TsunamiThe Great WaveChapter 5What 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.How a Tsunami FormsWhen 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.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.Earthquake Generated TsunamiMost tsunami are generated during shallow focus underwater earthquakes associated with sudden rise or fall of the seafloor, most commonly along subduction zones.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 HeightTsunami 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 EffectsRun 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.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.Chile Tsunami - May 1960On 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.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. The 2004 tsunami was the deadliest in recorded history.Tsunami HazardsDrowning 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.The Relevance of Hazard Prediction and Mitigation for the 2004 TsunamiA 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.Tsunami from Great Earthquakes in the Pacific NorthwestConvergence 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.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.How a Tsunami FormsVolcano Generated TsunamiTsunamis can be caused by volcanic processes that displace large volumes of water includingVolcanic earthquakes,undersea eruptions,pyroclastic flows,caldera collapse,landslides, laharsKrakatau Volcano, 1883Landslide and Rockfall Generated TsunamiWhen major fast-moving rockfalls or landslides enter the ocean, they can displace large amounts of water and generate tsunami.Lituya Bay, Alaska, 1958Tsunami from Volcano Flank Collapse and Submarine LandslidesThe 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. None have happened in historic time.Tsunami from Asteroid ImpactThe 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.TsunamiThe Great WaveChapter 5What 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.How a Tsunami FormsWhen 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. 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.Earthquake Generated
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