Classifying Waves
Anatomy of a Wave
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.
 Photo courtesy DigitalGlobe Tsunami strikes Sri Lanka, December 26, 2004 |
In order to understand tsunamis, let's first look at waves in general. Most of us are familiar with waves from days at the beach or local water park wave pools. Waves are made up of a crest (the highest point of the wave) and a trough (the lowest point of the wave). Waves are measured in two ways:
- The wave height is the distance between the crest and trough.
- The wave length is the horizontal distance between two consecutive wave crests.
 Photo courtesy U.S. Navy Anatomy of a normal wave |
The frequency of waves is measured by the time it takes for two consecutive waves to cross the same point. This is called the wave period.
Tsunamis and normal waves have all of the same parts and are measured in the same ways, but there are many differences between the two. The chart below shows some of the differences.
|
vs. Typical Wind-generated Wave | ||
| Wave Feature | Wind-generated Wave | Tsunami Wave |
| Wave Speed | 5-60 mph (8-100 kph) | 500-600 mph (800-1,000 kph) |
| Wave Period (time required for two waves to pass a single point in space) | 5 to 20 seconds apart | 10 minutes to 2 hours apart |
| Wave Length (horizontal distance between two waves) | 300-600 feet apart (100-200 meters apart) | 60-300 miles apart (100-500 km apart) |
The primary differences are size, speed and source. Let's look at what creates a normal wave.
Waves in the ocean are created by a number of things (gravitational pull, underwater activity, atmospheric pressure), but the most common source for waves is the wind.
 Photo courtesy DigitalGlobe Banda Aceh flooding, 2004 |
 Photo courtesy DigitalGlobe Banda Aceh Grand Mosque, before and after the 2004 tsunami |
When the wind blows across a smooth water surface, the air molecules grab water molecules as they are carried across the water by the wind. The friction between the air and water stretches the water's surface, creating ripples in the water known as capillary waves. The capillary waves move in circles. This circular motion of water continues vertically underwater, though the power of this motion decreases in deeper water. As the wave travels, more and more water molecules are collected, increasing the size and momentum of the wave. The most important thing to know about waves is that they do not represent the movement of water, but instead show the movement of energy through water.
In normal waves, the wind is the source of that energy. The size and speed of wind waves is dependant on the strength of the wind.
