Tsunamis are just long waves — really long waves. But what is a wave? Sound waves, radio waves, even “the wave” in a stadium all have something in common with the waves that move across oceans. It takes an external force to start a wave, like dropping a rock into a pond or waves blowing across the sea. In the case of tsunamis, the forces involved are large — and their effects can be correspondingly massive.
A tsunami is a series of extremely long waves caused by a large and sudden displacement of the ocean, usually the result of an earthquake below or near the ocean floor. This force creates waves that radiate outward in all directions away from their source, sometimes crossing entire ocean basins. Unlike wind-driven waves, which only travel through the topmost layer of the ocean, tsunamis move through the entire water column, from the ocean floor to the ocean surface.
A tsunami is a series of ocean waves that sends surges of water, sometimes reaching heights of over 100 feet (30.5 meters), onto land. These walls of water can cause widespread destruction when they crash ashore.
Most tsunamis–about 80 percent–happen within the Pacific
Ocean’s “Ring of Fire,” a geologically active area where tectonic shifts make volcanoes and earthquakes common.
Tsunamis race across the sea at up to 500 miles (805 kilometers) an
hour - about as fast as a jet airplane. At that pace, they can cross the entire expanse of the Pacific Ocean in less than a day. And their long wavelengths mean they lose very little energy along the way.
Large tsunamis are significant threats to human health, property, infrastructure, resources, and economies. Effects can be long-lasting, and felt far beyond the coastline. Tsunamis typically cause the most severe damage and casualties near their source, where there is little time for warning. But large tsunamis can also reach distant shorelines, causing widespread damage. The 2004 Indian Ocean tsunami, for example, impacted 17 countries in Southeastern and Southern Asia and Eastern and Southern Africa.
While Japan may have the longest recorded history of tsunamis, the sheer destruction caused by the 2004 Indian Ocean earthquake and tsunami event mark it as the most devastating of its kind in modern times, killing around 230,000 people. The Sumatran region is also accustomed to tsunamis, with earthquakes of varying magnitudes regularly occurring off the coast of the island.
Tsunamis are an often underestimated hazard in the Mediterranean Sea and parts of Europe. Of historical and current (with regard to risk assumptions) importance are the 1755 Lisbon earthquake and tsunami (which was caused by the Azores–Gibraltar Transform Fault), the 1783 Calabrian earthquakes, each causing several tens of thousands of deaths and the 1908 Messina earthquake and tsunami. The tsunami claimed more than 123,000 lives in Sicily and Calabria and is among the most deadly natural disasters in modern Europe. The Storegga Slide in the Norwegian Sea and some examples of tsunamis affecting the British Isles refer to landslide and meteotsunamis predominantly and less to earthquake-induced waves.
WHAT HAPPENS WHEN A TSUNAMI HITS LAND ?
A tsunami’s trough, the low point beneath the wave’s crest, often reaches shore first. When it does, it produces a vacuum effect that sucks coastal water seaward and exposes harbor and sea floors. This retreating of sea water is an important warning sign of a tsunami, because the wave’s crest and its enormous volume of water typically hit shore five minutes or so later. Recognizing this phenomenon can save lives.
A tsunami is usually composed of a series of waves, called a wave train, so its destructive force may be compounded as successive waves reach shore. People experiencing a tsunami should remember that the danger may not have passed with the first wave and should await official word that it is safe to return to vulnerable locations.
Some tsunamis do not appear on shore as massive breaking waves but instead resemble a quickly surging tide that inundates coastal areas.
Tsunami warnings come in different forms. There are official warnings issued by tsunami warning centers that are broadcast through local radio and television, wireless emergency alerts, NOAA Weather Radios, NOAA websites, and social media. They may also come through outdoor sirens, local officials, text message alerts, and telephone notifications. There may not be time to wait for an official warning, so it is important to be able to recognize natural tsunami warnings. These include strong or long earthquakes, a loud roar (like that of a train or an airplane) coming from the
ocean, and a sudden rise or fall of the sea level that is not related to the tide. Official and natural warnings are equally important. Be prepared to respond immediately to any tsunami warnings. Move quickly to a safe place by following posted evacuation signs. If you do not see an evacuation route, go to high ground or as far inland as possible.
SIGNS OF A TSUNAMI
- Learn the signs of a potential tsunami, such as an earthquake, a loud roar from the ocean, or unusual ocean behavior, such as a sudden rise or wall of water or sudden draining of water showing the ocean floor.
- Know and practice community evacuation plans. Some at-risk communities have maps with evacuation zones and routes. Map out your routes from home, work and play. Pick shelters 100 feet or more above sea level, or at least one mile inland.
- Create a family emergency communication plan that has an out-of-state contact. Plan where to meet if you get separated.
- Sign up for your community’s warning system. The Emergency Alert System (EAS) and National Oceanic and Atmospheric Administration (NOAA) Weather Radio also provide emergency alerts.
- Consider earthquake insurance and a flood insurance policy through the National Flood Insurance Program (NFIP). Standard homeowner’s insurance does not cover flood or earthquake damage.
SURVIVING A TSUNAMI OR EARTHQUAKE
- If there is an earthquake and you are in a tsunami area, protect yourself from the earthquake first. Drop, Cover, and Hold On. Drop to your hands and knees. Cover your head and neck with your arms. Hold on to any sturdy furniture until the shaking stops. Crawl only if you can reach a better cover, but do not go through an area with more debris.
- When the shaking stops, if there are natural signs or official warnings of a tsunami, move immediately to a safe place as high and as far inland as possible. Listen to the authorities, but do not wait for tsunami warnings and evacuation orders.
- If you are outside of the tsunami hazard zone and receive a warning, stay where you are unless officials tell you otherwise.
- Leave immediately if you are told to do so. Evacuation routes often are marked by a wave with an arrow in the direction of higher ground.
- If you are in the water, then grab onto something that floats, such as a raft or tree trunk.
- If you are in a boat, face the direction of the waves and head out to sea. If you are in a harbor, go inland.
WHAT IS THE WORST TSUNAMI IN RECENT HISTORY?
What has commonly been considered the worst tsunami in history occurred in the
Indian Ocean in 2004. It was the result of a 9.1 magnitude earthquake that occurred off the coast of Sumatra in Indonesia. It caused a wave that was a record-breaking 164 feet tall, resulting in around $10 billion in damage and the deaths of 230,000 people.
THE TOP THREE DEADLIEST TSUNAMIS
The top three worst tsunamis in history are the Sumatran tsunami of 2004, the Tōhoku tsunami in 2011, and the Lisbon earthquake and tsunami in 1755. Often, the worst tsunamis are not the largest. But, in these instances, waves reached incredible heights of more than 150 feet.
On March 11th, 2011, one of the worst tsunamis in recorded history hit the coast of Japan. The waves were recorded at 32 feet or 10 meters in height and traveling at around 500 miles per hour. The earthquake that caused this immense tsunami lasted for six minutes. It is commonly referred to as the “Great East Japan Earthquake.” It was the fourth most powerful earthquake recorded since modern record-keeping began in the 1900s.
Terrifyingly, residents in the affected area only had a few minutes of warning to escape from the impact zone of the wave. Even then, evacuation sites were heavily impacted, some of which were completely washed away. In 2021, official reports registered 19,747 deaths, with 2,556 people reported missing.
The earthquake and resulting tsunami also caused the Fukushima Daiichi nuclear disaster. Three of its reactors melted down, and radioactive discharge was released. This affected thousands of residents in the area and led to further evacuations. The earthquake resulted in the loss of somewhere between $14.5 to $34.6 billion. It is often referred to as the most expensive natural disaster in history.
RECORDED TSUNAMIS - 497 BC and 365 AD
As early as 426 BC the Greek historian Thucydides inquired in his book History of the Peloponnesian War about the causes of tsunami, and was the first to argue that ocean earthquakes must be the cause. The oldest human record of a tsunami dates back to 479 BC, in the Greek colony of Potidaea, thought to be triggered by an earthquake. The tsunami may have saved the colony from an invasion by the Achaemenid Empire.
Sea has suffered many cases of seismic activity, sinking
towns and cities along the southern coast of Europe. But the
one that figures in our Cleopatra
story affected the northern coast of Africa and Egypt, when
in 365 AD Alexandria
were washed into the sea and submerged, along with Cleopatra's
Temple and her mausoleum. Up until 2022, nobody knew for
sure where Cleopatra
VII's tomb was located, lost to the mists of time.
Despite, several archaeological teams on land, and marine
archaeologists diving all around Alexandria, the search
The Roman historian Ammianus Marcellinus (Res Gestae 26.10.15–19) described the typical sequence of a tsunami, including an incipient earthquake, the sudden retreat of the sea and a following gigantic wave, after the 365 AD tsunami devastated Alexandria.
LISBON EARTHQUAKE AND TSUNAMI — November 1st, 1755
Height: between 20 and 65 feet (6 and 20 meters)
Effect: Wide-ranging fires, thousands of deaths, the birth of seismology
On November 1, 1755, an earthquake occurred off the coast of Lisbon, Portugal. It resulted in a tsunami and fires. The surrounding cities, including Lisbon, were almost completely destroyed. Today, seismologists suggest that the earthquake occurred at a magnitude of 7.7 or more. The epicenter was around 200 km, or 120 miles, west southwest of Cape Saint Vincent. The death toll was estimated to be between 12,000 and 50,000 people.
Today, it is considered to be one of the deadliest earthquakes and tsunamis in history. Interestingly, the earthquake and resulting disasters inspired European enlightenment philosophers who wrote and speculated upon the natural disaster for years. It also led to the birth of seismology and, later, modern earthquake engineering.
KRAKATOA ERUPTION AND TSUNAMI — May 20th, 1883
Height: up to 150 feet (46 meters)
Effect: Thousands of deaths, destruction of Krakatoa, a volcanic winter
The famous eruption of Krakatoa off the coast of Indonesia resulted in a deadly earthquake and tsunami. It began on May 20th and lasted until August 27th. Around 70% of the island of Krakatoa and the surrounding islands were destroyed. The eruption itself is considered one of the most destructive volcanic events in recorded history. It was heard up to 1,900 miles away in Western Australia.
At least 36,000 deaths are attributed to the eruption and the tsunami that resulted. Terrifyingly, ships as far away as South Africa were impacted by the tsunami. Waves as high as 46 meters, or 150 feet, were recorded.
MEIŌ EARTHQUAKE AND TSUNAMI — September 20th, 1498
Height: up to 56 feet (17 meters)
Effect: Between 5,000 and 41,000 deaths and the destruction of the building that housed the Great Buddha at Kōtoku-in
The Meiō earthquake occurred off the coast of Nankaidō, Japan, on September 20th, 1498. Today, seismologists estimate that the earthquake had a magnitude of 8.6. The death toll associated with this horrifying natural disaster ranges from 5,000 up to 41,000 casualties. Famously, the resulting tsunami destroyed the building that housed the Great Buddha at t Kōtoku-in, although the statue itself survived.
HŌEI EARTHQUAKE AND TSUNAMI – October 28th, 1707
Height: up to 82 feet (25 meters)
Effect: Loss of 5,000 lives, possibly the last eruption of Mount Fuji
The Hōei earthquake was considered the largest earthquake in Japanese history until 2011. In combination with the resulting tsunami, an estimated 5,000 lives were lost. Scientists have also speculated that it may have triggered the last eruption of Mount Fuji, which occurred 49 days later.
SANRIKU EARTHQUAKE AND TSUNAMIS — June 15th, 1896
Height: 125 feet (38 meters)
Effect: Two tsunamis, loss of at least 22,000 lives
On June 15th, 1896, an 8.5 magnitude earthquake hit the coast of Japan. It was one of the most destructive in Japanese history. It resulted in two tsunamis that destroyed up to 9,000 homes and took at least 22,000 lives. It broke the record height at the time, reaching 125 feet or around 38 meters.
ARICA EARTHQUAKE AND TSUNAMI – August 13th, 1868
Height: 39 feet (12 meters) and 52 feet (16 meters)
Effect: 25,000 casualties, destruction of ships and towns
On August 13th, 1868, an earthquake somewhere between 8.5 and 9.0 hit the coast of Chile. Multiple tsunamis were produced recorded as far away as Hawaii, Japan, and Australia. The earthquake and resulting tsunamis caused around 25,000 casualties and destroyed numerous ships. Three ships anchored along the coast were carried around 800 meters or 2,600 feet inland. Several others were completely destroyed.
MESSINA EARTHQUAKE - December 28th 1908
The Messina earthquake was caused by an underwater landslide triggering an earthquake that killed about 123,000 people in Messina, Italy on December 28, 1908. The quake, also referred to as the 1908 Messina and Reggio earthquake, had its epicenter in the Strait of Messina which separates Reggio Calabria on the Italian mainland from the busy port city of Messina in Sicily. The quake was felt in a radius of 186 miles. This tsunami left Messina under extreme depopulation with only 19, 000 people remaining with an utmost 2,000 remaining in the old city center. The earthquake almost brought everything to the ground in Messina with 91% of the structures being damaged irreparably. It was the most destructive earthquake to hit Europe
TŌHOKU EARTHQUAKE AND TSUNAMI - March 11th 2011
This tragic tsunami which occurred on March 11, 2011, at the Pacific coast of Japan is the most powerful earthquake ever recorded to have hit Japan doubling as the fourth in the world since modern record-keeping began in 1900. The earthquake also caused a powerful tsunami that had its surge height estimated at 133 feet above sea level and traveled up to 6 miles inland. The tsunami was triggered by an earthquake leaving 18,550 people confirmed either dead or missing and left around 4.4 million households in northeastern Japan without electricity. An additional 1.5 million households were rendered waterless.
WHAT IS THE DIFFERENCE BETWEEN TSUNAMI AND A TIDAL WAVE?
Tsunamis and tidal waves refer to sea waves which are significantly different, although many people confuse the two. A tidal wave is a shallow wave which is caused by the interactions between the gravitational forces of
earth, the moon, and the sun. In the past, the name tidal wave was used to describe what is now known as a tsunami. On the other hand, a tsunami refers to an ocean wave that is generated by a powerful earthquake originating from under the ocean. Other phenomena that can set off tsunami include
submarine landslides, volcanic eruptions, glacial carving, and onshore landslides with an excessively large volume of debris falling into the water.
Tidal waves are experienced periodically and they depend on relative positions of the moon and
earth. This is the main reason why tides occur at different times each day. The gravitational force of the moon determines the height of the tidal wave, and that is why during the full and new moon, the tidal wave is highest. Similarly, during the quarter phases of the moon, the tidal wave is lowest. Coastal regions experience two low tides and two high tides each day. Due to the distance of the sun, its effect is minimal compared to the effects of the moon, which is significantly greater. As a result, tidal waves are largely predictable because they are affected by the atmosphere and they are more pronounced mainly along the river estuaries and in narrow bays. As a result of tidal waves, water levels could rise several feet high within hours. Tidal waves also could quickly fizzle out before reaching the coast.
NEW SCIENTIST MARCH 2011 - JAPAN IS BEST PREPARED FOR TSUNAMIS WITH SHELTERS
Of all nations, Japan was perhaps the best prepared to withstand a tsunami. But as last week’s terrible loss of life has shown, better shelters are sorely needed.
Building towers and berms – artificial embankments – that can provide high ground for residents of low-lying areas is difficult and expensive. Brahman Developments of San Juan,
Rico, has proposed an innovative and affordable alternative: a submarine-shaped reinforced concrete structure that would literally ride the waves, keeping up to 80 people safe inside.
Known as the STATIM Shelter System, the tubular structures would be tethered, but could float freely. They are designed to survive temporary submersion and to be self-righting. Built from reinforced concrete, they should also be sturdy enough to survive being battered with debris in a major tsunami. It’s like “an inland lifeboat”, says inventor Miguel Serrano of Brahman. “It’s not a fun ride, but you would survive.”
The company projects that each shelter would cost about $100,000 and could be equipped with communications systems, GPS and enough food and water for inhabitants to survive for up to a week without aid. They could be deployed every few blocks in densely populated areas. That’s an appealing prospect, given that many of those who perished in Japan were elderly people who lacked the mobility to flee to higher ground.
Japan already has some permanent structures designed as tsunami escape havens. Nishiki Tower in Mie prefecture, for instance, is five storeys tall and doubles as a public toilet, museum and storage space. While more such structures are needed, Japan is well ahead of the rest of the world.
In the US’s Pacific northwest, for instance, city leaders are just starting to plan for a tsunami that could be generated by a rupture of the Cascadia fault – which threatens an earthquake of magnitude 9.0 or greater.
Cannon Beach, Oregon, has an ageing city hall that it hopes to tear down and rebuild as a state-of-the-art tsunami refuge. The building would have sea walls to the front and rear, says Yumei Wang, a geotechnical engineer for the state, and sit upon concrete pillars reinforced with tension
steel cables. These would allow water and debris from a tsunami to pass underneath while local residents take refuge on the upper floor and roof.
Long Beach, Washington, meanwhile, is considering building a berm next to the town’s elementary school. It would normally provide a grassy slope for spectators to watch school sports, but provide shelter for up to 1000 people should a tsunami strike.
A nuclear weapon detonated below the ocean's surface can cause great devastation.
One need not look further than the underwater US nuclear weapons tests of the 1940s and 1950s, including operations "Crossroads Baker" and "Hardtack I Wahoo" to see why. These underwater fireballs — roughly as energetic as the bombs dropped on Hiroshima or Nagasaki in August 1945 — burst through the surface, ejecting pillars of seawater more than a mile high while rippling out powerful shockwaves.
Some warships staged near the explosions were vaporized. Others were tossed like toys in a bathtub, sinking them. Others sustained crushed or cracked hulls, crippled engines, and other damage from the shockwaves. And — notably— the explosions roughly doubled the height of waves to nearby atoll islands, flooding inland areas there.
Yet, it is unlikely that even the most powerful nuclear bombs could come close to unleashing a significant tsunami.
The energy in a large nuclear weapon is but a drop in the bucket compared to the energy of a [naturally]-occurring tsunami. So, any tsunami created by a
nuclear weapon couldn't be very large.
For example, the 2011 Tohoku earthquake and tsunami that killed more than 15,000 people in Japan released about 9,320,000 megatons (MT) of TNT energy. That's hundreds of millions of times greater than the bomb dropped on Hiroshima in 1945, and roughly 163,000 times greater than the Soviet Union's "Tsar Bomba" test of October 30, 1961: the most powerful nuclear detonation in history.
And, because of the small solid angle that would subtended by a nuclear-induced tsunami (in the direction of the shoreline), most of the energy would be wasted going back out to