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
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2. Managing risk
As stated earlier, risk necessitates a vulnerability to the natural hazard. Managing risk, therefore, entails a better understanding of the hazard in question, as well as a reduction of the concerned societies' vulnerability vis-à-vis the said hazard through the establishment of an operational early-warning system.
a) A better understanding of the hazard
Since reducing the frequency of tsunamis is not an option, we must instead work to reduce their possible impact by better understanding both the processes that provoke tsunamis and their mechanisms of propagation, followed by the setting up of an appropriate protection system.
Therefore, a better understanding of the hazard means being capable of not only understanding the phenomenon (how it manifests itself, its frequency and intensity, and the area affected), but also predicting it (in other words, specifying where and when it will occur).
As will be shown, understanding the hazard in order to consider the risk necessitates our calling upon diverse scientific fields, such as seismology, geography, oceanography, geology and biology.
As it turns out, it is essential to collect data allowing for a better understanding of the hazard's characteristics. To do this, we must rely upon not only eyewitness accounts and photographs, but also hydraulic and geographic records in order to determine the run-up level and the areas inundationed. That is why post-tsunami surveys are so important, for they allow for a close, reliable examination of the hazard, especially in sparsely populated regions.
Tsunami-mapping in French Polynesia rests, to a large extent, on meticulous observations made by scientists in the aftermath of a tsunami.
Understanding tsunamis also depends upon a correct understanding of their causes: earthquakes, landslides and volcanic eruptions. The data that must be collected is two-fold:
- On the one hand, data directly linked to a specific event (the localization and magnitude of a tsunami-generating earthquake, the localization of a landslide and the volume of displaced rocks, the localization of a volcanic eruption and the volume of rocks either expelled or displaced following the volcano's collapse, etc.); this information allows for a better understanding of the phenomenon.
- On the other hand, a more global understanding of the causes of tsunamis and their localization via the study of faults, active volcanoes and instable rocky zones along the seashore or underwater. For example, studying tsunami directivity allows scientists to better determine the concerned zones. Indeed, while a tsunami will spread outward in all directions from its source of origin, a large amount of its energy will propagate in a direction perpendicular to the fault zone. As a result, the longer the earthquake's rupture area, the greater the number of concerned zones. In addition, a zone outside the tsunami's angle of maximum energy will be relatively safe, even if located near the source of origin, while a zone within the angle of maximum energy will receive the full force of the tsunami, even if located thousands of kilometres away. Therefore, this data facilitates the prediction and mapping of tsunamis.
Insofar as the hazard is characterized by its intensity and frequency, it is important to have access to long series of data and to reconstruct and determine the scale of past events. To this end, several historical catalogues have been drawn up:
- An American catalogue covering the entire globe, established by the National Geophysical Data Center, part of the United States Department of Commerce's National Oceanic and Atmospheric Administration (NOAA 6 ( * ) );
- Two Russian catalogues, one covering the Pacific zone and the other the Mediterranean zone, established by the Russian Academy of Sciences;
- A European catalogue, financed by the European Commission within the framework of the Fifth Research Framework Programme, entitled The Genesis and Impact of Tsunamis on European Coasts (2001);
- An Italian catalogue;
- Studies concerning the West Indies carried out by O'Loughlin and Lander (2003), Lander et al. (2002), and Zahibo and Pelinovsky (2001).
It should be pointed out that this task proves to be quite difficult for ancient (and, sometimes, even recent) events, insofar as there exists little direct data and the events must be reconstructed from diverse documents (written texts, eyewitness accounts, photographs or drawings). This historical work is quite delicate and demands both a critical examination of the sources and a verification of the data's coherency, in order to use the information in the most pertinent and reliable manner. In addition, this work is never completed, since technological advances and the discovery of new sources are liable to provide additional information.
It is in this context that computer simulations play an important role .
Firstly, they allow scientists to test various hypotheses concerning the triggering and propagation of tsunamis.
The Nice tsunami of 16 October 1979 is a case in point: because simulations showed that the landslide observed in the area of the airport extension was insufficient to explain the magnitude of the observed waves, the scientists directed their research towards a second, much larger landslide. This hypothesis was later confirmed by underwater observations.
Likewise, simulations can be used to complement in situ observations and refine tsunami maps. For example, in French Polynesia, several simulations were carried out in the most vulnerable bays, in certain harbours and in the area of the airport, in order to best determine those areas concerned by tsunamis. Taking into account the population density along the coast, it is difficult for the Polynesian authorities to set strict regulations with regard to construction; they therefore opt for a precise delimitation of the evacuation zones.
Secondly, simulations allow scientists to "test" tsunamis in the zones considered vulnerable, but which lack reliable observations. Therefore, these simulations allow us to predict a possible tsunami, to determine its potential intensity and to take the appropriate precautionary measures. These simulations have the added advantage of being able to make decisions in the case of a tsunami without having to wait for a confirmation of the risk. Such simulations are all the more useful for those tsunamis endangering nearby zones, thereby greatly reducing the reaction time available.
To clarify, let us consider the following example: Suppose that an earthquake of magnitude 7.5 occurs off the coast of Japan. Taking into account its magnitude and its location at sea, there is a strong chance that the earthquake provokes a tsunami. However, in order to predict its amplitude and the height of the waves that will strike the coast, scientists must have instruments for measuring sea level (tsunamimeters) out at sea. If there are not enough of these instruments or if the earthquake is centred too close to the coast for the information provided by the tsunamimeters to be of any use (due to a lack of time), the concerned populations cannot be protected. On the other hand, if the authorities responsible for their safety benefit from similar (simulated) scenarios prior to the current event, they can take the necessary measures. 7 ( * ) As we will see, this is the solution that Japan has chosen to limit the impact of tsunamis on its population.
It should be pointed out that the quality of any given simulation depends in large part on the reliability of the data used. In particular, an excellent understanding of the concerned zone's bathymetry and coastal topography is essential for a correct analysis of the tsunami's propagation and amplification upon reaching the coast.
* 6 The NOAA is a federal agency dependent on the United States Department of Commerce. Its writ extends to all issues relative to the state of the oceans and atmosphere. In particular, it is in charge of evaluating the risk and limiting the impact of tsunamis.
* 7 In which case, the "only" risk run is that of issuing a false warning.