Estimateur de Risque de Tsunami

Estimate tsunami risk based on earthquake parameters and your coastal proximity.

Assessment

Comment les séismes génèrent des tsunamis

Les tsunamis sont des vagues océaniques générées par un déplacement soudain et à grande échelle de l'eau, le plus souvent causé par des séismes sous-marins le long des zones de subduction. Lorsqu'une section de croûte océanique est projetée vers le haut ou s'enfonce lors d'un grand séisme, elle déplace un volume d'eau énorme qui se propage vers l'extérieur sous forme d'une série de vagues à longue période. En haute mer, les vagues de tsunami se déplacent à des vitesses de 500 à 800 km/h (similaires à un avion de ligne) avec des hauteurs de vague de seulement 30 à 60 cm, les rendant quasiment indétectables. À l'approche des eaux côtières peu profondes, les vagues ralentissent et se compriment, provoquant une amplification dramatique de leur hauteur — un processus appelé shoaling — pouvant atteindre 10 à 30 mètres ou plus au rivage.

Tous les séismes ne génèrent pas de tsunamis. Les facteurs clés sont : le séisme doit être sous-marin (sous l'océan), superficiel (généralement moins de 70 km de profondeur), important (généralement M7,0 ou plus pour les tsunamis locaux, M7,5+ pour les régionaux), et doit impliquer un déplacement vertical significatif du fond marin. Les séismes décrochants, qui impliquent principalement un mouvement horizontal de la faille, génèrent rarement des tsunamis significatifs. Le mécanisme source de tsunami le plus dangereux est le chevauchement sur les méga-chevauchements de zone de subduction, comme lors des événements de l'océan Indien 2004 (M9,1), de Tōhoku 2011 (M9,1) et du Chili 1960 (M9,5).

Concepts clés de la science des tsunamis

Le temps d'alerte tsunami dépend de la distance : les tsunamis en champ proche peuvent arriver en 10 à 30 minutes, tandis que les tsunamis transocéaniques peuvent prendre des heures — le tsunami japonais de 2011 a atteint le Chili 22 heures plus tard.
Les séismes tsunamigéniques sont une classe spéciale d'événements à rupture lente qui génèrent des tsunamis disproportionnellement importants par rapport à leur magnitude, les rendant particulièrement dangereux pour les côtes proches.
La hauteur de montée — l'élévation verticale maximale atteinte par l'eau à terre — peut largement dépasser la hauteur de la vague au large en raison de la topographie côtière et des effets de résonance portuaire.
Le Centre d'alerte aux tsunamis du Pacifique (PTWC) et les centres d'alerte régionaux utilisent des bouées DART (capteurs de pression en haute mer) pour détecter et confirmer les tsunamis en temps réel.

Utilisations courantes

Comprendre si un séisme signalé a le potentiel de générer un tsunami en fonction de ses paramètres.
Exploration éducative de la relation entre les caractéristiques des séismes et la génération de tsunamis.
Évaluer l'exposition côtière au risque de tsunami pour la planification de voyages ou de déménagement.
S'informer sur les systèmes d'alerte aux tsunamis et l'importance de l'évacuation immédiate pour les événements en champ proche.

How to Use

1
Enter Earthquake Parameters
Input the earthquake magnitude, focal depth, and location. Tsunamis are most efficiently generated by shallow (< 50 km depth) thrust earthquakes with vertical fault displacement; the tool checks these criteria automatically.
2
Specify Your Coastal Location
Enter your coastal city or coordinates. The tool calculates your approximate distance from the source and identifies whether you are in a mapped tsunami inundation zone based on NOAA and national tsunami center data.
3
Read Your Risk Summary
Review the estimated wave arrival time, indicative wave height range, and evacuation tier. Treat all outputs as supplementary to official warnings from PTWC, NTHMP, or JMA, which must always take precedence.

About

Tsunami science sits at the intersection of seismology, physical oceanography, and coastal engineering. The word tsunami derives from the Japanese 津波 (tsu, harbor; nami, wave), reflecting Japan's millennia of devastating experience with these events. Despite their colloquial name 'tidal waves,' tsunamis have no connection to tidal forces; they are long-period gravity waves with wavelengths of 100–500 km in deep water and wave periods of 10–60 minutes, compared to wind-driven ocean waves with periods of seconds.

The physics of tsunami generation requires a mechanism that displaces a large volume of water vertically over a large area. Megathrust earthquakes accomplish this by the sudden elastic rebound of the overriding plate—the 2011 Tohoku earthquake caused the seafloor to rise by 5–8 meters over a 300 × 200 km area instantaneously, displacing an estimated 5 cubic kilometers of water. The resulting wave system propagates radially, with energy concentrated perpendicular to the fault strike. Directivity effects mean that the coast directly opposite the rupture typically receives the highest waves.

The Deep-ocean Assessment and Reporting of Tsunamis (DART) buoy network, operated primarily by NOAA, provides real-time sea-level measurements from the deep ocean floor. These buoys detect tsunami wave amplitudes of centimeters in the open ocean, enabling confirmation or cancellation of warnings within 15–30 minutes of a potentially tsunamigenic earthquake. Combined with coastal tide gauge networks and numerical propagation models, DART data allows warning centers to issue probabilistic wave height forecasts for coastal communities hours before wave arrival in distant-field scenarios. Local and regional tsunamis remain the most challenging problem in warning science because the lead times are measured in minutes.

FAQ

What earthquakes generate tsunamis?

Tsunamis are most commonly generated by shallow (< 50 km depth) submarine thrust earthquakes that produce significant vertical seafloor displacement, typically exceeding 1 meter over large areas. The key factors are: magnitude ≥ M7.5 (though some M7.0 events with favorable geometry have generated destructive tsunamis), a thrust (reverse) or oblique-thrust focal mechanism with a large vertical displacement component, shallow focal depth in the crust or uppermost mantle, and occurrence beneath ocean floor rather than continental crust. Strike-slip earthquakes—where motion is primarily horizontal, as on transform faults—rarely generate significant tsunamis. The 2018 Sulawesi earthquake was exceptional: a predominantly strike-slip event triggered localized submarine landslides that generated the devastating Palu Bay tsunami.

How fast do tsunamis travel and when do they arrive?

Tsunami propagation speed in open ocean is governed by the shallow-water wave formula: v = √(g × d), where g is gravitational acceleration and d is ocean depth. In the deep Pacific (average depth ~4,000 m), tsunamis travel at approximately 700–800 km/h—comparable to a commercial jet aircraft. As waves enter shallower coastal water, they slow dramatically (to 50–100 km/h near shore) while their amplitude increases through shoaling. This means a tsunami generated 4,000 km away arrives in about 5–6 hours in deep water, but can take 30–60 minutes to inundate a coast after entering shallow shelf waters. The Pacific Tsunami Warning Center (PTWC) issues initial bulletins within 3 minutes of detecting a significant seismic event.

How far inland can tsunami waves travel?

Tsunami inundation distance depends on wave height, coastal topography, and land elevation. The 2011 Tohoku tsunami waves reached heights of 40.5 m (at Miyako, Japan) and inundated up to 10 km inland across flat coastal plains, destroying the town of Rikuzentakata. In contrast, mountainous coastlines confine inundation to narrow strips. Maximum inundation run-up is generally defined as the highest elevation reached by wave water, measured as meters above mean sea level. NOAA and national emergency management agencies have mapped tsunami inundation zones for high-risk coastlines using numerical models calibrated against historical events. These maps define evacuation zones (A through E or Zone 1 through 3 depending on jurisdiction) for emergency planning.

Is a tsunami warning always issued after a large earthquake?

Warning centers issue tsunami warnings, advisories, and watches based on rapid seismic analysis within minutes of a significant event. PTWC monitors earthquakes globally and issues initial bulletins for any event M7.0+ in ocean basin settings within 3 minutes. However, the first message is often a 'tsunami information statement' that may not indicate confirmed wave generation—definitive warnings come after sea-level gauges and DART buoys confirm or deny wave propagation. Local tsunamis—generated by earthquakes within 50–100 km of shore—can arrive within 5–20 minutes, before official warnings are disseminated. In these cases, the natural warning is the shaking itself: the international standard guidance is that prolonged strong shaking near the coast (> 20 seconds) is itself a tsunami warning signal, and coastal residents should evacuate immediately without waiting for official messages.

Are there tsunamis caused by non-earthquake sources?

Tsunamis can be generated by submarine landslides, volcanic activity, meteorite impacts, and atmospheric pressure disturbances (meteotsunamis). Submarine landslides are the second most common cause: the 1958 Lituya Bay event in Alaska, triggered by an earthquake-induced rockslide, produced a 524-meter run-up—the tallest wave in recorded history. The 2022 Hunga Tonga-Hunga Ha'apai volcanic eruption generated an unusual meteotsunami-like pressure wave that propagated globally at the speed of sound in the atmosphere (~340 m/s), reaching Peru and Japan within hours. Volcanic island collapses—hypothesized for the Canary Islands—could theoretically generate Atlantic basin tsunamis, though the probability and magnitude of such events are debated in the scientific community.