About Tsunami                                                                                                                                       What does the word tsunami mean?  A tsunami is a Japanese word which translates as "harbor wave", now used internationally to refer to a series of waves traveling across the ocean with extremely long wavelengths ( up to hundreds of miles between wave crests in the deep ocean). When these waves approach shore, the speed of the wave decreases as they begin to "feel" the bottom. It is at this time that the height of the wave drastically increases. As the waves strike shore they may inundate low-lying coastal areas resulting in mass destruction and in many instances loss of life. Often a tsunami is incorrectly referred to as a tidal wave. Tidal waves are simply the periodic movement of water associated with the rise and fall of the tides produced by the gravitational attraction of the sun and moon. Tsunamis have no connection with the weather nor with tides.  What causes a tsunami?  Oceanographers often refer to tsunamis as seismic sea waves as they are usually the result of a sudden rise or fall of a section of the earth's crust under or near the ocean. A seismic disturbance can displace the water column, creating a rise or fall in the level of the ocean above. This rise or fall in sea level is the initial formation of a tsunami wave.  Tsunami waves can also be created by volcanic activity and landslides occurring above or below the sea surface. These types of activity produce tsunamis with much less energy than those produced by submarine faulting. The size and energy of these tsunamis dissipates rapidly with increasing distance from the source, thus resulting in more local devastation.  Could Nuclear testing create a tsunami? This is a difficult topic to research, because much of the information surrounding nuclear testing is classified. During the Cold War there was fear of tsunamis produced by the detonation of nuclear bombs on the continental shelf off the East Coast of the US. A nuclear bomb was never detonated on the shelf, however a huge explosion did generate a tsunami during World War I. Any large disturbance that displaces a large volume of water can be a potential cause of a tsunami.  How is a tsunami wave different from a normal wave?  The waves you see at the beach are generated by wind blowing over the sea surface. The size of these waves depends on the strength of the wind creating them and the distance over which it blows. Generally the distance between these waves, known as the wavelength, ranges from a couple of feet to perhaps a thousand feet. The speed of these waves as they travel across the ocean ranges from a few miles an hour up to sixty miles an hour in some instances.  Tsunami waves resulting from physical mechanisms ( see above question) behave much differently than wind generated waves. The magnitude of the disturbance causing the tsunami is the primary factor influencing the size and strength of the waves. The height of the wave when it is generated is very small, usually less than a few feet. The distance between successive wave crests or the wavelength however, is much larger than that of a normal wave and may be hundreds of miles apart. Depending on the depth of the water in which the tsunami is traveling, it may attain speeds of up to 500 miles an hour.  How does a tsunami behave as it approaches land?  When the waves of a tsunami approach land, their appearance and behavior become dependent on several local factors. Two of the most important factors are the topography of the seafloor and the actual shape of the shoreline. As a tsunami encounters shallow waters surrounding the shoreline, its height can increase from a meter or less to over 20 meters. Wave heights can also be increased when concentrated on headlands or when traveling into bays having wide entrances that become progressively more narrow. The presence of an offshore coral reef can dissipate the energy of a tsunami, decreasing the impact on the shoreline. Normal wind swell may ride atop of a tsunami wave thereby increasing wave height.  The image most people have of a tsunami is a large, steep wave breaking on the shore. This image is hardly if ever the case. Most tsunamis appear as an advancing tide without having a developed wave face, resulting in rapid flooding of low-lying coastal areas. Sometimes, a bore can form during which an abrupt front of whitewater will rapidly advance inland much similar to the tidal bore formed at the mouth of large rivers.  Another rare event that may result from a tsunami is a standing wave or seiche. A seiche occurs in bodies of water that are partially or completely enclosed, such as Hilo Bay, creating a standing wave that continually sloshes back and forth. The appearance of a seiche would be very similar to what happens when you place a glass of water on the table; the water rocks back and forth before settling. When a seiche is generated by a tsunami, subsequent tsunami waves may arrive in unison with a seiche resulting in an increase in the drawdown in sea level and a dramatic increase in wave height. Seiche waves may continue several days after a tsunami.  How long does it take a tsunami to reach land?  Once generated, a tsunami wave in the open ocean can travel with speeds greater than 500 miles an hour. These waves can travel across the Pacific Ocean in less than one day. Locally generated tsunamis can reach coastlines in just minutes.  How many waves are there in a tsunami?  A tsunami generally consists of a series of waves, often referred to as the tsunami wave train. The amount of time between successive waves, known as the wave period, is only a few minutes, in some instances, waves are over an hour apart. Many people have lost their lives after returning home in between the waves of a tsunami, thinking that the waves had stopped coming.  How are tsunami wave heights measured?  The wave height of a tsunami can be highly variable in a local area depending on the underwater topography, orientation to the oncoming wave, the tidal level, and the magnitude of the tsunami. Because direct physical measurement of a tsunami wave would be a life threatening event, the most common method for determining tsunami wave height is by measuring the runup, the highest vertical point reached by the wave. Runup heights are measured by looking at the distance and extent of salt-killed vegetation, and the debris left once the wave has receded. This distance is referenced to a datum level, usually being the mean sea level or mean lower low water level. The reference to mean lower low water is more significant in areas with greater tidal ranges such as in Alaska where a smaller tsunami wave can be more devastating during a high tide than a larger wave at low tide.  What is the "wrap-around" effect?  Whether a tsunami is generated in the North or South Pacific, it has the potential to effect all shores of the Hawaiian Islands. As large tsunami waves approach the islands, they may refract or bend around the islands and diffract through the channels between the islands as well. The ability of a tsunami wave to bend around and through the islands is called the wrap-around effect. During the wrap-around effect, the energy of the tsunami often decreases resulting in smaller wave heights. Sometimes tsunami waves will reflect off of a land mass instead or bending around, thereby increasing wave height of the approaching wave. Therefore, when a tsunami warning is issued from an earthquake in Chile, Alaska, or Japan, inhabitants along all shores of the Islands should take the necessary precautions.  Do all oceans have tsunamis?  Yes. Tsunamis have been recorded to occur in all the major oceans of the world. However, this phenomenon is mainly restricted to the Pacific basin, an area surrounded by volcanic island arcs, mountain chains and subduction zones earning the nickname the "ring of fire", as it is the most geologically active area on the planet. The amount of activity in this region makes it much more susceptible to submarine faulting and subsequent tsunami events, whereas the Indian and Atlantic oceans are far less geologically active, with some exceptions, and therefore the occurrence of tsunamis is rare.