How is a “Tsunami Warning” triggered?
On Friday, May 19, a powerful 7.7 magnitude earthquake struck the Pacific Ocean southeast of New Caledonia, prompting a “tsunami warning” that was lifted an hour later. We explain the triggering of a tsunami warning.
event detection
The tsunami is a phenomenon of geological origin. It is caused by the intrusion or disappearance of large amounts of geological material into the seabed, resulting in the displacement of large bodies of water. When it comes to earthquakes, we tend to speak of uplift or subsidence. Not all earthquakes produce tsunamis. Therefore, it is necessary to identify the event that is likely to trigger a tsunami: an underwater or coastal earthquake, a landslide, or a volcanic explosion. There are seismic and oceanographic monitoring networks to detect earthquakes and other potentially tsunami-triggering activities. Seismic sensors detect earthquakes and oceanographic buoys measure sea level changes. In Tahiti, for example, the Geophysics Laboratory (LDG) provides information on tsunamis that could hit the coasts of French Polynesia. This laboratory built the Polynesian seismic network and developed increasingly sophisticated methods for assessing the tsunamigenic potential of earthquakes.
Initial assessment
When an earthquake or other potentially dangerous event is detected, the data is analyzed to assess its magnitude, depth, location and potential risk of triggering a tsunami. In the event of an earthquake, therefore, it is important to measure the extent of the seismic fracture. The strength of a tsunami is determined by the location, size, and geometry of the seismic fracture. For the tsunami warning, a seismic signal, called phase W, allows for a quick assessment of these parameters during severe earthquakes. According to the seismological network, the information can be provided in 10, 20 or 30 minutes. On this point, the development is remarkable: until the late 2000s, it took at least several hours to obtain reliable information about the seismic source. Now a new, faster method developed by a Franco-American team has been implemented. It should be noted that after the tsunami of December 26, 2004, international cooperation with the aim of establishing a global tsunami warning and prevention system was significantly strengthened. The kick-off took place in the 3th World Conference on Prevention of Natural Disasters, held in Kobe (Japan) in January 2005: A plenary session was devoted to the creation of an alert mechanism against tsunamis in the Indian Ocean.
Tsunami threat confirmed
If the event is deemed large enough to pose a threat, tsunami monitoring centers like this will Pacific Tsunami Warning Center (PTWC) or the West Coast and Alaska Tsunami Warning Center (WCATWC) in the United States are responsible for confirming the presence of a tsunami using additional data and wave propagation models.
dissemination of the warning
As soon as the tsunami danger is confirmed, an alert will be issued to warn the authorities and the affected population. Alerts can be sent above Emergency warning systems, radios, televisions, sirens, mobile applications and other means of communication.
Speed of information is one of the goals of CENALT (Tsunami Warning Centre), which monitors strong earthquakes and tsunamis in the western Mediterranean and north-east Atlantic and alerts security forces in the event of a tsunami threat. Its operational objectives include:
« Broadcast an alert message to the French authorities and an information message to other foreign national and regional alert centers in the Mediterranean region, within 15 minutes of potential earthquake tsunami events (…) Broadcast messages confirming (or denying) the occurrence of a tsunami, giving arrival times and estimated offshore amplitudes, if any. The propagation time depends on the availability of tide data. »
Evacuation and Emergency Measures
Based on the tsunami warning, local authorities can trigger evacuation plans and take emergency measures to protect the lives and property of people in vulnerable coastal areas. It is imperative to follow the instructions of the authorities and stay safe in high or sheltered areas. Such was the case with the tsunami warning system first tested during the Tohoku earthquake in Japan in March 2011: a magnitude 9.1 earthquake. A first tsunami warning was issued very quickly, leaving only five minutes to evacuate the coasts. Then an estimate of the strength of the earthquake was made in 20 minutes, which allowed evacuation from the coasts of several countries. The Japan Meteorological Agency issued a tsunami warning for the entire Pacific coast of Japan, and the Pacific Tsunami Warning Center in Hawaii issued the same for Russia, the Marcus Islands and Mariana Islands, Guam, Taiwan, Indonesia and the Philippines. The warning has been extended to cover the entire west coast of the United States and Canada, from the Mexican border to Chignik Bay in Alaska.
And finally, some impressive figures (for tsunamis in the 20th century, according to the report (2007-2008) by Mr. Roland Courteau, commissioned by the Parliamentary Office for the Evaluation of Scientific and Technological Decisions): 77% of the tsunamis were in the Pacific generated, compared to 9% in the Mediterranean, 10% in the Atlantic Ocean and 4% in the Indian Ocean. What’s more, in the last century, 58 tsunamis claimed more than 26,000 lives (220,000 deaths in Indonesia in December 2024).
Figures that underscore the need for a tsunami warning system, which no doubt needs to become even more effective when we know that given sea level rise caused by climate change, more tsunamis are to be expected in the future… in 2021 The United Nations has figured that out Goal set to prepare all population groups for tsunamis by 2030.
