Visualizing a Safer Planet
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What is a
tsunami?Historically, seismic ocean waves have been incorrectly called "tidal waves." Actually, these events are not at all related to tides. Seismic ocean waves are now commonly referred to as tsunami (a Japanese word that translates to "harbor waves"). A tsunami is traditionally defined as a series of ocean waves with very long wavelengths that can travel great distances. However, tsunami can also occur in inland seas, such as the Mediterranean Sea, and in large lakes; geologic studies of Lake Tahoe in the U.S. indicate an active fault system which may produce a tsunami when the next earthquake occurs. In deep oceans, tsunami can reach speeds over 800 kph (500 mph). Tsunami wave heights average 9 meters (30 feet), but have been recorded over 30 meters (100 feet). Geologic evidence indicates that tsunami may reach heights as high as 300 meters (1000 feet). Interestingly, they are almost unnoticeable in the open ocean. Here, tsunami usually have wave heights of only a few feet (1 meter) and pose no threat to ships. When tsunami approach the coast, water and energy are compressed upward due to the shallow ocean bottom. Here, they can carry the largest ocean vessels several kilometeres inland, inundate coasts with flood water, and drag entire communities out to sea as they recede. tsunami can be generated by any event that displaces a large volume of ocean water, such as an earthquake, volcanic eruption, or landslide.
Where do tsunami occur?
Tsunami primarily threaten coasts around the Pacific Ocean, which has
highest number of recorded tsunami. Here, the islands of Japan and
Hawaii, as well as the Alaskan and South American coasts, are at
particularly high risk of tsunami. Although they are rare, tsunami do
occur in the Indian Ocean, Atlantic Ocean, Mediterranean Sea, and even
large lakes. Since most of the world's population lives on a coast,
tsunami potentially impact the lives of hundreds of millions of people.
It is important to note that while a tsunami may originate near the
coast of South America, it can travel across the entire Pacific Ocean,
potentially affecting locations as far away as Hawaii, Japan, and
Alaska.
When do tsunami occur?
Similar to earthquakes and volcanoes, there is not a tsunami "season".
Tsunami can occur any day at any time. They are caused by geologic
processes, not atmospheric processes, so they are not more common
during a certain time of the year, or a certain time of day.
How do we cope with
tsunami?
Because tsunami are unpreventable and often
unpredictable more than a few hours in advance, warning systems are
essential for informing people about tsunami threats. Currently, the
U.S. National Oceanic and Atmospheric Administration (NOAA) maintains a
network of tsunami detection buoys in the Pacific Ocean. These buoys
transmit data via satellite to the Pacific Tsunami Warning Center in
Hawaii. From here, scientists can issue alerts for any location in the
world. However, when an earthquake occurs close to the coast, warning
times may be as short as ten minutes. This provides very little time to
evacuate to higher ground. Accordingly, local tsunami monitoring
stations have been established in some of these areas, such as Japan.
Education for understanding the risks associated with tsunami can help communities be prepared for this often unknown and misunderstood hazard. Establishing evacuation routes and procedures can save lives when warning times are short. Ultimately, building inland and on higher ground is the best way to cope with tsunami in areas that are susceptible to these hazards. Currently, buildings can only be constructed to resist the force of small tsunami. In tsunami prone areas, seawalls have been constructed as barriers against these ocean waves. These massive structures can prevent small and moderate size tsunami from damaging towns. However, most seawalls provide only limited protection, since tsunami wave heights can be greater than the height of such barriers. Occasionally, the presence of a seawall or levee can result in just as much damage because they keep in the same water they were meant to keep out, if a tsunami spills over the top of the seawall. The result can be coastal flooding with extensive damage to property and loss of life.