What is a tsunami and how are they monitored?

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SHAFAQNA- A dangerous tsunami is a series of waves that can be triggered by any event that disrupts the sea floor in some way.

They are usually caused by underwater earthquakes, but can also be caused by underwater landslides, nuclear weapons testing and even asteroids falling into the sea.

So why do tsunamis occur, and how does the monitoring system work?

What causes a tsunami?

Tsunamis can occur when there is a sudden movement or change within the ocean.

The majority are caused by underwater earthquakes, but there are a number of other events capable of triggering large waves, according to James Goff, Professor of Tsunami Science at the University of New South Wales.

An underwater volcanic eruption is one such event. In 1883, the Krakatau eruption generated a wave 30 metres high.

Landslides, both under and above water, are also “hugely under recognised” triggers, Professor Goff said.

“The largest historic tsunami happened in 1958 when a landslide fell into a fjord in Lituya Bay in Alaska,” he said.

“That one generated a wave 30 metres high.”

Do tsunamis happen every time there is an underwater earthquake?

When a serious tsunami occurs the culprit is usually an underwater earthquake, but not all underwater earthquakes generate tsunamis.

The main culprits are shallow (not deep below the seafloor, within a few kilometres) and large magnitude (greater than at least magnitude 6.5) earthquakes that happen along a subduction zone — where the edge of one continental plate slips under another continental plate suddenly pushing it up, said Professor Goff.

Another kind of fault known as a strike-slip fault, where two plates slide horizontally past each other moving the seafloor sideways, can also generate a tsunami.

“This can shift underwater hills and mountains that shift water sideways … however this is less common,” Professor Goff said.

Subduction zones tend to be in deeper water, and up to a point, the deeper the water the more significant the event.

“You’re essentially moving a lot more water if it happens at a depth of 5 kilometres than if it happens in 100 metres,” Professor Goff said.

“If you have an uplift of 20 metres in 5 kilometres of water you’re displacing kilometres of water, and that energy, moving that amount of water, is what is going to generate the tsunami at some later point as it comes on land.”

An earthquake generated in a subduction zone will head in both directions, mainly at right angles to the fault line that created it.

“After a big underwater event in deep water, say about 5 kilometres, the wave is probably moving at around 800 kilometres an hour, [or] about the speed of a jet,” Professor Goff said.

“The wave settles quickly [after the original upheaval of water] and may not be very big out in the deep ocean, maybe only 50 centimetres.

“But when it gets closer to land, the shallow water slows it down and the waves behind begin to catch it up. Then the wave gets higher and bigger and slower.

“When it reaches land it will have slowed to anywhere between probably 5 and 40 kilometres an hour, but you’d have to be an Olympic athlete to outrun it at those speeds over several kilometres, even if it is slowing down”.

What does a tsunami look like as it approaches land?

When a tsunami reaches land it doesn’t look like a regular wave.

“They were originally called ‘tidal waves’ which was a horrible misnomer in some ways, but in other ways it acts a bit like that. In other words, the tide rises very, very quickly,” Professor Goff said.

As the water slows down near the shore, the entire sea level lifts up as water being pushed in from behind bunches up.

“And it’s a broken wave. You never get this towering, deep impact, Armageddon-type of thing that Hollywood gives you.

“It’s basically a broken mass of roiling wave coming at you.”

And it is not just one wave, but a series of waves separated by hours.

Unlike storm waves, which crash onto shore releasing their energy every few tens of seconds, the crests of tsunami waves can be hundreds of kilometres apart.

“Invariably the first wave is never the biggest. It’s usually the second, third or fourth and tsunami waves can keep inundating the coast hours after the first one has arrived,” said Professor Goff.

How big does a tsunami have to be to cause damage?

Tsunamis don’t need to be that big to cause damage.

“Obviously tsunamis are much more impressive when they are about 3 or 4 metres high, but you just need to start inundating the land,” Professor Goff said.

Low-lying coastlines, river mouths and flood plains are susceptible to inundation.

“A lot of Japanese coastline is very low lying, hence they have seawalls and that kind of thing,” he said.

Once a tsunami hits land it only needs to be around 90 centimetres to be dangerous.

“Even if it’s a small wave, the energy is huge. You won’t be able to stand up in a wave of 90 centimetres high,” Professor Goff said.

A tsunami doesn’t even need to inundate land to cause chaos, he added.

“You can imagine boats being moved around in harbours smashing into each other, they can smash into fuel tanks and catch fire.”

How are tsunamis monitored?

Following devastating Pacific-wide tsunamis in 1946 and 1960, a Pacific tsunami warning system was set up in Hawaii.

Now, similar warning systems operate in Alaska, the Caribbean and the Indian Ocean.

“When any event like this happens, these sensors all around the world instantaneously pick up a signal,” Professor Goff said.

In addition to earthquake sensors, ‘dart buoys’ or tsunami buoys float in the deep ocean in different places around the world.

“They are specially set up to detect and record the passing of a tsunami wave,” Professor Goff said.

“As soon as the wave passes that point, you know how long it’s taken to get from the source and how big the wave is at that point, so you have some idea of what’s coming.

“Islands have tide gauges too, which help track a wave across the Pacific.”

All that information is fired back automatically and processed through sophisticated computer software which works out, based on all the information coming in, where the earthquake happened, how far below the sea it occurred and what size the tsunami is likely to be.

When is a tsunami warning triggered?

Tsunamis happen often but many are very small. Certain criteria decide whether a warning will go out.

“Generally if an earthquake is over 6.5 and is happening at between 0 and 5 kilometres depth beneath the seafloor, they’ll send out a tsunami warning,” Professor Goff said.

That warning, he said, could go out within three to five minutes of the undersea earthquake and can give an early indication of its potential to cause a tsunami that may do damage.

Warnings go out to all countries in the area likely to be affected, but also to other warning systems around the world.

Once a warning comes in, it’s up to individual countries as how they handle the data, and warnings will change upward or downward as more data comes in and a better picture forms of the approaching tsunami.

How much warning do you get to evacuate?

Professor Goff said that depending on a country’s proximity to a subduction zone, there is a ‘window of death’ in which it has to decide how seriously to take the approaching threat and what to do.

“Australia is lucky as we’re a reasonable distance away from subduction zones so we generally have at least an hour or so to make a decision, but for some local sources, countries like New Zealand have much less than an hour,” he said.

“Indonesia [has] maybe half an hour and Chile [has] between 15 minutes and half an hour.”

Decisions have to be made very quickly about whether an area needs to be evacuated.

“These are decisions which have phenomenal implications. If you get it wrong and evacuate, say, the whole eastern seaboard of Australia, it will cost millions,” Professor Goff said.

To make decisions easier, countries work from modelling based on previous similar sized earthquakes in similar conditions to decide how severe the tsunami may be.

However, in the case of the Japanese 2011 tsunami, Professor Goff said the tsunami was much bigger than expected because such large events had not been modelled and were not expected.

“Japan is the most tsunami-aware and prepared country in the world but they never thought 2011 could be as bad as it was,” he said.

“Japan’s previous biggest event in the same region was the 869 AD Jogan Tsunami, which had been studied by lots of geologists.

“But for various reasons they underestimated the size of that event and so ran models for a smaller earthquake.

By Annie Hastwell and Genelle Weule

http://www.abc.net.au/

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