Geological identification of historical tsunamis

1
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detailed historical documents of tsunami flooding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).
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.
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.
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 Palaeontolog
ical -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 ?ber 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.