RECENT DEVELOPMENTS IN THE TSUNAMI RESEARCH IN GREECE: A SHORT REVIEW GERASSIMOS A. PAPADOPOULOS

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1 CHAPTER 7 RECENT DEVELOPMENTS IN THE TSUNAMI RESEARCH IN GREECE: A SHORT REVIEW GERASSIMOS A. PAPADOPOULOS Institute of Geodynamics, National Observatory of Athens, Athens, Greece In: S. L. SOLOVIEV, O. N. SOLOVIEVA, C. N. GO, K. S. KIM & N. A. SHCHETNIKOV TSUNAMIS IN THE MEDITERRANEAN SEA, 2000 B.C A.D., KLUWER, , Introduction The last ten years or so an intensive effort was made by several institutes to promote the tsunami research in Europe mainly thanks to the research projects GITEC ( ) and GITEC TWO ( ) supported by the EU DGXII as well as to other research initiatives, like a trilateral Japanese Greek Turkish (JGT) project ( ) focused particularly to the Aegean Sea tsunamis. Within the frame of these research activities the Institute of Geodynamics, National Observatory of Athens (NOAGI), collaborating with several institutes, performed tsunami research directed mainly to the following topics: 1.1. An instrumental Tsunami Warning System (TWS) in the Kythira Strait, South Aegean Sea, was installed and its experimental operation was tested A new tsunami catalogue of Greece and adjacent seas was compiled A pilot tsunami risk management study in the Heraklion coastal area, Crete Island, was performed Efforts were made in modelling numerically particular tsunami waves in the South Aegean Sea and in the Corinth Gulf Geological methods were developed to investigate and identify palaeotsunamis in particular coastal segments of the Aegean Sea, while submarine geological studies and geophysical prospecting were performed to 175

2 176 CHAPTER 7 understand better the mechanism of tsunamigenesis of the large 9 July 1956 catastrophic tsunami in the South Aegean Sea. 2. The Tsunami Warning System (TWS) in the Kythira Strait: state-ofthe-art The Kythira Strait TWS consists of two main subsystems: the seismograph subsystem and the tide gauge subsystem. The seismograph subsystem consists of five digital stations installed in several locations of the South Aegean Sea. These field stations are equipped with digitiser, GPS and 1 component, short period seismometers, and via modems and dedicated telephone lines are telemetrically connected with the Central Acquisition Unit (CAU) installed at NOAGI in Athens. The Autolocate software of the CAU is able to determine an earthquake event within about 30 sec after the last record provided that at least four records are available. The tide gauge subsystem consists of two pneumatic water level sensors installed in the area under study and connected telemetrically with the CAU in Athens. After the experimental testing of the system its further utilisation faces funding problems. It is possible to hope that these problems will be solved in the nearest future. 3. Revision of the tsunami catalogue for Greece and the adjacent regions The very long history of the Greek area makes equally long the historical record of seismicity and its associated phenomena like tsunami occurrence [Galanopoulos, 1960; Ambraseys, 1962; Antonopoulos, 1980c; Papadopoulos and Chalkis, 1984]. Therefore, a new revision of the catalogue of Greek tsunamis was required, using recently found historical documents and updating the information about the tsunamis till 1998 inclusively [Papadopoulos, 1998a]. This catalogue has been prepared in a completely new format elaborated by the GITEC and GITEC TWO research groups for the entire region of Europe. An example for Italy has been published by Tinti and Maramai [1996]. In this effort special attention has been given to particular tsunamigenic zones of Greece. For example, a list of historical earthquakes and tsunamis for the area of Kythira Strait was prepared by Papadopoulos and Vassilopoulou as a contribution to the databases supporting the TWS in that area. A similar list has been compiled for historical earthquakes and tsunamis reported in the Corinth Gulf. This is one of the main tsunamigenic zones of Greece, but not suitable for the development of the TWS because of its closed morphology and the very short travel times of the tsunami waves. The catalogue by Papadopoulos [1998a] supplies information on the historically

3 STUDY OF TSUNAMIS IN GREECE AND ITALY 177 reported tsunamis as well as on tsunamis observed in the present century. The list of historical earthquakes by Papadopoulos et.al., containing about 1,200 large and medium size events for the period 4th century B.C A.D. soon will appear. It is of interest that a variety of tsunami genesis causes can be recognised in the Corinth Gulf (Table). They are earthquakes (ER), earthquake coastal landslides (EL)or earthquake submarine slides (ES), earthquake associated events(ea), where the generation mechanism is not very clear, and gravitational slides(gs) triggered aseismically. One of the largest historically reported tsunami (373 b.c.) that occurred in the western part of the Corinth Gulf, has been particularly studied on the basis of the original historical documents available [Papadopoulos 1998b]. The main conclusion is that the historical reports are reliable and rather consistent. Therefore, there is no doubt now that the terrible tsunami of 373 B.C. was really generated by a large earthquake associated with the Eliki fault system, which reactivated on December 26, 1861 and on June 15, 1995 with tsunamigenic earthquakes. 4. Pilot tsunami risk mapping Mapping of the tsunami risk in a microzonation sense in a particular coastal segment has been one of the main efforts of NOAGI within GITEC and GITEC TWO. As an example was selected a 6-km long coastal segment to the west of Heraklion, the capital city of Crete, that was hit by strong tsunamis in c1628 b.c., 365 a.d., 1650 and A series of thematic maps of 1: 10,000 scale was produced as for the tsunami impact in relation to the natural environment, land use/land cover types, road network, functions and lifelines and socio economic and population parameters. Finally, a synthetic map that summarises the proposed tsunami risk management, i.e. the prevention and mitigation measures, was produced [Papadopoulos and Dermentzopoulos, 1998]. Each one of the maps is accompanied by detailed legend and tabular explanations aiming to facilitate the understanding of the maps construction and numerous methodological details. 5. Numerical simulations of tsunami waves An effort was made to simulate numerically the terrible historical tsunami of 365 A.D. generated by a large earthquake in the Kythira Strait. Simulations were performed for few different positions of the tsunami source under the assumption of mild bed slope and taking into account the vertical acceleration component of the water motion. The adopted model has the general form of Bussinesq equations for long, non-linear dispersive waves. This simulation

4 178 CHAPTER 7 gave too small wave amplitudes compared to the historically reported ones [Papadopoulos et al. 1997]. It was decided, therefore, to continue the effort of simulating this very important tsunami. On the other hand, the 6-m high damaging tsunami of February 7, 1963, that occurred aseismically in the west side of the Corinth Gulf, because of a gravitational coastal landslide, was simulated. The results obtained show that the theoretical run-ups for this tsunami are quite consistent with the observed ones [Koutitas and Papadopoulos 1998]. 6. Geological and Geophysical Studies The geological record of tsunamis was investigated in several coastal places of the South Aegean Sea in collaboration with the Tohoku University, Sendai, Japan, and the Middle East Technical University of Ankara, Turkey, within the frame of the JGT project, as well as with the Coventry University, United Kingdom, within GITEC. Tsunami sediment deposits, clearly attributed to the great tsunami generated by the Minoan eruption of Thira (Santorini) volcano in the South Aegean Sea in the 17th century B.C., were identified in coastal segments of Crete Island and of SW Turkey [Minoura et al., 2000]. On the other hand, several difficulties arisen in the geological investigation of palaeotsunamis has been shown by Dominey Howes et al. [2000]. They failed to identify tsunami sediment deposits in particular coastal segments on the east side of Thira, where extensive inundation caused by a large volcanogenic tsunami on September 30, 1650 has been historically well documented. Since one of the frequent tsunami causes in Greece is the seismic or aseismic, coastal or submarine mass slumping [Papadopoulos 1993a], a particular effort has been made to understand better the role of such mechanisms in the Greek tsunamigenesis. First of all, the magnitude distance relations for soil liquefaction and landslides due to seismicity were examined for Greece and world wide [Papadopoulos and Lefkopoulos, 1993]. The particular case of the July 9, 1956, large tsunami generated to the east of Thira, South Aegean Sea, has been studied on the basis of submarine geological and geophysical data obtained from the tsunamigenic source during ship cruises performed on 1992 and 1993 [Perissoratis and Papadopoulos, 1999]. More precisely, new evidence for a very recent sea floor sediment instability was presented from bathymetric, shallow and intermediate penetration seismic profiles and a number of gravity cores. Then, the generation of the large 1956 tsunami was attributed to the combined action of a submarine normal fault reactivated with the large earthquakes of Ms = 7.5 and Ms = 7.2 that ruptured the particular region in this date, and to associated massive sediment slump in the same region.

5 STUDY OF TSUNAMIS IN GREECE AND ITALY 179 TABLE 4. Parameters of the tsunamis observed in the Gulf of Corinth from the ancient times up to the present days. Key: y = year, m = month, d = day, h = hour, I = tsunami intensity. Codes for genesis causes are explained in the text. y m d h Genesis I cause 373 B.C. winter night ER A.D. 05 ER ER : ER : ER : ER : ER : ER ER : EA : EA : GS : EL : ER : ES : ES : GS? 2 One of the most critical parameters for modelling and understanding better the generation mechanism of the 1956 South Aegean tsunami is the maximum wave height, h, in the near field. In earlier reports it is supported that heights of m were observed [Galanopoulos, 1957]. However, more recent studies based on the field signature of the tsunami and on eyewitnesses accounts [Papadopoulos, 1996] as well as on the sediment record of the wave [Dominey - Howes, 1996a,b, Dominey - Howes et al., 2000] indicated that the earlier reported values of h possibly constitute an overestimation, and h ranging between 10 and 15 m is likely more realistic. However, strong submarine earthquakes are the main cause of the Greek tsunamis. Therefore, seismicity studies were not neglected in the effort to

6 180 CHAPTER 7 understand tsunami phenomena in Greece. As an instance, the magnitude - frequency relation and its consequences for the calculation of the mean recurrence times of large earthquakes and the validity of the characteristic earthquake model was examined for several seismotectonic segments of the Hellenic Arc [Papadopoulos et al., 1993]. Besides, the real tsunami nature of some abnormal see oscillations observed during the 20th century in the Aegean Sea was investigated [Papadopoulos, 1993b]. It has been concluded that although such oscillations are similar to tsunami waves they are rather of meteorological or even of unknown origin.