How the Japanese tsunami travelled
ONE OF THE BIGGEST recorded earthquakes in history has caused one of the most devastating tsunamis in Japan. Measuring 8.9 on the Richter scale, the offshore earthquake struck at a depth of 24.4 km and just 130 km off the coast of Japan near the worst-hit city of Sendai.
The resulting shock generated a tsunami – a series of monster waves that have inundated Japan’s east coast, up to 5 km inland. Tsunamis are so common in Japan, that they created the word: from ‘tsu’ meaning harbor and ‘nami’ meaning wave.
Earthquakes that occur under that sea that are larger than magnitude 7.0 and less than 30 km deep can generate tsunamis, which are larger when there is a substantive vertical displacement of two plates.
The islands of Japan sit on the boundary of the Pacific tectonic plate, which is being pushed under the Eurasian plate. It is a very active tectonic area of what’s known as the Pacific ‘Ring of Fire’. There have been seven earthquakes in Japan over magnitude 8 since 1891. In 1923 the great Kanto earthquake, which measured 7.9, killed 147,000 people.
Tsunami the speed of a jet plane
The massive amount of energy surges out into the ocean, like ripples in a pond. These tsunami waves can travel at the speed of a jet plane – about 800 km/h in deep, open water. Because the open ocean is so vast, the energy is spread across long distances – a wave can span hundreds of kilometres.
Tsunami wave heights in the open ocean are only a few centimetres – and up to 30 cm – which is why they are largely undetectable out at sea. However, as the wave train approaches shallow waters off the shore, the landforms concentrate the energy, transferring it upward and increasing the height of the waves. The tsunami waves that hit Japan had reported heights of 10 m.
This tsunami map shows how the wave train from the Japan earthquake travelled the globe. (NOAA)
Tsunami gave little warning
“Japan has very advanced tsunami warning systems, but the warning time was short, 10-15 minutes, since the earthquake was close to the coast, reducing evacuation times,” says Dr Tom Baldock, a coastal engineer at the University of Queensland.
“The earthquake was close to a region where the coastline is very low-lying, with a small coastal dune protecting even lower areas behind. Therefore, once the tsunami wave and water levels exceeds the dune height, the water flows over [it], much like it does over a weir or causeway.
“The wave front propagates inland quickly, picking up debris which becomes a major hazard in addition to the force from the water. The wave can also travel through harbours and then inland by overflowing canal and river banks. This appears to be the way the tsunami impacted on the coast around Sendai airport.”
The worst tsunami in recorded history was the December 26, 2004 one that killed about 230,000 people. It was generated by a magnitude 9.3 earthquake off the north-western coast of Indonesia.
The earthquake comes just over two weeks after the 6.9 magnitude earthquake that devastated Christchurch in New Zealand.