What is a tsunami - and what are the tell-tale signs?

Yahoo

14-05-2025

Greek authorities issued a temporary tsunami warning this week after a earthquake measuring 5.9 on the richter scale struck off the island of Crete.
The country's Ministry for Climate Crisis and Civil Protection posted on the 112 Greece account: "A magnitude 5.9 earthquake occurred 48km southeast of Kasos. Risk of possible Tsunami in your area. Move away from the coast immediately. Follow the instructions of Local Authorities."
The quake - which was felt as far away as Egypt - happened at a depth of 22 miles, near the island of Karpathos in the Mediterranean Sea, according to the Athens Geodynamic Institute. The warning was issued in the early hours of May 14, and no tsunami was recorded.
Greece is among Europe's most earthquake-prone countries, and is located on a number of fault lines. Earlier this year, an unprecedented level of seismic activity shook the holiday island of Santorini for weeks.
Tsunamis are giant waves caused by earthquakes or volcanic eruptions under the sea, and can also be triggered by large rock slides or meteorite impacts.
The name 'tsunami' is a Japanese word meaning 'harbour wave'.
Tsunamis can travel at hundreds of miles an hour, as fast as a jet aircraft, and are relatively small as they travel over the ocean, only losing speed and gaining height as they approach land.
Tsunami waves can be hundreds of feet high, according to the British Geological Society.
The British Geological Survey says: 'The wave only becomes dangerous when it approaches land because, due to bottom friction with the seabed, the speed decreases. This causes the wavelength to decrease too and, as it does so, the wave's elevation increases.
"As it strikes land it can be tens, if not hundreds, of metres high. With these elevations the wave can be massively destructive.'
Unlike earthquakes, experts tend not to use a standard 'scale' to warn of the size of a tsunami, instead looking at the location, magnitude and depth of the earthquake that caused the wave.
Scales such as the Papadopoulos-Imamura tsunami intensity scale measure tsunamis, but tend to be less widely used than, for example, the Richter scale for earthquakes.
Authorities issue warnings if tsunamis are likely to occur, based on recent earthquakes, but there are also 'natural tsunami warnings'.
The US National Weather Service says that if you experience any one of the following warning signs, it could be a sign that a tsunami is incoming.
The Weather Service says, 'Natural tsunami warnings include strong or long earthquakes, a loud roar (like a train or an airplane) from the ocean, and unusual ocean behaviour.
'The ocean could look like a fast-rising flood or a wall of water. Or, it could drain away suddenly, showing the ocean floor, reefs and fish like a very low, low tide.'
Tsunami warnings are typically issued at the earliest moment, based on seismic information (i.e. at the moment when an earthquake that might trigger a tsunami is detected).
The warning alerts the public that a tsunami may be on its way, and alert local emergency management officials to take measures such as evacuating areas or moving ships to deeper waters.
Organisations such as UN's the Intergovernmental Oceanographic Commission (IOC) work with governments to coordinate tsunami warnings.
Most tsunamis (around 80% of them) happen within the geologically active area in the Pacific Ocean known as the 'ring of fire'.
But tsunamis are not unknown in Britain, according to the British Geological Survey.
The BGS writes, 'Here, 8200 years ago off Norway, a massive submarine landslide generated a tsunami that was up to 30m high when it flooded northern Britain. This tsunami is best recorded in Shetland by sediments deposited as the land was flooded.
'In southern Britain, in the Dover Straits in 1580, it is believed that a cliff fall of the Chalk cliffs north of Dover caused a tsunami that flooded the coast of France.'
The IOC and UNESCO advise that people should get to higher ground or move inland if there is a tsunami warning, or signs of a tsunami.
People are advised to head to the third storey of a concrete and reinforced building if they cannot get to higher ground, or climb a strong tree if a building is not available.
If that fails, people are advised to look for something to use as a raft.
The IOC and UNESCO advise that people should stay out of danger zones until an all-clear is issued as tsunamis can last for hours.
Areas are at greater risk if they are less than 7 metres (23 feet) above sea level and within 2km (1.2 miles) of the coastline.
The first tsunami wave is often not the largest, the IOC/UNESCO advise, and waves can move several hundred feet inland, travelling up rivers and streams and 'wrapping round' islands to impact coasts not facing the wave.
The biggest tsunami ever recorded happened in Lituya Bay, Alaska in 1958 after an earthquake triggered a rock slide.
An enormous mass of rock measuring 2,400 feet by 3,000 feet and estimated to weigh as much as 90 million tons crashed into Gilbert Inlet, unleashing a huge tsunami wave.
The landslide generated what remains the tallest wave ever documented, reaching heights of up to 1,719 feet within the enclosed bay.
The 2004 Indian Ocean tsunami remains the deadliest natural disaster of the 21st century and the worst tsunami disaster in history.
Arriving on Boxing Day, an undersea earthquake triggered waves of up to 100ft high and killed an estimated 227,898 people.