Tsunami Facts

What is a tsunami?

A tsunami is a series of ocean waves with very long wavelengths (typically hundreds of kilometres) caused by large-scale disturbances of the ocean, such as:

• earthquakes
• landslide
• volcanic eruptions
• explosions
• meteorites

These disturbances can either be from below (e.g. underwater earthquakes with large vertical displacements, submarine landslides) or from above (e.g. meteorite impacts).

Tsunami is a Japanese word with the English translation: "harbour wave". In the past, tsunamis have been referred to as "tidal waves" or "seismic sea waves".

 The term "tidal wave" is misleading; even though 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 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 non-seismic event, such as a landslide or meteorite impact.

Tsunamis are also often confused with storm surges, even though they are quite different phenomena. A storm surge is a rapid rise in coastal sea-level caused by a significant meteorological event - these are often associated with tropical cyclones.

The physics of a tsunami

Tsunamis can have wavelengths ranging from 10 to 500 km and wave periods of up to an hour. As a result of their long wavelengths, tsunamis act as shallow-water waves. A wave becomes a shallow-water wave when the wavelength is very large compared to the water depth. Shallow-water waves move at a speed, c, that is dependent upon the water depth and is given by the formula:

where g is the acceleration due to gravity (= 9.8 m/s²) and H is the depth of water.

In the deep ocean, the typical water depth is around 4000 m, so a tsunami will therefore travel at around 200 m/s, or more than 700 km/h.

For tsunamis that are generated by underwater earthquakes, the amplitude (i.e wave height) of the tsunami is determined by the amount by which the sea-floor is displaced. Similarly, the wavelength and period of the tsunami are determined by the size and shape of the underwater disturbance.

As well as travelling at high speeds, tsunamis can also travel large distances with limited energy losses. As the tsunami propagates across the ocean, the wave crests can undergo refraction (bending), which is caused by segments of the wave moving at different speeds as the water depth along the wave crest varies.

What happens to a tsunami as it approaches land?

As a tsunami leaves the deep water of the open-ocean and travels into the shallower water near the coast, it transforms. If you read the "The physics of a tsunami" section, you will know that a tsunami travels at a speed that is related to the water depth - hence, as the water depth decreases, the tsunami slows. The tsunami's energy flux, which is dependent on both its wave speed and wave height, remains nearly constant.

Consequently, as the tsunami's speed diminishes, its height grows. This is called shoaling. Because of this shoaling effect, a tsunami that is unnoticeable at sea, may grow to be several metres or more in height near the coast.

The increase of the tsunami's waveheight as it enters shallow water is given by:

where h_s and h_d are waveheights in shallow and deep water and H_s and H_d are the depths of the shallow and deep water. So a tsunami with a height of 1 m in the open ocean where the water depth is 4000m would have a waveheight of 4 to 5 m in water of depth 10 m.

Just like other water waves, tsunamis begin to lose energy as they rush onshore - part of the wave energy is reflected offshore, while the shoreward-propagating wave energy is dissipated through bottom friction and turbulence. Despite these losses, tsunamis still reach the coast with tremendous amounts of energy. Depending on whether the first part of the tsunami to reach the shore is a crest or a trough, it may appear as a rapidly rising or falling tide.

Local bathymetry may also cause the tsunami to appear as a series of breaking waves.

Tsunamis have great erosion potential, stripping beaches of sand that may have taken years to accumulate and undermining trees and other coastal vegetation. Capable of inundating, or flooding, hundreds of metres inland past the typical high-water level, the fast-moving water associated with the inundating tsunami can crush homes and other coastal structures. Tsunamis may reach a maximum vertical height onshore above sea level, often called a run-up height, of tens of metres.

How are tsunamis measured or observed?

In the deep ocean, a tsunami has a small amplitude (less than 1 metre) but very long wavelength (hundreds of kilometres). This means that the slope, or steepness of the wave is very small, so it is practically undetectable to the human eye. However, there are ocean observing instruments that are able to detect tsunamis.

Tide Gauges

Tide gauges measure the height of the sea-surface and are primarily used for measuring tide levels. Most of the tide gauges operated by the Bureau of Meteorology's National Tidal Centre are SEAFRAME stations (Sea Level Fine Resolution Acoustic Measuring Equipment). These consist of an acoustic sensor connected to a vertical tube open at the lower end which is in the water.

The acoustic sensor emits a sound pulse which travels from the top of the tube down to the water surface, and is then reflected back up the tube. The distance to the water level can then be calculated using the travel time of the pulse. This system filters out small-scale effects like wind-waves and has the capacity to measure sea-level changes within 1mm accuracy.

The tide gauge at Cocos Island observed the tsunami on December 26th 2004 as it passed by the island, as shown in these observations made during December.

Satellites

Satellite altimeters measure the height of the ocean surface directly by the use of electro-magnetic pulses. These are sent down to the ocean surface from the satellite and the height of the ocean surface can be determined by knowing the speed of the pulse, the location of the satellite and measuring the time that the pulse takes to return to the satellite.

One problem with this kind of satellite data is that it can be very sparse - some satellites only pass over a particular location about once a month, so you would be lucky to spot a tsunami since they travel so quickly. However, during the Indian Ocean tsunami of December 26th 2004, the Jason satellite altimeter happened to be in the right place at the right time.

The picture below shows the height of the sea surface (in blue) measured by the Jason satellite two hours after the initial earthquake hit the region southeast of Sumatra (shown in red) on December 26, 2004. The data were taken by a radar altimeter on board the satellite along a track traversing the Indian Ocean when the tsunami waves had just filled the entire Bay of Bengal. The data shown are the differences in sea surface height from previous observations made along the same track 20-30 days before the earthquake, showing the signals of the tsunami.

Picture courtesy of NASA/JPL-Caltech

The DART System

In 1995 the National Oceanic and Atmospheric Administration (NOAA) began developing the Deep-ocean Assessment and Reporting of Tsunamis (DART) system. An array of stations is currently deployed in the Pacific Ocean. These stations give detailed information about tsunamis while they are still far off shore. Each station consists of a sea-bed bottom pressure recorder which detects the passage of a tsunami. (The pressure of the water column is related to the height of the sea-surface) .

The data is then transmitted to a surface buoy via sonar. The surface buoy then radios the information to the Pacific Tsunami Warning Center (PTWC) via satellite. The bottom pressure recorder lasts for two years while the surface buoy is replaced every year. The system has considerably improved the forecasting and warning of tsunamis in the Pacific.

The Indian Ocean tsunami of 26th December 2004

An undersea earthquake in the Indian Ocean on 26th December 2004 produced a tsunami that caused one of the biggest natural disasters in modern history. Over 200,000 people are known to have lost their lives.

The waves devastated the shores of parts of Indonesia, Sri Lanka, India, Thailand and other countries with waves reported up to 15 m high, reaching as far as Somalia on the east coast of Africa, 4500 km west of the epicentre. Refraction and diffraction of the waves meant that the impact of the tsunami was noticed around the world and sea-level monitoring stations in places such as Brazil and Queensland also felt the effect of the tsunami.

This animation (10.4Mb) was produced by scientists in the Bureau of Meteorology's National Tidal Centre. A numerical model was used to replicate the generation and propagation of the tsunami and it shows how the waves propagated around the world's ocean basins.

The earthquake took place at about 1am UTC (8am local time) in the Indian Ocean off the western coast of northern Sumatra. With a magnitude of 9.0 on the Richter scale, it was the largest since the 1964 earthquake off Alaska and equal fourth largest since 1900, when accurate global seismographic record-keeping began.

The epicentre of the earthquake was located about 250 km south-southeast of the Indonesian city of Banda Aceh. It was a rare megathrust earthquake and occurred on the interface of the India and Burma tectonic plates.

This was caused by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. A megathrust earthquake is where one tectonic plate slips beneath another, causing vertical motion of the plates. This large vertical displacement of the sea-floor generated the devastating tsunami, which caused damage over such a large area around the Indian Ocean.

The earthquake was also unusually large in geographical extent. An estimated 1200 km of faultline slipped about 15 m along the subduction zone over a period of several minutes. Because the 1,200 km of faultline affected by the quake was in a nearly north-south orientation, the greatest strength of the waves was in an east-west direction. Bangladesh, which lies at the northern end of the Bay of Bengal, had very few casualties despite being a populous low-lying country.

Due to the distances involved, the tsunami took anywhere from fifteen minutes to seven hours (for Somalia) to reach the various coastlines. (See this travel time map). The northern regions of the Indonesian island of Sumatra were hit very quickly, while Sri Lanka and the east coast of India were hit roughly two hours later. Thailand was also struck about two hours later, despite being closer to the epicentre, because the tsunami travelled more slowly in the shallow Andaman Sea off its western coast.

On its arrival on shore, the height of the tsunami varied greatly, depending on its distance and direction from the epicentre and other factors such as the local bathymetry. Reports have the height ranging form 2-3 m at the African coast (Kenya) up to 10-15 m at Sumatra, the region closest to the epicentre.

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What Is Frozen Fuel?

All by itself, methane isn't very exciting. It's a colorless, odorless gas and the simplest member of the alkane series of hydrocarbons. Its biggest claim to fame is that, as the main constituent of natural gas, it's useful as a source of energy.

Recently, however, geologists have discovered a type of methane that has piqued their curiosity. Part of its unusual character is how it exists in its natural state -- trapped inside a cage of ice. Even more intriguing is how much of this frozen methane seems to be locked away in the Earth's crust. Some estimates indicate that as much as 700 quadrillion (700 × 10¹⁵) cubic feet (20 quadrillion cubic meters) of methane are encased in ice and trapped in seafloor sediments all over the world [source: Tarbuck]. That's twice as much carbon as Earth's other fossil fuels combined.

That's not just mud (pictured). It's mud containing methane hydrate, icelike crystals that form at low temperatures and high pressure. And it could be a future energy source. See more alternative fuel pictures.

The discovery of this new type of methane, what scientists call methane hydrate, has led to two important questions. The first is pragmatic: Will it burn like ordinary methane? It turns out it will. If you take a piece of methane hydrate -- it looks like hard-packed snow -- and touch a lighted match to it, the sample will burn with a reddish flame. And if that's the case, it could be used to heat homes, fuel cars and generally power energy-hungry nations such as Japan, the United States, India and China. Recent data suggest that just 1 percent of Earth's methane hydrate deposits could yield enough natural gas to meet America's energy needs for 170,000 years [source: Stone].

The second question is partly an ethical consideration: Should we, as a global community trying fervently to develop clean, renewable energy, embrace one of the fossil fuels that got us into trouble in the first place? Science can't answer that question. It can, however, reveal the challenges and risks that face countries hoping to take advantage of methane hydrate. One of the most significant challenges is finding efficient ways to extract the frozen fuel. More troubling are potential catastrophes -- ranging from massive underwater landslides to a runaway greenhouse effect -- related to methane mining.

In this article, we'll explore all the positives and negatives of methane hydrate. We'll look at its relatively brief history, as well as how it fits in some possible future scenarios. And, of course, we'll examine the basic science behind this so-called "flammable ice."

 Fire and Ice: The Chemistry of Methane Hydrate

Frozen fuel is the catchy name for a family of substances known as gas hydrates. The gas in question is natural gas, a mixture of hydrocarbons, such as methane, propane, butane and pentane. Of these, methane is by far the most common component and one of the most-studied compounds in chemistry.

Like all hydrocarbons, methane contains only two elements -- carbon and hydrogen. It is an example of a saturated hydrocarbon, or a molecule composed entirely of single bonds and therefore the maximum number of hydrogen atoms allowed. The general formula for saturated hydrocarbons is C_nH_2n+2. Methane only has one carbon atom, so its chemical formula is CH₄. Chemists describe this shape as a tetrahedron.

Methane is a colorless, odorless, combustible gas produced by bacterial decomposition of plant and animal matter. It forms in a process shared by all fossil fuels. First, marine plants and animals die and fall to the seafloor. Next, mud and other seafloor sediments cover the decomposing organisms. The sediments put a great deal of pressure on the organic matter and begin to compress it. This compression, combined with high temperatures, breaks down the carbon bonds in the organic matter, transforming it into oil and natural gas.

Generally, this methane -- what geologists describe as "conventional" methane -- is located beneath the Earth's surface. To get to it, workers must drill through rock and sediment and tap into the methane deposits to release the gas. Then they pump it to the surface, where it's transported through pipes across the country.

Methane can also form unconventionally if the sediments producing it are located about 1,640 feet (500 meters) below the ocean surface.

The near-freezing temperatures and high pressure of these conditions causes the methane to become encased in ice. The methane doesn't bond chemically with the water. Instead, each tetrahedral methane molecule sits inside a crystalline shell made of ice. This unique substance is known as methane hydrate, and as soon as it reaches warmer temperatures and lower pressures, the ice melts away, leaving behind pure methane.

Geologists discovered naturally occurring methane hydrate only recently, but chemists have known about it for years.

A Brief History of Methane Hydrate

The history of gas hydrates can be traced back to Humphrey Davy, a chemist from Cornwall, England, who identified chlorine as an element in 1810.

Davy and his assistant, Michael Faraday, continued to work with chlorine throughout the early 1800s, mixing the green gas with water and cooling the mixture to low temperatures.

It's very likely that Davy observed the strange solid that resulted as chlorine atoms became encased in ice crystals, but Faraday gets official credit for the discovery. In 1823, Faraday issued a report describing the strange substance and called it chlorine clathrate hydrate. Other types of clathrates, each involving a guest compound locked inside the lattice structure of a host, were soon discovered, but they remained a laboratory curiosity.

Then, in the 1930s, natural-gas miners began to complain of an icelike material clogging pipelines exposed to cold temperatures. Scientists determined that this material was not pure ice, but ice wrapped around methane. They wasted no time trying to find ways to prevent hydrates from forming and turned primarily to chemicals, such as methanol or monoethylene glycol. Since then, mining companies have added these materials to their natural-gas pipelines to inhibit hydrate formation.

In the 1960s, scientists discovered that methane hydrate, or "solid natural gas," existed in the Messoyakha gas field in western Siberia. This was significant because naturally occurring gas hydrates had never been found before. Geologists and chemists arrived in the vast basin and began to study the conditions in which the hydrates were forming. They found that sub-permafrost sediments were rich in hydrates and began to look for similar deposits in other high-latitude regions. Soon, another team of researchers found methane hydrate in sediments buried deep below the North Slope of Alaska.

Based on these early findings, the U.S. Geological Survey (USGS) and the Department of Energy National Energy Technology Laboratory conducted extensive research between 1982 and 1992, revealing that methane hydrate deposits could be found in offshore sediments as well. Suddenly, what had once been a curiosity and an industrial nuisance looked like it might be a significant resource. In the mid-1990s, Japan and India took the lead in methane hydrate research, with the goal of finding more deposits and developing ways to extract the trapped methane economically. Scientists have since discovered methane hydrate deposits in numerous locations, including the Mackenzie River delta in Canada and the Nankai Trough off the coast of Japan.

Up next, we'll consider the impact methane hydrate could have on the world's energy supply.

 The Potential of Frozen Fuel

Once scientists began looking for methane hydrate deposits, they weren't disappointed. They found them beneath Arctic permafrost and  beneath the seafloor, especially in areas where one tectonic plate slides over another. These regions are known as subduction zones because the edge of one plate moves beneath another.

For example, off the coast of Washington and Oregon, the Juan de Fuca plate is sliding underneath the North American plate. Like a piece of wood being drawn across the blade of a plane, the sediments, including hydrates, of the Juan de Fuca plate are removed by the rocky crust of the North American plate. This creates a ridge of hydrates that runs parallel to the coast.

Hydrate deposits have also been found in regions where large ocean currents meet. Blake Ridge is a formation located off the coast of South Carolina, in water ranging from 6,562 to 15,748 feet (2,000 to 4,800 meters) deep. Geologists believe the ridge formed during the Oligocene epoch, about 33.7 to 23.8 million years ago.

The Greenland Sea opened up during this time, allowing huge amounts of cold, dense water to flow south along the Atlantic coast. As this cold water ran headlong into warm water being carried northward on the Gulf Stream, the currents slowed down and dropped large amounts of sediment. Organic material buried in these sediments eventually gave rise to a large amount of methane hydrate.

How much of this frozen fuel exists at Blake Ridge and other sites around the world? Some estimates put the amount of methane locked away in hydrates at anywhere from 100,000 trillion to 300,000,000 trillion cubic feet (2,832 trillion to 8,495,054 trillion cubic meters). Compare that to the 13,000 trillion cubic feet (368 trillion cubic meters) of conventional natural gas reserves remaining on the planet, and you can understand why jaws in the scientific community have dropped [source: Collett].

Of course, finding the hydrate deposits is one thing. As we'll see in the next section, getting them out -- and doing it safely -- is another thing entirely.

 The Risky Business of Mining Methane Hydrate

The potential rewards of releasing methane from gas hydrate fields must be balanced with the risks. And the risks are significant. Let's start first with challenges facing mining companies and their workers. Most methane hydrate deposits are located in seafloor sediments. That means drilling rigs must be able to reach down through more than 1,600 feet (500 meters) of water and then, because hydrates are generally located far underground, another several thousand feet before they can begin extraction. Hydrates also tend to form along the lower margins of continental slopes, where the seabed falls away from the relatively shallow shelf toward the abyss. The roughly sloping seafloor makes it difficult to run pipeline.

Even if you can situate a rig safely, methane hydrate is unstable once it's removed from the high pressures and low temperatures of the deep sea. Methane begins to escape even as it's being transported to the surface. Unless there's a way to prevent this leakage of natural gas, extraction won't be efficient. It will be a bit like hauling up well water using a pail riddled with holes.

Believe it or not, this leakage may be the least of the worries. Many geologists suspect that gas hydrates play an important role in stabilizing the seafloor. Drilling in these oceanic deposits could destabilize the seabed, causing vast swaths of sediment to slide for miles down the continental slope. Evidence suggests that such underwater landslides have occurred in the past (see sidebar), with devastating consequences. The movement of so much sediment would certainly trigger massive tsunamis similar to those seen in the Indian Ocean tsunami of December 2004.

But perhaps the biggest concern is how methane hydrate mining could affect global warming. Scientists already know that hydrate deposits naturally release small amounts of methane. The gas works itself skyward -- either bubbling up through permafrost or ocean water -- until it's released into the atmosphere. Once methane is in the atmosphere, it becomes a greenhouse gas even more efficient than carbon dioxide at trapping solar radiation. Some experts fear that drilling in hydrate deposits could cause catastrophic releases of methane that would greatly accelerate global warming.

Does that make methane from hydrate fields off-limits? This is the question scientists from all over the world are trying to answer.

The Future of Frozen Fuel

In 1997, the U.S. Department of Energy (DOE) initiated a research program that would ultimately allow commercial production of methane from gas hydrate deposits by 2015. Three years later, Congress authorized funding through the Methane Hydrate Research and Development Act of 2000. The Interagency Coordination Committee (ICC), a coalition of six government agencies, has been advancing research on several fronts. Much of what we know about the basic science of methane hydrate -- how it forms, where it forms and what role it plays, both in seafloor stabilization and global warming -- has come from the ICC's research.

Interesting ideas about how to extract the methane from hydrates efficiently are also emerging. Some experts propose a technique in which miners pump hot water down a drill hole to melt the hydrate and release the trapped methane. As the methane escapes, it is pumped to the seafloor through a companion drill hole.

From there, submarine pipelines carry the natural gas ashore. Unfortunately, such pipelines would need to travel over difficult underwater terrain. One solution is to build a production facility on the seafloor so it is situated near the hydrate deposits. As methane escapes from the heated sediments, workers in the plant would refreeze the gas to form "clean" methane hydrate. Submarines would then tow the frozen fuel in huge storage tanks to shallower waters, where the methane could be extracted and transported safely and efficiently.

Is all of this necessary? Won't renewable energy sources make it a waste of time to pursue another nonrenewable fossil fuel so vigorously? Realistically, fossil fuels will still be an important component of the world's overall energy mix for decades to come. According to the Energy Information Administration (EIA), total U.S. natural gas consumption is expected to increase from about 22 trillion cubic feet (0.622 trillion cubic meters) today to about 27 trillion cubic feet (0.76 trillion cubic meters) in 2030. Global natural gas consumption is expected to increase to 182 trillion cubic feet (5.15 trillion cubic meters) over the same period [source: EIA]. Tapping into the methane locked away in hydrates will obviously play a key role in meeting that demand.

That means the frozen fuel from methane hydrate can buy more time as scientists search for alternatives to power our planet. Think of it as an important stepping-stone in our transition to cleaner, greener energy sources.

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Japan's Tsunami Rips Icebergs Double the Size of Manhattan

When icebergs break off into the polar seas, scientists usually have to work backwards to figure out why--they try to piece the clues together to figure out what caused an event that already happened. But in March, NASA scientists were able to follow the wake of the Japan tsunami over 8,000 miles, through the Pacific and Southern Oceans, until it snapped off several icebergs from Antarctica--icebergs that together are about as big as not one but two Manhattans (the island, not the drink). 

              Icebergs have long been suspected to have a link with seismic activity, but the creation of icebergs is usually a sudden and mostly unpredictable event, the culmination of lots of pressure over decades or centuries. 

             The Tohoku Tsunami, triggered by the earthquake off the coast of Japan this past March, was a tremendous enough seismic event that cryosphere specialists immediately knew that tracking the wave could provide the first visual proof that this connection exists.

             Tohoku didn't disappoint. Eighteen hours after the tsunami struck Japan, the wave--now only about a foot high--met the Sulzberger ice shelf, a 260-feet-thick sheet of ice extending from Antarctica's land mass towards New Zealand. The Sulzberger ice shelf is no brittle sheet of ice, either, having not moved in nearly half a century.

             Despite the relatively short height, the continuous pressure was massive enough to snap off several huge pieces of ice, one of which is about four by six miles in surface area--roughly the same size as the other chunks combined, and close to Manhattan's 23-square-mile surface area.

            Using satellite imagery (including some help from MODIS), the scientists were able to see the calving, or breaking off, in nearly real-time.  It's definitive proof that a big enough tsunami can have huge effects, not just on the immediate site but even a hemisphere away. 

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A Tsunami

 

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.

                  These awe-inspiring waves are typically caused by large, undersea earthquakes at tectonic plate boundaries. When the ocean floor at a plate boundary rises or falls suddenly it displaces the water above it and launches the rolling waves that will become a tsunami.

         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 may also be caused by underwater landslides or volcanic eruptions. They may even be launched, as they frequently were in Earth’s ancient past, by the impact of a large meteorite plunging into an ocean.

         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.

         Occur most frequently in the Pacific and East Indies. One of the largest tsunami:
In the Atlantic Ocean, the earthquake of November 1, 1755 that hit Lisbon
The wave reached a height of 7 feet from the normal level, affecting coastal areas including the Netherlands.

           
         According database NOOA (National Oceanic and Atmospheric Administration), there are:

     

              2400 tsunami sources:  -81% Pacific Ocean,

                                                  -9% Mediterranean Sea

                                                  -7% Atlantic Ocean

                                                  -2% Black Sea

                                                  -1% Indian Ocean

              7,000 locations where tsunamis can trigger:

                                                

                                                 -89% Pacific Ocean

                                                 -6% Atlantic Ocean

                                                 -3% Mediterranean Sea

                                                 -2% Indian Ocean

  

           Causes of tsunamis:

                     landslides - events: 64; 4,6%; total dead: 14661

                     earthquakes - events: 1171; 82,3%; total dead: 390929

                     volcanic eruptions - events: 65; 4,6%; total dead: 51643

                     unknown - events: 121; 8,5%; total dead: 5364

                TOTAL: events: 1422; 100%; total dead: 462597


           A tsunami can deliver: 
                  - destruction of life, damage of all sorts, spread of diseases, destruction of particular ecosystems.


           We can reduce the energy of a tsunami by - development of offshore breakwaters, that can produce preventive breaking of the waves, prevent invasion of water in urban areas.

 
            In deep ocean, tsunami waves may appear only a foot or so high. But as they approach shoreline and enter shallower water they slow down and begin to grow in energy and height. The tops of the waves move faster than their bottoms do, which causes them to rise precipitously.

                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.

The best defense against any tsunami is early warning that allows people to seek higher ground. The Pacific Tsunami Warning System, a coalition of 26 nations headquartered in Hawaii, maintains a web of seismic equipment and water level gauges to identify tsunamis at sea. Similar systems are proposed to protect coastal areas worldwide.

         Video below shows a tsunami provoked by a testing submerged hydrogen bomb:

                    

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