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How an earthquake becomes a tsunami

 

waves breaking on shore CreditPexelsHuge Waves Breaking on the Seashore. Photo Source: Pexels

 

The complex relationship between earthquakes and tsunamis has long puzzled scientists, particularly in the context of subduction zones where oceanic and continental plates meet. The powerful megathrust earthquakes that occur in these regions have the potential to generate devastating tsunamis, making it crucial to understand the processes that drive this transformation. In a significant breakthrough, an international research team, including Prof. James Foster from the Institute of Geodesy at the University of Stuttgart, conducted groundbreaking research in an underwater-earthquake zone off the coast of Alaska. Their use of cutting-edge technology and innovative autonomous vessels provided unprecedented insights into this natural phenomenon. In this article, we explore how earthquakes evolve into tsunamis and the exciting developments in this field.

The Dynamics of Megathrust Earthquakes

Megathrust earthquakes are among the most powerful seismic events on Earth. These earthquakes occur at subduction zones, where an oceanic tectonic plate is forced beneath a continental plate. The relentless collision and friction between these colossal plates result in a build-up of stress and strain. Eventually, this pent-up energy is released in the form of a massive earthquake.

One such remarkable event was the Chignik earthquake, which rocked the coast of Alaska on July 28, 2021. With a magnitude of 8.2, it became the seventh-strongest earthquake in U.S. history. This seismic tremor occurred 32 kilometers below the ocean floor, underscoring the raw power generated by the incessant movement of tectonic plates.

The Subduction Zone Enigma

Despite our awareness of the destructive potential of megathrust earthquakes in subduction zones, the mechanisms driving these seismic events and their connection to tsunamis have remained a topic of scientific intrigue. The primary challenge lies in the ocean floor's inaccessibility for direct measurements, making it difficult to understand how and when an earthquake can trigger a tsunami.

To address this knowledge gap and enhance our ability to predict the likelihood of an earthquake leading to a tsunami, a dedicated team of researchers, under the leadership of Benjamin Brooks from the United States Geological Survey, embarked on an ambitious expedition. Their mission was to explore the seafloor off the coast of Alaska, focusing on the aftermath of the Chignik earthquake.

Cutting-Edge Technology and Autonomous Vessels

wave glider CreditUofStuttgartA picture of a wave glider equipped with GNSS
and acoustic measurement tools for seafloor monitoring.
Photo credits: University of Stuttgart https://www.uni-stuttgart.de/
en/university/news/all/How-an-earthquake-becomes-a-tsunami/

A pivotal aspect of this research venture was the deployment of cutting-edge technology and autonomous vessels. These advanced vessels, known as wave gliders, are equipped with state-of-the-art instrumentation, including a global navigation satellite system (GNSS) and acoustic measuring devices. Wave gliders are capable of navigating on the water's surface, making them exceptionally well-suited for the challenging conditions of the open ocean.

One noteworthy contributor to the development of these autonomous vessels was Prof. James Foster from the Institute of Geodesy at the University of Stuttgart. These wave gliders played an indispensable role in collecting invaluable data and observations in the underwater earthquake zone off Alaska.

Precise Measurements to the Nearest Centimeter

topography bathymetry Alaska subduction zone RefScienceAdvancesTopography and bathymetry of the Alaska subduction zone’s Semidi section.
Image reference: Science Advances
https://www.science.org/doi/10.1126/sciadv.adf9299

The groundbreaking element of this research was the precision of the measurements obtained. The team managed to capture the movements within the subduction zones with unprecedented accuracy, down to the nearest centimeter. This level of precision enabled researchers to focus specifically on the shallow segments of the slip zones, as these regions are particularly crucial in determining whether a tsunami may be generated.

Understanding the Significance of Shallow Zones

Shallow parts of subduction zones are where the magic happens, scientifically speaking. In these regions, the oceanic plate grinds against the continental plate, building up immense pressure and tension. This is the point where the potential for a massive release of energy is at its highest, setting the stage for a tsunami.

The Quest for Deeper Insights

While this research has provided significant insights into the dynamics of subduction zones, there is a yearning for a more profound understanding. Prof. James Foster, a central figure in this research, expressed a desire to measure the movements of the seafloor at even greater depths, specifically at depths ranging from 3,000 to 4,000 meters. These depths are directly above the shallowest part of the fault system, and exploring this uncharted territory represents the next frontier in tsunami research.

Overcoming the Challenges

The primary challenge of obtaining measurements at these extreme depths is the limitation of existing geodetic systems. However, there is optimism on the horizon. Researchers anticipate acquiring a device equipped with sensors capable of taking geodetic measurements at these extreme depths. With this cutting-edge technology, scientists are poised to gain direct access to the deepest sections of tsunamigenic faults, advancing our understanding of the processes that transform earthquakes into tsunamis.

Conclusion

The journey from an earthquake to a tsunami is a complex and intricate process that involves immense geological forces and precise conditions. Recent advancements in technology and international collaboration have provided a significant leap forward in our quest to understand this natural phenomenon.

As researchers continue to push the boundaries of knowledge and as technology evolves, we edge closer to demystifying the intricacies of how an earthquake can lead to a tsunami. This newfound understanding promises not only to enhance our ability to predict and respond to these catastrophic events but also to safeguard lives and reduce the impact of tsunamis on coastal communities worldwide. It is a testament to the relentless pursuit of knowledge and the relentless spirit of exploration that drives scientific progress.

Source: https://www.science.org/doi/10.1126/sciadv.adf9299
https://phys.org/news/2023-06-earthquake-tsunami.html

 

Why are locally generated tsunamis so dangerous?

 

A locally generated tsunami may reach a nearby shore in less than ten minutes. There may not be sufficient time for tsunami warning centers, such as the Pacific Tsunami Warning Center, or for local authorities to issue a warning. For people living near the coast, the shaking of the ground from an earthquake is nature's natural warning that a tsunami may be imminent. Observation of unusual ocean changes, or the hearing of loud ocean roars are also nature's natural tsunami warning signs. If you sense of these and are near the occean, evacuate immediately inland and to higher ground.  
For tsunamis from more distant sources, however, tsunami warning centers play an important role in ensuring public safety. Accurate warnings of when a tsunami might arrive are possible because tsunamis travel at a known speed. Warning centers will work with local authorities to advise them on whether a tsunami will impact their coasts.

 

           sensing_a_tsunami_thumbnail

Where and how frequently are tsunamis generated?


Tsunamis are disasters that can be generated in all of the world's oceans, inland seas, and in any large body of water. Each region of the world appears to have its own cycle of frequency and pattern in generating tsunamis that range in size from small to the large and highly destructive events.  Most tsunamis occur in the Pacific Ocean and its marginal seas. The reason is that the Pacific covers more than one-third of the earth's surface and is surrounded by a series of mountain chains, deep-ocean trenches and island arcs called the "ring of fire" - where most earthquakes occur (off the coasts of Kamchatka, Japan, the Kuril Islands, Alaska and South America). Many tsunamis have also been generated in the seas which border the Pacific Ocean. Tsunamis are generated, by shallow earthquakes all around the Pacific, but those from earthquakes in the tropical Pacific tend to be modest in size. While such tsunamis in these areas may be devastating locally, their energy decays rapidly with distance. Usually, they are not destructive a few hundred kilometers away from their sources.  

That is not the case with tsunamis generated by great earthquakes in the North Pacific or along the Pacific coast of South America. On the average of about half-a-dozen times per century, a tsunami from one of these regions sweeps across the entire Pacific, is reflected from distant shores, and sets the entire ocean in motion for days. For example, the 1960 Chilean tsunami caused death and destruction throughout the Pacific. Hawaii, Samoa, and Easter Island all recorded runups exceeding 4 m; 61 people were killed in Hawaii. In Japan 200 people died. A similar tsunami in 1868 from northern Chile caused extensive damage in the Austral Islands, Hawaii, Samoa and New Zealand. 

Although not as frequent, destructive tsunamis have been also been generated in the Atlantic and the Indian Oceans, the Mediterranean Sea and even within smaller bodies of water, like the Sea of Marmara, in Turkey. In 1999, a large earthquake along the North Anatolian Fault zone, generated a local tsunami, which was particularly damaging in the Bay of Izmit. 

In the last decade alone, deadly tsunamis have occurred in Chile (2007, 2010), Haiti (2010), Indonesia (2004, 2005, 2006, 2010), Japan (2011), Peru (2001), Samoa - American Samoa - Tonga (2009), Solomons (2007). Of these, only Indonesia (2004) and Japan (2011) caused deaths at distant shores.

 

 

What determines how destructive a tsunami will be near the origin and at a distant shore?


Tsunamis arrive at a coastline as a series of successive crests (high water levels) and troughs (low water levels) - usually occurring 10 to 45 minutes apart. As they enter the shallow waters of coastlines, bays, or harbors, their speed decreases to about 50-60 km/h.  For example, in 15 m of water the speed of a tsunami will be only 45 km/h. However 100 or more kilometers away, another tsunami wave travels in deep water towards the same shore at a much greater speed, and still behind it there is another wave, traveling at even greater speed.  

As the tsunami waves become compressed near the coast, the wavelength is shortened and the wave energy is directed upward - thus increasing their heights considerably. Just as with ordinary surf, the energy of the tsunami waves must be contained in a smaller volume of water, so the waves grow in height. Even though the wavelength shortens near the coast, a tsunami will typically have a wavelength in excess of ten kilometers when it comes ashore. Depending on the water depth and the coastal configuration, the waves may undergo extensive refraction - another process that may converge their energy to particular areas on the shore and thus increase the heights even more. Even if a tsunami wave may have been 1 meter of less in the deep ocean, it may grow into a huge 30-35 meter wave when it sweeps over the shore.  

Thus, tsunami waves may smash into the shore like a wall of water or move in as a fast moving flood or tide - carrying everything on their path. Either way, the waves become a significant threat to life and property. If the tsunami waves arrive at high tide, or if there are concurrent storm waves in the area, the effects will be cumulative and the inundation and destruction even greater. The historic record shows that there have been many tsunamis that have struck the shores with devastating force, sometimes reaching heights of more than 30-50 meters. For example, the 1946 tsunami generated by an earthquake off Unimak island in Alaska's Aleutian Islands, reached heights of more than 35 meters, which destroyed a reinforced concrete lighthouse and killed its occupants.  

Finally, the maximum height a tsunami reaches on shore is called the runup. It is the vertical distance between the maximum height reached by the water on shore and the mean sea level surface.  Any tsunami runup over a meter is dangerous. The flooding by individual waves will typically last from ten minutes to a half-hour, so the danger period can last for hours. Tsunami runup at the point of impact will depend on how the energy is focused, the travel path of the tsunami waves, the coastal configuration, and the offshore topography.  

Effects on Islands
Small islands with steep slopes usually experience little runup - wave heights there are only slightly greater than on the open ocean. This is the reason that islands with steep-sided fringing or barrier reefs are only at moderate risk from tsunamis.  

However, this is not the case for islands such as the Hawaiian or the Marquesas. Both of these island chains do not have extensive barrier reefs and have broad bays exposed to the open ocean. For example, Hilo Bay at the island of Hawaii and Tahauku Bay at Hiva Oa in the Marquesas are especially vulnerable. The 1946 Aleutian tsunami resulted in runup, which exceeded 8 m at Hilo and 10 m at Tahauku; 59 people were killed in Hilo and two in Tahauku. Similarly, any gap in a reef puts the adjacent shoreline at risk. The local tsunami from the Suva earthquake of 1953 did little damage because of Fiji's extensive offshore reefs. However, two villages on the island of Viti Levu, located on opposite gaps in the reef, were extensively damaged and five people were drowned.
 

Tsunami wave height increase as they reach the shore due to the shallowing of seafloor.

tsuwavecharacteristics 08.03.11

What is a tsunami?


The phenomenon we call tsunami is a series of large waves of extremely long wavelength and period usually generated by a violent, impulsive undersea disturbance or activity near the coast or in the ocean. When a sudden displacement of a large volume of water occurs, or if the sea floor is suddenly raised or dropped by an earthquake, big tsunami waves can be formed.  The waves travel out of the area of origin and can be extremely dangerous and damaging when they reach the shore.  

The word tsunami (pronounced tsoo-nah'-mee) is composed of the Japanese words "tsu" (which means harbor) and "nami" (which means "wave"). Often the term, "seismic or tidal sea wave" is used to describe the same phenomenon, however the terms are misleading, because tsunami waves can be generated by other, non seismic disturbances such as volcanic eruptions or underwater landslides, and have physical characteristics different of tidal waves. The tsunami waves are completely unrelated to the astronomical tides - which are caused by the extraterrestrial, gravitational influences of the moon, sun, and the planets. Thus, the Japanese word "tsunami", meaning "harbor wave" is the correct, official and all-inclusive term. It has been internationally adopted because it covers all forms of impulsive wave generation.

Tsunami waves often look like walls of water and can attack the shoreline and be dangerous for hours, with waves coming every 5 to 60 minutes. The first wave may not be the largest, and often it is the 2nd, 3rd, 4th or even later waves that are the biggest. After one wave inundates, or floods inland, it recedes seaward often as far as a person can see so the seafloor is exposed. The next wave then rushes ashore within minutes and carries with it many floating debris that were destroyed by previous waves. When waves enter harbors, very strong and dangerous water currents are generated that can easily break ship moorings, and bores that travel far inland can be formed when tsunamis enter rivers or other waterway channels.

1983 Japan Sea Tsunami  1983 japansea tsunami big

What are the factors of destruction from tsunamis?


There are three factors of destructions from tsunamis: inundation, wave impact on structures, and erosion. Strong, tsunami-induced currents lead to the erosion of foundations and the collapse of bridges and seawalls. Flotation and drag forces move houses and overturn railroad cars. Considerable damage is caused by the resultant floating debris, including boats and cars that become dangerous projectiles that may crash into buildings, break power lines, and may start fires. Fires from damaged ships in ports or from ruptured coastal oil storage tanks and refinery facilities, can cause damage greater than that inflicted directly by the tsunami. Of increasing concern is the potential effect of tsunami draw down, when receding waters uncover cooling water intakes of nuclear power plants. 

ptmpier1462 crop bigDestruction of Hilo, Hawaii harbor pier during 1946 Aleutians Islands tsunami.

Can asteroids, meteorites or man-made explosions cause tsunamis?


Fortunately, for mankind, it is indeed very rare for a meteorite or an asteroid to reach the earth.  Although no documented tsunami has ever been generated by an  asteroid impact, the effects of such an event would be disastrous.  Most meteorites burn as they reach the earth's atmosphere.  However, large meteorites have hit the earth's surface in the distant past. This is indicated by large craters, which have been found in different parts of the earth.  Also, it is possible that an asteroid may have fallen on the earth in prehistoric times - the last one some 65 million years ago during the Cretaceous period.  Since evidence of the fall of meteorites and asteroids on earth exists, we must conclude that they have fallen also in the oceans and seas of the earth, particularly since four fifths of our planet is covered by water.  

The fall of meteorites or asteroids in the earth's oceans has the potential of generating tsunamis of cataclysmic proportions. Scientists studying this possibility have concluded that the impact of moderately large asteroid, 5-6 km in diameter, in the middle of the large ocean basin such as the Atlantic Ocean, would produce a tsunami that would travel all the way to the Appalachian Mountains in the upper two-thirds of the United States. On both sides of the Atlantic, coastal cities would be washed out by such a tsunami. An asteroid 5-6 kilometers in diameter impacting between the Hawaiian Islands and the West Coast of North America, would produce a tsunami which would wash out the coastal cities on the West coasts of Canada, U.S. and Mexico and would cover most of the inhabited coastal areas of the Hawaiian islands.  

Conceivably tsunami waves can also be generated from very large nuclear explosions. However, no tsunami of any significance has ever resulted from the testing of nuclear weapons in the past. Furthermore, such testing is presently prohibited by international treaty.

Click the link below to view a movie that shows a physics-based computer simulation of the tsunami generated by the impact of the Chicxulub asteroid 65 million years ago. This asteroid impact is thought to responsible for the extinction of the dinosaurs. https://youtu.be/Dcp0JhwNgmE

Why aren't tsunamis seen at sea or from the air?


In the deep ocean, tsunami wave amplitude is usually less than 1 m (3.3 feet). The crests of tsunami waves may be more than a hundred kilometers or more away from each other. Therefore, passengers on boats at sea, far away from shore where the water is deep, will not feel nor see the tsunami waves as they pass by underneath at high speeds. The tsunami may be perceived as nothing more than a gentle rise and fall of the sea surface.

The Great Sanriku tsunami, which struck Honshu, Japan, on June 15, 1896, was completely undetected by fishermen twenty miles out to sea. The deep-water height of this tsunami was only about 40 centimeters when it passed them and yet, when it arrived on the shore, it had transformed into huge waves that killed 28,000 people, destroyed the port of Sanriku and villages along 275 km of coastline. For the same reason of low amplitude and very long periods in the deep ocean, tsunami waves cannot be seen nor detected from the air.  From the sky, tsunami waves cannot be distinguished from ordinary ocean waves.
 

 

How do submarine landslides, rock falls and underwater slumps generate tsunamis?


Less frequently, tsunami waves can be generated from displacements of water resulting from rock falls, icefalls and sudden submarine landslides or slumps. Such events may be caused impulsively from the instability and sudden failure of submarine slopes, which are sometimes triggered by the ground motions of a strong earthquake. For example in the 1980's, earth moving and construction work of an airport runway along the coast of Southern France, triggered an underwater landslide, which generated destructive tsunami waves in the harbor of Thebes.  

Major earthquakes are suspected to cause many underwater landslides, which may contribute significantly to tsunami generation. For example, many scientists believe that the 1998 tsunami, which killed about 2200 people and destroyed Sissano and nearby villages along the northern coast of Papua-New Guinea, was generated by a large underwater slump of sediments, triggered by an earthquake.  

In general, the energy of tsunami waves generated from landslides or rock falls is rapidly dissipated as they travel away from the source and across the ocean, or within an enclosed or semi-enclosed body of water - such as a lake or a fjord. However, it should be noted, that the largest tsunami wave ever observed anywhere in the world was caused by a rock fall in Lituya Bay, Alaska on July 10, 1958. Triggered by an earthquake along the Fairweather fault, an approximately 40 million cubic meter rock fall at the head of the bay generated a wave, which reached the incredible height of 525-meter runup (~1750 feet) on the opposite side of the inlet. A initial huge solitary wave of about 180 meters (600 feet) raced at about 160 kilometers per hour (100 mph) within the bay debarking trees along its path. However, the tsunami's energy and height diminished rapidly away from the source area and, once in the open ocean, it was hardly recorded by tide gauge stations. Only two persons died and three boats were destroyed in Lituya Bay. In nearby Yakutat Bay, 6.1 meter runup was measured and three persons died.

Shown below is a Lituya Bay time sequence simulation illustrating the formation of the tsunami (red) from the landslide, its propagation across the bay, and then extreme run up the facing shore slope (from left to right).   Numerical modeling can provide many insights on the behavior of tsunamis; in the images, you can see many details, including fingers and eddy trails of water that are formed as part of the initial landslide splash.

LituyaBayFrames


 

 

What is a mega-tsunami and can it happen today?


The following is a position paper that was issued by the Tsunami Society concerning the occurrence of Mega-Tsunamis:

The mission of the Tsunami Society includes "the dissemination of knowledge about tsunamis to scientists, officials, and the public". We have established a committee of private, university, and government scientists to accomplish part of this goal by correcting misleading or invalid information released to public about this hazard. We can supply both valid, correct and important information and advice to the public, and the names of reputable scientists active in the field of tsunami, who can provide such information.

Most recently, the Discovery Channel has replayed a program alleging potential destruction of coastal areas of the Atlantic by tsunami waves which might be generated in the near future by a volcanic collapse in the Canary Islands. Other reports have involved a smaller but similar catastrophe from Kilauea volcano on the island of Hawai`i. They like to call these occurences "mega tsunamis". We would like to halt the scaremongering from these unfounded reports. We wish to provide the media with factual information so that the public can be properly informed about actual hazards of tsunamis and their mitigation.

Here are a set of facts, agreed on by committee members, about the claims in these reports:

- While the active volcano of Cumbre Vieja on Las Palma is expected to erupt again, it will not send a large part of the island into the ocean, though small landslides may occur. The Discovery program does not bring out in the interviews that such volcanic collapses are extremely rare events, separated in geologic time by thousands or even millions of years.

- No such event - a mega tsunami - has occurred in either the Atlantic or Pacific oceans in recorded history. NONE.

- The colossal collapses of Krakatau or Santorin (the two most similar known happenings) generated catastrophic waves in the immediate area but hazardous waves did not propagate to distant shores. Carefully performed numerical and experimental model experiments on such events and of the postulated Las Palma event verify that the relatively short waves from these small, though intense, occurrences do not travel as do tsunami waves from a major earthquake.

- The U.S. volcano observatory, situated on Kilauea, near the current eruption, states that there is no likelihood of that part of the island breaking off into the ocean.

- These considerations have been published in journals and discussed at conferences sponsored by the Tsunami Society.

Some papers on this subject include:

"Evaluation of the threat of Mega Tsunami Generation From ....Volcanoes on La Palma ... and Hawaii", George Pararas-Carayannis, in Science of Tsunami Hazards, Vol 20, No.5, pages 251-277, 2002.

"Modeling the La Palma Landslide Tsunami", Charles L. Mader, in Science of Tsunami Hazards, Vol. 19, No. 3, pages 160-180, 2001.

"Volcano Growth and the Evolution of the Island of Hawaii", J.G. Moore and D.A.Clague, in the Geologic Society of America Bulletin, 104, 1992.

Committee members for this report include:

Mr. George Curtis, Hilo, HI (Committee Chairman)

Dr. Tad Murty, Ottawa, Canad

Dr. Laura Kong, Honolulu, HI

Dr. George Pararas-Carayannis, Honolulu, HI

Dr. Charles L. Mader, Los Alamos, NM

All can comment on this or other tsunami matters.

For information regarding the Tsunami Society and its publications, visit: http://www.tsunamisociety.org/.

For general and educational material on tsunamis, check: www.tsunami.org.

  

How do volcanic eruptions generate tsunamis?


Although relatively infrequent, violent volcanic eruptions represent also impulsive disturbances, which can displace a great volume of water and generate extremely destructive tsunami waves in the immediate source area. According to this mechanism, waves may be generated by the sudden displacement of water caused by a volcanic explosion, by a volcano's slope failure, or more likely by a phreatomagmatic explosion and collapse/engulfment of the volcanic magmatic chambers. One of the largest and most destructive tsunamis ever recorded was generated in August 26, 1883 after the explosion and collapse of the volcano of Krakatoa (Krakatau), in Indonesia. This explosion generated waves that reached 135 feet, destroyed coastal towns and villages along the Sunda Strait in both the islands of Java and Sumatra, killing 36,417 people. It is also believed that the destruction of the Minoan civilization in Greece was caused in 1490 B.C. by the explosion/collapse of the volcano of Santorin in the Aegean Sea.

 

Diagram of how a volcanic eruption can generate a tsunami.

Diagram of how a volcanic eruption can generate a tsunami. Geoscience Australia.

Diagram source: Geoscience Australia.

 

When you look at the risk of a tsunami in your community, you need to consider the different sources in your area/region that can generate a tsunami. Volcanoes are one source that can produce tsunamis as high as those produced by the largest earthquake. They can be caused by mechanisms such as volcanic earthquakes, undersea eruptions, pyroclastic flows, caldera collapse, landslides, lahars, phreatomagmatic eruptions, lava bench collapse, and airwaves from large explosions. There have been 110 eruptions that caused tsunamis' (NGDC/WDS). Below are a few examples of volcanic eruptions that have caused a tsunami:

  • In 1792, the eruption of Mount Unzen in Japan produced a destructive landslide generating a 
        165-foot tsunami. The death toll from the disaster is estimated at over 15,000 people, 
        making it the most deadly volcanic eruption in Japan's history.
  • The 1883 eruption of Krakatoa in Indonesia caused by *pyroclastic flows entering the water 
        (A base surge resulting from collapse of the eruption column) that produced run-up 
        heights of 120 feet and killed over 26,000 people and many costal villages destroyed.
  • The 1980 Eruption of Mount St. Helens in Washington (USA) caused partial collapse of the 
         volcano's flank and an avalanche into Spirit Lake producing a 780-foot tsunami.


Citations:
 
National Geophysical Data Center / World Data Service (NGDC/WDS): Significant Volcanic Eruptions Database. National Geophysical Data Center, NOAA. doi:10.7289/V5JW8BSH

*1883 eruption of Krakatoa: Volcanologists Self & Rampino had the correct source sorted out in 1981 (Science), though a convincing numerical model confirming that source was not published until Maeno & Imamura in 2011 (JGR).

 mount st helens 1982 2007

How does tsunami energy travel across the ocean and how far can tsunamis waves reach?


Once a tsunami has been generated, its energy is distributed throughout the water column, regardless of the ocean's depth. A tsunami is made up of a series of very long waves. The waves will travel outward on the surface of the ocean in all directions away from the source area, much like the ripples caused by throwing a rock into a pond. The wavelength of the tsunami waves and their period will depend on the generating mechanism and the dimensions of the source event. If the tsunami is generated from a large earthquake over a large area, its initial wavelength and period will be greater. If the tsunami is caused by a local landslide, both its initial wavelength and period will be shorter. The period of the tsunami waves may range from 5 to 90 minutes. The wave crests of a tsunami can range from a few to a hundred kilometers or more apart as they travel across the ocean. As the waves approach the coast, their wavelength decreases and wave height increases.

On the open ocean, the wavelength of a tsunami may be as much as two hundred kilometers, many times greater than the ocean depth, which is on the order of a few kilometers. In the deep ocean, the height of the tsunami from trough to crest may be only a few centimeters to a meter or more - again depending on the generating source. Tsunami waves in the deep ocean can travel at high speeds for long periods of time for distances of thousands of kilometers and lose very little energy in the process. The deeper the water, the greater the speed of tsunami waves will be.  

For example, at the deepest ocean depths the tsunami wave speed will be as much as 800 km/h, about the same as that of a jet aircraft. Since the average depth of the Pacific ocean is 4000 m (14,000 feet) , tsunami wave speed will average about 200 m/s or over 700 km/h (500 mph). At such high speeds, a tsunami generated in Aleutian Islands may reach Hawaii in less than four and a half hours. In 1960, great tsunami waves generated in Chile reached Japan, more than 16,800 km away in less than 24 hours, killing hundreds of people.


Tsunamis slow down but grow in size as they come ashore.
Tsu Wave Characteristics big
 
 
TsuWaveCharacteristics ft

How do earthquakes generate tsunamis?


By far, the most destructive tsunamis are generated from large, shallow earthquakes with an epicenter or fault line near or on the ocean floor.  These usually occur in regions of the earth characterized by tectonic subduction along tectonic plate boundaries.  The high seismicity of such regions is caused by the collision of tectonic plates.  When these plates move past each other, they cause large earthquakes, which tilt, offset, or displace large areas of the ocean floor from a few kilometers to as much as a 1,000 km or more.  The sudden vertical displacements over such large areas, disturb the ocean's surface, displace water, and generate destructive tsunami waves.  The waves can travel great distances from the source region, spreading destruction along their path.  For example, the Great 1960 Chilean tsunami was generated by a magnitude 9.5 earthquake that had a rupture zone of over 1,000 km.   Its waves were destructive not only in Chile, but also as far away as Hawaii, Japan and elsewhere in the Pacific.  It should be noted that not all earthquakes generate tsunamis.  Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.
Most tsunamis are generated by shallow, great earthquakes at subductions zones.  More than 80% of the world's tsunamis occur in the Pacific along its Ring of Fire subduction zones. subduction_zone_gen_big

When a great earthquake ruptures, the faulting can cause vertical slip that is large enough to disturb the overlying ocean, thus generating a tsunami that will travel outwards in all directions.

TsuGeneration_EQ2

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