SENAT
Report n° 117 (2007-2008) by M. Roland COURTEAU, Senator (for the parliament office for the evaluation of scientific and technological choices) - Appendix to the minutes of the 7 December 2007 session
Disponible au format Acrobat (21 Moctets)
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N° 488
CONSTITUTION OF 4 OCTOBER 1958 THIRTEENTH TERM |
N° 117
REGULAR MEETING OF 2007-2008
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Registered with the Presidency of the National Assembly
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Appendix to the minutes of the
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PARLIAMENT OFFICE FOR THE EVALUATION OF
SCIENTIFIC AND TECHNOLOGICAL CHOICES (OPECST)
REPORT
on
Evaluating and preventing the tsunami risk for
France's metropolitan and overseas coasts
by
M. Roland COURTEAU, Senator
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Filed with the Bureau of the National Assembly by Mr Claude BIRRAUX First Vice-Chairman of the OPECST |
Filed with the Senate Bureau par M. Henri REVOL Chairman of the OPECST |
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INTRODUCTION
The Parliamentary Office for the Evaluation of Scientific and Technological Choices (OPECST) has already had the opportunity to examine the subject of natural risks and their prevention, thanks to the work carried out by our colleague, Mr. Christian Kert, Deputy, author of two benchmark reports , the first of which 1 ( * ) dealt with earthquakes and ground movements and the second of which 2 ( * ) dealt with other natural hazards: hazards due to the weather, avalanches, inundations , droughts, forest fires, volcanism, collapsing mines and underground cavities.
In addition, following the dramatic Indonesian tsunami of 26 December 2004, our colleague, Mr. Christian Kert, organized public hearings before the National Assembly on 17 February 2005. These hearings were again held on 18 March 2005 in Port-la-Nouvelle (in the Aude département ), in cooperation with your rapporteur and our colleague Mr. Jacques Bascou, Deputy. This work shined a light on a risk that had up until then generated little concern in France. These hearings also allowed us to take stock of the situation with regard to tsunami-detection research and international cooperation in the field of tsunami prevention and early-warning systems.
On 22 March 2005, the OPECST was commissioned to prepare a report by the Bureau of the National Assembly, in accordance with Article 6 (section B) of edict no. 58-1100 of 17 November 1958 concerning earthquake- and tidal wave-related hazards in the Mediterranean Sea. Your rapporteur was put in charge of this study, the title of which was later modified following the feasibility study.
The title of the study was modified following the presentation of the feasibility study.
Firstly, the term "tidal wave" turned out to be inappropriate, since it refers to a meteorological phenomenon, while tsunamis are always seismic in origin.
Secondly, an analysis of the tsunami risk limited to the Mediterranean area proved too narrow in scope, in so far as tsunamis can occur in any large ocean basin and France is present in every ocean of the world, via its overseas départements and territories.
The final report therefore deals with evaluating and preventing the risk posed by tsunamis to all French coasts, both in metropolitan France and overseas.
Your rapporteur first examined in detail the characteristics of this hazard and concluded that in order to decrease our vulnerability to tsunamis, a sophisticated early-warning system must be established.
Indeed, tsunamis are relatively rare phenomena compared to other natural events, such as storms and inundations; however, they often have a devastating impact on coastal populations. As a result, their prevention (or, at the very least, the limitation of their impact) demands a dense system of instruments for measuring earthquakes and sea level, a dependable, high-speed data-transmission system, and a preestablished, operational plan for alerting the concerned populations. The effectiveness of any early-warning system ultimately depends upon an informed, aware population, that must also be able to adopt the appropriate responses.
These significant budgetary and logistical constraints, combined with the fact that tsunamis most frequently occur in the Pacific Ocean, have led to diverse measures for the management of this risk. These measures vary according to the particular ocean basin: while the Pacific states began to set up an international warning system in the second half of the 20th century, efforts to manage the tsunami risk in the other ocean basins are much more recent and directly linked to the shock of the tsunami of 26 December 2004.
On the one hand, it was necessary to recognize the vulnerability of all the oceans and seas. Statistically speaking, the Indian Ocean is considered the safest (large) ocean basin with regard to tsunamis, since it accounted for only 4% of tsunamis generated during the 20th century. However, the 2004 tsunami claimed more victims than all other (known) tsunamis combined since ancient times.
On the other hand, it was necessary to explain to the concerned governments that, even if the risk is rare, the public no longer accepts to be left unprotected when a warning system that saves human lives could be set up.
Under the aegis of the United Nations, it was therefore decided in 2005 to create a tsunami warning system for the Indian Ocean, the Caribbean and the Mediterranean/Northeast Atlantic zone.
Two years later, this project has achieved unequal results: while the assessment is generally positive for the Indian Ocean, the setting up of an early-warning system for the Caribbean and Mediterranean/Northeast Atlantic zone has fallen far behind schedule due to the wait-and-see policy of the concerned countries.
Due to the dispersal of its overseas territories, France is particularly sensitive to the risk of tsunamis. Indeed, it began to set up its tsunami warning system for French Polynesia in the 1960s. Following the Sumatra tsunami, France pushed strongly for the setting up of an early-warning system to cover the three other ocean basins, particularly the Indian Ocean.
Nevertheless, it is clear that once the initial shock had faded away, this movement quickly ran out of steam, due to a lack of political will and insufficient funding. While the last meeting dedicated to the setting up of an early-warning system for the Mediterranean and the North Atlantic represented a real break from the waiting game that France had been playing for over a year, no concrete decision has yet been made and numerous questions remain concerning the structure of the national tsunami warning system, its geographical coverage and, above all, the means it is to be allocated.
Your rapporteur will therefore make structural proposals concerning the four ocean basins, as well as recommendations for each sea/ocean, in order for France to quickly set up a national tsunami warning centre for the Mediterranean, the Caribbean and the Indian Ocean.
I. WHAT IS A TSUNAMI?
Your rapporteur would first like to consider in detail the following questions: What is a tsunami? How are tsunamis generated? How do they manifest themselves? How can this natural phenomenon become a major risk for coastal populations?
A. A NATURAL HAZARD
Tsunamis are phenomena of geological origin, whose frequency varies depending on location.
1. A phenomenon of geological origin
If one excludes those very rare tsunamis generated by a man-made explosion or the impact of a meteorite, it can be said that tsunamis are always of geological origin . They are caused by the sudden penetration or retreat (when speaking of earthquakes, the terms "uplift", "upheaval", "sinking" and/or "subsidence" are often used) of a large quantity of geological material in the ocean depths, resulting in the displacement of a large mass of water.
a) The different causes
Three natural phenomena are liable to cause a tsunami: underwater and coastal earthquakes, landslides and volcanic eruptions.
(1) Underwater earthquakes
On the surface, an earthquake manifests itself by ground vibrations. It is caused by the fracturing of rocks deep below the surface. This fracturing results from the release of a great accumulation of energy, creating or reactivating faults 3 ( * ) , when the rocks' rupture threshold has been reached.
The earth's crust is made up of several large lithospheric plates which move about in relation to one another. While the plates normally move from a few millimetres to a few centimetres per year, in those regions on two plates boundary, this movement is discontinuous. The faults can remain locked during long periods of time, while the plates' regular movement (convergence, divergence and sliding) continues. This locking of the plate boundaries is the cause of local and regional deformations, such as the buckling of the plates on either side of the oceanic trenches.
In brief, the situation is as follows: the region of the blocked fault is progressively deformed (slow elastic deformation) as it accumulates energy, up until the time this energy is suddenly released, resulting in a seismic rupture; with the subsequent relaxing of the tectonic constraints, the fault is once again blocked, and the same seismic cycle begins all over again.
There are three types of fault line:
- Normal faults: the horizontal displacement provoked by the slip corresponds to an extension of the crust (E), with one of the blocks sinking downward relative to the second.
- Reverse faults: the horizontal displacement provoked by the slip corresponds to a shortening of the crust (R), with one of the blocks overlapping the second.
- Strike-slip faults: this third type of fault corresponds to a horizontal shift along a vertical fault surface.
Source: IPGP
For a submarine earthquake to generate a tsunami, it must result in a vertical movement of the sea floor. Therefore, the hypocentre must be situated at a depth of less than 100 km. In addition, it must have a magnitude of at least 6.5.
Subduction (or reverse-fault) earthquakes are particularly dangerous, because the activated faults are often very long and an earthquake's magnitude is proportional to this length, as well as the seismic sliding along the fault line. The tsunami that ravaged Indonesia on 26 December 2004 was provoked by an earthquake with a magnitude of 9.3, which occurred off the northwestern tip of the island of Sumatra. At this location, the Indo-Australian Plate converges with and sinks below the Eurasian Plate. The fault ruptured along 1,200 km. This rupture lasted 9 minutes and caused water displacements of 15 to 25 metres. With a plate convergence speed of 6 cm/year in this region, the last major earthquake would have taken place between 400 and 600 years ago.
Normal-fault earthquakes are provoked by much smaller fault lines (with a maximum length of 200 to 300 km) and, therefore, are of a weaker magnitude. However, since the angles are generally steeper (30° to 40°, as compared to 10° to 20° for subduction earthquakes), the tsunami risk is not negligible. The underwater earthquake of 21 November 2004 with a magnitude of 6.3, which occurred some ten kilometres south of the Iles des Saintes, generated a tsunami that hit Guadeloupe and Martinique.
One could easily suppose that strike-slip fault earthquakes do not generate tsunamis, since the sliding is horizontal. However, although the tsunami risk is, in fact, low, it still exists and depends on the angle of the underwater fault. A perfect example is the Izmit earthquake in Turkey, which provoked a local tsunami.
* 1 "Les techniques de prévision et de prévention des risques naturels : séismes et mouvements de terrain", report no. 261 (Senate) and no. 2017 (National Assembly) by Mr. Christian Kert, Deputy, on behalf of the Parliamentary Office for the Evaluation of Scientific and Technological Choices (1995).
* 2 "Les techniques de prévision et de prévention des risques naturels en France", report no. 312 (Senate) and no. 1540 (National Assembly) by Mr. Christian Kert, Deputy, on behalf of the Parliamentary Office for the Evaluation of Scientific and Technological Choices (1999).
* 3 A discontinuity or fracture in the earth's crust, showing evidence of relative movement between the two blocks of rock separated by the fault.