January 4, 2005 > The Tsunami Phenomenon
The Tsunami Phenomenon
by Venkat Raman
A tsunami is a simple concept taken to gigantic proportions. Any disturbance in a fluid causes energy to be dissipated across the fluid through wave action. Sound is a common example where vibrations of some material like vocal chords or musical instruments cause sound energy to travel through air.
Similarly, when there is a disturbance in the depths of the Earth's oceans, it results in a release of energy that is dissipated through wave propagation. The common forms of disturbance are earthquakes on the ocean floor, volcanic action out of deep ocean floor, or landslides along the ocean floor. The disturbance should be deep within the ocean to involve the entire body of the ocean water in the process. Surface phenomena like hurricanes also cause turbulence and destruction, but their behavior is different from tsunamis, which are deep ocean phenomena.
Taking the recent South Asian earthquake and the ensuing tsunami as an example, we can describe the process of a tsunami. It is typical that a tsunami is caused by an earthquake along the ocean floor. It is also typical that the placement of such quakes is relatively close to some shore rather than being in the middle of the ocean far away from all shores. Though every such quake technically starts a tsunami, small quakes cause small disturbances and the resulting tsunamis can be indistinguishable from normal turbulence and wave action on the ocean surface. The recent South Asian earthquake was such a powerful one that it has shrunk the Earth slightly and has caused the earth's rotation to speed up. It is no surprise that a major tsunami ensued in its wake.
When a significant earthquake occurs, movement of landmass on the floor of the ocean immediately affects the entire depth of the ocean directly above it. So we can think of one vertical slice of the ocean water suddenly rising as compared to the slice right next to it. This first phase of a tsunami is termed "Initiation." This imbalance cannot last so the process of equalization of these slices starts immediately. The potential energy of the slice that just rose needs to be dissipated. This dissipation starts when the first tsunami splits into two tsunamis and start to travel in opposite directions perpendicular to the fault line that caused the earthquake. This is termed the "Split" phase and happens a few minutes after initiation.
One of these tsunamis, the local tsunami, has a short distance to landfall and the other, the distant tsunami, a relatively long distance, often thousands of miles before such landfall. The distant tsunami travels much faster than the local tsunami. Speeds of 500 mph have been reported for the distant tsunami in the recent episode. Since water is a fluid and the energy involved is pretty high, this energy does not dissipate very much during travel through the deep ocean. When tsunamis travel, the entire water column, from the surface to the floor, participates in the travel. In the deep ocean, the extent of disturbance at the surface is very minimal, not different from normal waves. So a boat or a ship on the surface may not feel anything different when a tsunami passes it.
When the tsunami approaches land, however, the continental slope starts a rise in the ocean floor and hence progressively impedes the propagation of the tsunami in the depths of the water column. This has the effect of kicking up the water column resulting in a rise in the ocean surface as well as turbulence in the entire water column. Irregularities in the ocean floor and presence of coral reefs further add to the turbulence and dissipation of energy. However, this dissipation is not enough to slow down massive tsunamis very much. This phase of propagation to the land is termed "Amplification" as the wave increases in amplitude on approaching the shore.
Finally, when the tsunami goes on shore, the combination of increased potential energy due to climbing over the land as well as going through obstructions on the land like buildings and other features help dissipate all the energy carried by the tsunami. Since the water came in unnaturally, it needs to flow back into the ocean to fill the void created there due to this action. The interaction of a tsunami and the land features and this flow back is complex. As a result, this onslaught of the ocean on land can come multiple times before dying down completely. This final phase is termed "Runup."
The time delay between the occurrence of the event that causes the tsunami (the earthquake in our example) and the landfall of the tsunami will depend on the distance to shore from the origin of the event. The nearby shore is likely affected in a manner of minutes whereas the distant shores have many hours before landfall. If it were possible to know that a tsunami is on its way, it would be possible to evacuate the shoreline possibly in plenty of time to avoid massive loss of lives. Property destruction is inevitable, but is not as devastating as loss of lives. Tsunami warning systems are in its infancy. Many parts of the world do not have them. The key is to identify false alarms while not overlooking the detection of severe instances. The problem with false alarms is that people will resist paying attention to true alarms, with devastating consequences.
There is plenty of information on the mechanics of tsunamis and case studies of past tsunamis. There are also recommendations on what one can do to suspect the approach of a tsunami and take action to survive it. A few tidbits on tsunamis:
* A tsunami can occur on any coastline. There is no place on the Earth where a tsunami is guaranteed not to occur. The Pacific coastline is certainly a candidate for this occurrence. In fact, the Pacific Ocean is a more prolific producer of tsunamis. Indian Ocean tsunamis are rare.
* A tsunami resulting from a disturbance far away can strike a shore quite unexpectedly. Since local weather is not related to such distant geological phenomena, an otherwise normal and clear day can be marred by the onslaught of a tsunami.
* It is possible for the water line to recede quickly and drastically prior to a significant tsunami. When water so recedes, usually there will be about five minutes or so before the strike of the first wave. Seek high ground in a hurry.
* When visiting shoreline (any beach) always keep in mind the possibility of a tsunami. If news is accessible, pay attention to any reports of earthquakes or volcanic actions and try to determine if there is a direct path across the ocean from that location to where you are. If so, be on extra alert.
* If you are in a boat or on a ship in the open ocean and a tsunami alert is issued, the safest place to be is right where you are. If you are in a harbor, get out to the open sea if there is time. Being close to land is very dangerous.
* Tsunamis don't normally arrive at land as giant breaking waves. They behave more like a very rapidly rising tide, rising well beyond normal high tide levels.
If all of these precautions tend to make you paranoid, keep things in perspective. The chance of death during a normal drive through town is greater than that due to a tsunami. So, just take it easy and have fun at the beach!
Additional reading materials are available on the Internet. Some of the links that can be useful in understanding more about tsunamis are:
The author wishes to acknowledge help from Praveena Raman in gathering background materials for this article and the US Geological Survey website for providing pictorials depicting different phases of a tsunami.