What is a Tsunami? A tsunami is a series of waves (called a "wave train") generated in a body of water by an abrupt disturbance that vertically displaces the water column. Earthquakes, landslides, volcanic eruptions, volcanic flank collapse/submarine landslides, and large asteroid impacts all have the potential to generate a tsunami.
The term tsunami comes from the Japanese language meaning ‘ harbor wave’ .
Tsunamis are also called ‘ seismic sea waves’ .
Tsunamis have been historically referred to as tidal waves .
(Refer to pg. 99)
How a Tsunami Forms (Refer to pg. 99, 114-116)
How a Tsunami Forms When the water is displaced, water from the surrounding rushes in to fill the depression, forming a series of high speed (up to 870km/hr), flat, spread out waves (average wavelength of 360km). In deep water tsunami waves are nearly undetectable. (Refer to pg. 99, 114-116)
How a Tsunami Forms As the leading waves of a tsunami approach a shoreline, friction with the sea floor slows the waves down. This compresses the wavelength and increases the wave height. The waves surge onto shore as a rapidly rising flood of water with great destructive power. (Refer to pg. 99, 114-116)
How a Tsunami Forms Earthquake Generated Tsunami (Refer to pg. 104-105) Most tsunami are generated during shallow focus underwater earthquakes associated with sudden rise or fall of the seafloor, most commonly along subduction zones. Tsunami Animation
Most destructive tsunamis occur in Pacific Ocean. The borders of the Pacific Ocean are dominated by active subduction zones that produce frequent violent earthquakes .
Velocity and Wave Height (Refer to pg. 115-116) 1957 Aleutian Tsunami Tsunami waves in the open ocean are low and far apart but move at velocities of several hundreds of kilometers per hour. They slow and build much higher in shallow water near the coast, especially in coastal bays.
Coastal Effects (Refer to pg. 114-116) Run up is the height to which a tsunami wave rushes up onshore. Driftwood, trees, and the remains of boats, houses and cars are swept up by the incoming wave, and commonly mark the upper limit of tsunami run-up. The first run-up of a tsunami is often not the largest. The inundation can extend inland by 1000 feet (305 m) or more, covering large expanses of land with water & debris. run-up animation Above: Katukurunda, South of Kalutara on the West Coast: Tsunami run-up height of over four meters near the beach. Left: The Indian Ocean coastline near Phuket, Thailand. The changes along the coast from the 2004 tsunami are obvious where the vegetation has been stripped away.
As the wave recedes into the trough before the next wave, the onshore water and its debris flow back offshore. The time between the trough and the next tsunami wave is often more than a half hour. Below: Maximum recession of tsunami waters at Kata Noi Beach, Thailand, before the 3rd, and strongest, tsunami wave. Above: A broad offshore beach is exposed at Kalutara, Sri Lanka as the first wave of the tsunami drains back to the ocean.
Chile Tsunami May 1960 On May 22, 1960 the largest earthquake on record struck the coast of Chile with a Mw of 9.5. The earthquake ruptured along a 1,000 km length of the subduction zone. In Chile, the earthquake and the tsunami that followed took more than 2,000 lives. From Chile the tsunami radiated outward, killing 61 people in Hilo, Hawaii and 122 on the island of Honshu, Japan. Left: Stuck to the subducting plate, the overriding plate gets squeezed. This movement goes on for decades or centuries, slowly building up stress. Right: An earthquake along a subduction zone happens when the leading edge of the overriding plate breaks free and springs seaward, raising the sea floor and the water above it. This uplift starts a tsunami. (Refer to pg. 87-88, 102-104)
Left: This tide gauge record shows the tsunami waves in Hilo, Hawaii, fifteen hours following the Chilean Earthquake. Right: This low lying area of downtown Hilo was destroyed by the Chilean tsunami. Left: The tide gauge at Onagawa, Japan, recorded a dramatic drop in sea level as the Chilean tsunami arrived. (Refer to pg. 102-104, 118) Above: tsunami in Hilo, Hawaii, 1960.
On December 26, 2004 at 07:58:53 local time in the Indian Ocean there was an undersea earthquake; known as the Sumatra-Andaman earthquake As the pictures illustrate, the Indian Plate subducts under the Burma Plate at the Sunda Trench. With the earthquake, an estimated 745 miles of faultline slipped along the subduction zone. The vertical rise of the seafloor by several meters during the earthquake produced the tsunami. South East Asia Tsunami December 26, 2004
Because of the distances involved, the tsunami took anywhere from fifteen minutes to seven hours to reach the various coastlines (Left). As the map shows, the tsunami devastated the shores of Indonesia, Sri Lanka, Thailand, India, Malaysia, Burma, and Bangladesh with waves up to 100 feet. Maldives, Somalia, Kenya and Tanzania were also affected (Right). The total energy of the tsunami waves was about five megatons of TNT (more than twice the total explosive energy used during all of WWII, including the two atomic bombs). NYTimes Animation ASIA'S DEADLY WAVES
The 2004 tsunami was the deadliest in recorded history.
The U.S. Geological Survey records the toll as 283,100 killed, 14,100 missing, and 1,126,900 people displaced. 2004 Tsunami Satellite Images
Drowning in the incoming waves.
Being thrown against solid objects.
Being carried back out to sea in the outgoing wave.
Being hit by debris carried by the wave.
(Refer to pg. 117-118)
The Relevance of Hazard Prediction and Mitigation for the 2004 Tsunami The water line suddenly retreated A simple program of public education and awareness of the potential hazards could have saved many lives. The magnitude of the tsunami disaster could have been mitigated with a proper disaster preparedness plan and a functioning early warning system. Human destruction of coral reefs, coastal mangrove trees, and sand dunes that had formerly protected some coastal areas was believed to be a significant factor in the loss of life and damage. Northern part of Banda Aceh, Sumatra before (above) and after (right) the tsunami
Tsunami from Great Earthquakes in the Pacific Northwest (Refer to pg. 119-122) Above and Right: A sequence of peat, sand, and mud are a geologic record of tsunami. Radiocarbon dating of organics in buried soils along the coast of the Pacific Northwest indicate tsunami with recurrence intervals ranging from 300 to 900 years. Radiocarbon dating places the last of those events around 1700. Above and Left: Convergence of the North American and Juan de Fuca plates causes bulging of the N.A. plate off the coasts of Oregon, Washington, and S. British Columbia. Uplift rates are ~4mm/yr. and eastward transport is ~30mm/yr.
How a Tsunami Forms Volcano Generated Tsunami Krakatau Volcano, 1883 Left: Krakatau Before the 1883 eruption (top) and after (bottom). The most devastating volcanic tsunami recorded was that produced by the eruption of Krakatoa in 1883. It is believed that following earlier eruptions, cold seawater entered the magma chamber and super heated steam built tremendous pressure which, in turn, resulted in the large explosion of the volcano and caldera collapse. Water rushed into the caldera generating huge tsunami waves. These waves destroyed most of the coastal settlements in the Sunda Strait between Java and Sumatra and killed 36,000 people. (Refer to pg. 105, 111)
Tsunamis can be caused by volcanic processes that displace large volumes of water including
How a Tsunami Forms Landslide and Rockfall Generated Tsunami (Refer to pg. 105, 109) When major fast-moving rockfalls or landslides enter the ocean, they can displace large amounts of water and generate tsunami. Lituya Bay, Alaska, 1958 Lituya Bay, located within Glacier Bay National Park, Alaska, was the site of one of the largest tsunamis ever recorded. On July 7, 1958, a magnitude 7.5 earthquake occurred along the Fairweather Fault causing a large landslide of rock and glacial ice into the head of the bay creating a 150m high tsunami wave which swept across the bay. The tsunami sent water as far as 3,600 feet inland (see left and above. Examination of the forested shoreline of Lituya Bay show two much higher trimlines produced by earlier tsunami (above right).
How a Tsunami Forms Tsunami from Volcano Flank Collapse and Submarine Landslides (Refer to pg. 111-114) The ridges that radiate outward from the top of a shield volcano and become the sites of most eruptions break the volcano into three enormous segments. Rapid collapse of one of these segments into the ocean can displace thousands of cubic kilometers of water and generate tsunami hundreds of meters high (left). None have happened in historic time. Scarps of such collapsed segments become giant coastal cliffs (below).
How a Tsunami Forms Tsunami from Asteroid Impact (Refer to pg. 114) The impact of a large asteroid into the ocean would generate large tsunami that would radiate outward from the impact site…CANNONBALL! Scientists have found traces of an asteroid-collision event 3.5BYA that they say would have created a giant tsunami that swept around the Earth several times, inundating everything except the mountains and exterminating all primitive life. First a hot steam of molten rock and water followed by the massively destructive tsunamis would have destroyed most primitive life. After that, years of incredibly cold winters would have conspired to kill nearly everything else.
Mitigation of tsunami destruction involves two main approaches: tsunami warning systems , and inundation maps . Tsunami Hazard Mitigation
International Tsunami Warning System
ITWS includes 31 seismic stations & > 60 tide stations
A world network of seismographs locates the epicenter of major earthquakes.
Pressure sensors pick up subtle pressure changes as tsunami waves pass by and Buoys transmit the data to warning centers via satellite (right).
The ITWS issues watches and warnings to the media and to local, state, national, and international officials and NOAA Weather Radio broadcasts tsunami information directly to the public.
Mapping of potential inundation areas . An inundation map shows areas of potential tsunami flooding, based on elevation and orientation with respect to the open ocean. The figure to the right shows an example of such a map -- the green areas show likely places of inundation.