Learning about tsunamis How a tsunami occurs
A tsunami is created when the seafloor moves due to an earthquake
When an earthquake occurs offshore, the seafloor near its hypocenter rises and falls. This creates movement through the entire water column, which generates a large wave in the surrounding area. This is what is known as a tsunami.
In normal waves driven by the wind, only the water near the surface of the sea is in motion.
In a tsunami, however, the entire water column—which is to say, the water from the seafloor right up to the surface—is displaced over a large area. The wave that approaches the land is like a wall of water, making the destructive power of a tsunami immensely different from that of normal waves.
The rise and fall of the seafloor occurs at tectonic plate boundaries
When an oceanic and a continental plate converge, the oceanic plate subducts (sinks) and drags the leading edge of the continental plate with it. Large amounts of elastic energy build up in the continental plate, as if it wants to spring back to its original position. When this build-up reaches its limit, the continental plate suddenly springs back, producing what is known as a megathrust earthquake, or kaikogata jishin in Japanese.
Meanwhile, local earthquakes, officially termed epicentral earthquakes, occur when the land slips along active fault lines. The 2011 off the Pacific Coast of Tohoku Earthquake was a subduction zone earthquake, whereas the 1995 Great Hanshin Earthquake (Kobe Earthquake) and the 2016 Kumamoto Earthquake were epicentral earthquakes.
The Japanese archipelago is surrounded by four tectonic plates
The earth’s surface is covered in tectonic plates whose thickness ranges from about 15 kilometers to up to 200 kilometers. New plates are constantly forming under the sea to create the seabed, and they push their way toward continental plates.
Earthquakes occurs when tectonic plates collide with each other.
Miyagi Prefecture lies in the place where the Pacific Plate is slipping under the North American Plate. The Great East Japan Earthquake occurred due to a rupture along a wide area of this plate boundary.
Tsunamis are terrifying What distinguishes them from other waves?
“I had thought that a riverbed or seabed becomes visible when a tsunami is on its way, but I was mistaken”
Some people think that the tide always recedes before a tsunami is on its way. But this isn’t true.
Depending on the way the earthquake occurred and the spatial relationship between the coast and the place the tsunami started, a large tsunami can strike the coast without any warning. The tsunami associated with the 2003 Hokkaido Earthquake and the tsunami that struck the coasts of Sri Lanka and India after the 2004 Indian Ocean Earthquake were both large-scale tsunamis that struck without the sea receding in the moments before.
“The tsunami swept in, in overlapping layers of waves”
A tsunami is more than one wave. It’s a series of waves. A tsunami spreads out in all directions from the epicenter of the earthquake. Some tsunamis strike the coast after rebounding off ridges on the seafloor or shorelines in faraway places. Multiple waves can overlap, and the second or third waves can be higher than the first. Also, a tsunami carries many drifting objects that can collide with people and cause injury.
“I didn't expect the tsunami to approach from the river” “Suddenly, there was black water everywhere”
Tsunamis can travel up rivers. The tsunami that came with the Great East Japan Earthquake traveled up the Abukuma, Natori, Nanakita, and Kitakami Rivers in Miyagi Prefecture, for example. At the Kitakami River, it traveled 50 kilometers upstream from the river’s mouth. There are also cases where the tsunami inundated the land from the coast while also traveling up a river and and overtopping its banks. The people living in the basin were flooded from two directions.
“We’ve had so many tsunami warnings in the past”
“Most of them were false alarms,
so I thought this would be the same, so I didn’t feel any urgency about evacuating.”
A tsunami can strike even if the area you’re in has not been hit by an earthquake. In the 1960 Great Chilean Earthquake, which happened on the far side of the Pacific Ocean, a six-meter-high tsunami struck the coast of Japan. Another example is the 1896 Sanriku Earthquake and tsunami, which caused a giant tsunami even though the tremors were slight, only a seismic intensity level of 2 or 3 on the Japanese seismic scale.
“After an eerie silence, I saw a black wall of water rushing toward me” “The world looked completely different”
The height of a tsunami can change hugely depending on the landforms along the coast, like headlands or islands. Distinctive topographical features can affect it too—things like the shape of the tip of a headland or the inner part of a V-shaped bay. There are many bays like this along the ria coast of the Sanriku area.
Onagawa Town in Miyagi Prefecture is an example of a bay where the wave grew extremely high as it traveled up the bay. It rose to 34.7 meters.
At Kesennuma City’s Koyo High School, located near Cape Iwaisaki, the tsunami reached the fourth floor of the school building.
Meanwhile, on Oshima Island in Kesennuma, the tsunami was almost 20 meters high. The wave merged in the central part of the island, inundating the lowlands so completely that the elevated areas of Oshima appeared to be three separate islands.
“It all happened so suddenly, before I could even figure out what was going on”
“In a fraction of a second, I had water up to my shoulders”
A tsunami moves insanely fast. You can’t watch it coming before trying to escape. There’s simply not enough time. The wave gets taller and taller very quickly as it approaches the coast.
A tsunami even a little over two meters in height is able to wash away a wooden house. Even with a tsunami that is a mere 20 centimeters in height, there is a risk of being washed some 500 meters toward the ocean should one’s feet get caught in the drawback.
