Tsunamis: Cause and Effect

by

Ashley Swensen, Rozhin Torabi, Suwanta Shah, Michelle Owen, Carly Bierman

Abstract

The purpose of this project was to determine and elaborate upon the history, causes,

physics, prevention, and detection of tsunamis. A tsunami is defined as: “a very high, large wave

in the ocean that is usually caused by an earthquake under the sea and that can cause great

destruction when it reaches land.” (Mirriam-Webster). This essay discusses the effect of

tsunamis past and present, as well as the natural processes which cause them such as

earthquakes, volcanoes, and landslides. Also discussed are the physical properties of a tsunami

and how they can contribute to the size and propagation of the wave. New preventative measures

are also being employed to aid in protection from tsunamis. This essay will mention some of

these new techniques, including flood gates, tsunami walls and water channels. Modern

instruments are now used in tsunami detection, which can aid in early preparation. These

instruments and procedures will also be discussed.

History

The Greek historian Thucydides suggested in his late 5th century BC History of

Peloponnesian War, that tsunamis were related to submarine earthquakes, but the understanding

of tsunami’s nature remained slim until the 20th century and much remains unknown. Major areas

of current research include trying to determine why some large earthquakes do not generate

tsunamis while the other smaller one does; trying to accurately forecast the passage of tsunamis

across the oceans; and also to forecast how tsunami waves interact with specific shorelines.

While Japan may have the longest recorded tsunamis, the sheer destruction caused by

2004 Indian Ocean earthquake event mark it as the most devastating of its kind in the modern

times, killing around 230,000 people. The Sumatran region is not unused either, 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 ten thousand deaths and the 1908 Messina earthquake and

tsunami. The tsunami claimed more than 123,999 lives in Sicily and Calabria and is among the

deadliest 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.

According to an article in Malian Gulf, Greece (426 BC), this is the first historic tsunami,

as earlier as events do not have written records (prehistoric). In summer of 426 BC, a tsunami hit

the Malian Gulf between the northwest tip of Euboea and Lamia. The Greek historian

Thucydides (3.89.1-6) described how the tsunami and a series of an earthquake affected the

raging Peloponnesian War (431-404 BC) and, for the first time in the history of natural science,

correlated quakes with waves in terms of cause and effect.

On the night of July 9, 1958 an earthquake along the Fair Weather Fault in the Alaska

Panhandle loosened about 40 million cubic yards (30.6 million cubic meters) of rock high above

the northeastern shore of Lituya Bay. This mass of rock plunged from an altitude of

approximately 3000 feet (941 meters) down into the waters of Gilbert Intel. The impact

generated a local tsunami that crashed against the southwest shoreline of Gilbert Intel. The wave

hit with such power that it swept completely over the spur of land that separates Gilbert Intel

from the main body of Lituya Bay. The wave the continued down the entire length of Lituya

Bay, over La Chaussee Spit and into the Gulf of Alaska. The force of the wave removed all trees

and vegetation from elevations as high as 1720 feet (524 meters) above sea level. Millions of

trees were uprooted and swept away by the wave. This is the highest wave that has ever been

known.

The 9.1 magnitude earthquake off the coast of Sumatra was estimated to occur at a depth

of 30 km. The fault zone that caused the tsunami was roughly 1300 km long, vertically

displacing the sea floor by several meters along that length. The ensuring tsunami was as tall as

50 m, reaching 5km inland near Meubolah, Sumatra. This tsunami is also most widely recorded,

with nearly one thousand combined tide gauge and eyewitness measurements from around the

world reporting a rise in wave height, including places in the US, the UK and Antarctica. An

estimated US dollar of 10 billion of damages is attributed to the disaster, with around 230,000

people reported dead.

Lately, in the 16 September 2015 magnitude 8.3 off Illapel, Chile earthquake (31.570

degree south, 71.654 degree west, depth 25 km) occurred at 2255 UTC and generated a tsunami

that was observed all over the Pacific region and caused damage locally. According to news

reported, there were at least 13 dead due to the earthquake ground shaking. In February 2010, a

magnitude 8.8 Mw located near the central coast of Chile generated a tsunami that caused 156

Fatalities. According to the data provided we can also see that due to the advanced technologies

to detect tsunamis, nowadays it has resulted to less destruction and people are being aware of it.

Causes

What is seismic activity? Seismic activity is the frequency and size of an earthquake that

happens over time in a certain area. When the ocean is raised or dropped, the waves are formed

by gravitational forces. Waves of particularly large size are called Tsunamis. Tsunamis can occur

in any body of water that has tectonic plates, chunks of earth’s crust, rubbing together. It can

even occur in lakes, but most occur in the sea or ocean. When the tectonic plates rub together, it

activates an earthquake.  When the tectonic plates rise or drop underwater, this forms a wave.

The wave is then pushed outward until it can dissipate (see figure 1). Earthquakes are the main

cause of tsunamis, but there are others; volcanos, landslides, and meteorites.

Earth has many tectonic plates which help form the ground. These plates move at a slow

and steady rate. Earthquakes are generated in a subduction zone where the plates are being

forced down into the mantle, the layer beneath the earth’s crust, by the tectonic plates. This

makes the plates “stuck.” The stuck plates continue to descend into the mantle and cause a slow

motion of distortion, which then forms an accumulating energy. The energy can actually

accumulate over long periods of time, even decades! An earthquake has that energy until the

overriding plates exceeds frictional forces of the two stuck plates. This results in the plates

snapping back into unrestraint positions. A sudden motion like this causes an enormous shove

underwater; while this “shove” is forming, there is an overlying of water being pulled to the

plates causing water from shorelines to pull away. Then that water travels outward from the

earthquake. Some of the water travels towards ocean basins or even returns to the shorelines that

were pulled back from where it originally was. Most people assume that tsunamis are single

waves, but they actually have “wave trains” or multiple waves. Tsunamis can start as regular

tides and then slowly grow until you find larger waves; soon after, even bigger waves. These

waves then pound repeatedly into the shoreline. In Chile 1960, an earthquake produced a tsunami

that reached all the way to Hawaii in about 15 hours and Japan in less than 24 hours. Figure 2

shows how enormous tsunamis can actually be. Earthquakes are one cause of tsunamis, but

volcanoes can also be the cause.

When a volcano violently ruptures, it can displace a great volume of water and generate

tsunamis. When volcanoes have slope failure or phreatomagmatic explosion, the volcano moves

towards the water and displaces the water from the volcano (see figure 3). On August 26th, 1883,

one of the largest destructive tsunamis was recorded after an explosion and collapse of the

volcano of Krakatoa (Krakatau) in Indonesia. The waves of this tsunami were 135 feet in height,

destroying any villages or coastal towns along Sunda Strait in both of the islands of Java and

Sumatra; 36,417 people were killed during this event. Tsunamis from volcanoes are devastating,

but there are also tsunamis caused by landslides.

When large masses of land are displaced on a slope, a landslide occurs. Landslides are

actually triggered by earthquakes. The rapidly moving landslide mass enters in the water causing

water displacement and the waves turns into tsunamis. The potential of landslides can create

tsunami waves at close and very great distances. For example, a rock avalanche, or rock falls;

also a type of landslide, caused a regional tsunami in Tidal Intel, Glacier Bay National Park,

Alaska.  On July 9, 1958, an earthquake with the magnitude of 7.9 triggered this rock slide on the

Fairweather Fault in Lituya Bay, Alaska. The landslide created a wave that was 524 meters on

the shore and sent 30 meter high wave through Lituya Bay, sinking two of three fishing boats

and killed two people (see figure 4).

Submarine landslides create a displacement of water behind or ahead of the rapidly

moving underwater landslide. When a slide fails to hold it displaces water creating a tsunami.

There are about four steps in how underwater slides create tsunamis. First an earthquake has to

occur; resulting in a surface rupture. Second, sediment loading happens. This occurs by steep

slopes becoming loaded with too much sediment. Third, the sea level changes when the cause of

sediment or methane hydrates become unstable. And lastly, gas hydrates or methane hydrates,

are buried in the continental shelf and become unstable which results in a blow-out. These steps

can also generate tsunamis around the Atlantic and the Pacific Rim.

It’s a very slim chance that a meteorite can cause a tsunami. If a tsunami does occur it’s

because a meteorite slammed in the ocean and form giant waves. If a meteorite were to hit a

continental shelf it could cause a huge wave taking hours to recede. Many scientists think that

meteorites are over hyped.

Physics

Much physics is involved in the creation and dissipation of a tsunami. These include

gravity, Archimedes’ Principle, the law of conservation of momentum and of course, wave

motion. It is important to learn and understand the physical properties of these monstrous waves.

We will cover each of these fundamentals of physics and more by showing how they contribute

to tsunamis.

Gravity is the universal force acting upon all objects. Although distance decreases the

effects of gravity on a body, it can never be completely free of its influence. We are most

acquainted with gravity as the force that keeps us anchored to Earth’s surface. The magnitude of

the force can change depending upon where you are in the universe, as objects with a larger mass

have a larger pull. As for ocean tides, they are caused by the gravitational pull between the Earth

and Moon. Remember that according to Newton’s third law of motion that for every action, there

is an equal and opposite reaction. This pulling force creates “tidal bulges” (see figure 5). When

the moon is facing one side of the Earth, the difference in gravity pulls more water and matter

toward that point. Because of Newton’s third law, the water on the other side of the Earth pulls

back, creating a tidal bulge on each side of the Earth. These sides experience high tide, while the

perpendicular cross-section of the Earth experiences low tide (Hewitt, 2014 pg. 167-169). While

tides are not the cause of tsunamis, or a major contributor (we would be in big trouble if they

were!) they can have an effect at the time of impact (Physics of Tsunamis). For instance, if a

tsunami were to strike a coastal area that was experiencing high tide, there would be a higher

concentration of water to contribute to the wave in that area, possibly lending to more

destruction.

Another factor contributing to tsunamis is Archimedes’ Principle. This principle was

discovered by the Greek scientist Archimedes in 3rd century B.C. It states that: “An immersed

object is buoyed up by a force equal to the weight of the fluid it displaces” (Hewitt, 2014 pg.

250). But, why is this important in the formation of tsunamis? When a large amount of water is

displaced from its natural state, it will attempt to regain equilibrium. For instance, if a meteorite

were to strike the ocean, the amount of fluid displaced would be equal to the weight of the

meteorite. Most meteors are disintegrated by Earth’s atmosphere before they even make it to the

surface. However, there are some cases where very large meteorites have struck, and in cases

such as these, the volume of water displaced would have to be very large to accommodate

something so massive. Waves would begin to propagate from the point of impact in an attempt to

achieve equilibrium, which can generate a tsunami.

Similarly, the law of conservation of momentum is a major factor when it comes to the

height and speed of a tsunami. Momentum is “The product of the mass and the velocity of an

object” (Hewitt, 2014 pg. G-10). The law of conservation of momentum states that: “In the

absence of a net external force, the momentum of an object or system of objects is unchanged”

(Hewitt, 2014 pg. 98). This becomes a problem when the waves get closer to shore. Wave speed

is proportional to the depth of the water. The greater the depth, the faster the wave, and the same

holds true for the opposite. As a wave approaches shallow water, it begins to slow, but due to the

conservation of momentum, if the waves speed (velocity) decreases, then its height (mass) must

increase. Unless it is slowing due to an outside net force, the momentum of the system cannot be

changed. This is a major contributor when it comes to the size of a tsunami, and it also explains

why they are generally smaller in the open ocean than at a shoreline. This effect is known as

“shoaling” (see figure 6).

Lastly, wave motion is a component of physics that can be used to describe and measure

tsunamis. Waves transfer energy, not matter. Energy, in regard to tsunamis, can come from many

sources, but the wave through which it is distributed is called a transverse wave. This means that

the motion of the medium (in our case, water) travels perpendicular to the direction the wave

travels. This “up and down” motion produces crests at the top of each wave, and troughs at the

bottom. The speed of a tsunami is determined by the wavelength and the frequency (Hewitt,

2014 pg. 362). According to the National Oceanic and Atmospheric Association: “Wind-

generated waves usually have period…of five to twenty seconds and a wavelength…of about

100 to 200 meters (300 to 600 ft). A tsunami can have a period in the range of ten minutes to two

hours and a wavelength in excess of 300 miles (500 km).” This means that tsunamis can achieve

wave speeds of up to 150 mph (250 kmh). Remember that as a wave approaches land, its height

(mass) begins to increase. One of the most memorable tsunamis of this generation was the Indian

Ocean tsunami on December 26th, 2004. Data suggests that the waves reached up to 20 to

30 m (65 to 100 ft). (Gibbons, H., Gelfenbaum, G., 2005).

Prevention

A tsunami can strike anywhere along most of the U.S. coastline. The most destructive

tsunamis have occurred along the coasts of California, Oregon, Washington, Alaska, and Hawaii.

If a major earthquake or landslide occurs close to shore, the first wave in a series could reach the

beach in a few minutes, even before a warning is issued. Areas are at greater risk if they are less

than 25 feet above sea level and within a mile of the shoreline. Drowning is the most common

cause of death associated with a tsunami.

Tsunami waves and the receding water are very destructive to structures in the run-up

zone. Other hazards include flooding, contamination of drinking water, and fires from gas lines

or ruptured tanks. Tsunami waves do not resemble normal sea waves. Rather than appearing as a

breaking wave, a tsunami may instead initially resemble a rapidly rising tide, and for this reason

they are often referred to as tidal waves.

Major areas of current research include trying to determine why some large earthquakes

do not generate tsunamis while other smaller ones do; trying to accurately forecast the passage of

tsunamis across the oceans; and also to forecast how tsunami waves interact with specific

shorelines.

Once a tsunami has been formed, there is no way to stop it but with an effective warning

system in place, people can be evacuated. And reducing the damage caused by a tsunami is

certainly achievable. Tsunami walls, flood gates, and channels are three measures that can be

taken to that end.

The tsunami's impact was even worse because it hit some of the poorest parts of the

affected countries. The solution now - along with an end to local conflicts - must be a

reconstruction plan that does more than just rebuild poverty. It must create better access to safe

water, health and education, reduce hunger, and lower child mortality rates.

Japan is prone to tsunamis and has built numerous tsunami walls along the coast of

heavily populated areas. The tsunami walls are designed to absorb some of the energy of the

tsunami and redirect some of the water back towards the open ocean. The walls may be as high

as 14 feet.

Flood gates and channels can help redirect some of the incoming water from a tsunami,

but a large tsunami will easily overwhelm any of the measures that are used to reduce its

strength. Some European countries, including the UK and Sweden, have taken a lead in untying

humanitarian food aid. The rest of Europe is slowly following suit.

Safety and Detection

Every coastal area is at risk for a Tsunami. More than three quarters of all Tsunamis

occur in the Pacific Ocean. The Aleutian Islands, Alaska, Chile, Philippines, and Japan are all

likely places for a Tsunami to occur. The Pacific Ocean is nicknamed “The Ring of Fire”

because 80% of tsunamis occur in the Pacific Ocean. Luckily, scientists can detect Tsunamis by

monitoring the seismic activity. Seismic waves travel faster than the actual tsunami, which

makes them a great warning signal. The best way to prepare for a Tsunami is early detection

because once a tsunami has started, there is no stopping it. Areas at risk for tsunamis have

warning systems in place so that the residents can be alarmed and evacuate. Areas near the

Pacific Ocean have a tsunami warning system that was established in Hawaii in 1949. This

system is called the Pacific Tsunami Warning System at the International Tsunami Warning

Center. It is made up of seismic-monitoring stations and deep-ocean tsunami detection buoys

that can warn people when a Tsunami is detected. The deep-ocean detection buoys record

changes in elevation as well as the speed of a wave (National Geographic, 2016).

When a tsunami is detected a warning is sent out to all areas affected by the National

Weather Service alerts the Emergency Evacuation System (EAS). All broadcasters including the

radio, television, cable, and weather radio receivers receive the emergency signals (Buehner,

n.d.). There are also sirens on the tops of telephone poles in some areas that produce a loud

sound. When a tsunami is detected the National Oceanic and Atmospheric Association (NOAA)

either announces a Tsunami watch or a Tsunami warning. (Tsunami Education and Awareness,

2009).

During a tsunami watch people should listen to the warning alarms and announcements

so that they are notified of the warnings and advisories. People are encouraged to have a natural

disaster safety kit, so after the warning you should make sure your kit is stocked. The next step

would be to locate family members and go over evacuation plans. It is important to secure and

anchor items around their house and workplace if there is enough time. Tsunamis can destroy

and carry away items, but if items are secured they will not be swept away as easily. The last

thing to do in a tsunami warning is to be prepared to evacuate at any time, because tsunamis

travel quickly (Tsunami Education and Awareness, 2009). See figure 7 for the tsunami detection

process.

It is important to listen to the advice from the local authorities because there are different

advisories depending on the strength and speed of the tsunami. Local authorities will advise

people to seek higher ground. Keep in mind that roads may be destroyed during a tsunami so a

lot of the time you will need to seek higher elevation by foot. Some people like to try to watch

the wave, but this is very dangerous and people need to remember that if you can see the wave,

then you are in danger. Scientists cannot predict the exact height of local effects of the tsunami,

therefore it is best to go as far inland as possible. Do not return to your home unless the local

advisors announce that the tsunami is over. Some tsunamis may last for several hours. Just

because one large wave hit does not mean that the tsunami is over. Sometimes there is an even

bigger wave that follows after the first wave (Lincoln, n.d.).

Many citizens worry about living in areas where tsunamis are likely to happen. However,

they can prepare and learn survival strategies. The first survival tip is to prepare your home with

food and survival kits. Many people think that they will die from the initial earthquake before the

tsunami even comes, but this is false. Most people are killed during the actual Tsunami. One of

the biggest natural warning signs of a tsunami are the rapid rise and fall of the coastal waters.

Some people do not realize that this is a warning sign and will actually walk out to the low tide

waters. This is very dangerous! If the tide is suddenly low this is a sign of a tsunami. It is

important to pay attention to the natural warning signs of a tsunami.

The effects of a tsunami can range from minor to devastating. Small tsunamis happen

almost every day, it is the big tsunamis that we need to worry about. The destruction is mainly

caused by two mechanisms. The smashing force of the water and the volume of water draining

back to the ocean taking many debris with it. Tsunamis destroy everything in their path and can

leave islands unrecognizable. People have had to learn how to clean up and survive after a

tsunami. After a large tsunami people need to find food and water. Often times a neighboring

country will send aid to the area in need. Many fresh water lines are destroyed from tsunamis and

sewage lines are also destroyed causing contamination. It is difficult for people to stay healthy

because of the contaminated water. Many people get infections and diseases from the

contamination. Medical aid is also needed after a tsunami. Clean up and reconstruction are major

cost problems after a tsunami. Debris needs to be cleaned up and unsafe buildings need to be

demolished. It is a big ordeal and takes a lot of time. Another effect of a tsunami is the

psychological damage to the survivors. Many survivors suffer from grief and depression after a

tsunami. Homes are lost, family member are injured or dead, and businesses are lost. This is a

traumatic event and many people don’t fully recover from the psychological damage (Folger,

2012).

Figures

Figure 1: Tectonic plates. Seismic activity during an earthquake causing a tsunami

Figure 2: Size of a tsunami. Map of the Chilean tsunami of 1960. (Chile 1960)

Figure 3: Volcanic activity. Water being displaced as volcano collapses in toward the ocean.

Figure 4: Landslides. Map of landslide that caused Alaskan tsunami (Lituya Bay, Alaska 1958).

Figure 5: Tides. High and low tidal bulges created by gravitational forces between Earth and Moon (photograph courtesy of EarthSky.org)

Figure 6: Conservation of momentum. Shoaling effect as momentum is conserved during propagation of wave toward shoreline. (photograph courtesy of Chandra, 2015)

Figure 7: The detection process of a Tsunami (Tsunami Education and Awareness, 2009).

Figure 8: Destruction after a tsunami (National Geographic, 2016)Works Cited

Physics: Ashley Swensen

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Causes: Michelle Owen

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Safety and Detection: Carly Bierman

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