The meteorological department said a huge 7.5-magnitude earthquake struck eastern Indonesia last week, issuing a tsunami warning, although there was no immediate information on any injuries or damage. It was later found out that the quake had a magnitude of 7.3, according to the US Geological Survey.
Tsunami (pron: 'soo-nar-me' pron.') is a Japanese term that combines the words 'tsu' (harbour) and 'nami' (wave). Tsunamis are waves that occur when the ocean surface moves suddenly owing to earthquakes, landslides on the seafloor, land collapsing into the water, massive volcanic eruptions, or a meteorite impact in the ocean.
Tsunamis were once known as tidal waves, but this name is no longer widely used because tsunamis have nothing to do with tides (which are driven by the gravity of the Earth, Moon and Sun). Although some tsunamis resemble a fast rising or falling tide at the coast, they can also include one or more violent breaking waves in other scenarios.
They do, in fact. Tsunamis can occur when the seafloor deforms suddenly and vertically displaces the water above it. Tectonic earthquakes are a type of earthquake related to the deformation of the earth's crust; when they happen beneath the sea, the water above the deformed area is pushed from its equilibrium position.
As the displaced water mass, which acts under the pull of gravity, tries to recover equilibrium, waves emerge. A tsunami occurs when significant portions of the seafloor rise or fall.
At plate borders, large vertical motions of the earth's crust can occur. Plates collide at fault lines, which are known as faults. Denser oceanic plates, for example, slip under continental plates towards the Pacific Ocean's borders in a process called subduction. Tsunamis are very effective at being generated by subduction earthquakes.
The majority of tsunamis are caused by shallow, large earthquakes near subduction zones. The Pacific Ring of Fire subduction zones account for more than 80% of the world's tsunamis. When a significant earthquake occurs, the process of faulting can create vertical slip, substantial enough to upset the overlying water, resulting in a tsunami that travels in all directions.
Based on past data and processes of the Pacific Tsunami Warning Centre, the following are basic criteria for when earthquake magnitude can produce tsunamis.
Earthquakes of this magnitude are very unlikely to trigger a tsunami.
Tsunamis from earthquakes of this magnitude are unusually devastating. Small variations in sea level may be seen in the area around the epicentre, though. In this magnitude range, tsunamis capable of causing damage or casualties are uncommon, but they have happened owing to secondary impacts such as landslides or underwater slumps.
Tsunamis could be generated by earthquakes of this magnitude, especially near the epicentre. Small sea level fluctuations may be visible at considerable distances. Tsunamis that can cause damage over long distances are uncommon in the magnitude range.
Near the epicentre, destructive local tsunamis are probable, as are severe sea level changes and destruction across a larger region. It's worth noting that after a magnitude 9.0 earthquake, an aftershock of magnitude 7.5 or more is possible.
Although the magnitude of an earthquake is a significant component in tsunami creation, that is not to say that there are no other important factors to consider.
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