EXPERT CONSULTATION MEETING – TSUNAMI RISK IN EUROPE
STATUS, GAPS AND NEEDS RELATED TO EARLY WARNING SYSTEMS
GREEK– DRAFT PAPER
G.A. Papadopoulos1 and Ch. Koutitas2
1 – Institute of Geodynamics, National Observatory of Athens
2 – Department of Civil Engineering, University of Thessaloniki
1. Is there a tsunami risk in Europe?
European coastal segments were hit in the past by large, destructive tsunamis. Hundreds
of both local and regional tsunami waves have been reported in documentary sources
from the antiquity up to now. Other historic or pre-historic events were revealed from
palaeotsunami methods based on geological observations. Tsunamis are mainly generated
by submarine earthquakes while some events were reported to have been caused by
volcanic eruptions and locally by landslides. Therefore, tsunami waves should not be
neglected as a potential source of risk that threaten coastal communities of the European
and particularly of the Mediterranean Sea. However, the frequency of tsunami occurrence
is quite different in different coastal zones. The highest rate of tsunami occurrence is
observed in the east Mediterranean Sea where the seismicity is also the highest in Europe.
Medium rate is observed in the central and western sides of the Mediterranean Sea, while
in the Marmara Sea, in the North Sea, in the Black Sea as well as in the Atlantic coasts of
Europe the rate of tsunami generation is low.
2. What do we know from the past?
Data contained in The New European Tsunami Catalogue, one of the main achievements
of the EU-funded GITEC (1992-95) and GITEC-TWO (1996-98) projects, indicate that
more than 250 tsunami events were reported in the Mediterranean Sea in about the last
3500 years. However, the data basis is statistically complete only for the last few
centuries. The largest known destructive events, assigning intensity 5 or 6 in 6-grade
intensity scale, took place (1) in the Thera (Santorini) volcanic island complex, Cyclades,
South Aegean Sea, Greece, in association with volcanic eruptions on c.1630 B.C.,
1650A.D. and tectonic earthquakes on 09.07.1956, (2) in the Hellenic Arc on 21.07.365,
03.08.1303, ?.04.1609, and 31.01.1741, (3) in the Maliakos and Corinth Gulfs, central
Greece on 426B.C. and 373B.C., (4) in the Levantine Sea on 09.07.551 and 25.11.1759
and (5) in the Messina straits-Calabria region on 06.02.1783 and 28.12.1908. The
statistical completence in the last few centuries does not implies that all the historical
tsunamis are known. Therefore, more research in both the historical archives and the
geological field is needed in order to reveal tsunami events that remained unidentified so
far and to enrich the existing data basis.
3. How to assess and predict tsunami risk?
In natural disaster science, it is generally accepted that the risk (R), that is the expected
impact of a potentially damaging or disastrous natural event on a particular region, is a
convolution of three parameters: hazard (H), vulnerability (VU) and value (VA):
R = H * VU * VA (1)
where hazard (H) is a measure of the probability for the natural event to occur in a given
time window, vulnerability (VU) is a measure of the degree of resistance of the
anthropogenic environment (structures, population etc.) to the natural event, and value
(VA) is the economic value exposed to the natural hazard. Hazard is a description of only
the natural process and does not include components of the impact of it. In particular, the
assessment of the tsunami hazard involves several parameters like the frequency of
tsunami occurrence, the different mechanism of tsunami generation and the
identification of the potential tsunamigenic sources, the propagation of the tsunami waves
from the source to the threatened coastal zones as well as the inundation of the coastal
zones. As for the vulnerability, it is increasing with the decrease of the degree of
resistance of the anthropogenic environment. In this sense, the assessment of the tsunami
risk in a particular coastal segment or region depends on the assessment of hazard,
vulnerability and value: risk is an increasing function of H, VU and VA.
a) Tsunami Occurrence
In the Mediterranean Sea, the recurrence of damaging or destructive waves, that is of
tsunami intensity equal to or larger than 4, 5 and 6, in 6-grade intensity scale, is on the
order of 10, 30 and 90 years, respectively. The most frequent tsunami generation is noted
in the east Mediterranean Sea and particularly in the Western and Eastern segments of the
Hellenic arc as well as in the Corinth Gulf, Greece. Infrequent but large or strong events
were described in Cyclades, South Aegean Sea, the Messina straits-Calabria in South
Italy, as well as in the coasts of Algeria (Alboran Sea), Cyprus, Israel, Lebanon and
Palestine (Levantine Sea), and of Marmara Sea. In the area of Greece, the recurrence of
tsunami intensity equal to or larger than 4, 5 and 6 is on the order of 15, 60 and 250
years, respectively.
b) Tsunami Generation Mechanisms
In Greece and in general in the Mediterranean Sea the earthquake activity is the main
cause of tsunami waves. The earthquakes capable to generate tsunamis, however, have
some particular properties: they are strong (magnitude around 6 or larger), shallow (focal
depth less than about 40km) events associated with submarine or near-coast dip-slip fault
motions. These properties make a very important knowledge background for the
formulation of warning algorithms in real time conditions. However, submarine volcanic
eruptions and coastal or submarine landslides are additional mechanisms that may
generate tsunamis from time to time. The Thera (Santorini) active volcano in the South
Aegean Sea is a typical source of large volcanigenic tsunamis. On the other hand, the
Corinth Gulf, Central Greece, is a typical area where the high rate of earthquake
occurrence is combined with the frequent occurrence of seismic or aseismic landslides
and result in a high rate of local but powerfull tsunamis. Since Corinth Gulf is a closed
sea area the design of instrumental tsunami warning systems should take into account the
local nature of the phenomena and the very short travel times of the waves.
c) Identification of Potential Tsunamigenic Sources
The identification of potential tsunamigenic areas constitutes a corner-stone in the effort
to mitigate tsunami risk on the basis of instrumental warning systems and other actions.
The past tsunamicity of the Mediterranean Sea is rich enough to provide data for the
construction of a reliable tsunami zonation map. On the basis of the past tsunami history
sixteen main tsunamigenic zones have been identified (Fig. 1 and Table 1). Most of them
are located in the east Mediterranean Sea and particularly in Greece. This is due to that
Greece, being located in the front of convergence of the African and Eurasian
lithospheric plates, is characterized by the highest seismicity in the western Eurasia, that
is from Caucasus to the Atlantic Ocean and from Africa to the north pole. However,
additional sources that are potentially tsunamigenic may have not been identified and,
therefore, more research effort is needed towards this aim. Submarine active fault
segments are of particular interest as for their possible association with future strong,
regional tsunamis. Coastal and submarine landsliding masses also bear some interest as
possible agents of local tsunami generation.
d) Tsunami Propagation Properties
The tsunami travel times in the Mediterranean Sea are relatively short due to that most of
the tsunamigenic sources are lying close to the coasts. Therefore, the expected travel
times may range from a few minutes up to about two hours at maximum. In addition, the
tsunami wave attenuation in the Mediterranean Sea is much stronger than that in the
Pacific Ocean. These two properties imply that only local and regional tsunamis are
expected to be observed in the Mediterranean Sea. On the contrary, no distant or
transoceanic tsunamis are expected and never such tsunamis were observed in the past
although several megatsunamis were produced in historical times as well as during the
20th century. As an instance, reliable documentary sources leave no doubt that the large
365 and 1303 tsunamis, that were caused by large earthquakes in the Hellenic Arc,
propagated towards remote places of the east Mediterranean Sea, like Alexandria, North
Egypt on 365 and Akko, Israel, on 1303. Similarly, the 9 July 1956 tsunami, generated by
a magnitude 7.5 tectonic earthquake in the South Aegean Sea and inundated many places
of the Aegean Sea with heights up to at least 15m. However, the tsunami did not
propagated to more remote places. The three tsunamis mentioned were regional,
megatsunamis given that they inundated many places of the east Mediterranean Sea with
high wave amplitudes. However, they did not propagated to the western side of the
Mediterranean Sea, that is they were not transoceanic tsunamis. The propagation
properties of the Mediterranean Sea tsunamis indicate the importance for developing
mainly local and regional tsunami warning systems.
e) Inundation Maps and Tsunami Intensity
The technology of the tsunami numerical simulation provides important possibilities for
the determination of the zones expected to be inundated by future tsunami waves.
Inundation mapping may include the area of flooding as well as the wave height and
direction of water flow. However, inundation maps do not describe the expected impact
of the wave. Therefore, there is need to develop further this technology so that to
translate expected tsunami inundation to expected tsunami impact. One of the most
practically applicable parameter to describe the tsunami impact is the tsunami intensity.
Traditional tsunami intensity scales developed since the 20’s are 6-grade scales.
However, they are not detailed and sensitive enough to describe adequately the several
components of the tsunami impact. More recently, a 12-grade scale was introduced by
following the long seismological tradition in this field. The new scale needs further tests
and calibration on the basis of actual tsunami impact data.
f) Identification of Coastal Vulnerabilities
No standard methodology has been generally adopted for the assessment of the
vulnerability of the anthropogenic environment to the impact of the natural hazards
including the tsunami hazard. Parameters that may be used include types and properties
of the engineering structures, population density, the land use/land cover peculiarities and
the existence or not of critical facilities. For example, in a pilot study developed as part of
the GITEC and GITEC-TWO projects for the tsunami risk assessment in a coastal
segment of Crete island, Greece, such parameters were semi-quantitatively introduced for
the vulnerability assessment. The timing of the event, that is the season, the day and the
time of occurrence, make a very important function for the vulnerability assessment.
There is no doubt that there is important room for improvements of the methodology
mainly towards making it more quantitative.
g) Evaluation of Potential Economic and Social Losses
The quantitative evaluation of the expected economic and social losses from natural
hazards is a very difficult task. Again no standard methodologies have been generally
adopted. A direct measure of the economic impact could be the local GNP of the area
considered. Other parameters that could be taken into account may include the population
density, the road network and other infrastructures, the critical facilities and again the
timing of the event.
4. What exists for the moment in Greece
4.1 Instrumental Networks
4.1.1 Real-Time Seismograph Networks
In Greece, a well-established national seismograph system consisting from about 30
permanent, on-line stations is operated on a 7/24 basis by the Institute of Geodynamics,
National Observatory of Athens (NOAGI). All the stations are equipped with BB
seismometers. The seismic signals recorded by the stations are transmitted to NOAGI in
real-time via dedicated telephone lines. Scientific and technical staff is on duty on a 7/24
basis and announce publicly moderate and strong earthquakes in routine times ranging
between 10 and 15 minutes by processing manually the data. The earthquake
announcement is also transmitted directly to governmental bodies like the General
Secretary of Civil Protection and the Earthquake Planning and Protection Organization.
This system is capable to transmit simultaneously and in real-time up to 13 different
geophysical signals via the same telephone lines. Recently, GPS signals started to be
transmitted from some of the stations. In addition, following international standards a
system for the automatic process of the data is under development in NOAGI. It is
expected that in a time prospect of about two years from now this system may provide the
possibility to routinely announce earthquakes within about 5 minutes from their
generation. NOAGI also operates seismic stations of international networks, like
MEDNET, under the supervision of INGV, Italy, and GEOFON under the supervision of
GFZ, Potsdam, Germany. Moreover, NOAGI has participated in research projects which
incorporated seismic monitoring with Ocean Bottom Seismographs (OBS) in several
seismogenic places of Greece.
As a part of GITEC-TWO, NOAGI developed an experimental tsunami warning system
in the South Aegean Sea consisting of five additional, temporary digital seismographs
and two tide-gauges connected on-line with NOAGI (see more details below).
In Greece, other academic institutes maintain local seismograph networks. However, they
either have no personnel on duty for the monitoring of their area or their capability is
only limited. Currently a new project was submitted with the aim to unify the local
networks with the NOAGI national network by developing special software which is
expected to make compatible the presently different networks. It is expected that the
government will approve this important 3-year project.
4.1.2 Tide-Gauge Network
A system of about 20 analog tide-gauge stations is operated by the Greek Navy mainly
for oceanographic purposes related to national security aspects. No data transmission is
made by this system. As a part of GITEC-TWO, NOAGI developed an experimental
tsunami warning system in the South Aegean Sea consisting of five additional, digital
seismographs and two tide-gauges connected on-line with NOAGI (see more details
below).
4.1.3 Experimental Tsunami Warning System
On 1998, as a part of GITEC-TWO, NOAGI developed an experimental tsunami warning
system in the South Aegean Sea consisting of two subsystems: one seismograph
subsystem, incorporating five digital seismographs additional to those of the national
system, and one sea-level changes subsystem incorporating two new, digital tide-gauges
equipped with pressure meters. Both subsystems were connected with NOAGI with
dedicated telephone lines that transmitted in real time the seismic and tide-gauge signals.
The experiment proved successful and important know-how was obtained.
4.2 Tsunami Hazard Assessment
4.2.1 Tsunami Simulation
The main research group involved in tsunami modeling in Greece is that of the Dept. of
Civil Engineering, University of Thessaloniki, which in collaboration with NOAGI
simulated several tsunamis observed in Greece. Other groups (e.g. from Japan and
Norway) also tried to simulate Greek tsunamis. The most important result is that the
algorithms used reproduce well-enough the observed wave heights in the near-field but
usually they fail to reproduce wave heights observed in the far-field. Therefore, it is of
great importance to investigate further the factors that control this inconsistency. The
most important of them include source properties, bathymetry and simulation techniques.
4.2.2 Tsunami Hazard Assessment – Sources
Although the historical record of tsunamis is incomplete, the existing data basis is
adequate enough to define the main tsunamigenic zones not only in Greece but in the
Mediterranean Sea in general (Fig. 1 and Table 1). However, what remains unknown is
why only some of the characteristic earthquakes occurring in the same seismic fault or
plate boundary segment produce tsunamis while others do not. For example, in the
eastern Hellenic Arc, it is historically and geologically well-documented that in the
Rhodes island region from the five large earthquakes that took place in the last six
centuries only three were tsunamigenic while the other two were not. This is of extreme
importance for the tsunami hazard assessment and, therefore, further research for
understanding better the properties of the seismic tsunamigenic sources is needed.
4.2.3 Tsunami Hazard Assessment – Recurrence
The recurrence of large tsunamis is only roughly estimated in a few tsunamigenic
regions. This is due to that the historical record is incomplete. Important improvement is
expected from further research in the historical documentation but also from the
application of the palaeotsunami method for the identification of past tsunamis from
geological field techniques combined with analytical laboratory methodologies. The
palaeotsunami method has been applied very successfully in the Aegean Sea, the Corinth
Gulf and the Marmara Sea by NOAGI in collaboration with Japanese and Turkish
institutes. About ten strong palaotsunami events were identified in the last ten years.
4.2.4 Vulnerable Areas Identification
In Greece the most tsunami-vulnerable coastal areas are those that are highly involved in
tourist activities mainly in the broad region of the South Aegean Sea (e.g. Cyclades island
complex, Crete island, Dodecanese island complex), the south part of the Ionian Sea
(west and south Peloponnese and nearby islands) and the Corinth Gulf, central Greece.
5. What can already be said today with available knowledge, methods and techniques,
networks?
Thanks to the GITEC, GITEC-TWO and the Greek-Japanese tsunami project (1996present), a remarkable crucial mass of human potential has been created incorporating
senior researchers and students from a few institutes. The main group that is constantly
active and coordinate the Greek tsunami research efforts is that of the Institute of
Geodynamics, National Observatory of Athens (NOAGI). Research links have also been
established between NOAGI and European and Japanese tsunami groups. On 2003 the
Tsunami Commission of the International Union of Geodesy and Geophysics decided to
ask from NOAGI to organize the 22nd International Tsunami Symposium. In fact, the
Symposium will take place from 27 to 29 June, 2005 in Crete and is expected not only to
become an important event but also to increase the links between the Greek tsunami
community with the international tsunami community.
Thanks to the international projects the studies of the tsunami community working in
Greece the last ten years or so did great progress and now covers nearly all the aspects of
tsunami science and technology: cataloguing, generation mechanisms, wave simulation,
paleotsunami identification, hazard and risk assessment. A significant gap is noticed in
the development of instrumental tsunami warning systems and only one experimental
effort was made on 1998.
Unfortunately the topic of tsunamis has been rather neglected from the national research
plans in the past years and it seems that the last Indian Ocean event did not attracted
significant interest at governmental level. On the contrary, as regards the tsunami hazard
in Greece, the interest of the general public but also of official authorities (e.g.
Embassies) or mass media of other European countries is great.
6. Is there a need for tsunami detection and warning systems?
From the past tsunami history in Greece it results that the tsunami risk is considerable
and, therefore, no doubt remains that a systematic effort should be undertaken for the
establishment of a system for tsunami detection and warning. It should be noted,
however, that although such systems constitute very powerful and useful tools for early
tsunami warning they should certainly be complemented by additional actions like hazard
and vulnerability assessment, tsunami scenarios based on wave simulation, people
education and civil protection plans.
7. Where are actually the relevant competences?
An operational tsunami warning system should be constructed around the following main
components:
- seismograph system,
- tide-gauge/ pressure meters system,
- data communication links,
- personnel on 7/24 duty.
In Greece, NOAGI operates the national seismograph system with personnel on 7/24
basis and on-line real-time communication links (details were given above). What is
further needed are (i) the increase of the number of seismograph stations, and (ii) the
minimization of the time needed for the earthquake determination. In addition, an effort
started by NOAGI on 1998 to deploy a tide-gauge network as well. The existing
capabilities of other institutes for the monitoring of the seismicity and sea-level changes
were already explained above.
8. Which research areas need to be targeted? What are still the research gaps,
problems, needs and priorities for short and long term perspective?
The knowledge on the tsunami phenomena and on the risk mitigation methodologies has
been drastically improved that last 15 years or so. However, major gaps still exist and
among them the most important include: (i) understanding better why some earthquakes
produce tsunamis and some other similar earthquakes do not, (ii) thorough sensitivity
analysis of the wave simulation techniques, (iii) testing of scientifically reliable and
operationally efficient tsunami early warning system, (iv) further development of
vulnerability and risk assessment.
9. How should tsunami warning be part of a multi hazard system?
A tsunami warning system as described above is a multi hazard system since it
incorporates earthquake detection, tsunami detection, and recording of sea- level changes,
which is useful for the monitoring of meteorological hazards like storm surges.
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