9
NEEDS AND PERSPECTIVES OF TSUNAMI RESEARCH IN EUROPE
S.TINTI
Università di Bologna,
Dipartimento di Fisica, Settore di Geofisica,
Viale Carlo Berti Pichat, 8, 40127, Bologna, Italy
Abstract
Tsunamis are a serious threat for all coastal European countries for they attack countries in
the north, such as Norway and Great Britain, as well as countries in the southern belt from
Portugal to Greek and Turkey. In the last decade tsunami research in Europe gained strong
impulse from mutual collaboration among various groups of several countries chiefly under
the sponsorship of the European Union. This led to remarkable progress in physical
understanding, in numerical modelling, and in defining and applying standards for data
catalogues, and allowed also to better define the objectives still to be achieved and the
research to undertake in the next future. This paper will mostly focus on the main issues
that have to be faced by future research projects and that demand the establishing of
stronger link between experts of physical and social sciences: local tsunamis induced by
near-shore earthquakes, tsunamis generated by mass failures and by flank collapses of
volcanoes, implementation of instrumental networks and early warning systems, tsunami
disaster management, incorporation of the tsunami impact factor in policies of sustainable
development of coastal communities.
1. The Past and the Future
Research developments on tsunamis in the last decade have enlarged the recognition that
tsunamis can constitute a serious danger for European coasts (see [1] in this volume; see
also [2] and [3]). The belt of the southern European countries from Portugal, to the west, to
Greece and Turkey, to the east, is mostly affected by tsunamis generated by local
earthquakes, and, though less frequently, by volcanic lateral collapses and eruptions.
Northern countries may be hit by remotely generated tsunamis, or, as is the case of
Norway, by tsunamis caused by local mass failures (ice blocks, rock-falls, landslides, etc).
Historical reports date back more than 2000 years for Mediterranean countries and show
that tsunamis affected disastrously the European coastal towns and economy several times.
Therefore tsunamis, like all other natural events that can be the cause of large disasters, are
and must be a topic of interest not only for scientists, but also for society, and studies on
tsunamis in Europe are expected to contribute both 1) to the scientific goal of a better
understanding of the physical processes, and, equally importantly, also 2) to the objective
of defending the European coasts from tsunami attacks, by favouring a correct and wise
policy of tsunami hazard evaluation and risk mitigation, which should be suitably
integrated in the policy of protection of European society from natural disasters. Both
A. C. Yalçıner, E. Pelinovsky, E. Okal, C. E. Synolakis (eds.),
Submarine Landslides and Tsunamis 9-16.
@2003 Kluwer Academic Publishers. Printed in Netherlands
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goals, in order to be achieved, need co-operation on a national and on an international level
and a multidisciplinary approach involving scientists of different branches of research,
engineers, technicians, experts in civil protection business and in social impact of large
events. These concepts have been already discussed and established in the past decade,
during which relevant international projects on tsunamis involving several European
countries were launched and financed by the European Union, such as GITEC [4], GITECTWO [5], INTAS [6], BIGSETS [7]. This financial policy has to continue in the next years,
and to be further encouraged to launch joint projects involving collaboration with countries
outside Europe (e.g. USA, Japan, northern African countries, and other countries that are
members of ITSU) that will be highly beneficial. The topics that are of uttermost interest
for Europe and that could form the basis for joint research and activities can be summarised
as follows: local tsunamis induced by near-shore earthquakes, tsunamis generated by mass
failures and by flank collapses of volcanoes, implementation of instrumental networks and
early warning systems, tsunami disaster management and incorporation of the tsunami
impact factor in policies of sustainable development of coastal communities.
2. Tsunamigenic Near-Shore Earthquakes
European and national tsunami catalogues ([1], [2], [3]) show that most of the European
tsunamis are local tsunamis caused by earthquakes with fault surface located near-shore or
in the coastal region. This means that the co-seismic deformation involves zones both onland and underwater, and that the shore region may experience uplift or subsidence, or
both, with a permanent change of the shoreline position and a permanent regression or
ingression of the sea (see e.g. the impressive uplift of the Phalasarna harbour in Crete that
exceeded 6.5m and was provoked by the tsunamigenic earthquake hitting Crete in 365AD:
the uplift caused the disruption of the economic activity of the coastal town and the
abandon of the harbour [8]). The changes in surface morphology associated with
earthquakes large enough to generate sizeable tsunamis are remarkable and complex, and
generally they cannot be entirely explained by simple double-couple point-like or
rectangular-fault seismic source models. Non-uniformity of slip or more complicated fault
geometries have to be invoked. Quite often a rather complicated picture has to be
considered including multiple rupturing taking place in the source region, since beyond the
main fault or faults also a number of ancillary faults are involved: these happen to be
destabilised by the main series of shocks and, being usually shallow, may induce surface
displacements that locally have magnitude as large as or even larger than the ones
associated with the main rupture processes. An exemplary case is the complex deformation
pattern of the recent tsunamigenic 17 August 1999 Izmit earthquake occurred along the
Northern Anatolian Fault in Turkey. The total surface rupture was found to extend about
145-km and to consist of five segments separated by releasing step-overs and was due to a
strike slip mechanism with dextral offset up to 5.2 m measured along the Sapanca-Akyazı
segment [9], [10]. But it was also accompanied by very shallow normal faulting, with about
2m-offest, observed in the south-eastern Izmit bay region, that could have had effects in
producing the tsunami [11], [12]. Another relevant aspect is that near-shore and coastal
zones are characterised by large topographic and bathymetric slopes that are prone to
gravitational instability. Therefore earthquakes here may trigger submarine or coastal mass
failures that can act as additional sources of tsunami. It is worth recalling that the Italian
tsunami that was responsible for the largest number of ascertained victims was very likely
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due to the huge rock-fall that involved the seaward flank of Mount Pacì, at Scilla on the
Tyrrhenian side of Calabria. The collapse occurred during a period of intense seismic
activity and took place in the night of 6 February 1783 following an aftershock of the
catastrophic I=XI (MCS scale) 5 February 1783 earthquake. The tsunami was local, but
very lethal, since took more than 1500 human lives according to the abundant coeval
documents [2], [13]. The above considerations prove that seismic sources in the coastal
areas are expected to be complex sources of tsunamis, and that usually approximating them
by simplistic sources would be unsatisfactory, since it would miss important features and
leave relevant observations unexplained.
3. Tsunamis Induced by Mass Failures
Tsunamis can be produced by masses entering the sea from altitudes above sea level or by
submarine landslides. Recently it has been recognised that this generation mechanism was
overlooked by the tsunami research in the past that in countries watered by the Pacific
Ocean was probably biased toward tsunamis induced by large earthquakes in the offshore
subduction zones and toward the effects of tele-tsunamis. But today, especially after the
example of the catastrophic Papua New Guinea 1998 tsunami induced by a huge submarine
slump triggered by a coastal earthquake [14], general consensus has been reached on the
need to intensify the theoretical developments and to enlarge the basis of observational data
for tsunamis induced by masses moving underwater. Mass failures can be caused by
seismic load, or more simply by gravity load, but can also be associated with volcanic
eruptions. In Europe such events are rare, but they are known to have affected and to have
potential to affect most coastal countries (see [1], [4], [5]), and there are even regions, such
as Norway, where this mechanism is largely predominant and occasionally disastrous (see
the case of the 1934 slide tsunami in the Tafjord fjord [15]), attracting therefore most of the
attention of the people involved in tsunami research. Theoretical investigations on waves
generated by moving underwater bodies ([16], [17], [18], [19], [20]) help understand the
basic process of energy transfer from the body to the wave, but the interaction is certainly
very complex and demands for more sophisticated models to simulate the slide, slump or
rockfall dynamics (see the Lagrangian approach used by Tinti et al. to model the
tsunamigenic volcanic slide of 1988 in the Vulcano island, south Tyrrhenian, Italy [21], and
to model the catastrophic 1963 slide displacing most of the water of the Alpine reservoir of
the Vajont valley [22], [23]) and to simulate the 2D and/or 3D water wave excitation and
propagation ([24] and [25]). This is a field where important progresses are needed, and
hopefully expected in the near future.
4. Tsunamigenic Collapses of Volcanic Cones
Tsunamis associated with debris avalanches and with collapses or eruptions of marine or
submarine volcanoes are quite rare, but may be catastrophically destructive, since the
volumes involved are very huge (up to hundreds of cubic kilometers) and the excited waves
can be giant and transoceanic. Several such volcanic islands or underwater cones are of
interest for European countries. The most impressive mass failures are expected to take
place in the Canary islands (Spain) in the Atlantic Ocean, where at least nine giant
landslides have been identified by geologists, that are similar to those observed in the
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Hawaiian archipelago ([26] and [27]), and where signs of instability have been recently
observed in the volcano Cumbre Vieja in the highly populated island of La Palma [28]:
mass collapse could concern the west flank of the volcano with a volume in the order of
500 km3. Evidence of repeated collapses has been collected also for Stromboli volcano in
the Aeolian Islands in the Tyrrhenian Sea, Italy. The last one produced the Sciara del Fuoco
scar in north-western flank of the volcano with 1-1.8 km3 volume involved [29].
Interestingly, a very strong structural similarity can be noticed between Stromboli and the
Nishi-yama volcano in the Oshima-Ohshima Island (Japan Sea), as far as regards the scar
morphology, the volcanic cone shape, and the underwater topography. The last lateral
collapse of this is volcano occurred in historical times during a series of eruptions in 1741
and produced a rather well documented tsunami that attacked the west Japan and was
observed in the entire Japan Sea ([30], [31], [32]). The volcanic island of Santorini in the
south Aegean sea is known to have been the source of tsunamigenic explosions: the last
one was the submarine explosion of the Mount Columbo in 1650 AD producing a tsunami
observed in the Aegean sea and in northern Crete ([1], [33], [34]), while the largest one is
the eruption culminating in the collapse of the Thera volcano in the Minoan times: this
tsunami was speculated to have destroyed all coastal towns of Crete and of the Aegean
islands and, consequently, to have caused the irreversible decline of Minoan civilisation.
Deposits of paleotsunamis of volcanic origin have been recently identified in various places
of the Aegean Sea coasts and attributed to the activity of Thera [35].
5. Instrumental Networks and Early Warning Systems
The most relevant obstacle to the development of knowledge on tsunamis is the endemic
lack of instrumental data. This is true world-wide, but it is especially true for Europe where
even the traditional gauges to record tides are too few and mostly not organised in
centralised data networks. What is needed is investment of resources to place proper
instrument on the coasts involved by tsunamis sources as well as at sea. It has to be stressed
that not only gauges to record tsunamis are needed, but also instruments to monitor the
tsunami sources (active coastal and submarine faults, zones with unstable coastal segments
or unstable underwater sediment masses, active volcanoes prone to be tsunamigenic, etc).
The basic elements of a system of warning for tsunamis are the detection of the source
and the verification of the tsunami occurrence, with both phases that have to be
accomplished soon enough to leave time to launch a useful alert and to take proper actions
before the tsunami attacks the target to protect. In case of tsunamis produced by
earthquakes an efficient seismic network based on broad-band instruments and a system of
coastal and/or offshore buy-based gauges to measure sea level can provide the data needed
to recognise that a tsunami has been generated and to assess the tsunami size and
propagation fronts through suitable processing (see e.g. the DART system devised and
implemented by PMEL for northern Pacific [36], and the TREMORS system conceived by
LDG-CEA (Paris) originally for Tahiti and installed in several countries, [37] and [5]).
Teletsunamis occurring very far from the target coasts can be identified expectedly with
great efficiency by such systems, since travel times from the source to the target exceed
several tens of minutes, and there is time to collect, transmit and process data with reliable
algorithms. Most of the damaging tsunamis of seismic origin that have occurred in Europe
are local tsunamis. The only examples of transoceanic tsunamis are the case of the 1755
Lisbon earthquake tsunami that was observed even in Norway and in the Caribbean Sea
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[1], and very likely the 365 AD Crete tsunami that was reported to be seen in the entire
eastern Mediterranean as well as in southern Adriatic and the Ionian Sea. Tsunamis
generated by coastal near-shore sources attack very soon. This means that we must accept
the challenge of devising special tsunami early-warning systems with reaction time as low
as a few minutes (1-3 min) to alert and protect the target that in this case is the local
communities. The idea is that any coastal community that is located close to a seismic
source with tsunamigenic potential should be provided with a local early warning system,
capable to detect the event almost at the inception time. Processing time must be minimised
by collecting only essential data (such as the signals from strong motion accelerometers)
and by adopting very simple detection algorithms based on expert systems, which are able
to identify the onset of an anomaly [38] (in this optics the tsunami is viewed as an
anomalous departure from the background noise). The local systems are not an alternative
to the traditional systems, but rather they complement each other on a regional level: the
former ones are only yes- or no-systems (since their goal is to detect whether or not a
tsunami has been generated and is on the point to attack the local community), while the
latter has the wider scope to assess the size and the propagation path of the tsunami and to
alert the communities that are more remote from the source. It should always be borne in
mind however that appropriate educational policies of the coastal population are an
indispensable prerequisite to reduce the impact of the waves.
6. Scenarios of Disasters
A very useful tool often adopted to plan prevention actions and to reduce natural disaster
impact is that of considering scenarios of future large events: for example, a large
earthquake triggering slumps and tsunami, a large volcanic eruption producing killing
pyroclastic flows and tsunami, etc. These events are usually quite complex, and need very
sophisticated modeling and data bases to simulate the physical evolution of the processes
and their impact on land and on social communities. Since the results concerning a given
scenario are to be used by a very large community of people, the maximum effort is needed
to provide the users with tools that are simple and practical, but most importantly, to clarify
the assumptions that are at the basis of the elaboration of the scenario, and the reliability of
the results. This latter issue is crucial, especially for scenarios concerning infrequent
events, such as catastrophic tsunamigenic processes, since it is always difficult and often
even impossible to elaborate statistical approaches and to use concepts such as probability
of occurrences or return times. An example could help understand this point. If we consider
the scenario of a large explosion of the volcano Cumbre Vieja in the Canary islands, entailing a giant collapse of the island of La Palma and if we model the giant tsunami attacking
locally the archipelago and remotely the coasts of the Atlantic ocean, including the
Caribbean sea countries and the eastern coast of the United States, what is relevant is to
provide scientific and technical details on the simulation, and moreover to provide some
hints concerning how often such an event is expected to occur in a given future period of
time. If this information is missing, the scenario may be a very important scientific
contribution, but looses much of its practical value, since it gives the society no elements
on the basis of which to take decisions and to formulate a correct policy of future
developments. How to reduce or to eliminate this drawback is not an easy task and is
certainly one of our major commitments for the future.
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7. Conclusions
The topics that have been briefly addressed in the previous sections constitute a serious
commitment for researchers involved in the tsunami field and, more generally, in the
natural hazards sciences. They are a problem not only for Europe, but for all other countries
exposed to tsunami attacks. The solution can be approached successfully only if a network,
formal and informal, of collaboration will be established among various research groups
world-wide, and if experience and data can be freely exchanged. This is an essential point
that is especially relevant for rare events: the whole earth should be seen as a natural
laboratory and the occurrence of a tsunami should be seen as a source of experimental data
accessible to all researchers. Beyond raw data, collaboration should involve methods,
models and elaboration of new concepts. What is even more important, scientists should try
to close the gap between earth and physical science and technique on one side, and social
sciences on the other side. Only if people pertaining to these worlds will make any efforts
to speak a common language with common words and common meanings, and to have
common targets, progress on protection of coastal communities from disastrous events will
be significant and development plans will be compatible with natural processes and
resources.
Acknowledgements
This contribution was carried out on funds from the Gruppo Nazionale di Difesa dai
Terremoti (GNDT) of the Istituto Nazionale di Geofisica e Vulcanologia (INGV) and from
the Ministero dell’Istruzione, dell’Università e della Ricerca (MIUR), formerly named
Ministero dell’Università e della Ricerca Scientifica e Tecnologica (MURST).
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