Coseismic Coastal Movements Associated with Strong Submarine Paleoearthquakes in the Eastern Segment of the Hellenic Arc: Observations from Rhodes Isl. (Greece)

Thursday, 18 December 2014
Ioanna Triantafyllou1 and Gerassimos A Papadopoulos1,2, (1)National Observatory of Athens, Athens, Greece, (2)Inst Geodynamics, Athens, Greece
The eastern segment of the Hellenic Arc and Trench system is characterized by the occurrence of large earthquakes and tsunamis known from both the historical and geological record particularly in Rhodes Isl. (Greece). Historical sources maintain direct evidence of coseismic uplift in the eastern coast of Rhodes, e.g. the very large earthquake of 12 October 1856 (M~7.5), caused ground uplift while the sea receded permanently. Similar observations have been made regarding the earthquake of c. 227 BC who caused the collapse of Colossus in Rhodes and the tsunamigenic earthquake of c. AD 142. Such observations, supported by the instrumental record of seismicity, make realistic the suggestion that the historical earthquakes had their epicenters offshore but close to Rhodes city. However, in SW Asia Minor at the opposite side of Rhodes, coastal subsidence was reported as a result of the strong earthquake sequence of February-April 1851. The occurrence of strong tsunamis after some earthquakes is an independent evidence for significant, submarine coseismic fault displacement very likely in the Rhodes Abyssal Plain of water depth up to 3 km. On the other hand, geological observations have indicated that the eastern side of Rhodes has systematically uplifted during the Holocene with uplift amplitude increasing from S to N with average velocity ranging from 0 in Prasonisi at south to 1 mm/yr at the NE side of the island where coseismic uplift was reported historically. We compare the historical rate of uplift with the geologically estimated rate. To this aim we compiled a new catalogue of historical earthquakes that caused coseismic uplift in the coastal zone of Rhodes city and adopted that they had their epicenters offshore but very close to the city. From damage descriptions maximum macroseismic intensity was assigned to each one of the earthquakes. Then intensity was converted to earthquake magnitude from empirical relationships found for instrumental Greek earthquakes. The geological and historical uplift rates were compared and seismotectonic implications were discussed from the point of view of the possible surplus or deficit of the historical uplift versus the geological uplift and the resulting potential stress accumulation.