Q: What is a tsunami?
A: The National Ocean Service defines them as such: “Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Out in the depths of the ocean, tsunami waves do not dramatically increase in height. But as the waves travel inland, they build up to higher and higher heights as the depth of the ocean decreases. The speed of tsunami waves depends on ocean depth rather than the distance from the source of the wave. Tsunami waves may travel as fast as jet planes over deep waters, only slowing down when reaching shallow waters. While tsunamis are often referred to as tidal waves, this name is discouraged by oceanographers because tides have little to do with these giant waves.”
Q: What causes a tsunami?
A: While more common in the Pacific Ocean, such a wave can strike any coast at any time, putting all coastal regions at risk. They occur as a direct result of a violent eruption or earthquake under the surface of the ocean. According to the National Ocean Service, about 80% of such waves are caused by earthquakes. But they can be caused by volcanoes, landslides, certain types of weather, and even asteroids or comets colliding with the ocean.
Q: How do tsunamis form?
A: When a large and sudden displacement occurs in the ocean (think a large earthquake below or near the ocean floor, or a volcanic eruption, or converging tectonic plates), the force of such a disruption creates waves that radiate outward in all directions from their source. This is what makes such waves so unpredictable and volatile. For example, an earthquake in one corner of the Pacific Ocean can cause a massive wave to strike a coastline thousands of miles away.
Q: How to survive a tsunami?
A: From a disaster preparedness standpoint, there is very little that one can do to “prepare” for such an event. The best advice is:
1). Outfit yourself in such a way that you have as much warning as possible before a wave strikes. For example, invest in an emergency radio that picks up the NOAA weather report, an automated, 24-hour network of F.M. weather radio stations that monitor weather hazards.
2). Even more importantly, have an evacuation route planned. Remember, if you live in a coastal area, you may not have much warning before a wave strikes. Even if you do get an alert on your radio, cell phone, or T.V., you may only have a few minutes to evacuate. So gather up your family, leave your possessions behind, and follow your evacuation route. Seek high ground and travel inland, if you can.
There’s no “battening down the hatches” and waiting out such a wave. You must get away from the coast as soon as possible.
Q: Where do tsunamis occur?
A: As touched on earlier, such waves can occur anywhere there is an ocean coast. However, they are most likely to happen in the Pacific Ocean and near Indonesia, as the Pacific Rim bordering the ocean has many active submarine earthquake zones.
Q: What does a tsunami look like?
A: It does not necessarily look like a traditional wave, with curling, cresting movement. Rather, most massive wave events will look like a wall of water moving forward or just a steadily, rapidly rising water level.
Q: How fast do tsunamis travel?
A: The speed and force depend quite a bit on the depth of the ocean. If one forms in very deep water, it can move extremely fast, over 500 mph. And because the wave moves within the water, mariners out at sea will not usually notice as one passes beneath them. The immense force of the water and the rising water level is not typically noticeable until it reaches shallow water and coastal areas.
Q: How long does a tsunami last?
A: Not last for very long. Most last for five minutes to two hours. However, just because water levels from a recede after a wave hits does not make it safe to return to a coastal area. Large events may involve more than one wave/water level rise, and some have even been recorded as lasting for several days. That’s why it is important not to return to a tsunami site until after emergency response officials have said it is safe to do so.
Q: What was the biggest tsunami?
A: The 1958 Lituya Bay Earthquake and Megatsunami was the largest by far. This was a massive wave that climbed almost 200 feet above sea level up the shore and more than 1,700 feet inland, caused by a severe earthquake and landslide in the region. (See the Sources section under “Earthquake Alaska” for an excellent article on this mass destruction event!)
Q: How big is a tsunami?
A: Though rare, some have been recorded as reaching more than 100 feet in height, a mega wave that surges inland, far beyond where waves usually break on the shore. However, most such waves are usually relatively short in height (10 feet high or so), which unfortunately hides their danger and destructive force.
Q: How far can a tsunami travel?
A: Thousands of miles. Such wave events can travel across an entire ocean’s span before reaching landfall. Perhaps the most famous example of this was a devastating earthquake in a coastal region of western South America, yet which caused a massive wave event to strike Hawaii, thousands of miles away.
Q: How often do tsunamis occur?
A: There are usually 2-3 such waves that occur per year.
Q: What does tsunami mean?
A: The word means “harbor wave” in Japanese.
Q: Can tsunamis be predicted?
A: No, mainly because earthquakes are usually what cause such wave events, and earthquakes themselves cannot be predicted more than a few seconds or short minutes before they occur.
Q: How do earthquakes cause tsunamis?
A: According to one scientific paper on such an event, “Most tsunamis are caused by large earthquakes on the seafloor when slabs of rock move past each other suddenly, causing the overlying water to move. The resulting waves move away from the source of the earthquake event.”
Q: What can cause a tsunami?
A: One of their unique attributes is how different they are from wind-driven waves. While a wind-driven wave (like one caused by a hurricane) only travels along the topmost layer of the ocean, a tsunami moves through the entire water column, from the ocean floor to the ocean surface. This is why such an event will often appear like a massive water ripple rather than a wave.
Q: What to do in a tsunami?
A: Seek higher ground, and move away from the coast. Flee away from the coast and above the ocean level.
Q: How tall can a tsunami get?
A: Most waves are usually around ten feet high, but they have been recorded as reaching thirty feet high or higher, and they can travel up to 1,000 feet inland, sometimes climbing up entire hillsides in doing so.
Q: What is the biggest tsunami ever?
A: The biggest such wave event was a wave that crashed 1,720 feet inland in Libya Bay in Alaska in 1958.
Q: Can a hurricane cause a tsunami?
A: Yes, it’s possible. Hurricanes can cause ocean sediments to move about, which could trigger an undersea landslide, which could then cause a massive wave event to form.
Q: Has there ever been a tsunami in California?
A: Yes. Dozens of such wave events have crashed upon the shores of California.
Q: How are tsunamis measured?
A: The National Oceanic and Atmospheric Administration has a special DART program (Deep-ocean Assessment and Reporting of Tsunamis). This system locates waves as soon as they occur within the deep ocean using buoys and other deepwater equipment to monitor sea-level changes and forces moving through the water.
However, though some measurement and graphing systems have been proposed, there is no current, universally accepted method for measuring the severity of such events. And that’s mostly because the severity will be entirely different based on where it strikes. For example, a wave may cause utter devastation in a coastal city just a few miles away from the earthquake source. But, conversely, that very same wave may not cause much damage at a coastal city several hundred miles away from the source.
As a general rule, most oceanographers measure such waves either by the severity of the damage they cause (if that damage is mostly uniform, strike-point to strike-point) or they measure the event by the height of the wave created by it.
Q: How far inland can a tsunami travel?
A: This varies considerably, but flooding can extend inland by as much as 1,000 feet.
Q: What is a tsunami wave?
A: Such is a massive wave, a veritable wall of water that moves towards the coast and inland, sometimes travels 1,000 feet or more inland.
Q: Are tsunamis predictable?
A: Most such wave events are caused by earthquakes and are therefore not easily predicted.
Q: Has a tsunami ever it the U.S.?
A: Yes. Hundreds of such wave events have hit the U.S. However, there have not been many deaths, at least not compared to natural disasters in the Pacific Ocean near Asia, Southeast Asia, and the Indian Ocean.
Q: How many tsunamis happen a year?
A: Approximately two such wave events occur each year.
Q: Can volcanoes cause tsunamis?
A: Yes. This is not as common as earthquakes causing such events, but volcanoes can cause massive wave crashes.
Q: Has there ever been a tsunami in Florida?
A: Yes, but such events are very, very rare. Almost all massive wave events that occur in the United States occur on the west coast.
Q: Why are tsunamis so dangerous?
A: Unlike hurricanes that are hazardous within the ocean and approach land, a massive wave event only becomes dangerous when it comes near a coastal region. As the wave enters shallow water, it slows to 20 to 30 mph. Then, three things occur that make the wave disastrous:
The wavelength decreases.
The heigh increases.
The currents intensify.
Because such a wave involves a sudden rush of water from the ocean up onto the coast and further inland, it can cause immense damage to human life, property, and entire, local economies. For example, it can destroy boats in harbors and shallow water; it can ruin cars parked near the coast, and it can even flood, damage, and destroy buildings.
Such a wave may only be 10 feet high, but the force of the rushing water can pick up and move virtually anything in its path, including entire houses and smaller buildings. Furthermore, some waves have been recorded as reaching 100 feet high or more, which causes them to act as a massive wave and a surging rise in ocean water level, all at the same time.
Q: Can you surf a tsunami?
A: No. Such a wave does not occur like a tidal wave, or windblown wave does, and it would be foolish and potentially lethal to try and surf a tsunami wave.
Q: What happens after a tsunami?
A: The water sucks back out into the open ocean, often dragging cars, trees, buildings, and people with it.
Q: Can a tsunami happen in a lake?
A: It’s highly unlikely that this would occur, but large lakes have been known to experience smaller versions of oceanic tsunamis.
Q: Can a tsunami hit Los Angeles?
A: Yes, it’s possible, has occurred before, and all undoubtedly happen again.
Q: Does Hawaii get tsunamis?
A: Yes. Hawaii has experienced a significant percentage of all such wave events that have occurred in the United States.
Q: How do you know when a tsunami is coming?
A: There may be a noticeable earthquake, a loud roaring sound, and the coastal water will likely seem to suck away from the coastline before coming back in force.
Q: How is a tsunami generated along a subduction zone?
A: According to one research paper, “A subduction zone is a boundary between tectonic plates that are part of the Earth’s shell. These plates descend or subduct under an adjacent plate. Sudden movement of the plates causes them to stick and generate an earthquake. The overriding plate gets squeezed as it sticks to the subducting plate. This pushes the leading edge down and the rear area bulges upward. Slowly building stress over centuries the movement continues. When an overriding plate breaks free and rises seaward the seafloor rises and lifts the water above it creating an earthquake along the subduction zone. A tsunami is generated. As the bulge end of the plate collapses it thins out and lowers coastal areas. The tsunami races off in two different directions, one, growing in size towards landfall nearby, the other to a distant shore.”
Q: How long does it take for a tsunami to form?
A: That depends entirely on the location of the wave. As for the event that forms it, it can happen in an instant. But the crucial factor as to the speed of a forming wave is how close people are to the source point of the wave. For example, suppose a coastal city or populated area is just a few miles from the source point. In that case, the wave could arrive in a matter of minutes, giving locals little time to prepare or evacuate.
Conversely, a coastal area that is hundreds or thousands of miles away from the source point could still be severely affected by the wave, but locals may have several hours to evacuate.
Q: How many waves are in a tsunami?
A: This varies, event to event, but most such events have several waves in them.
Q: What type of plate boundary are most tsunamis associated with?
A: Convergent plate boundaries are the type of place boundary most commonly associated with such wave events.
Q: What event happens underwater to cause a tsunami?
A: Usually, an earthquake is what causes such an event.
Q: When was the last tsunami in Oregon?
A: 1964 was the year of the last serious wave event in Oregon.
Q: Can underwater landslides cause tsunamis?
A: Yes. According to the USGS, “Tsunamis can be generated on impact as a rapidly moving landslide mass enters the water or as water displaces behind and ahead of a rapidly moving underwater landslide.”
Q: Do tsunamis affect marine life?
A: Yes. Particularly marine life that lives close to shore. Entire ecosystems can be destroyed and removed by such a wave event.
Q: Has there ever been a tsunami in San Diego?
A: Yes. More than two dozen such wave events have been recorded as striking San Diego since recording began in 1806.
Q: How can tsunamis be detected?
A: Weather scientists use seismographs to record such wave events.
Q: How long after earthquake does tsunami hit?
A: That depends on how close the coast is to where the earthquake occurs. If the earthquake occurs very close to a coastal area, a massive wave may crash upon that coast within minutes. However, serious earthquakes out in the open ocean can cause massive waves that travel thousands of miles and take hours to reach the shore.
Q: How to protect ourselves from tsunami?
A: Unlike as is the case with many other natural disasters, it does not make sense to shelter in place if a massive wave is heading your way. Rather, seek higher ground, and move inland. Of course, moving inland is always ideal. But if you cannot move inland, seek higher ground where you are.
Q: What are the different types of tsunamis?
A: Such wave events do come in different shapes and sizes, though they should not be confused with tidal waves. Although both are sea waves, a tsunami and a tidal wave are two very different and unrelated phenomena. A tidal wave is a shallow water event caused by the gravitational interactions between the Sun, Moon, and Earth. In addition, tidal waves can be influenced by wind.
On the other hand, Tsunamis are ocean waves triggered by a massive event underneath the ocean’s surface. One of the key differences between the two is that a tsunami involves a sudden rise in the ocean’s water level up onto a coastal area. In contrast, tidal waves usually do not do this.
Q: What are the signs of a tsunami?
A: NOAA has listed the following signs as being strong indicators of an approaching, mass wave event, including a strong earthquake that causes difficulty standing, a rapid rise or fall of the water along the coast, a load ocean roar, and the rapid departure of coastal mammals and birds.
Q: What not to do in a tsunami?
A: Do NOT stay near the coast or on the beach to watch the wave come in. Seek high ground, and try to move inland as far as possible.
Q: Where do tsunamis occur in the U.S.?
A: All of the coastal regions of the United States are at risk for such waves. Large wave events have struck American coasts, and they will undoubtedly occur again. Here are a few from relatively recent history:
In 1946, a tsunami with a wave height cresting between 33 and 55 feet struck Hawaii, killing 159 people.
In 1964, a magnitude 9.2 earthquake in the Gulf of Alaska generated a tsunami that caused damage and loss of life in Alaska, Hawaii, California, Oregon, and Washington.
A 1929 magnitude 7.3 earthquake in the Grand Banks area of the Caribbean Sea caused a small tsunami that caused minor damage as far north as South Carolina.
Q: Can animals sense tsunamis?
A: Yes! Some scientists believe dogs can sense such wave events, and coastal birds have repeatedly been spotted fleeing the beach right before such a wave occurs.
Q: Can you dive under a tsunami?
A: No. Such a wave event does not just occur on the surface of the ocean. It also occurs throughout the entire depth of the ocean, particularly as the ocean gets shallower.
Q: How can we prevent tsunami?
A: We cannot prevent such wave events from occurring, but we can better prepare for them and respond to them when they do occur.
Q: How do humans adapt to tsunamis?
A: Did you know that 40% of the world’s population lives within 100 kilometers of an ocean coast? Though massive wave events might not be as common as other disasters like earthquakes, tornados, or hurricanes, tsunamis have the potential to affect countless lives and create incomprehensible damage. Tsunamis are a legitimate concern for billions of humans on all continents of our planet. They’re a constant disaster threat that poses a serious risk for those who live in coastal regions. That’s why it is important to know what they are and how to prepare for them. Humans who live in coastal regions must innovate better, safer, and more efficient evacuation plans or build and develop coastal cities on higher ground to adapt to them.
Q: How do scientists study tsunamis?
A: Scientists use seismographs to study such wave events and the earthquakes that create them.
Q: What can a tsunami destroy?
A: Virtually anything. Cars, boats, humans, animals, trees, other plants, even entire buildings and homes can be destroyed.
Q: What can you do to help tsunami victims?
A: If you are not near a high-risk coastal area, you can donate to organizations that help support victims. If you live near a coastal area, you can receive first responder training and volunteer in the relief efforts if such a wave event ever strikes your area.
Q: Why does the water recede before a tsunami?
A: Such a wave event is much like a tidal wave, only much more forceful, hence the distinction. But as the wave approaches the shore, the force of the wave pulls the water in front of it back into the wave, building force and making the wave even more powerful.
Q: Do tsunamis crest?
A: Yes. Such waves do not always look like traditional tidal waves. But they do have a crest.
Q: Do’s and don’ts during tsunami?
A: If you are at home, DO turn off all water, gas, and electricity and flee to higher ground. Do NOT go down to the beach to watch the wave come in.
Q: How are tsunamis categorized?
A: They are categorized and recorded by the magnitude of the largest wave in the set, whichever one that happens to be.
Q: How could a tsunami affect a coastal ocean ecosystem?
A: Drastically. The base of such a wave can completely change the topography of the seafloor. It erodes the seafloor sediments and can completely devastate the “benthic” layer or the sea bottom ecosystem.
Q: How to survive a tsunami on an island?
A: Seek higher ground as quickly as possible while moving away from the water’s edge.
Q: How is climate change affecting tsunamis?
A: While such wave events have the potential to be extremely devastating, the silver lining (if there is any) with this natural disaster is that they are relatively infrequent. Major tsunamis that have the potential to cause serious damage usually occur once every 10-12 years on average, with smaller, less destructive events having the possibility to occur more frequently.
As for the future, it is thought that climate-induced sea level rises will make tsunamis worse. It is unclear if tsunamis will become more frequent in the future. Still, many scientists believe that rising sea levels will only exacerbate the destruction wrought by waves, as levees and break walls are already overtaxed.
Q: How many tsunamis have happened in the last 10 years?
A: While these are not as common as other natural disasters like floods, earthquakes, tornadoes, and hurricanes, tsunamis do happen. There have been about two dozen recorded tsunamis that have occurred in the last decade.
Q: How to prepare before a tsunami?
A: If you are aware that a wave is inbound, the best thing is to evacuate immediately and seek higher ground. Do not stay near the coast. Instead, flee away from the coast and seek higher ground as soon as possible.
Q: How to recover from a tsunami?
A: Recovering from a tsunami is sometimes a slow and arduous process, given how much damage can be caused by them. Some basic tips include:
Avoid disaster areas the may still be dangerous.
Avoid debris in the water.
Check yourself and others for injuries and seek first aid.
Assist in rescue efforts, but only in such a way that does not put yourself in danger.
Stay out of buildings with water in them, as the waves can cause floors and walls to crack and collapse.
Q: How to reduce tsunami damage?
A: Preventing or reducing wave damage is a science in and of itself. Washington University offers a full college course on the subject. Citing their information, there are four key ways to prevent and reduce damage:
“1. Avoid Inundation Areas: Site Buildings or infrastructure away from hazard area or locate on a high point.
2. Slow Water: Forests, ditches, slopes, or berms can slow down waves and filter out debris. The success of this method depends on correctly estimating the force of the tsunami.
3. Steering: Water can be steered to strategically placed angled walls, ditches and paved roads. Theoretically, porous dikes can reduce the impact of violent waves.
4. Blocking: Walls, hardened terraces, berms and parking structures can be built to block waves. The house and household in a wide community context such as whether it is a majority or minority situation, the conflict situation and ownership issues.”
Q: Are tsunamis becoming more frequent?
A: Some scientists think so. A warming planet is likely to lead to a range of events, including more frequent earthquakes, volcanic eruptions, giant landslides, and wave events.
Q: Are tsunamis tidal waves?
A: No. Although both are sea waves, they are distinctly different. According to the United States Geological Survey, “A tidal wave is a shallow water wave caused by the gravitational interactions between the Sun, Moon, and Earth (“tidal wave” was used in earlier times to describe what we now call a tsunami.) A tsunami is an ocean wave triggered by large earthquakes that occur near or under the ocean, volcanic eruptions, submarine landslides, or by onshore landslides in which large volumes of debris fall into the water.”
Q: Can a nuke create a tsunami?
A: It’s unlikely that a single nuke could cause a massive wave, but a line of explosions denoted off the coast could indeed create a destructive tsunami.
Q: Can a tsunami happen without an earthquake?
A: Yes. Other weather events and world factors can cause tsunamis, like a landslide or a volcanic eruption.
Q: Can the Great Lakes have a tsunami?
A: Yes! Shockingly enough, yes. According to one weather expert, “Tsunamis, also known as tidal waves, are almost exclusively believed to be an event that occurs in the world’s oceans and seas. Generally, a tsunami is caused by an Earthquake, volcanic eruption, or other underwater explosions such as landslides, glacier movement or even meteorite impacts. But tsunamis on the Great Lakes, known as ‘meteotsunamis’, can also happen. These are described as a rather unfamiliar phenomenon, but they have actually been happening often over the years.”
Q: Why is tsunami water black?
A: The answer is quite disgusting. According to the India Water Portal, “The earthquake and the tsunami is a natural phenomenon. BUT the black water is manmade! The black colour is putrefying sewage that accumulates on the ocean floors off the coast line of the continents where humans inhabit. The black tsunami illuminates a painfully unnoticed reality that the great majority of the people on our planet ignore. Storm water, domestic faeces and industrial sewage have been discharged into our rivers, lakes and oceans for more than two thousand years. The Romans discharged contaminated water into rivers and oceans. The black mud spreads to cover the beds of oceans, lakes, rivers and manmade channels. These become aquatic death zones created by man.”
Q: What is an orphan tsunami?
A: An orphan tsunami is a tsunami wave event that occurs, but the scientists who study it cannot accurately find or determine the parent earthquake that caused it. It is a mystery event, essentially.
Q: What is considered a tsunami?
A: To be classified as such a natural disaster, a wave has to meet the following qualifications, according to NOAA, “A tsunami is a series of waves caused by earthquakes or undersea volcanic eruptions…. Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea. Out in the depths of the ocean, tsunami waves do not dramatically increase in height.”
Q: What is the fear of tsunamis called?
A: The fear of tsunamis, or waves in general, is called “Cymophobia.”
Q: What is the main reason of tsunami?
A: According to NOAA, “Most tsunamis are caused by earthquakes on converging tectonic plate boundaries. Since 1900, over 80% of likely tsunamis were generated by earthquakes. However, tsunamis can also be caused by landslides, volcanic activity, certain types of weather, and—possibly—near-earth objects (e.g., asteroids, comets) colliding with or exploding above the ocean.”
Q: What is the scientific name for tsunami?
A: “Tsunami” is a Japanese word. According to Washington University, “Tsunami is a Japanese word with the English translation, ‘harbor wave.’ Represented by two characters, the top character, ‘tsu,’ means harbor, while the bottom character, ‘nami,’ means ‘wave.’ In the past, tsunamis were sometimes referred to as ‘tidal waves’ by the general public, and as ‘seismic sea waves’ by the scientific community. The term ‘tidal wave’ is a misnomer; although a tsunami’s impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term ‘seismic sea wave’ is also misleading. ‘Seismic’ implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact.”
Q: When was the last tsunami in the U.S.A.?
A: In total, there have been 72 recorded wave events that have hit the United States since 1737, with a total of 548 deaths. Virtually all of these have occurred in the Pacific Ocean, with most regions affected being Alaska and Hawaii. The most recent wave to hit the United States was a landslide in Icy Bay, Alaska. No fatalities occurred.
Q: Where and how frequently are tsunamis generated?
A: According to data compiled by NOAA and the Global Historical Tsunami Database, “Tsunamis that cause damage or deaths near their source occur approximately twice per year. Tsunamis that cause damage or deaths on distant shores (more than 1,000 kilometers, 620 miles, away) occur about twice per decade.” In summary, massive wave events are uncommon, but they are deadly.
Q: Where do tsunamis occur most often and why?
A: The Pacific Ocean has the most recorded instances of tsunami waves, particularly near Indonesia. This is because the Pacific Rim borders the Pacific Ocean and has many active submarine earthquake zones. However, more recently, tsunamis have been recorded in the Mediterranean Sea region and are expected in the Caribbean Sea.
Q: Where does the word tsunami come from?
A: Tsunamis, in rough English translation, means “harbor wave.”
Q: Which instrument is used to measure tsunami?
A: Scientists use seismographs to measure the strength, intensity, speed, and size of tsunamis.
Q: Why is tsunami called killer wave?
A: They are called as such because, unlike tidal waves, tsunamis can cause mass deaths when they reach land.
Q: How are tsunamis classified?
A: Tsunamis are classified by their strength and intensity. According to the experts, “A small movement from a convergent boundary will produce a tsunami of weak to moderate strength, while a major movement from the same boundary will produce a tsunamis of significant magnitude. Tsunami are usually named for the landmass they impact, or city or town they inflict damage upon. The recent Fukishima tsunamis that impacted Japan was named for the nuclear reactors that were overrun with salt water from the ocean. Footage of the tsunamis show it reaching far inland, destroying houses, fields, and part of the Japanese population that lived there. While at sea, it is hard to gauge the strength of a tsunamis, as there is more room for the water to swell in.”
Q: How big is a tsunami on average?
A: According to the data, most tsunamis cause a sea rise of no more than 10 feet. However, a tsunami wave can reach heights of 30 feet or more. Keep in mind, though, 40% of the world’s population lives in coastal areas. Just in the United States alone, 127 million people live near sea level. This is why tsunamis are so dangerous. Though a tsunami wave may only cause a sea-level rise of 10 feet, that 10 feet of rise and the fact that a wave can travel up to 1,000 feet inland can be devastating for human habitation.
Q: How deadly is a tsunami?
A: Very deadly! Not only are tsunamis deadly in the initial rush of water that comes surging up onto the land, but when the water recedes into the ocean, it can drag people out into the open water and drown them.
Sources:
http://streamer.co.il/articles/view/cymophobia-the-fear-of-waves-and-water?lang=en#
http://courses.washington.edu/larescue/precedents/prevention.htm