Geological identification of historical tsunamis in the Gulf of Corinth, Greece Stella Kortekaas1, G.A. Papadopoulos2, A. Ganas2 and A. Diakantoni3 (1) Coastal Geomorphology and Shoreline Management Unit, Université du Littoral-Côte d'Opale, France. (2) Institute of Geodynamics, National Observatory of Athens, Greece. (3) Dept. of Historical Geology and Paleontology, National and Capodistrian University of Athens, Greece. Introduction Geological identification of tsunamis is important for risk assessment studies, especially in areas where the historical data set is limited or absent. But even in areas with a well-documented tsunami history, like the Gulf of Corinth, the geological record can be used to obtain a data set of past tsunamis extending far beyond the instrumental and historical records. However, despite a sharp increase in palaeotsunami studies in recent years, many problems remain in the identification of tsunami deposits. A major problem is to distinguish them from geological evidence resulting from other coastal flooding events, like storms. Gulf of Corinth The Gulf of Corinth is a tsunami prone area due to its high seismicity and the high sedimentation rate in combination with the steep bathymetry, which create favourable conditions for submarine landslide generation. The historical documentation of tsunamis in the Gulf of Corinth is one of the richest in the world and extends back to the 4th century BC. The detailed historical documents of tsunami flooding were used as a reference system for the correlation of the time and place of occurrence of the geological identified events. Two sites were selected to study the geological evidence left by historical tsunamis: Aliki, situated on the south coast of the Gulf of Corinth and Kirra located on the north coast. Both areas are reported to have been flooded during tsunamis in the past and are vulnerable to tsunami flooding due to their morphology and position. Furthermore, they are suitable for tsunami sediment preservation because of the low energy depositional environments. Using stratigraphical, sedimentological, microfossil analyses episodes of marine flooding were identified in both sites. Kirra The Kirra site is situated east of Itea and consists of a salt marsh situated on a flat, low-lying coastal plain formed by two rivers. The salt marsh is separated from the sea by a modern coastal road with houses and a beach containing sand and pebbles. Detailed descriptions exist of the flooding of this area by a tsunami triggered by an earthquake (M6.5) on December 26, 1861: “…In Itea, the port of the Krissaic area, there were 5 waves. Because the coast is very flat in this area, all the houses near the beach were flooded up to 5-6 feet high. The first wave inundated only 3-4 ‘paces’ inland, the second wave 6-8 paces, but the third wave 75 paces…” (Schmidt 1875). “…At Itea, on the opposite coast of the Gulf of Corinth, the sea advanced 35 m inland flooding the port a number of times, causing little damage. However, at nearby Kirra the sea advanced a long distance inland, up to Agorasia, submerging a large area of low-lying cultivated land, including Angali…” (Ambraseys and Jackson 1997). Aliki The Aliki site is situated east of Aegion and consists of a lagoon surrounded by salt marshes. The lagoon is protected from the sea by a narrow beach barrier consisting of gravel and sand. The Aegion coast is reported to have been flooded repeatedly by tsunamis in the past e.g. 373 BC, AD 1402, 1742, 1748, 1817, 1861, 1888, 1963 and 1996 (Papadopoulos 2000). ca. 467040 years BP 1 2 3 4 Tsunami vs. storm deposits Both tsunamis and storms are high-energy events that may leave marine traces in the coastal sediment sequences. There are a number of characteristics that have been found in tsunami deposits from all over the world. Although they are not exclusive evidence of the tsunami-origin of a deposit, they can be used as diagnostic criteria (Kortekaas 2002). However many of these characteristics only indicate the high-energy conditions or marine source of the deposit and therefore they are likely to be found in storm deposits as well. The main differences between tsunami and storm deposits are: -Tsunami deposits extend further inland than storm deposits. -Boulders are reported to have been deposited during storms, however these are or isolated boulders or sometimes boulder fields, while in tsunami deposits boulders may occur within a sand matrix. -Tsunami deposits may show bi-directional imbrication, associated with runup and backwash. ca. 150 years BP Results of Aliki The stratigraphy consits of clay and silt with a few sand layers and pebbles, but no clear stratigraphic evidence of marine flooding was found. However, results of the foraminiferal analysis show an increase in marine foraminifera at 0.1 cm below mean high water level (MHW), suggesting marine inundation. A combination of 210Pb and 137Cs dating analyses provided an age of ca. 150 years for this level. Which could therefor correspond to the tsunami of 1861 which caused extensive flooding of the Aegion coast (Smidt 1875). Results Kirra The stratigraphy at this site consists of clay and silt, containing four sand layers of varying thickness. The top sand layer shows sand dykes reaching up into the overlying silts, suggesting liquefaction. This may be the result of the Fokis earthquake of 1870, during which extensive liquefaction occurred in the Kirra area (Ambraseys and Pantelopoulos 1989). The second sand layer contains large angular pebbles at its base. The third sand layer consists of fine sand becoming finer inland and the last sand layer consists of medium to coarse sand, fining up and containing shell fragments. All sand layers, except for the top layer contain foraminifera and other microfossils indicating a marine origin. A shell from the bottom sand layer yields a calibrated radiocarbon age of ca. 4780 BP. Unfortunately no dates are available for the other sand layers. Nevertheless, using a constant sedimentation rate, rough age estimations could be made of ca. 489 BP, 1393 BP and 3011 BP for sand layer 1, 2 and 3 respectively. Foraminifera in core 4 (in %). Salt marsh species are indicated in red, brackish species in green and marine species in blue. Conclusion To compare the diagnostic criteria for tsunami identification with the results of Kirra and Aliki: Kirra: Stratigraphical: -thins inland -fines inland -inland extent = ca. 200 m Sedimentological: -no boulders or intra-clasts found -fining upward -poorly sorted Palaeontological: -marine microfossils -mixture of marine and marsh foraminifera in layer 4 -shell fragments Aliki: A mixture of marine and marsh foraminifera was the only evidence present at this site. References: Ambraseys, N.N. and Jackson, J.A. 1997. Seismicity and strain in the Gulf of Corinth (Greece) since 1694. Journal of Earthquake Engineering, 1, 3, 433-474. Ambraseys, N.N. and Pantelopoulos, P. 1989. The Fokis (Greece) earthquake of 1 August 1870. European Earthquake Engineering, 1, 10-18. Kortekaas, S. 2002. Tsunamis, storms and earthquakes: distinguishing coastal flooding events. PhD-thesis Coventry University, UK. 171p. Papadopoulos, G.A. 2000. A new tsunami catalogue of the Corinth Rift: 373 B.C.-A.D. 2000. In: Papadopoulos, G.A. (ed.) Historical earthquakes and tsunamis in the Corinth Rift, central Greece. National Observatory of Athens, Institute of Geodynamics. Publication no. 12, 122-126. Schmidt, J.F.J. 1875. Studien όber Erdbeben. Leipzig. 324p. No evidence was found for the well documented 1861 tsunami at Kirra, but the sand layers discovered show that extreme marine flooding events have occurred in this area before historical times. Although many of the diagnostic criteria for tsunami identification were found in the sand layers, it is not possible to exclude storm surges, because the characteristics exclusively found in tsunami deposits were not present. The geological evidence found at Aliki is very limited. However, the age of the event horizon, which corresponds to a known tsunami, favours a tsunami origin. Finally, because the geological traces of the historical tsunamis in the Gulf of Corinth are very subtle, the best evidence for a tsunami origin is an accurate date which corresponds to a known tsunami. The availability of detailed historical information including eyewitness descriptions of the tsunami flooding and reports of the coastal changes induced may assist identification considerably, as they can be compared with the geological evidence found. Consequently, the interpretation of pre-historical tsunamis will always imply a certain degree of uncertainty, because dating control is not possible. However, with better knowledge of recent and historical tsunami deposits, identification of such deposits will become more reliable.