We are very sad to receive news from Japan's terrible earthquake. Our thoughts are with the Japanese as they try to bring normalcy to their lives. It took less than a minute for hundred of thousands of people to enter a new devastating reality.
We are floating away in the Southern Seas somewhat disconnected from the world. We have e-mail contact as well as a wonderful satellite phone, the morale phone, to keep in touch with loved ones. We get a couple of short daily summaries from the NY TImes and USA Today, so even though we are partially isolated, we do not escape the grip of news like the ones from Japan.
One of my cabin mates is married to a Japanese woman who was in Tokyo during the catastrophe. She and her family are doing well, but it is hard to imagine what they will go through in the coming months. Best of luck for all of them.
As you have heard, the earthquake produced a tsunami whose effects where felt on the coasts of North America (Canada, USA and México). I have been asked through our forum if we felt the effects of the tsunami here at sea. We have not, and there are a couple of reasons for this. I will attempt to describe the basic physics of a tsunami. For that, we need to understand the basics of water waves because a tsunami is a wave that propagates on the surface of the water. Please let me know if these mini lessons are welcome in the journal and if they are too simple or too complicated.
As a wave passes by, water particles at the surface move in a circular way: up, forward, down and backwards, to return pretty much to where the motion began. Next time you are on a beach, either on a lake or the sea, find a bird floating on the water and see that they move just like that: up, forward, down and backwards.
This motion diminishes with depth. Particles just below the surface move in smaller circles than the ones on the surface. The circles are smaller and smaller until we reach a depth level in which there is barely any motion. The depth at which there is barely any motion is roughly 1/6 to 1/7 of the length of the wave (the length of the wave, or wavelength, is the distance between two crests, see drawing below). We call these waves 'Deep water waves'.
Circular motion on a deep water wave. The wavelength ,which is the distance between two crests, is 7 times bigger the depth at which the water moves perceptibly in circles.
When the depth of the water is 6 to 7 times smaller than the length of the wave, the water particles in contact with the sea or lake floor are still moving; they no longer move in circles but in ellipses. The waves change by the interaction with the floor. Oceanographers call these 'shallow water waves' and say that the waves feel the bottom. The waves reduce their speed as the depth decreases. They end up contracting in length, which forces them to grow in the vertical (increase in amplitude). If the depth continues to shallow, the waves keep growing until they can no longer stay vertical and break. These are the breaking waves that you see on the beach that surfers love.
Deep water waves on the left become shallow water waves as the depth decreases. The wavelength is reduced by the contraction of the wave, but the amplitude increases until the wave breaks (right).
How can we know if waves feel the bottom? By comparing their length to their depth. As I said before, if the depth is about 6 or 7 times smaller than the length of the wave, then the wave is a shallow one. Let us take a quick look at what type of waves we find in the oceans in general.
The dominant open ocean waves have a length of 150 m. These waves remain deep water waves until they reach a depth of about 25 meters (25 m = 150m / 6). Most waves on the beach on a day without big swells will feel bottom at a shallower depth. Scuba divers know that on a day with no big swells they need to go below 30 or 40 feet (10 to 13 m) in the Pacific Ocean to avoid wave motions.
Tsunamis are shallow water waves even when the water depth is 5 km (about the average depth for the Pacific Ocean) because they can be 100 km long. They are very long, but have a very small amplitude when found in open waters. Tsunamis, which means 'harbor wave' in japanese, are extremely destructive in coastal areas because their amplitude grows as the depth decreases, like regular ocean waves. The difference is that they contract in length a lot more, therefore their amplitude increases a lot more, reaching tens of meters. How much they increase in amplitude depends on the local bathymetry. They might grow a lot more in a certain bay than another near one.
Tsunamis are shallow water waves. The boat on the left does not feel the tsunami as it passes by because the tsunami is very long with a small amplitude in deeper waters. The amplitude increases in the coast where water is shallower, until it breaks and travels as a wall of water.
There are two reasons why we would not be able feel the tsunami where we are. The tsunami generated in Japan propagated mostly towards the east, no the south were we are. And even if it had come this way, it was so long and flat in open waters that we would not have felt anything. That is why the tsunami alerts tell people to take their boats into deeper waters, where tsunamis are not destructive.
You have heard that the tsunami crossed the Pacific Ocean at the speed of a jet liner. How can a wave move so fast, when the ones we see on the beach move so much slowly?
Deep water waves move at different speed according to their lengths (in science talk this is said to be a dispersive wave). The longer the wave the faster it moves. On the other hand, all shallow water waves move at the same speed in the same depth (non-dispersive), since their speed depends basically on the depth of the water (the speed also depends on the gravitational force, but gravity is essentially constant along the oceans. For the mathematically inclined: v = √(gh), where v = speed, g = gravitational acceleration, h = depth of water and √ = square root).
Any shallow water wave at 5 km of depth, using round numbers, will move at 220 m/s (v=√(10m/s2 * 5 000 m) = 223 m/s). That is a very large speed! We can transform this speed into something we are more familiar with : 220 m/s ≈ 800 km/h ≈ 500 miles/h. The Pacific Ocean is deeper than that in many places where tsunamis travel even faster.
I hope this helps you understand a little bit more about tsunamis. I wish I had note been compelled to write about tsunamis in the journal, because it would mean that that there would not have been such a strong earthquake in Japan.