higher ranging from 5.9 - 12.1 mm/yr (Vlachou et al., 2011). On the contrary, Parcharidis et al. (2009) applied the Interferometric Point Target Analysis (IPTA) using a dataset of 42 ERS-1 and 2 scenes to detect ground deformation.
The authors concluded that the area of Rio is subsiding with a rate ranging between -1.8 mm/year and -6.2 mm/year, whereas the area of Antirio is stable or is subsiding with rates less than -2 mm/year.
The Rio fortress is located at the northern tip of the Rio peninsula (Achaea, Peloponnese) and along with its twin, the Antirio fortress (Aetolia-Acarnania, West Greece), was intended to protect the entrance of the Western Corinth Gulf. The first fortification wall of Rio castle was built by the Ottoman Sultan Bayezid II in 1499. It was smaller in area and with a double circuit and moat. In 1532 it was captured by the Spaniards and Andrea Doria, and retaken by the Turks. In 1603 the Knights of Malta caused significant damage. In 1687 it was taken by the Venetians, under Francesco Morosini. Drastic repairs were made and it received the form it has today: new towers - bastions, strengthened ramparts. The fortress was seized by the Ottomans in 1715, and remained there until 1828, when after a siege surrendered it to French General Maison. Then it was repaired and handed over to the Greeks. The castle was used as a prison between 1831 and 1912. During World War II
the Germans were established there. No excavations have been carried out in the area of the Fortress (source: http://odysseus.culture.gr/h/2/ gh251.jsp?obj_id=1610).
The Rio fortress has an irregular polygonal shape with a perimeter of 1000 m and is surrounded by a fortification wall (Fig. 1, 3a). The north end of the wall coincides with the tip of the Rio peninsula. The coastal eastern and western sides of the wall are surrounded by a sand pebble coast 25 m wide, while only a small section 130 m long at the east side is washed by the sea. The south side of the wall is bounded by a moat, which communicates with the sea and fills with water.
The base of the fortification wall consists of two up to four courses of orthogonal blocks, 0.25 m to 0.40 m thick. The superimposed structure of the wall is built of rubble stones connected with mortar. The depth of the seabed where the wall rests is -1.65 m in the section of the eastern Ottoman wall before the sandy shore, up to -1.80 m and -1.90 m at the south and north sides of the eastern Ottoman bastion, respectively, and -1.40 m to -1.50 m at the eastern Venetian wall (Fig. 1, 2a, b, c, d, e, 3b, f, g, i). An earlier sea level stand has eroded the base of the wall of the eastern Ottoman bastion, having formed an erosional cavity (Fig. 2b, 3c).
The coastline of the Ottoman period is located at a depth of 2.50 m, in a distance 7.80 m from the base of the Ottoman bastion. It was at the seaward end of the base of the coastal circular construction, 3.50 m in diameter (Fig. 2b, 3d), in the center of which the lower part of a 1.0 m diameter column is observed (Fig. 2b, 3e).
According to historical evidence, the eastern Ottoman bastion was constructed in 1499 and remained intact despite of the successive reconstructions of the fortress. Considering the maximum measured depth of -1.90 m at the base of the Ottoman bastion, a minimum rate of RSL rise of 3.70 mm/yr can be deduced for the last 517 years. The respective maximum rate of RSL rise is 4.85 mm/yr, as results from the current depth of the Ottoman coastline. From the depth of the base of the Venetian wall added in 1687, we can infer a RSL rise rate of 4.55 mm/yr for the last 329 years.
The observed slowdown is probably due to an intervening period of stability, during which the tidal notch at -1.18 m was formed on the Venetian wall (Fig. 3h).
Fig. 1: Google image of the Rio fortress with the measured depths of the seabed at selected positions where the Ottoman and Venetian fortification walls rest.
Fig. 2: Cross-sections at the submerged sections of the the eastern Ottoman wall and bastion (a, b) and eastern Venetian wall and bastion (c, d, e).
Fig. 3: Views of the eastern fortification wall of Rio fortress: (a) the Ottoman bastion (in front) and the Venetian wall (to the left), (b) the submerged section of the Ottoman wall, (c) the erosional cavity on the base of the Ottoman bastion, (d) the circular coastal construction, (e) the lower part of the column, (f) the northern end and (g) the base of the Venetian wall, (h) the tidal
notch formed on the Venetian wall, and (i) the submerged corner of the Venetian bastion. [Photos (a) and (h) were taken by Fabrizio Antonioli]
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