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Recognizing a Tsunami
Tsunami is Japanese for "harbor wave" and describes the series of waves commonly caused by an earthquake or seismic event.
What does a tsunami look like when it arrives at the coast? Look at these pictures and decide which are photos of tsunamis and which are photos of other types of waves.
Question 1
Is this a tsunami? (Choose the best answer.)
The correct answer is b).
No. This is a large breaking wave that's a part of regular ocean swell. Although tsunamis sometimes create large breaking waves, more often they appear in other forms.
Question 2
Is this a tsunami? (Choose the best answer.)
The correct answer is a).
Yes. This is an unusual sudden retreat of water from the shore that can accompany tsunamis.
Question 3
Is this a tsunami? (Choose the best answer.)
The correct answer is a).
Yes. This is a strong current pushing a river upstream caused by a tsunami.
Question 4
Is this a tsunami? (Choose the best answer.)
The correct answer is b).
No. This is a regular ocean wave crashing on the rocks. The waves around it show no sign of a tsunami.
Question 5
Is this a tsunami? (Choose the best answer.)
The correct answer is a).
Yes. These surging waves are common in tsunamis.
Question 6
Is this a tsunami? (Choose the best answer.)
The correct answer is b).
No. This is high ocean swell at sea, but not a sign of a tsunami.
Question 7
Is this a tsunami? (Choose the best answer.)
The correct answer is b).
No. These are regular short-wavelength ocean waves. However, a tsunami might look like this at sea, with short wavelength waves rippling on top of it.
Question 8
Is this a tsunami? (Choose the best answer.)
The correct answer is a).
Yes. This effect, in which water rises very rapidly, often occurs in a tsunami.
Question 9
Is this a tsunami? (Choose the best answer.)
The correct answer is a).
Yes. This wave produced by water surging up a river against a current is called a bore, and tsunamis can produce them.
Arrival at Shore
As you could see in the previous exercise, tsunamis don't all look the same when they arrive at shore. Why? It has to do with the shape of tsunami waves. Every wave has a crest and a trough. When the trough comes first, the sea level gets lower, so it withdraws from the shore.
Suddenly beach is visible where there was water before. Stranded fish flop around and can lure reckless beachgoers closer to danger.
When the crest comes first, the water can appear like a swiftly rising tide or like a single or series of surges of water. In either case, the volume of water that follows can far exceed anything produced by tides—remember that because of a tsunami's long wavelength, the wave can be hundreds of kilometers from crest to crest.
Only occasionally does a tsunami arrive like a massive wall of curling water—the way most people think of tsunamis.
A tsunami is usually not just one wave. Tsunamis arrive as a series of waves at unpredictable intervals, making the coastline dangerous for many hours. And—importantly—the first wave is not necessarily the biggest.
Tsunamis sometimes have other odd appearances. They can produce standing waves, a phenomenon often seen in rivers, where the crest actually remains in one place while the water runs through it.
Whirlpools may also suddenly appear when the front of the tsunami turns and retreats while the rest is still coming in.
Do an Internet search for "japan tsunami whirlpool" to view the amazing whirlpool from the March 2011 tsunami.
Inundation and Runup
Inundation refers to the horizontal distance inland that a tsunami reaches. Inundation is usually measured perpendicular to the shoreline and provides an estimate of how far the flooding extended. In the 2011 tsunami in Japan, the inundation measured almost 10 kilometers (6 miles) inland in some areas.
While inundation represents the tsunami's horizontal reach, a tsunami's vertical reach is called runup. Runup is the difference between how high the tsunami reaches on land and the sea level prior to the wave's arrival. Tsunami waves increase in height as they approach shore.
Tsunami damage in Japan following March 2011 earthquake and tsunami. The bottom floors of multistory building have been washed away by waves.
Once the waves meet the shoreline, runups can be very large. For instance, a wave that is 2 m (6.5 ft) high off shore may reach two to five times that height on shore.
Successive waves can push the water even higher and farther.
Topography refers to the shape and features of the land. Once waves wash up onto shore, topography helps determine where inundation and runup will be largest. Features such as funnel-shaped bays or steep walled valleys can heighten waves and increase runup.
Retreat
When the wave exhausts its momentum and turns back to sea, it's still capable of destruction. It's now armed and dangerous—a swirling mass of boards, tree limbs, furniture, and cars.
And It may be moving faster on its way back out to sea, especially if it is retreating downhill.
Nature's Warning Signs
Tsunamis can come ashore quickly, so it's important to know the signs and be prepared so that you react immediately. Test your knowledge by correctly identifying the Tsunami Signs listed here.
I know a tsunami might be occuring if...
Summary
A tsunami is a series of waves typically caused by an earthquake or other seismic event below or near the ocean floor.
Incoming tsunamis can have different appearances, but typically they do not look like a towering, curling wall of water.
A tsunami might arrive crest-first as a swiftly rising tide or surge of water, or trough-first, in which the water recedes first, then quickly and suddenly rushes shoreward.
Tsunami waves arrive in unpredictable intervals–and the first wave is not necessarily the biggest.
Tsunamis can travel far inland. This horizontal reach of a tsunami called inundation. The vertical reach of a tsunami is called runup .
Topography refers to the features and shape of the land. These features can funnel the water through valleys, bays, and up rivers, causing extremely high waves.
Retreating waves are at least as dangerous as incoming waves due to their fast movement and accompanying debris.
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