Tsunamis

A tsunami is a series of travelling ocean waves of extremely long length generated primarily by earthquakes occurring below or near the ocean floor. Underwater volcanic eruptions and landslides can also generate tsunamis. In the deep ocean, the tsunami waves propagate across the deep ocean with a speed exceeding 800 kilometers per hour ([km], ~500 miles per hour), and a wave height of only a few tens of centimeters (1 foot [ft]) or less. Tsunami waves are distinguished from ordinary ocean waves by their great length between wave crests, often exceeding a 100 km (60 miles [mi]) or more in the deep ocean, and by the time between these crests, ranging from 10 minutes to an hour. As they reach the shallow waters of the coast, the waves slow down and the water can pile up into a wall of destruction tens of meters (30 ft) or more in height. The effect can be amplified where a bay, harbor or lagoon funnels the wave as it moves inland. Large tsunamis have been known to rise over 30 meters (100 ft). Even a tsunami 3-6 meters (m) high can be very destructive and cause many deaths and injuries.

Tsunamis can wreak havoc on coastal populations and landscapes. The December 26, 2004, tsunami in the Indian Ocean claimed some 150,000 lives and cleared the landscape on millions of acres of oceanfront terrain. Here are some measures you can take to avoid trouble if you're caught in a tsunami.[1]

According to UNESCO, tsunami waves can flood or inundate low lying coastal areas.  Tsunami inundation is the horizontal, inland penetration of waves from the shoreline.  Flooding can extend inland by 300 meters (~1000 feet) or more, covering large expanses of land with water and debris.   Inundation distances can vary greatly along the shorelines, depending on the intensity of the tsunami waves, the undersea features, and the land topographic elevations.  One coastal community may see no damaging wave activity, while another nearby community can be attacked by large and violent waves.  When the tsunami reaches the coast and moves inland, the water level can rise many meters.  The first wave may not be the largest in the series of waves.[2]

Safety tips  

• When in coastal areas, stay alert for tsunami warnings.
• Plan an evacuation route that leads to higher ground.
• Know the warning signs of a tsunami: rapidly rising or falling coastal waters and rumblings of an offshore earthquake.
• Never stay near shore to watch a tsunami come in. Move inland to higher ground immediately and stay there. This is nature's tsunami warning and it should be heeded. You should move away immediately.
• Stay away from the beach. Never go down to the beach to watch a tsunami come in. If you can see the wave you are too close to escape it.
• A tsunami is not a single wave, but a series of waves that can come ashore for hours. The first wave may not be the largest. Stay out of danger areas until an "all-clear" is issued by a recognized authority.

Important Facts to Know about Tsunamis^[3]  

• Tsunamis that strike coastal locations in the Pacific Ocean Basin are most always caused by earthquakes. These earthquakes might occur far away or near where you live.
• Some tsunamis can be very large. In coastal areas their height can be as great as 30 feet or more (100 feet in extreme cases), and they can move inland several hundred feet.
• All low-lying coastal areas can be struck by tsunamis.
• A tsunami consists of a series of waves. Often the first wave may not be the largest. The danger from a tsunami can last for several hours after the arrival of the first wave.
• Tsunamis can move faster than a person can run.
• Sometimes a tsunami causes the water near the shore to recede, exposing the ocean floor.
• The force of some tsunamis is enormous. Large rocks weighing several tons along with boats and other debris can be moved inland hundreds of feet by tsunami wave activity. Homes and other buildings are destroyed. All this material and water move with great force and can kill or injure people.
• Tsunamis can occur at any time, day or night.
• Tsunamis can travel up rivers and streams that lead to the ocean.

If you are on land  

Be aware of tsunami facts. This knowledge could save your life! Share this knowledge with your relatives and friends. It could save their lives!

• If you are in school and you hear there is a tsunami warning, you should follow the advice of teachers and other school personnel.
• If you hear there is a tsunami warning, you should make sure your entire family or travel group is aware of the warning. If you are in a tsunami evacuation zone. Move in an orderly, calm and safe manner to the evacuation site or to any safe place outside your evacuation zone. Follow the advice of local emergency and law enforcement authorities.
• If you are at the beach or near the ocean and you feel the earth shake, move immediately to higher ground, DO NOT wait for a tsunami warning to be announced. Stay away from rivers and streams that lead to the ocean as you would stay away from the beach and ocean if there is a tsunami. A regional tsunami from a local earthquake could strike some areas before a tsunami warning could be announced.
• Tsunamis generated in distant locations will generally give people enough time to move to higher ground. For locally-generated tsunamis, where you might feel the ground shake, you may only have a few minutes to move to higher ground.
• High, multi-story, reinforced concrete hotels are located in many low-lying coastal areas. The upper floors of these hotels can provide a safe place to find refuge should there be a tsunami warning and you cannot move quickly inland to higher ground. Local Civil Defense procedures may, however, not allow this type of evacuation in your area. Homes and small buildings located in low-lying coastal areas are not designed to withstand tsunami impacts. Do not stay in these structures should there be a tsunami warning.
• Offshore reefs and shallow areas may help break the force of tsunami waves, but large and dangerous wave can still be a threat to coastal residents in these areas. Staying away from all low-lying areas is the safest advice when there is a tsunami warning.

If you are on a boat  

• Since tsunami wave activity is imperceptible in the open ocean, do not return to port if you are at sea and a tsunami warning has been issued for your area. Tsunamis can cause rapid changes in water level and unpredictable dangerous currents in harbours and ports.
• If there is time to move your boat or ship from port to deep water (after a tsunami warning has been issued), you should weigh the following considerations:
• Most large harbours and ports are under the control of a harbour authority and/or a vessel traffic system. These authorities direct operations during periods of increased readiness (should a tsunami be expected), including the forced movement of vessels if deemed necessary. Keep in contact with the authorities should a forced movement of vessel be directed.
• Smaller ports may not be under the control of a harbour authority. If you are aware there is a tsunami warning and you have time to move your vessel to deep water, then you may want to do so in an orderly manner, in consideration of other vessels. Owners of small boats may find it safest to leave their boat at the pier and physically move to higher ground, particularly in the event of a locally-generated tsunami. Concurrent severe weather conditions (rough seas outside of safe harbour) could present a greater hazardous situation to small boats, so physically moving yourself to higher ground may be the only option.
• Damaging wave activity and unpredictable currents can affect harbours for a period of time following the initial tsunami impact on the coast. Contact the harbour authority before returning to port making sure to verify that conditions in the harbour are safe for navigation and berthing.

What to Do After a Tsunami[4]  

• The following are guidelines for the period following a tsunami:
• Stay away from flooded and damaged areas until officials say it is safe to return.
• Stay away from debris in the water; it may pose a safety hazard to boats and people.
• Save yourself - not your possessions

More useful links:  

http://www.stopdisastersgame.org/en/pdf/Tsunami_fact-sheet.pdf

http://ioc3.unesco.org/itic/

http://www.fema.gov/hazard/tsunami/

http://gamapserver.who.int/mapLibrary/app/searchResults.aspx

http://portal.unesco.org/science/en/ev.php-URL_ID=6011&URL_DO=DO_TOPIC&URL_SECTION=201.html

http://www.fema.gov/hazard/tsunami/index.shtm

Monster Waves (from Eduplace.com)

Tsunami! (from Discoveryschool.com)

Seismic Sea (From Utah Science)

Tsunami Curriculum, Humboldt Earthquake Education Center, Department of Geology, Humboldt State Univ