Transcript Kortekaas - Historical Tsunami Evidence

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.
Gulf of Corinth
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
Kirra
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 The detailed historical documents of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.
from other coastal flooding events, like storms.
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:
Evidence
morphological
stratigraphical
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.
sedimentological
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).
geochemical
palaeontological
Tsunami
-wash-over fans behind breached
barriers
-thins inland and becomes
discontinuous
-fines inland
-erosional lower contact
-large inland extent
-boulders
-one or more fining upward
sequences, sometimes
homogeneous
-intra-clasts from underlying
material
-loading structures at base
-bi-directional imbrication
-poorly sorted (particle size
ranging from mud to boulders)
- sedimentary structures very
seldom found
-increase in geochemical elements
indicating marine origin
-marine fossils
-increased diversity (mixture
marine and brackish fossils)
-relative well/poorly preserved
fossils
-plant fragments
-shell rich units
-rafting light material
-buried plants at base
Storm
-wash-over fans behind breached
barriers
-thins inland
-fines inland
-erosional basal contact
-relative smaller inland extent
-boulder deposition has been
reported
-fining upward or homogeneous
“…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).
-not found
-not found
-unidirectional imbrication
-relatively better sorted
-sedimentary structures more
common
-similar signature is expected
because of marine origin
-marine fossils
-mixture of marine and fresh
water fossils
-poorly preserved fossils
1
2
3
-plant fragments
-shell fragments
-not found
-not found
4
ca. 467040 years BP
Tsunami vs. storm deposits
Results of Aliki
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.
ca. 150 years BP
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).
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 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
Foraminifera in core
4 (in %). Salt marsh
species are indicated
in red, brackish
species in green and
marine species in
blue.
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.
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.
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.