An Introduction to Tsunami

The 11 March 2011 earthquake near Honshu, Japan triggered a tsunami that spread across the Pacific Ocean. The tsunami arrived an hour before I gave the first tsunami lecture for the Natural Catastrophes class. In that context, it seems an appropriate time to provide an introduction to tsunami.

Introduction to Water Waves

Waves are generated when energy is added into the medium, propagated by restoring forces, until energy is dissipated through eliminating forces.

Waves transport energy without (really) transporting matter. A particle of water gets transported around and around in orbitals: a sea gull floating on the ocean’s surface will bob up and down without actually moving anywhere. Motion attenuates with depth, until below the wave base (located at a depth of half the wavelength) where essentially no motion is felt.

Waves in water deeper than the wave base are deep-water waves. The wave behaviour is completely unaffected by the sea floor, and wave speed is proportional to wavelength.

Wave behaviour in water shallower than the wave base is impacted by the sea floor. The friction between the bottom and the wave drags, flattening the orbitals. In water much shallower than the wave base (depth < 1/20 wavelength), the orbitals are flattened so motion is horizontally back-and-forth. These are shallow-water waves, and wave speed is proportional to water depth.

Tides vs. Tsunami vs. Wind Waves

Waves can be classified by the generating forces:

  1. Gravity (from the moon & sun) creating tides.
  2. Disturbances (pebbles into ponds, meteor impacts into oceans, earthquakes abruptly moving sea floors, icebergs calving from glaciers…) creating tsunami and seiche.
  3. Wind creating ripples, swell, and chop.
Tides vs. Tsunami

Tides vs. Tsunami. Numbers are approximate to give an idea of the scale of differences.

Tides are huge wavelength, enormous period waves. Tides are so large, it only takes two wavelengths to wrap around the planet, and you already knew that because we get two crests (high tides) and two troughs (low tides) per day. Comparatively speaking, tsunami have merely large wavelengths and long periods. Average ocean depth is a few kilometers. Since a few kilometers is much, much, much smaller than hundreds or thousands of kilometers, both tides and tsunami are shallow-water waves. The wave speed is dependent on water depth, with the wave slowing down as it enters even shallower coastal waters.

Wind waves vs. Tsunami waves

Wind waves vs. Tsunami waves. Numbers are approximate to give an idea of the scale of differences.

Wind waves are much, much smaller than tsunami. The relatively tiny wavelengths mean that wind waves behave like deep-water waves in open ocean, transitioning into shallow-water waves near the shore. Wind waves near the beach collapse into breakers (plunging surf curls to spilling foam) that are confined to the beach. Tsunami look like a rapidly rising tide, and may inundate far inland. Either the crest or trough of a tsunami may arrive first; if sea floor is suddenly exposed, run

Tsunami Characteristics

Tsunami characteristics with respect to depth.

Tsunami characteristics change with depth.

Tsunami in open ocean travel about the same speed as a jet, slowing down to highway speeds as they approach the shore. Like any other shoaling wave, the wavelength decreases, and the wave height increases. In open ocean, a tsunami is just a few meters of height difference spread over a few hundred kilometers. This is pretty much impossible to observe under all the wind waves, and is why the safest place for a boat to be during a tsunami is in open ocean.

11 March 2011 tsunami damage in Japan. Photography source: National Post

Tsunami damage by floating, dragging, sliding, scouring, and overturning both during advance and retreat. The first wave may not be the largest wave, and interference effects between waves advancing and retreating add to the chaos. Additional damage can come from fires (caused by breakage during the wave impact, and spread by flaming debris or oil slicks), and from disruption of cooling facilities at power plants.

 

Tsunami Forecasting

Tsunami Alert Levels

Tsunami Alert Levels

Warning Centers employ a pair of watchstanders all day, every day to create and distribute Information Statements, Watches, Advisories, and Warnings in response to triggering events. Roughly speaking, the alert levels translate to:

  • Information Statement – Something happened somewhere. Don’t worry about it.
  • Watch- Pay attention.
  • Advisory- Don’t go swimming.
  • Warning – Run!

Tsunami are shallow-water waves, so speed depends on depth. We know global bathymetry quite well, so scientists are good at predicting tsunami arrival times. However, it is very difficult to judge how large a tsunami will be until it comes onshore. This happened for the 2010 Chilean tsunami: when it arrived in Hawaii, the waves were only a few centimeters tall.

Forecasting models of wave height and arrival times have been pre-generated for all sorts of likely triggering events (including major earthquakes in various locations). The models are updated with data from the DART buoys, and from data as the tsunami arrives onshore, and the results are used to update any standing alerts.

Emergency Response

Tsunami Hazard Zone

Tsunami Hazard Zone

If you are in a coastal region and feel strong shaking, run uphill. Do not wait for official warnings; you have only a few minutes before a locally-generated tsunami may arrive.

If the ocean floor is suddenly and unusually exposed, run uphill. This is the draw-down from the trough of a tsunami arriving first, and you have as little as five minutes to get to safety.

Listen to official alerts. If your region is under a watch, keep the news on. If your region is under an advisory, you may be instructed to move watercraft out of harbours and you should certainly avoid going swimming. If your region is under a warning, follow evacuation orders. Tsunami evacuation routes are marked pretty much the same the world over: a blue sign with a white plunging breaker.

Tsunami Vertical Refuge sign

Tsunami Vertical Refuge sign

If you do not have sufficient time to get to high ground, you can get into a vertical refuge (a tall, strong building, or even as last-resort, a tree). At absolutely worst case, you can try holding on to something that floats.

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