Tsunamis are ocean waves produced by earthquakes or underwater landslides. The word is Japanese and means "harbor wave," because of the devastating effects these waves have had on low-lying Japanese coastal communities.
Tsunamis are often incorrectly referred to as tidal waves, but a tsunami is actually a series of waves that can travel at speeds averaging 450 (and up to 600) miles per hour in the open ocean. In the open ocean, tsunamis would not be felt by ships because the wavelength would be hundreds of miles long, with an amplitude of only a few feet. This would also make them unnoticeable from the air. As the waves approach the coast, their speed decreases and their amplitude increases. 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 and cause many deaths or injuries.
According to the National Oceanic and Atmospheric Administration's (NOAA) International Tsunami Information Center, all of the Pacific basin nations face a risk of tsunami. Between 1692 and 1998, 13 major tsunamis occurred in different parts of the world, and over 197,000 lives were lost. The December 2004 Tsunami in Asia claimed over 150,000 lives. Greater public awareness and early warning systems have the potential of saving lives, but some tsunamis can arise and strike shore so rapidly that current monitoring systems are incapable of providing adequate warning for evacuation.
Most tsunamis are caused by large earthquakes at the seafloor, when large slabs of rock are forced to move past each other. Earthquakes cause parts of the seafloor to raise or drop suddenly - the overlying water must move too. The resulting wave moves outwards and away from its source. Underwater landslides can cause tsunamis, and so can land that slumps into the ocean. Landslides happen when slopes become too steep to withstand gravity.
Less commonly, volcanic eruptions can initiate a tsunami - this happens in several ways: if an underwater volcano erupts, the hot lava may heat the surrounding seawater quickly and explosively; massive flows of volcanic debris such as ash can travel down the side of a land volcano and into the ocean, pushing water outwards as it does so; the top of an underwater volcano may collapse downwards, so that the overlying water also drops when compared to water that is further away
Large meteorite impacts that occur at sea can trigger tsunamis too.
If a major earthquake is felt, 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. Associated risks include flooding, contamination of drinking water, fires from ruptured tanks or gas lines, and the loss of vital community infrastructure (police, fire, and medical facilities).
From an initial tsunami-generating source area, waves travel outward in all directions much like the ripples caused by throwing a rock into a pond. As these waves approach coastal areas, the time between successive wave crests varies from 5 to 90 minutes. The first wave is usually not the largest in the series of waves, nor is it the most significant. Furthermore, one coastal community may experience no damaging waves while another, not that far away, may experience destructive deadly waves. Depending on a number of factors, some low-lying areas could experience severe inland inundation of water and debris of more than 1,000 feet.
The Pacific Tsunami Warning Center (PTWC) is responsible for providing warnings to international authorities, Hawaii, and U. S. territories within the Pacific basin. All tsunamis are potentially dangerous, even though they may not damage every coastline they strike.
The PTWC may issue the following bulletins, available through their web site:
- WARNING: A tsunami was or may have been generated, which could cause damage; therefore, people in the warned area are strongly advised to evacuate.
- WATCH: A tsunami was or may have been generated, but is at least two hours travel time to the area in watch status. Local officials should prepare for possible evacuation if their area is upgraded to a warning.
- ADVISORY: An earthquake has occurred in the Pacific basin, which might generate a tsunami. WC/ATWC and PTWC will issue hourly bulletins advising of the situation.
- INFORMATION: A message with information about an earthquake that is not expected to generate a tsunami. Usually only one bulletin is issued.
Be familiar with the tsunami warning signs. A strong earthquake near the coast may generate a tsunami. A noticeable rapid rise or fall in coastal waters is also a sign that a tsunami is approaching.
Tsunamis most frequently come onshore as a rapidly rising turbulent surge of water choked with debris. They are not V-shaped or rolling waves, and are not "surfable."
Tsunamis may be locally generated or from a distant source. In 1992, the Cape Mendocino, California, earthquake produced a tsunami that reached Eureka in about 20 minutes, and Crescent City in 50 minutes. Although this tsunami had a wave height of about one foot and was not destructive, it illustrates how quickly a wave can arrive at nearby coastal communities and how long the danger can last. In 1957, a distant-source tsunami generated by an earthquake in the Aleutian Islands in Alaska struck Hawaii, 2,100 miles away. Hawaii experienced $5 million in damages from that tsunami.
Plan for a Tsunami
Tsunami-specific planning should include the following:
- Learn about tsunami risk in your community. Contact your local city office. Find out if your home, school, workplace or other frequently visited locations are in tsunami hazard areas. Know the height of your street above sea level and the distance of your street from the coast or other high-risk waters.
- Plan an evacuation route from your home, school, workplace, or any other place you'll be where tsunamis present a risk. If possible, pick an area 100 feet above sea level or go up to two miles inland, away from the coastline. If you can't get this high or far, go as high as you can. Every foot inland or upwards may make a difference. You should be able to reach your safe location on foot within 15 minutes. After a disaster, roads may become impassable or blocked. Be prepared to evacuate by foot if necessary. Footpaths normally lead uphill and inland, while many roads parallel coastlines. Follow posted tsunami evacuation routes; these will lead to safety.
- Practice your evacuation route. Familiarity may save your life. Be able to follow your escape route at night and during inclement weather. Practicing your plan makes the appropriate response more of a reaction, requiring less thinking during an actual emergency situation.
What to Do if You Feel a Strong Coastal Earthquake
If you feel a strong earthquake when you are on the coast:
- Drop, cover, and hold on. You should first protect yourself from the earthquake.
- When the shaking stops, gather your family members and evacuate quickly.
- Leave everything else behind. A tsunami may be coming within minutes. Move quickly to higher ground away from the coast.
- Be careful to avoid downed power lines and stay away from buildings and bridges from which heavy objects might fall during an aftershock.
- Return home only after local officials tell you it is safe. A tsunami is a series of waves that may continue for hours. Do not assume that after one wave the danger is over. The next wave may be larger than the first one