Discussion»Statements»Rosie's Corner» Do all earthquakes of a certain magnitude generate TSUNAMIS if they occur close enough to ocean waters? If not why not?
I'm glad I have bought my own home. I 'd hate to have to rent from a landlady capable of generating a tsunami! :-)
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[Also from by subduction displacement, possibly the more common cause; with the earthquake being the jolt transmitted through the rock, of that movement.]
ANY magnitude Shuhak? Oh dear. So anyplace at any time it's possible? Yikes. Thank you for your reply and Happy Tuesday to thee and thine! :) Have we ever had one? If so I don't recall it!
A tsunami of that type is generated by an earthquake below the Continental Shelf.
(They can also result from massive mud-slides down the Continental Slope, the edge of the Shelf, but more rarely. Also from under-sea, explosive volcanic eruptions but these are very rare, with Krakatau, in the late 19c, as the last major one.)
Simplified somewhat..... This how it all works in the majority of cases.
Plate Tectonics.
The Earth's rock surface is broken into huge "Plates" floating on the molten but extremely viscous Mantle below.
Those forming the ocean floors are the Mantle's solidified surface, like ice on a pond. Only they are also being added to by lava from the Mantle oozing up through the fissures between the plates, pushing them apart and creating huge ridges along the sea floor.
The Continents are much thicker plates also floating on the Mantle, and surrounded by the ocean floor. The continent's true shape is not the familiar coastline, but the outline out at sea of a nearly flat coastal ledge, the Continental Shelf, submerged to relatively shallow depth. The edge of the Shelf gives way to the steeper Continental Slope down to the ocean floor. Atlas maps that indicate sea depths by shades of blue, depict the Shelf as the irregular, distinct band of light blue all along the coast.
As the continents continue their slow Drift around the globe, driven by convection currents in the Mantle that is also widening parts of the ocean floor, the ocean floors have to accommodate this. The only option is what is called 'Subduction' (= 'carried below'), wherein that part of the sea-floor is forced to bend downwards and into the Mantle below the Continent, creating in the process a very deep trench along the foot of the Slope.
You asked a short while ago about the Marianas Trench and its deepest part, the Challenger Deep. This is a subduction trench.
Now, umpteen miles thick rock slabs are not going to slide on each other without a fight despite the gigantic forces at play here. Although lubricated by the water and wet sea-bed ooze trapped between the two plates, their motion is one of stick-slip. They stick until the accumulated stress is enough to jerk the Ocean Floor plate forwards and downwards to its eventual doom of melting back into the Mantle.
That sudden jolt is what we feel as the Earthquake.
The centre of the slip is called the Focus of the earthquake; and the point of maximum effect on the surface above is its Epicentre.
The 'quake can also be caused by a similar stick-slip movement of rock against rock, but this time onshore, as the Continental Drift stresses are relieved by sudden movements on Faults (fractures) within the plates themselves.
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So to Tsunami - its Japanese name is the ancient "ordinary" language term in that country due to it suffering so many, and is the official, international geological term for such waves.
A large tsunami comes from the sudden movement of the sea-floor: as the rock is displaced it displaces the water above it. So it needs its own epicentre to be under the sea. An earthquake on land will not create a tsunami.
The wave races across the ocean as a long, low swell; and is harmless there. It is only when it approaches a shore that it becomes so dangerous. As it enters shallow water its roots drag on the sea-bed. This impedance converts it from a true wave in which each particle of water or a boat on it simply moves in a small circle without travelling anywhere; to the destructive, tumbling mass of water propelled forwards with great force, overwhelming everything it can.
''''' So to answer your question, yes an earthquake can trigger a tsunami, but has to be more than "close to" the ocean. It has to be under it!
As for wave amplitude and power, that is a function of the earthquake's own displacement.
One of the best sea-defences against a tsunami is all natural, a long stretch of wide mangrove swamps... but that's not prevented developers being allowed to destroy them to build tourist resorts in some of the Asian countries most at risk from these waves.
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Nature's not finished with us yet. Pushing sea-floor rock under the continent also leads to the sort of explosive, very destructive volcanic eruptions associated with this Plate Tectonics process.
Oh, how fast do continents drift? It varies a lot around the world but the Atlantic is widening at about 25mm / one-inch a year, mean speed.
I feel as if I just attended an advanced class in earthquakeology Durdle. Thank you for the very comprehensive and clearly stated reply. I did not know a lot of what you shared but mostly I did not know that Tsunamis can only be generated by earthquakes in the ocean. So an earthquake along the San Andreas Fault in California can't possibly trigger a Tsunami. Have you ever experienced an earthquake in your hometown? Are there areas of the world more prone to them? Since the earth is covered with so much water odds are there will be a lot of them. Oh. Has there ever been earthquakes beneath rivers or is that only beneath oceans? What about earthquakes beneath lakes? Happy Tuesday to thee and thine m'dear! :) I'm honored that you remember my fascination with the Marianas Trench. 7 Miles deep...the deepest part of anywhere on earth. Going down that far in a bathysphere or whatever would be amazing but I get claustrophobic and the thought that much water overhead scares me just thinking about it. What is the pressure that deep?
This post was edited by RosieG at August 17, 2021 1:33 AM MDT
The science is Geology - that specifically about earthquakes is Seismology.
Major earthquakes of the type I described are limited to specific areas of active Plate Tectonics, but that includes most of the Pacific coastlines, to a lesser extent the Mediterranean and Middle East; and Himalaya to the North of India.
The British mainland experiences a surprisingly large number of earthquakes. I read somewhere we have more than almost anyone else! They are though very mild, rarely causing any damage, and most are only really detected on seismographs, which record the time, duration and nature of the vibrations.
Some are from old mine working collapsing, but the natural ones are stress-relieving slips on faults in the NW European Continental Plate on which the British Isles rest.
I wonder if some of the stresses are from pressure of the widening Atlantic's floor as the continent drifts North-eastwards. Some are thought to be lingering traces of adjustment to the loss of weight of ice-sheet in the last glaciation, tilting the island very slowly: SE England sinking, NW Scotland rising.
Though the British Isles are that - islands - they are on the same slab of Crust as France, Germany, Scandinavia etc. The North and Irish Seas and the English Channel are just low bits, under the sea.
I've not experienced an earthquake but one small tremor was detected only a few miles from me. I forget the year it happened.
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Earthquakes can happen well inland, because the Crustal Plates are not single entities under the entire continent. They are fractured (the faults); but also the present continents are made up of fragments of much older ones that split apart and re-combined in different ways over very many millions of years.
So essentially the planet's surface, land and under-sea, is an irregular crazy-paving of slabs of rock continually shuffling around. The sea-floor slabs are thinner than the continents, and of a different rock, but still involved in this stately waltzing around; growing, fracturing, subducting and re-melting....
The Indian sub-continent positively raced North at rates calculated as up to a metre a year; then it hit Southern Asia and burrowed its way into that continent, pushing a huge chunk of it upwards to create the Himalayan Mountains. It has not stopped, either. The mountains are still rising, very slowly; and the awful earthquake suffered by Nepal a few years ago was an effect of this active example of Continental Collision.
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An earthquake under a river or lake is not likely to create a tsunami but the movements responsible may have other effects, such as modifying the levels to divert the flow or create a barrier behind which a lake will form. I may be wrong but believe this happened on the Mississippi, in the 19C.
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I'm not sure I'd want to take a trip to the floor of the Challenger Deep, either. You would not see much anyway. It is completely dark well before that depth, so what is visible is only within the range of the craft's lamps. They don't penetrate very far because the deep ocean water is foggy with particles sinking very slowly to the sea-bed. I think the oceanographers call it "snow" but it is mainly fish droppings from the more populous water high above! There is some animal life even down there though, as small worms etc., and micro-organisms.
The craft used on the first descent was called a "bathyscaphe", a sort of mini-submarine floating independently from the survey-ship; but without motors and propellors. A bathysphere was used in much shallower water as it was just a steel sphere with an access hatch and small portholes, lowered by cable from the ship.
Modern sea-bed studies still use manned submersibles but many are carried out by remotely-controlled craft fitted with cameras, and operated by cables from the ship. It avoids having to accommodate people; and if disaster strikes you might lose a very expensive piece of equipment, but not kill anyone.
The name "Challenger" honours the British ship, HMS Challenger, that discovered the Mariana Trench, measuring depths down to 26850 feet with a weighted sounding-rope, in 1875. The first descent was by Jacques Pickard (Swiss) and Lieutenant Don Walsh (US Navy), in 1960; into the deepest part. They reported a "fish" living on the sea-bed, now thought to be another animal, a sea-cucumber.
Pressure down there?
Maximum depth = 10 929 metres.
At 1 bar (= 1 standard sea-level atmospheric pressure) per 10m depth, that gives a water pressure of
10 929 / 10 = 1093 Bar.
In Imperial units: 35 856 feet depth (about 7 miles), and 15 853 lbs/square-inch.
I think you use 1Ton = 2000lbs; the UK Ton = 2240lbs.
So that pressure is either 7.93 or 7.08 Ton/sq. in; respectively, depending on the definition of the Ton.
Shuhak 