On the morning of December 26, 2004, a M9.1 earthquake ruptured approximately 1,300 kilometers of the seafloor along the Sunda Trench off the western coast of Sumatra. Within minutes, the displaced ocean floor set in motion a series of waves that radiated outward across the Indian Ocean. Over the next seven hours, those waves struck coastlines in 14 countries, from Indonesia to Somalia, killing approximately 228,000 people and displacing more than 1.7 million. No tsunami warning system existed in the Indian Ocean.
Not all earthquakes produce tsunamis, and not all tsunamis are caused by earthquakes. But the overwhelming majority of destructive tsunamis — including the deadliest in recorded history — originate from large submarine earthquakes along subduction zones. Understanding the connection between earthquakes and tsunamis, the physics that governs wave generation and propagation, and the warning systems designed to save lives is essential knowledge for the hundreds of millions of people who live along tsunami-vulnerable coastlines worldwide.
How Earthquakes Generate Tsunamis
A tsunami is fundamentally different from a wind-driven ocean wave. Wind waves affect only the surface of the ocean and have wavelengths measured in meters to hundreds of meters. A tsunami, by contrast, involves the displacement of the entire water column — from the ocean surface to the seafloor — and has wavelengths that can exceed 100 to 200 kilometers. This means that in the deep ocean, a tsunami wave behaves as a shallow-water wave despite traveling through water thousands of meters deep, because its wavelength is so much greater than the water depth.
The generation process requires vertical displacement of the seafloor over a large area. When an earthquake occurs at a subduction zone — where one tectonic plate is forced beneath another — the overriding plate, which has been compressed and dragged downward by friction along the plate interface, suddenly snaps upward during the earthquake. This uplift pushes the overlying water column upward, creating a bulge on the ocean surface. Simultaneously, the area where the plate is being subducted may drop, creating a corresponding trough. This combined uplift and subsidence creates the initial tsunami wave profile.
Conditions Required for Tsunami Generation
Not every submarine earthquake generates a destructive tsunami. Four conditions must generally be met:
Large magnitude: The earthquake must be large enough to displace a significant volume of water. In practice, this usually means M7.5 or greater, though exceptions exist. Earthquakes smaller than M7.5 occasionally produce local tsunamis, particularly when they trigger submarine landslides. The USGS and NOAA begin tsunami threat evaluation for any submarine earthquake of M7.0 or greater.
Vertical seafloor displacement: The earthquake must produce significant vertical motion of the seafloor. Thrust (reverse) faults at subduction zones are the most efficient tsunami generators because their slip is primarily vertical. Normal faults can also produce tsunamis but are less common in the deep ocean. Strike-slip faults, which move primarily horizontally, generally do not produce significant tsunamis — though the 2018 Palu, Indonesia earthquake was a notable exception (discussed below).
Submarine location: The earthquake must occur beneath the ocean, or at least partially beneath the ocean, to displace the water column. An earthquake that ruptures entirely onshore, regardless of magnitude, will not generate a tsunami (though it can generate seiches in enclosed water bodies).
Shallow depth: Shallower earthquakes concentrate their energy closer to the seafloor surface, producing greater vertical displacement. Most tsunamigenic earthquakes occur at depths of less than 70 km. Intermediate and deep earthquakes, which occur at 70–700 km depth, are too far below the seafloor to produce significant vertical displacement at the ocean bottom, even if they are very large.
Tsunami Physics: From Open Ocean to Shore
Speed, Wavelength, and Height
In the deep ocean, tsunami speed is governed by a simple equation: v = √(g × d), where g is gravitational acceleration (9.8 m/s²) and d is ocean depth. In the Pacific Ocean, where average depth is approximately 4,000 meters, this yields a speed of approximately 200 m/s, or about 713 km/h — roughly the cruising speed of a commercial jet aircraft. In the open ocean, the tsunami's wavelength extends from 100 to more than 200 km, and its height is typically less than 1 meter. Ships at sea rarely notice a passing tsunami.
Shoaling
As the tsunami approaches the coast and water depth decreases, the physics change dramatically. The front of the wave slows as it enters shallower water (since speed depends on depth), while the back of the wave, still in deeper water, continues at higher speed. This compresses the wave's energy into a shorter wavelength, and the water must go somewhere — so the wave height increases. This process is called shoaling.
A tsunami that is 0.5 meters high in 4,000 meters of water can grow to 10 meters or more as it crosses the continental shelf and enters shallow coastal waters. The amplification depends on the coastal bathymetry (underwater topography), shoreline shape, and the specific characteristics of the incoming wave. Narrow bays, inlets, and harbors can further amplify wave heights through focusing and resonance effects.
Run-up and Inundation
When the tsunami reaches shore, it does not typically arrive as a single, towering breaking wave (as often depicted in movies). Instead, it usually manifests as a rapid, powerful surge of water — like a suddenly rising tide moving at high speed. The water may advance inland for hundreds of meters to several kilometers, depending on the coastal topography and wave energy. The maximum vertical height reached by the water above normal sea level is called run-up, while the maximum horizontal distance reached is called inundation.
The leading wave is not always the largest. Tsunamis arrive as a series of waves (a "wave train"), and the second or third wave is often the most destructive. The interval between successive waves can range from 10 minutes to over an hour, meaning people who return to the coast after the first wave may be caught by subsequent, larger waves.
Cross-section diagram — Tsunami Wave Propagation from Open Ocean to Coast. Show the seafloor at a subduction zone with fault displacement, the initial wave formation over the rupture zone (amplitude <1m, wavelength ~200 km), wave travel across deep ocean (speed ~700 km/h), wave crossing continental shelf (speed decreasing, height increasing), and final run-up at coast (height 5-30m+). Include annotations showing depth, speed, wavelength, and height at each stage. Data source: NOAA Center for Tsunami Research, general tsunami physics.
Warning Systems: From Detection to Evacuation
Pacific Tsunami Warning Center (PTWC)
The Pacific Tsunami Warning Center was established in 1949 in Hawaii following the devastating 1946 Aleutian Islands tsunami, which killed 159 people in Hawaii (including 96 in Hilo). The PTWC monitors seismic activity throughout the Pacific Basin and issues tsunami warnings, watches, and advisories to Pacific Rim countries. It is operated by NOAA's National Weather Service.
In 2004, the PTWC's area of responsibility was expanded to include the Indian Ocean and Caribbean as an interim measure while regional warning centers were being developed. Today, the PTWC serves as the primary warning center for the Pacific and provides backup for other ocean basins.
DART Buoy Network
The Deep-ocean Assessment and Reporting of Tsunamis (DART) system, developed by NOAA's Pacific Marine Environmental Laboratory, is the backbone of modern tsunami detection. Each DART station consists of a pressure sensor anchored on the seafloor at depths of 1,000 to 6,000 meters and a surface buoy that relays data to satellites in real time.
The seafloor sensor detects changes in water pressure caused by the passage of a tsunami wave overhead. Because tsunami wavelengths are so long, even a wave less than 1 centimeter high in the deep ocean produces a measurable pressure change. The DART system can detect tsunamis within minutes of the earthquake and provide critical data for forecasting wave heights at distant coastlines.
As of recent deployment, the global DART network includes approximately 60 stations positioned along tsunami-prone coastlines and across major ocean basins, with the highest density in the Pacific Ocean.
Pacific Ocean DART Buoy Locations and Historical Tsunami Propagation Paths. Show the approximate locations of DART buoy stations across the Pacific, Indian, and Atlantic basins. Overlay historical propagation paths for the 1960 Chile, 2004 Indian Ocean, and 2011 Tohoku tsunamis showing travel times in hours. Data source: NOAA National Data Buoy Center, NOAA Center for Tsunami Research. Features: Buoy locations marked as points, travel time contours (isochrones) for each historical event, subduction zones highlighted.
Indian Ocean Tsunami Warning System
Before 2004, no coordinated tsunami warning system existed in the Indian Ocean. The 2004 disaster prompted the international community, coordinated by the UNESCO Intergovernmental Oceanographic Commission (IOC), to establish the Indian Ocean Tsunami Warning and Mitigation System (IOTWS). The system became operational in phases beginning in 2006 and includes:
- Regional tsunami warning centers in Australia (Joint Australian Tsunami Warning Centre), India (Indian National Centre for Ocean Information Services), and Indonesia (BMKG)
- A network of seismometers, DART buoys, and coastal tide gauges across the Indian Ocean basin
- National tsunami warning centers in each participating country
- Community-level preparedness programs, including evacuation route signage and education campaigns
Global Warning System Coverage
| Warning System | Year Est. | Region | Operated By | Primary Detection |
|---|---|---|---|---|
| Pacific Tsunami Warning Center (PTWC) | 1949 | Pacific Basin (also backup for Indian, Atlantic, Caribbean) | NOAA/NWS (USA) | Seismometers + DART buoys |
| Japan Meteorological Agency (JMA) | 1952 | Japan and surrounding waters | JMA (Japan) | Dense seismic and coastal networks; warnings within 2-3 minutes |
| Alaska Tsunami Warning Center (now NTW) | 1967 | Alaska, U.S. West Coast, Atlantic, Gulf of Mexico, Caribbean | NOAA/NWS (USA) | Seismometers + DART buoys |
| Indian Ocean Tsunami Warning System (IOTWS) | 2006 | Indian Ocean Basin | UNESCO IOC coordination; regional centers in Australia, India, Indonesia | Seismometers, DART buoys, tide gauges |
| NEAMTWS (Northeast Atlantic, Mediterranean, Connected Seas) | 2005 | Mediterranean, NE Atlantic | UNESCO IOC; centers in France, Greece, Italy, Turkey, Portugal | Seismometers, tide gauges, limited DART coverage |
| CARIBE-EWS (Caribbean) | 2005 | Caribbean Sea, adjacent regions | UNESCO IOC; PTWC provides backbone | Seismometers, DART buoys, tide gauges |
Case Study: 2004 Indian Ocean Tsunami
The Earthquake
At 00:58:53 UTC on December 26, 2004, a M9.1 earthquake nucleated at a depth of approximately 30 km beneath the Indian Ocean, off the northwest coast of Sumatra, Indonesia. The earthquake ruptured approximately 1,300 km of the plate boundary along the Sunda Trench, where the Indian-Australian Plate subducts beneath the Burma Plate. The rupture propagated northward over approximately 8 to 10 minutes — one of the longest fault ruptures ever recorded instrumentally.
The earthquake caused vertical uplift of the seafloor of up to 5 meters over an area roughly 1,300 km long and 100–150 km wide. This massive displacement of the water column generated tsunami waves that radiated in all directions from the rupture zone.
The Waves
The tsunami struck the nearest coast of Sumatra within 15 to 30 minutes of the earthquake, with wave heights reaching 30 meters or more in parts of Aceh Province. The city of Banda Aceh, located at the northern tip of Sumatra near the rupture zone, was devastated. The waves penetrated as far as 5 km inland in some areas.
The tsunami then crossed the Indian Ocean, striking the coasts of Thailand (~2 hours after the earthquake), Sri Lanka and India (~2 hours), the Maldives (~3.5 hours), and the east coast of Africa (~7 hours). Even distant Somalia, approximately 5,000 km from the epicenter, recorded wave heights of several meters and 298 deaths.
The Toll
The final death toll was approximately 228,000 across 14 countries, making it one of the deadliest natural disasters in recorded history. Indonesia suffered the greatest losses, with an estimated 167,000 deaths. Sri Lanka lost approximately 35,000, India approximately 16,000, and Thailand approximately 8,000 (including more than 2,000 foreign tourists).
The disaster occurred in a region with no tsunami warning system and minimal public awareness of tsunamis. Many victims had no concept of what a tsunami was and did not recognize natural warning signs. In some areas, the initial withdrawal of the sea — a common precursor to tsunami arrival — drew people toward the water to collect stranded fish, placing them directly in the path of the incoming wave.
However, some communities survived through traditional knowledge. On Simeulue Island, located just 60 km from the earthquake epicenter, oral traditions from the 1907 tsunami taught residents to flee to high ground when they felt strong shaking and the sea withdrew. Only 7 of Simeulue's 78,000 residents died in the 2004 tsunami.
Case Study: 2011 Tohoku, Japan Tsunami
The Earthquake
On March 11, 2011, at 14:46 local time, a M9.1 earthquake struck off the Pacific coast of northeastern Japan (the Tohoku region). The earthquake ruptured approximately 500 km of the Japan Trench subduction zone at a depth of about 29 km. GPS and seafloor measurements showed that the Pacific Plate lurched westward by up to 50 meters — the largest coseismic slip ever measured. The seafloor was uplifted by as much as 10 meters over the rupture area.
Japan's Earthquake Early Warning (EEW) system detected the earthquake within seconds and broadcast alerts to millions of people before the strongest shaking arrived — providing critical seconds for people to take protective action against the earthquake itself. However, the system initially underestimated the earthquake's magnitude (issuing an initial estimate of M7.9 rather than M9.1), which led to an initial tsunami warning height estimate of 3 meters — later revised upward repeatedly as the true scale became apparent.
The Waves
The JMA issued its first tsunami warning just 3 minutes after the earthquake. The initial warnings called for waves of 3 to 6 meters. As data from DART buoys and coastal tide gauges poured in, the warnings were updated — eventually calling for waves exceeding 10 meters.
The tsunami struck the Tohoku coast within 30 minutes of the earthquake. The waves were staggering: run-up heights reached 40.5 meters at Miyako, Iwate Prefecture — the highest tsunami run-up measured in Japan's modern record. Waves overtopped and destroyed seawalls that had been designed for smaller tsunamis. The town of Rikuzentakata was virtually erased. The seaside town of Minamisanriku lost much of its population despite evacuation efforts.
Fukushima Daiichi
The tsunami waves reached the Fukushima Daiichi Nuclear Power Plant approximately 50 minutes after the earthquake. The plant's seawall was designed for a maximum tsunami height of 5.7 meters. The actual waves were approximately 14 to 15 meters high. The tsunami flooded the plant's emergency diesel generators, which were needed to power cooling systems for the reactor cores after the earthquake had knocked out offsite power. Without cooling, three of the plant's six reactor cores melted down over the following days, leading to hydrogen explosions and radioactive contamination. The Fukushima disaster was classified at Level 7 on the International Nuclear Event Scale — the highest level, shared only with the 1986 Chernobyl disaster.
Impact and Lessons
The earthquake and tsunami killed approximately 18,500 people, the vast majority from drowning. Economic losses exceeded $235 billion, making it the costliest natural disaster in history at the time. Approximately 470,000 people were displaced.
The 2011 disaster reshaped tsunami engineering and policy worldwide. It demonstrated that historical records and computational models had underestimated the potential size of tsunamis in the Tohoku region. Japan subsequently revised its tsunami hazard assessments, raised coastal defenses, and expanded vertical evacuation structures. The disaster also prompted nuclear safety reviews globally and led to the permanent closure of some nuclear power plants in Japan and other countries.
Other Notable Earthquake-Generated Tsunamis
1960 Chile: The First Transpacific Tsunami Warning Test
The M9.5 earthquake in Valdivia, Chile on May 22, 1960 — the largest earthquake ever recorded — generated a tsunami that devastated the Chilean coast and then crossed the Pacific Ocean. The waves reached Hawaii approximately 15 hours later, killing 61 people in Hilo despite warnings from the Pacific Tsunami Warning Center. The tsunami continued across the Pacific, killing 142 people in Japan — more than 17,000 km from the epicenter — approximately 22 hours after the earthquake. This event demonstrated that tsunamis can be deadly across an entire ocean basin and led to the expansion of the Pacific tsunami warning system.
1964 Alaska: Crescent City, California
The M9.2 Great Alaska earthquake on March 27, 1964 generated a tsunami that devastated coastal communities in Alaska and propagated southward along the West Coast. Crescent City, California, approximately 2,600 km from the epicenter, was struck by multiple waves, the largest arriving as the fourth wave approximately 4.5 hours after the earthquake. Eleven people were killed in Crescent City — some of whom had returned to the waterfront after earlier, smaller waves, not realizing that later waves could be larger.
2018 Palu, Indonesia: An Unexpected Tsunami
On September 28, 2018, a M7.5 earthquake struck near Palu, on the island of Sulawesi, Indonesia. The earthquake occurred on the Palu-Koro Fault, a strike-slip fault — a fault type not normally associated with significant tsunamis because the displacement is primarily horizontal. Nevertheless, the earthquake generated tsunami waves up to 11 meters in Palu Bay. The likely mechanism involved submarine landslides triggered by the earthquake and possibly some vertical displacement associated with fault geometry complexities. The tsunami struck within minutes, leaving virtually no time for warning. Combined with the earthquake itself and associated landslides and liquefaction, the disaster killed over 4,300 people. The event prompted a reexamination of tsunami hazard assessments for strike-slip fault environments.
Deadliest Earthquake-Generated Tsunamis in Recorded History
| Date | Event/Location | Earthquake Magnitude | Maximum Wave Height | Estimated Deaths | Key Factor |
|---|---|---|---|---|---|
| Dec 26, 2004 | Indian Ocean (Sumatra) | M9.1 | ~30 m (Aceh) | ~228,000 | No warning system in Indian Ocean |
| Mar 11, 2011 | Tohoku, Japan | M9.1 | 40.5 m (Miyako) | ~18,500 | Tsunami exceeded seawall design heights |
| Nov 1, 1755 | Lisbon, Portugal | Est. M8.5–9.0 | Est. 6–15 m | Est. 10,000–100,000 (total incl. earthquake and fire) | Struck major European capital; triggered fires and wave |
| Aug 27, 1883 | Krakatoa, Indonesia | Volcanic (VEI 6) | ~30+ m | ~36,000 | Volcanic eruption and collapse, not tectonic earthquake |
| May 22, 1960 | Chile (Valdivia) | M9.5 | ~25 m (local); ~10 m (Japan) | ~1,000–2,200 (Chile); 61 (Hawaii); 142 (Japan) | Transpacific propagation |
| Jun 15, 1896 | Sanriku, Japan | M~8.5 (tsunami earthquake) | ~38 m | ~22,000 | Weak shaking preceded large tsunami |
| Mar 27, 1964 | Alaska (Prince William Sound) | M9.2 | ~67 m (local max run-up) | 131 (including Crescent City, CA) | Transpacific propagation; multiple waves |
| Sep 28, 2018 | Palu, Sulawesi, Indonesia | M7.5 | ~11 m | ~4,300 | Strike-slip fault + submarine landslide; no warning time |
| Dec 28, 1908 | Messina, Italy | M7.1 | ~12 m | ~75,000 (total incl. earthquake) | Struck densely populated Strait of Messina |
Cascadia Subduction Zone: The Pacific Northwest's Tsunami Threat
The Cascadia Subduction Zone (CSZ) extends approximately 1,000 km from Cape Mendocino in northern California to Vancouver Island in British Columbia. It is the boundary where the Juan de Fuca Plate subducts beneath the North American Plate. On January 26, 1700, the CSZ ruptured in an estimated M~9.0 earthquake that generated a tsunami recorded across the Pacific — including in Japan, where historical records document an "orphan tsunami" (a tsunami with no associated local earthquake) that struck the coast on January 27, 1700. Japanese records provided the precise dating of the Cascadia event, which was confirmed by geological evidence of coastal subsidence, tsunami sand deposits, and drowned forests along the Pacific Northwest coast.
Geological evidence indicates that great Cascadia earthquakes (M8.7–9.2) recur at intervals averaging approximately 500 years, with significant variability (200 to 800+ years between events). The current interval since 1700 is 326 years — within the range where another great earthquake is considered possible.
A future Cascadia megathrust earthquake would generate a tsunami similar in mechanism to the 2011 Tohoku event. Modeling by NOAA and state agencies suggests that tsunami waves could reach the outer coast of Washington, Oregon, and northern California within 15 to 30 minutes of the earthquake — leaving very little time for evacuation. Waves of 6 to 12+ meters are projected for exposed coastlines, with higher amplification in some bays and inlets.
The limited warning time for near-field Cascadia tsunamis makes natural warning signs critically important. If people on the coast feel strong shaking lasting more than 20 seconds, or observe a sudden, unusual withdrawal of the sea, they should immediately move to high ground without waiting for an official warning.
What causes earthquakes Cascadia Subduction Zone Ring of Fire Tsunami safety guide 2004 Indian Ocean earthquake 2011 Japan earthquake and tsunami
Tsunami Preparedness
Know Your Zone
Most coastal communities in tsunami-prone areas have mapped tsunami inundation zones based on worst-case scenario modeling. These maps are available from NOAA, state emergency management agencies, and local government websites. If you live, work, or vacation in a coastal area, determine whether your location falls within a mapped tsunami inundation zone and identify evacuation routes to high ground.
Natural Warning Signs
The most reliable natural warning signs of an approaching tsunami include:
- Strong or prolonged earthquake shaking: If you are on the coast and feel an earthquake that lasts more than 20 seconds or is strong enough to make it difficult to stand, move immediately to high ground. Do not wait for an official warning — for near-field tsunamis, there may not be time for one.
- Unusual ocean withdrawal: A rapid, dramatic withdrawal of the sea — exposing seafloor that is normally submerged — can indicate that a tsunami trough is arriving before the crest. This withdrawal may precede wave arrival by 5 to 10 minutes.
- Unusual roaring sound: A tsunami may be preceded by a sound described as similar to a freight train.
Vertical Evacuation Structures
In low-lying coastal areas where high ground is not accessible within the available warning time, vertical evacuation structures provide an alternative. These are reinforced concrete buildings (or purpose-built towers) designed to withstand tsunami forces, with designated refuge floors above the expected inundation level. Japan has built hundreds of vertical evacuation structures along its Pacific coast. In the United States, the Ocosta Elementary School in Westport, Washington — completed in 2016 — includes a rooftop tsunami evacuation structure designed to shelter the school's students and nearby residents. It was the first purpose-built tsunami vertical evacuation structure in North America.
NOAA Tsunami Warning Center — Official warnings and information Pacific Tsunami Warning Center