Ahmad Zaid's Articles

Tsunami’s are Ocean Waves produced by earthquakes occurring under the ocean. The word tsunami originated in Japan because of the large amount of tsunamis that have devastated Japan’s coastline. On the open ocean, tsunamis cannot be felt by ships because the wavelength would be hundreds of miles long, with an amplitude of only a few feet. This means that you can only see the tsunami as it approaches land because the waves start to bunch up in the shallowing water and the only place the water has to go it up. That means that detecting tsunamis at sea or from the air is impossible until its too late. Unusual wave heights have been known to be over 100 feet high. However, waves that are 10 to 20 feet high can be very destructive ( as in the Indian Ocean Tsunami) and cause many deaths or injuries.
 Tsunamis are mainly generated by earthquake-induced movement on the ocean floor. Landslides, volcanic eruptions, and even meteorites can also generate tsunamis. If a major earthquake happens near a costal area, a tsunami could reach the beach in a few minutes, even before a warning is issued. Areas at greatest risk are less than 25 feet above sea level and within one mile of the shoreline. Most deaths caused by a tsunami are because of drowning.
From an initial tsunami generating source area, waves travel outward in all directions like the ripples caused by throwing a rock into a pond. As these waves approach coastal areas, the time between successive waves can vary from 5 to 90 minutes. The first wave is usually not the largest in the series of waves, but can be. Tsunamis can also travel up to 2 miles up rivers and large streams inland. Many people think that there are specific tsunami danger zones but tsunamis can strike any coastal area.
Many people think that a tsunami can be seen on the horizon from miles away but the wave actually starts about a quarter mile from the beach. So when you hear a tsunami warning you need to get to high ground as fast as possible because you don’t know how fast the tsunami will get there after the warning. You can never be too safe from tsunamis, every tsunami is different, you should get as far away from the coast as possible and wait for waves to stop.

Subduction Zones are Potential Tsunami Locations Most tsunamis are caused by earthquakes generated in a subduction zone, an area where an oceanic plate is being forced down into the mantle by plate tectonic forces. The friction between the subducting plate and the overriding plate is enormous. This friction prevents a slow and steady rate of subduction and instead the two plates become "stuck". Accumulated Seismic Energy As the stuck plate continues to descend into the mantle the motion causes a slow distortion of the overriding plage. The result is an accumulation of energy very similar to the energy stored in a compressed spring. Energy can accumulate in the overriding plate over a long period of time - decades or even centuries. Earthquake Causes Tsunami Energy accumulates in the overriding plate until it exceeds the frictional forces between the two stuck plates. When this happens, the overriding plate snaps back into an unrestrained position. This sudden motion is the cause of the tsunami - because it gives an enormous shove to the overlying water. At the same time, inland areas of the overriding plate are suddenly lowered. Tsunami Races Away From the Epicenter The moving wave begins travelling out from where the earthquake has occurred. Some of the water travels out and across the ocean basin, and, at the same time, water rushes landward to flood the recently lowered shoreline. Tsunamis Travel Rapidly Across Ocean Basis Tsunamis travel swiftly across the open ocean. The map below shows how a tsunami produced by an earthquake along the coast of Chile in 1960 traveled across the Pacific Ocean, reaching Hawaii in about 15 hours and Japan in less than 24 hours. Tsunami "Wave Train" Many people have the mistaken belief that tsunamis are single waves. They are not. Instead tsunamis are "wave trains" consisting of multiple waves. The chart below is a tidal gauge record from Onagawa, Japan beginning at the time of the 1960 Chile earthquake. Time is plotted along the horizontal axis and water level is plotted on the vertical axis. Note the normal rise and fall of the ocean surface, caused by tides, during the early part of this record. Then recorded are a few waves a little larger than normal followed by several much larger waves. In many tsunami events the shoreline is pounded by repeated large waves.

Tsunami generation images by USGS.