Lecture 3 - Tsunamis Tsunami is Japanese for harbour wave...

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Tsunamis T sunami is Japanese for “harbour wave”. They are produced by the sudden displacement of water. Events capable of triggering tsunamis: -earthquakes that cause uplift of the seafloor -landslides -volcano flank collapse -meteorites
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Historic Tsunamis Dat e Cause Approximate Human Casualties 1755 Lisbon Earthquake (M 9.0), Portugal 20,000 1883 Krakatoa Volcanic Eruption, Indonesia 36 000 2004 Sumatra Earthquake (M 9.1), Indonesia 230,000 2011 Tohoku Earthquake (M9.0), Japan 15 000
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Earthquake-Triggered Tsunamis Earthquakes can cause tsunamis in two ways: -by displacement of the seafloor -by triggering a landslide that enters water Generally, an earthquake must be of at least M7.5 in order to trigger a tsunami. Tsunamis develop in a 4 stage process.
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Earthquake-Triggered Tsunami Development Stage 1: Displacement of the seafloor sets waves in motion that transmit energy upward and outward. When the waves reach the surface of the water, they spread outward.
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Earthquake-Triggered Tsunami Development Stage 2: The waves move rapidly across the open ocean (they can reach speeds of over 500 km/h). The spacing of the wave crests is very large (it can be more than 100 km) The height (amplitude) of the waves is often small (less than 1 m). Passengers on ships in the ocean rarely even notice tsunamis passing beneath them
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Earthquake-Triggered Tsunami Development Stage 3: As the tsunami approaches land, the water depth decreases. This results in the water ‘piling up’ and causes these effects: -decrease in wave speed -decrease in spacing of the waves -increase in wave amplitude
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Earthquake-Triggered Tsunami Development Stage 4: As the tsunami impacts land, waves can reach heights of dozens of meters. The wave speed at this time can be up to 50 km/h making them impossible to outrun. During some tsunamis, the water first recedes from the shore and exposes the seafloor.
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Earthquake-Triggered Tsunami Development
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Tsunami Event A tsunami event consists of a series of large waves reaching shore that can last for several hours. Run-up: The maximum horizontal and vertical distances that the largest wave of a tsunami reaches as it travels inland. The run-up essentially describes the geographic area impacted by a tsunami.
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Types of Tsunamis Distant tsunami: A tsunami that travels thousands of kilometres across the open ocean. On remote shorelines across the ocean, reduced energy lessens the impact. They are also called tele-tsunamis. Local tsunami: A tsunami that affects shorelines a few km to about 100 km from its source. Because of this short distance, local tsunamis provide little warning.
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Regions at Risk Coasts located near subduction zones or across oceans from subduction zones are most at risk.
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