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POSSIBLE TSUNAMI DEPOSITS DISCOVERED ON THE BULGARIAN BLACK
SEA COAST AND SOME APPLICATIONS
B.K. RANGUELOV
Geophysical Institute, BAS.
Acad. G.Boncev str. bl.3, Sofia 1113, Bulgaria.
Abstract
The recently discovered tsunami deposits on the Northern Bulgarian Black Sea coast
supports the idea that these shore lines have been flooded by local tsunamis in the past. The
dating of the discovered deposits and inclusions in them as well as the levelling of their
recent positions show that these deposits have been formed may be during the significant
earthquake (M>7.0, approximate coordinates: 28.3E, 43.3N) generated the giant landslide
occurred in I-st century BC. The effects of this landslide can be still observed on the
seashore. The obtained data combined with others (such as tsunami catalogue for the Black
sea, the recurrence graphs, the refraction models, the zoning of the tsunami generating
sources and vulnerable areas, etc.) can be useful for the tsunami hazard assessment of the
Black sea. This possibility is slightly mentioned using the previous investigations of the
author and the results obtained about the newly discovered deposits.
1. Introduction
Many new investigations have been performed during the mid 90s of the last century for the
tsunami research of the Black Sea region. A lot of new data, maps and interpretations have
been made for the tsunami zoning purposes. For the first time tsunami deposits have been
discovered in the outcrops located on the northern part of the Bulgarian coast. Different
observations have been made. Geodesy positioning according to the recent sea level helps
to estimate the supposed tsunami height [1]. A large amount of data has been collected and
processed during the performance of the GITEC Project with EU [2]. Calculations and
modelling of different parameters of the tsunamis as well the tsunamigenic sources and
vulnerable areas locations have been performed [3, 4]. The travel times of the tsunamis
from different sources, zones of shadows due to the refraction, tsunami energy distribution
according to the geometry of the coast and bottom and other important values have been
calculated [5,6]. Many calculations of the probabilistic tsunami risk assessment and the
expected negative consequences from the tsunami flooding have been done and the average
repeatability estimated for the different time periods and the different run-up’s [7, 8]. New
data and interpretations about the fractal properties of the coastal line and the respective
observed run-ups have been obtained using the method of fractal analysis as well [9]. All
these calculations and models have been verified by the data of observed tsunamis
described in old chronics or registered by the recent equipment. Several observed cases
A. C. Yalçıner, E. Pelinovsky, C. E. Synolakis, E. Okal (eds.),
Submarine Landslides and Tsunamis 237-242.
@2003 Kluwer Academic Publishers. Printed in Netherlands
238
support the recognition of the estimated tsunami run-ups [10, 7]. All these data can be
combined and used in case of the tsunami hazard assessment of the seashore.
2. Observations
The discovery and the observations of the well-preserved possible tsunami deposits have
been made during the field expedition - summer time of 1996 [1]. The area under
investigations is located on the northern part of the Bulgarian coast near Varna City. The
discovery has been made by chance following the digging works of the local house builders.
Several points with the visible tsunami deposits have been recognized as the probable sites
in a small bay located north of Varna City and south from the most powerful seismic source
at the region (fig.1.).
Figure 1. Sketch of the location of the tsunami deposits’ sites pointed by vertical crosses and numbered
consequently. Site 1 – distance 4 meters from the sea shore line (height – 4.25 meters above the water
level); site 2 – 503 meters from the seashore (height – 5.15 meters); site 3 – 1215 meters from the seashore
(height – 8.50 meters); site 4 – 204 meters from the seashore (height – 2.75 meters)
The layers have been carefully investigated. Usually the deposited sands over the hardcarbonized sandstone compose them. The sands include bigger sandstone pieces (size
between 2-3 and 10-15 cm.) brought by the visible lower levels of the beach gravel. Often
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in this suspected tsunami deposits the shells of different mollusks can be observed. The
sites
Figure 2. Sketches of the cross-sections of some selected sites (1-4) of fig.1. Stratigraphy and the position of the
discovered tsunami layers are presented:1-soil layers; 2 - marine sediments of gravel, sands, some shells of
Mollusks and microfossils (tsunami deposits!); 3 - hard rock basement (carbonized sandstone); 4 measuring point above sea level. The thickness is in cm.
are well visible, with clear stratification of the layers. At the lower part – gravel with rough
sands can be seen followed by the rough grains and the finest fraction over all. At some
places the preserved back drive channels of the water can be observed (for example on site
2). The correlation of the positioned level of the deposits can be follow to all sites. The
deposits have different thickness according their space position – distance from the recent
seashore and the respective heights. The interpolation of the level’s positions performed for
the different sites fits very well most of them. This shows that the deposits have not been
moved from their original places. Some samples from the located sites have been collected.
The sampling points have been positioned by the leveling measurements to the recent water
level (fig.2).
Photographs have been taken of almost all discovered sites. After the samples have been
collected and investigated in the laboratory of Coventry Polytechnic, the microfossil and
radiocarbon analysis have been performed. The results show that these deposits can be
recognized as a result of the local tsunamis generated by a big earthquake occurred in I-st
century BC. This event is famous, with the subsided underwater city of Bizone (old Greek
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colony) and the heavy very big monolith blocks slide down in the water. They are visible
even at the recent seashore. The estimated magnitude is M>7.0 and approximate
coordinates – 28.3E, 43.3N [11, 10, 12, 13]. The radiocarbon dating method C14 was
performed in the Laboratory of Radioactivity and environment in Sofia, on three samples
and gives reasonable dating in average of about 2000 years. The selected samples consist of
many fragments and whole shells from the recent Black Sea molluscs. The main species
belong to the Cardium edule, Tapes rugatus and Donax julianae [14]. The specimens have
been found at almost all localities (No’s 1-3). The results of the special microfossil analysis
performed together with the specialists of the Coventry polytechnic show the presence of
the skeletons of the microfossils. Skeletons of the microfossils Foraminifera and Ostracoda
have been recognised [15]. “The Foraminifera species present were Haynesina depressula,
Elphidium williamsoni and Anunonia becarii. These three benthic species in general are
tolerant of brackish conditions” [15]. The levelling measurements on the described sites
established the tsunami run-ups according to the recent water level reached up to 7-8 meters
[7]. The age of the deposits very well coincides with the reported strong earthquake in I-st
century BC. These deposits confirmed the described by the old chroniclers large flooding,
probably generated by tsunamis [16, 11].
3. Results and applications
The tsunami deposits discovered on the Bulgarian Black Sea coast support the idea of the
local flooding generated by tsunamis in the historical times [11]. On the fig.3 the
Figure 3. Approximate run-up tsunami heights according the return period for various places (according [17]):
J - Japan, H - Hilo bay (Hawaii), SF - San Francisco, M - Mediterranean, BS - Black Sea. The dashed line
show the same for the tsunami sources in the aquatory of BS, the solid one - including offshore tsunami
sources. The dashed vertical bars show the extreme values observed for a certain period. For the Black Sea,
the extreme value is obtained due to the new discovered paleotsunami deposits. The position of the vertical
bars is according the appearance of the extreme heights of tsunamis.
extreme value has been added on the repeatability graph established earlier [7, 17] for the
Black Sea.
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The data from this graph have been used about the model calculation for the empirical
Black Sea tsunami zoning [8]. The results of the investigations of the tsunami deposits and
the tsunami hazard for the Bulgarian Black Sea coast are reflected on the created map of the
Geological Hazards in Bulgaria (scale 1:500 000) [18]. All tsunami effects on the Bulgarian
part of the Black sea coast have been outlined and possible tsunami influence such as
generating sources as well as flooding areas from near and far-field tsunami sources are
reflected [5]. A selection of some most tsunami vulnerable areas of the coast have been
made for target calculations. The tsunami influence due to the refraction is estimated [3].
The results show that the most dangerous for the coastal facilities are the local tsunami
sources. The far field sources can produce huge tsunamis, but their energy dissipates in the
sea and only the local sources can produce significant influence on the coast [19]. This is
due to the specific location of the tsunami generating sources (mostly earthquakes and very
rare other generators such as landslides (both - surface and submarine), stonefalls, etc.,
which can generate tsunamis. On the other hand the geometry of the bottom and the coastal
line and the reflections due to those bathimetry peculiarities can create strong auto focusing
effects to the local specific sites attacked by the waves [3]. As it was shown in the previous
investigations, the tsunami energy is concentrated by the local geometry peculiarities of the
coast and the sea bottom [6, 10, 9]. The newest tsunami zoning made by an empirical
approach [8] shows the possibility of combined influence - tsunami plus sashes (or
meteosurges) which can amplify the tsunami flooding effects. May be the discovered
tsunami deposits also reflects such combined influence. The established height from the
tsunami deposits position reaches 7-8 meters up and thus provides the data to suggest the
big sea surges, which can occur in the Black sea. The newest achievements can help the
reliability of the adopted or suggested solutions. Up to now there is no systematic search for
similar deposits. They are intended in a near future. It seems very probable that some other
promising places (for example near Crimea coasts and near Varna City south coast) also
contain tsunami deposits [13]. Several deposits of sapropel breccia have been discovered
according the oil prospecting in the regions. Usually they are indicators of the fast and
massive flooding. The necessity to try to find them is obvious, for future science
investigations. They may help the dating of other local tsunamis as well. Unfortunately the
activated landslides on the Bulgarian coast during April 1997 disturbed the observational
points and changed dramatically all observed levels of the discovered deposits’ sites. This
means that no more real position of the deposits can be recognised [12]. This fact shows
that similar investigations must be performed with highest accuracy on the collected data,
materials and samples.
4. Conclusions
Using new and sophisticated methods of microfossil analysis, precise levelling and
radiocarbon dating the new discovered probable tsunami deposits are reported. The age of
the deposits estimated by the carbon 14 method correlated with the time consequences of
the strongest earthquake and tsunami reported in the I-st century BC. The depth origin of
the deposits is confirmed by the microfossil analysis. This new fact applied to the existing
repeatability relationships can help the tsunamizoning of the Black Sea coasts and the
justification of the existing tsunami catalogue of the same sea.
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