How Tsunamis Work

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On December 26, 2004, a massive underwater earthquake off the coast of Indonesia's Sumatra Island rattled the Earth in its orbit. The quake, measuring 9.0 on the Richter scale, was the largest one since 1964. Dozens of aftershocks with magnitudes of 5.0 or higher occurred in the following days. But the most powerful and destructive aftermath of this devastating earthquake was the tsunami that it caused. The death toll reached higher than 220,000, and many communities suffered devastating property damage

Introduction:

The word "tsunami" comes from the Japanese words tsu (harbor) and nami (waves). A tsunami is a wave or series of waves in the ocean that can be hundreds of miles long and have been known to reach heights of up to 34 ft (10.5 m). These "walls of water" travel as fast or faster than a commercial jet. The massive December 26, 2004 tsunami traveled 375 miles (600 km) in 75 minutes. That's 300 mph (480 kph). These walls of water are capable of inflicting massive damage along coastal lands.

The Birth of a Tsunami:

The most common causes of tsunamis are underwater earthquakes. To understand underwater earthquakes, you must first understand plate tectonics. The theory of plate tectonics suggests that the lithosphere, or top layer of the Earth, is made up of a series of huge plates. These plates make up the continents and seafloor. They rest on an underlying viscous layer called the asthenosphere.
Think of a pie cut into eight slices. The pie crust would be the lithosphere and the hot, sticky pie filling underneath would be the asthenosphere. On the Earth, these plates are constantly in motion, moving along each other at a speed of 1 to 2 inches (2.5-5 cm) per year. The movement occurs most dramatically along fault lines (where the pie is cut). These motions are capable of producing earthquakes and volcanism, which, when they occur at the bottom of the ocean, are two possible sources of tsunamis.

When two plates come into contact at a region known as a plate boundary, a heavier plate can slip under a lighter one. This is called subduction. Underwater subduction often leaves enormous "handprints" in the form of deep ocean trenches along the seafloor.
In some cases of subduction, part of the seafloor connected to the lighter plate may "snap up" suddenly due to pressure from the sinking plate. This results in an earthquake. The focus of the earthquake is the point within the Earth where the rupture first occurs, rocks break and the first seismic waves are generated. The epicenter is the point on the seafloor directly above the focus.
When this piece of the plate snaps up and sends tons of rock shooting upward with tremendous force, the energy of that force is transferred to the water. The energy pushes the water upward above normal sea level. This is the birth of a tsunami. The earthquake that generated the December 26, 2004, tsunami in the Indian Ocean was a 9.0 on the Richter scale -- one of the biggest in recorded history.
Hitting the Water

Once the water has been pushed upward, gravity acts on it, forcing the energy out horizontally along the surface of the water. It's sort of the same ripple effect you get from throwing a pebble in the water, but in reverse: The energy is generated by a force moving out of rather than into the water. The energy then moves through the depths of the water and away from the initial disturbance.
The tremendous force created by the seismic disturbance generates the tsunami's incredible speed. The actual speed of the tsunami is calculated by measuring the water depth at a point in time when the tsunami passes by. The speed is the square root of the product of acceleration of gravity and the quantity of water depth, or:
t = square root (g x d)
t = tsunami speed in meters per second
g = acceleration of gravity (32 feet/10 meters per second/per second)
d = quantity of water depth

A tsunami's ability to maintain speed is directly influenced by the depth of the water. A tsunami moves faster in deeper water and slower in shallower water. So unlike a normal wave, the driving energy of a tsunami moves through the water as opposed to on top of it. As a result, as a tsunami moves though deep water at hundreds of miles an hour, it is barely noticeable above the waterline. A tsunami is typically no more than 3 feet (1 meter) high until it gets close to shore.
Once a tsunami gets close to shore, it takes its more recognizable and deadly form.

When a tsunami reaches land, it hits shallower water. The shallow water and coastal land acts to compress the energy traveling through the water. This starts the transformation of the tsunami.

2004 Tsunami

On December 26, 2004, the world's most powerful earthquake in more than 40 years struck deep under the Indian Ocean off the west coast of Sumatra, triggering a massive tsunami. One of the things that made this event particularly destructive is that the tsunamis struck relatively well-populated areas in the middle of the tourist-packed holiday season. Here is a timeline of the disaster:
12:59 am - A massive 9.0 earthquake occurs in the Indian Ocean off Sumatra, Indonesia. The quake is so large it is felt in the neighboring countries of Thailand, Malaysia and Singapore. Huge buildings in the Thai capital of Bangkok shake under the force of the earthquake and its aftershocks. Bangkok is 1,242 miles (nearly 2,000 km) from where the earthquake took place.
1:07 am - After the quake, stations in Australia alert the NOAA Pacific Tsunami Warning Center of the earthquake and the potential tsunami threat. There are widely conflicting reports from different sources about the size of the quake. Different reports put the earthquake at magnitudes varying between 6.6 and 8.9. At the same time, an Indonesian radio station reports the death of nine villagers as the result of a tidal wave.

2:27 am - The massive waves hit Kalmunai, Sri Lanka.
2:30 am - Kattankudy is hit. By now, almost the entire east coast of Sri Lanka is under 9 feet (2.7 meters) of water.
2:40 am - Over the next 15 minutes, Batticaloa, Mullaitivu and Trincomalee, Sri Lanka, are hit. Yala, Thailand, is also struck by the tsunami. Though it has not yet been reported, more than 15,000 people have died.
2:57 am - News wire services release the first report: "Earthquake sets off big waves." Reports of heavy damage and fatalities began to come in from Thailand's Phukit resort area. The official death toll is still nine. All reports are unclear, and the numbers are unconfirmed.

3:00 am - An AFP news correspondent in Colombo, Sri Lanka, gets a phone call from reporters in Trinco: "The sea is coming in." In the same moment, Valvettiturai and Hambantota are hit. Almost 7,000 people are washed out to sea.
3:15 am - A Washington Post correspondent reports a tsunami hitting Weligama, Sri Lanka. The Sri Lankan provinces of Matara, Galle and Panadura are also hit. Another 5,000 people die.
3:20 am - Loaded with European tourists, the Sri Lankan resort Rae of Kalutara is hit. Satellite imagery reveals the water reaching 500 yards (460 meters) in from the shore line.
3:30 am - The AFP news correspondent in Colombo gets a call from Matara indicating that a second round of waves is coming. Waves hit the Indian coast. PTWC begins getting wire reports from the Internet about Sri Lankan casualties. Negombo, Sri Lanka, is hit.
3:46 am - AFP news reports massive casualties and numbers of homeless in Sri Lanka.
4:11 am - Rapidly rising water levels in India damage the coastline. Some small tremors are felt.
5:00 am - PTWC advises the U.S. Pacific Command in Hawaii of the potential threat of more tsunamis in the western Indian Ocean.
5:13 am - Sri Lanka deploys the military and asks India for help.
5:41 am - The Prime Minister of Thailand orders the evacuation of three major provinces, including Phukit.
6:09 am - In a few hours, the tsunami has all but crossed the ocean, flooding Male, the capitol of Maldives.
7:15 am - The PTWC advises the U.S. State Department on the continuing threat of tsunamis in Madagascar and Africa. In the next few hours, organized relief efforts begin.