A tsunami is a sequence of ocean waves generated by sudden displacement of a large water volume, typically from undersea earthquakes along a Subduction zone, but also from submarine landslides, volcanic processes, and, rarely, meteor impacts; the term derives from Japanese (tsu, “harbor,” + nami, “wave”), and “tidal wave” is discouraged because tides are not the cause NOAA Ocean Service, USGS, NOAA Tsunami Story.

Physical characteristics

• –In deep water, tsunami speeds depend primarily on depth and can approach 500–800 km/h (300–500 mph), while wave heights are typically small and wavelengths can exceed 100 km; as waves shoal near shore, speed decreases and height increases, producing dangerous run-up and currents NOAA Ocean Service, Encyclopaedia Britannica, USGS Water Science School.
• –Tsunami periods commonly range from 5 to 60 minutes, and the first wave is not necessarily the largest; multiple arrivals over hours are typical Encyclopaedia Britannica, USGS Water Science School.
• –Run-up is the elevation water reaches onshore above a reference sea level, and inundation distance is the horizontal penetration inland; these metrics are used to quantify coastal impact USGS, UNDRR Tsunami Glossary.

Causes and triggering mechanisms

• –Most tsunamis originate from vertical seafloor displacement during large megathrust earthquakes; additional sources include submarine and subaerial landslides, volcanic explosions or flank collapses, and, in rare cases, meteorite impacts USGS, NOAA NCEI—Introduction to Global Historical Tsunami Data.
• –Landslide-generated waves can produce extreme local run-up, exemplified by the 1958 Lituya Bay event in Alaska, where a rockslide triggered a maximum run-up of about 524 m (1,720 ft), the highest ever recorded USGS, Natural Hazards and Earth System Sciences.
• –Long-wave events driven by atmospheric pressure disturbances are termed meteotsunamis; while meteorological in origin, they share similar wave dynamics and hazards in confined basins NTHMP/NOAA (see fact-sheet resources), NOAA Tsunami Program.

Global occurrence and distribution

• –Historical catalogs show a strong concentration in the Pacific Ocean, especially around the Pacific Ring of Fire, with roughly 70–78% of known events occurring in the Pacific during the instrumental era; events also occur in the Indian, Atlantic, and Mediterranean basins NOAA NCEI—Global Historical Data, UNESCO‑IOC Tsunami Programme.
• –Teletsunamis (distant-source tsunamis) can cross entire ocean basins within hours; coastal impact varies with source, bathymetry, and coastline geometry NOAA Ocean Service, USGS Water Science School.

Detection and warning systems

• –Modern warning relies on rapid earthquake characterization, deep-ocean and coastal sea-level measurements, and forecast models. NOAA’s DART (Deep-ocean Assessment and Reporting of Tsunamis) network uses bottom pressure recorders and moored buoys to detect centimeter-scale waves in deep water and relay data via satellite for real-time forecasting NOAA NDBC—DART Description, NOAA PMEL, NOAA NDBC—System Overview.
• –The United States operates two 24/7 warning centers: the National Tsunami Warning Center (NTWC) in Alaska and the Pacific Tsunami Warning Center (PTWC) in Hawaiʻi. They issue standardized alert products (Warning, Advisory, Watch, Information Statement) and guidance domestically and provide international support within regional systems NWS Tsunami Warning Centers, Tsunami.gov.
• –Internationally, regional systems coordinated by UNESCO’s IOC operate in the Pacific, Indian Ocean, Caribbean, and the North-eastern Atlantic/Mediterranean; the Indian Ocean system was established after 2004 and became fully operational in 2013 under regional providers UNESCO‑IOC Indian Ocean, UNESCO.

Impacts and hazard metrics

• –Destructive effects include rapid inundation, strong currents in harbors and channels, debris impact, scour and foundation failure, and prolonged hazardous oscillations; damage often extends far inland on low-lying coasts USGS Water Science School, NOAA Education—Tsunamis.
• –Preparedness emphasizes recognizing natural warnings (strong/shaking earthquake, sudden sea level fall or rise, loud roar), moving immediately to high ground, and following official alerts. In the U.S., the National Tsunami Hazard Mitigation Program supports hazard mapping, evacuation planning, and community readiness NTHMP/NWS, NOAA Tsunami Program.

Historic tsunamis

• –1755 Lisbon earthquake of 1755: An Atlantic megathrust event generated waves reported about 6 m at Lisbon and up to ~20 m at Cádiz, with effects observed as far as the Caribbean, illustrating long-range transoceanic impact Encyclopaedia Britannica.
• –1960 Valdivia (Chile): The M9.5 event—the largest instrumentally recorded earthquake—produced a trans-Pacific tsunami that killed 61 in Hilo, Hawaiʻi, and 139 in Japan, with widespread Pacific impacts NOAA Science On a Sphere, NOAA NCEI, Encyclopaedia Britannica.
• –2004 Indian Ocean: The M9.1–9.3 Sumatra–Andaman earthquake generated the deadliest recorded tsunami, with approximately 227,000–229,000 dead or missing, maximum measured run-up around 51 m in Aceh, and impacts across at least 14–15 countries; the disaster spurred creation of the Indian Ocean warning system NOAA Science On a Sphere—Run‑ups, UNESCO, UNESCO‑IOC.
• –2011 Great East Japan (Tōhoku): An M9.0–9.1 megathrust earthquake off Tōhoku generated a tsunami exceeding 30 m locally, causing nearly 20,000 deaths and triggering the Fukushima Daiichi nuclear accident; extensive field and instrumental records made it one of the best-studied events USGS (Seismological Research Letters summary), USGS NEIC overview.

Data, archives, and research

• –The NOAA/WDS Global Historical Tsunami Database catalogs >2,200 source events and >27,000 run-up observations spanning 2000 B.C. to present, informing hazard assessments and models NOAA NCEI—Global Historical Data, NOAA Tsunami Archive.
• –Operational forecasting integrates real-time observations (DART, tide gauges) with numerical models to estimate arrival times, amplitudes, and coastal impacts for decision support at warning centers NOAA NDBC—DART Description, NWS Tsunami Warning Centers.