Introduction

What is a Tsunami?

      The word Tsunami comes from the Japanese 'tsu' meaning harbor and 'nami' meaning wave.  Tsunamis are giant waves caused by the rapid displacement of an oceanic water column on a vast scale.  These giant waves are usually triggered by earthquakes, volcanic eruptions, mass movements, and sometimes meteorite impacts.  Tsunamis generated in deep water, travel with a small amplitude and very long wavelengths in the deep open ocean, losing very little energy as they 'ripple' out from a source.  The reason for the minimal loss of energy is the lack of contact with the ocean floor.  Figure 1.2 shows how normal coastal waves travel with a circular motion, due to the increase in wavelength of open ocean tsunami waves there is no rotational motion or contact with the bottom.  Tsunamis can travel at speeds upwards of 700 km/hr in the Deep Ocean, with wavelengths reaching thousands of kilometers.  (see figure 1.1 below for comparison of a tsunami wave and regular ocean waves.)
      Upon reaching continental shelf or island coastlines where ocean depth decreases, the wave begins to lose energy in the transformation into amplitude or height.  This creates towering waves upwards of 30 meters high.  These giant waves become unstable as shoreline runs up to land and reach a threshold amplitude.  Once this threshold is reached the wave comes crashing down upon the land it has come in contact with, often flooding areas up to 300 meters inland.  Similarly, landslide generated tsunamis create a massive wave from a water column displacement taking place at or below sea level.  These waves, however, when generated at an existing coastline reach a maximum nearly instantly and are quickly running up shore.

                                                                                                                                                            Figure 1.2:   Particle motion open ocean waves (http://scubageek.com/articles/wwwparticle.html)
Figure 1.1 Regular wave formation compared to an open ocean tsunami and a tsunami                                .
upon reaching coastline. (virtual vacationland, www.bigelow.org/virtual/index_buoy.html)


Why Study Tsunamis?

      Tsunamis are vital for study in the current day and age, as the devastation they reap can cause hundreds of thousands of deaths from single waves.  The largest concern about tsunamis is that they are unpredictable.  The reason for the unpredictability of Tsunamis is that the causes of them are just as unpredictable.  Earthquakes, mass movements and volcanic eruptions are all unpredictable events with serious consequences.    Since the massive devastation that took place in 2004 when over 200,000 people died on the Indian Coast, the pacific tsunami warning system has dedicated time and energy into furthering a warning system as soon as a seismic event is unleashed.  Currently, the system is based on high frequency alarms that are satellite operated and only give people a limited amount of time to get to higher ground.  The Pacific Tsunami Warning System was set in 1949 after a tsunami hit Hawaii and Alaska killing 165 people in 1946.  Primarily the warning system is centered on seismic data received whenever an earthquake or seismic event takes place.  This information is relayed via buoys that are placed throughout the pacific monitoring sea level information measurements in the open ocean.  If a seismic event occurs and water amplitude shows distinct differences from normal activity, a signal is relayed from buoys to satellites and back to observation stations that pass warning to threatened coastal areas.  There is no way to tell from an event right away if it will cause a tsunami, only the possibility.


D.A.R.T Buoy system. Method of tsunami warning (http://tatnews.org/tourism_news/images/tsunami_early/b-dart.jpg)
To see an animation on how the D.A.R.T. Buoy system is used click here (Animation courtesy of NOAA (National Oceanic and Atmospheric Administration))


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