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)
b) The role of operational warning systems
Tsunami warning systems are meant to reduce the vulnerability of populations to this hazard. For them to be effective, three conditions must be met:
- the warning system is fast, reliable and operational;
- measures for protecting the population are the subject of a preestablished plan;
- the population is informed of the tsunami risk.
Let us now examine each of these conditions in detail.
In order to be operational , the warning system must be capable of detecting a tsunami early on, of predicting its propagation, its time of arrival and the height of its waves along the threatened coasts, and of transmitting this information to the authorities responsible for civil protection.
Tsunamis are detected via measuring instruments. Seismometer networks allows scientists to locate the epicentre and focus of an earthquake and to measure its magnitude, in order to determine if the latter can provoke a tsunami. 8 ( * ) In the event of the answer being yes, the data gathered by the tsunamimeters and tide gauges allows scientists to confirm the presence of a tsunami and to refine the information concerning its amplitude. Therefore, the quick detection of a tsunami requires not only a sufficient number of measuring devices and networks, but also networks with advanced means of communication for the real-time transmission of their data. As for the early-warning centre, it must not only have access to this data, it must also be capable of processing and analyzing the data; this necessitates round-the-clock monitoring, seven days a week.
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The role of tsunamimeters and tide gauges A tsunamimeter is a pressure sensor installed at sea and capable of detecting waves of very low amplitude (a few centimetres). Indeed, when a wave passes by, the pressure increases due to the greater volume of water located above the sensor. Tsunamimeters are used not only to detect tsunamis, but also to predict the development and impact of both regional and distant tsunamis. There are two types of tsunamimeter: - The first type of sensor is linked to an underwater cable which transmits its data. Such a device has the advantages of being less expensive to maintain and having little chance of being damaged. However, there are two limits to this system: firstly, the tsunamimeter cannot be installed very far from the coast (150 km max); secondly, a violent earthquake can snap the cable. Such cable-linked tsunamimeters are principally used by the Japanese. - The second type of sensor is installed on the seafloor and transmits its data via an acoustical link to a buoy on the surface which then relays the data via satellite to the warning centre. The Americans began to develop this type of device starting in 1997 with their Deep-Ocean Assessment and Reporting of Tsunamis (DART) buoys, within the framework of their national programme for limiting the impact of tsunamis. These sensors are impressively precise, for they are capable of detecting waves of only one centimetre in depths of 6,000 metres. In addition, they can be installed in the middle of the ocean, thereby allowing for a real forecasting of tsunami events. However, they are very expensive to install and maintain: according to the information gathered by your rapporteur, this type of instrument costs between €70,000 and €200,000, its installation costs €100,000, and its obligatory annual visit costs between €50,000 and €70,000; what's more, the device must be replaced every 5 to 10 years. A tide gauge is a device which measures the sealevel at a specific location. It is generally situated in a harbour and sometimes combined with a GPS station. This instrument is most often used to measure tides. In addition, its data is rarely transmitted in real-time, but rather stored and recovered once a day or once a month. Insofar as they are installed along the shore, they cannot be used to forecast tsunamis affecting the same zone. However, they are still useful for two reasons. Firstly, they are an integral part of the early-warning system, providing precious information to the neighbouring countries/regions/islands, as well as to the civil protection services that can then take immediate safety measures in the case of strong waves being detected. Secondly, the measurements (wave amplitude, number and time of arrival) are later used to reconstruct the phenomenon and by the simulation models. For all that, their integration into the early-warning system necessitates their being able to transmit their data in real-time. |
When a tsunami risk is detected, the information must be quickly transmitted to the authorities in charge of civil security, so that they can take the necessary measures.
Taking into account the limited amount of time available (anywhere between a few minutes and a few hours), the chain of command and the civil protection plan cannot be improvised.
The authorities chosen to receive the warning messages must be clearly identified. That is why in the warning system coordinated by the Intergovernmental Oceanographic Commission (IOC), each country must designate a focal point: an organization charged with receiving these messages. For an effective transmission of this information, the said organization must provide round-the-clock monitoring, seven days a week. In the case of an alert, it is responsible for informing the civil security services.
In addition, emergency plans must be established and tested beforehand and each person's role and responsibilities clearly defined. In general, these plans rely on both evacuation maps based on past tsunamis and inundation maps provided by numerical simulations.
The population, therefore, will be encouraged to play an active role in protecting itself from the impact of a tsunami: depending on the amplitude of the tsunami, the population will have to evacuate the beaches and/or certain other low-lying coastal zones and seek refuge either on higher ground or in a sufficiently high and strong building. In some cases, the population will have to wait several hours before being able to return to the coast. What's more, if it feels an earthquake or hears a siren, the population must be capable of making the right decisions. Therefore, an early-warning system is only effective if the population is informed and well-aware of the tsunami phenomenon. A policy of prevention is therefore essential and can be divided into two necessary parts:
- Educating children at school, with both theoretical instruction (e.g., understanding tsunamis, eyewitness accounts) and practical instruction (e.g., evacuation exercises);
- Regular communication of the tsunami risk via the publishing of pamphlets and books, the holding of conferences, the inauguration of tsunami-specific signals, or organizing exercises simulating the arrival of a tsunami and the evacuation of the endangered zone.
Most of the persons interviewed by your interlocuteur pointed out that this policy of raising the population's awareness of the tsunami risk often represents the weak link in the early-warning chain. Not only must it be continually repeated to remain effective, but the population's receptiveness depends on its perception of the danger and its cultural and social behaviour. However, a tsunami is a relatively rare phenomenon and therefore of little weight in the collective consciousness, especially among the young. On the other hand, preventive measures, such as evacuations, have a heavy impact, because they can paralyze the economy of an entire region during several hours. Therefore, many warning systems have been designed in order to both protect the population and avoid false alarms (deemed catastrophic economically and financially, and which undermine the credibility of the policy seeking to limit the impact of tsunamis).
Therefore, it would seem that the policies for managing tsunami risk cannot be uniform: to be acceptable, suitable and long-lasting, they must take into account the concerned population's behaviour, which can be deeply rooted in its culture, tradition and social practices, thereby determining which measures would be accepted by the population.
* 8 Seismometers can also be used to detect volcanic eruptions and landslides.