Safety first for tsunamis 2
If you are on a boat:
Keep in contact with the authorities should a forced movement of vessel be directed.
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 harbor) 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 effect harbors for a period of time following the initial tsunami impact on the coast. Contact the harbor authority before returning to port making sure to verify that conditions in the harbor are safe for navigation and berthing.
Tsunami Facts: How They Form, Warning Signs, and Safety Tips
In the wake of yesterday's tsunami in the Solomon Islands, National Geographic News looks at how the killer waves are caused, what the warning signs are, and how to respond when a tsunami threatens.
• A tsunami is a series of great sea waves caused by an underwater earthquake, landslide, or volcanic eruption. More rarely, a tsunami can be generated by a giant meteor impact with the ocean.
Scientists have found traces of an asteroid-collision event that they say would have created a giant tsunami that swept around the Earth several times, inundating everything except the tallest mountains 3.5 billion years ago. The coastline of the continents was changed drastically and almost all life on land was exterminated. (Read the story.)
• Tsunami (pronounced soo-NAH-mee) is a Japanese word. Tsunamis are fairly common in Japan, and many thousands of Japanese have been killed by them in recent centuries.
• An earthquake generates a tsunami if it is of sufficient force and there is violent movement of the earth to cause substantial and sudden displacement of a massive amount of water.
• A tsunami is not a single wave but a series of waves, also known as a wave train. The first wave in a tsunami is not necessarily the most destructive. Tsunamis are not tidal waves.
• Tsunami waves can be very long (as much as 60 miles, or 100 kilometers) and be as far as one hour apart. They are able to cross entire oceans without great loss of energy. The Indian Ocean tsunami traveled as much as 3,000 miles (nearly 5,000 kilometers) to Africa, arriving with sufficient force to kill people and destroy property.
Scientists say that a great earthquake of magnitude 9 struck the Pacific Northwest in 1700 and created a tsunami that caused flooding and damage on the Pacific coast of Japan. (Read the story.)
As Fast as a Commercial Jet
• Where the ocean is deep, tsunamis can travel unnoticed on the surface at speeds up to 500 miles an hour (800 kilometers an hour), crossing an ocean in a day or less. Scientists are able to calculate arrival times of tsunamis in different parts of the world based on their knowledge of water depths, distances, and when the event that generated them occurred.
• A tsunami may be less than a foot (30 centimeters) in height on the surface of the open ocean, which is why they are not noticed by sailors. But the powerful shock wave of energy travels rapidly through the ocean as fast as a commercial jet. Once a tsunami reaches shallow water near the coast, it is slowed down. The top of the wave moves faster than the bottom, causing the sea to rise dramatically.
• Geological features such as reefs, bays, river entrances, and undersea formations may dissipate the energy of a tsunami. In some places a tsunami may cause the sea to rise vertically only a few inches or feet. In other places tsunamis have been known to surge vertically as high as 100 feet (30 meters). Most tsunamis cause the sea to rise no more than 10 feet (3 meters).
The Indian Ocean tsunami of December 2004 caused waves as high as 30 feet (9 meters) in some places, according to news reports. In other places witnesses described a rapid surging of the ocean.
Flooding can extend inland by a thousand feet (300 meters) or more. The enormous energy of a tsunami can lift giant boulders, flip vehicles, and demolish houses. Knowledge of the history of tsunamis in your area is a good indicator of what is likely to happen in a future tsunami event.
• Tsunamis do not necessarily make their final approach to land as a series of giant breaking waves. They may be more like a very rapidly rising tide. This may be accompanied by much underwater turbulence, sucking people under and tossing heavy objects around. Entire beaches have been stripped away by tsunamis.
Many witnesses have said a tsunami sounds like a freight train.
• The 2004 Indian Ocean tsunami could rank as the most devastating on record. More than 200,000 people lost their lives, many of them washed out to sea.
The most damaging tsunami on record before 2004 was the one that killed an estimated 40,000 people in 1782 following an earthquake in the South China Sea. In 1883 some 36,500 people were killed by tsunamis in the South Java Sea, following the eruption of Indonesia's Krakatoa volcano. In northern Chile more than 25,000 people were killed by a tsunami in 1868.
• The Pacific is by far the most active tsunami zone, according to the U.S. National Oceanic and Atmospheric Administration (NOAA). But tsunamis have been generated in other bodies of water, including the Caribbean and Mediterranean Seas, and the Indian and Atlantic Oceans. North Atlantic tsunamis included the tsunami associated with the 1775 Lisbon earthquake that killed as many as 60,000 people in Portugal, Spain, and North Africa. This quake caused a tsunami as high as 23 feet (7 meters) in the Caribbean.
• The Caribbean has been hit by 37 verified tsunamis since 1498. Some were generated locally and others were the result of events far away, such as the earthquake near Portugal. The combined death toll from these Caribbean tsunamis is about 9,500.
• Large tsunami waves were generated in the Marmara Sea in Turkey after the Izmit earthquake of 1999.
• An earthquake is a natural tsunami warning. If you feel a strong quake do not stay in a place where you are exposed to a tsunami. If you hear of an earthquake be aware of the possibility of a tsunami and listen to the radio or television for additional information. Remember that an earthquake can trigger killer waves thousands of miles across the ocean many hours after the event generated a tsunami.
• Witnesses have reported that an approaching tsunami is sometimes preceded by a noticeable fall or rise in the water level. If you see the ocean receding unusually rapidly or far it's a good sign that a big wave is on its way. Go to high ground immediately.
Many people were killed by the Indian Ocean tsunami because they went down to the beach to view the retreating ocean exposing the seafloor. Experts believe that a receding ocean may give people as much as five minutes' warning to evacuate the area.
• Remember that a tsunami is a series of waves and that the first wave may not be the most dangerous. The danger from a tsunami can last for several hours after the arrival of the first wave. A tsunami wave train may come as a series of surges that are five minutes to an hour apart. The cycle may be marked by a repeated retreat and advance of the ocean. Stay out of danger until you hear it is safe.
Survivors of the Indian Ocean tsunami reported that the sea surged out as fast and as powerfully as it came ashore. Many people were seen being swept out to sea when the ocean retreated.
• A tsunami surge may be small at one point of the shore and large at another point a short distance away. Do not assume that because there is minimal sign of a tsunami in one place it will be like that everywhere else.
• Tsunamis can travel up rivers and streams that lead to the ocean. 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.
• It's always a good idea to keep a store of emergency supplies that include sufficient medications, water, and other essentials sufficient for at least 72 hours. Tsunami, earthquake, hurricane—an emergency can develop with little or no warning.
Advice for Sailors
• NOAA advises that since tsunami wave activity is imperceptible in the open ocean, vessels should not return to port if they are at sea and a tsunami warning has been issued for the area. Tsunamis can cause rapid changes in water level and unpredictable, dangerous currents in harbors and ports. Boat owners may want to take their vessels out to sea if there is time and if the sailors are allowed to do so by port authorities. People should not stay on their boats moored in harbors. Tsunamis often destroy boats and leave them wrecked above the normal waterline.
• Heightened awareness of the potential for a tsunami to inundate the U.S. western coastline has caused NOAA, the U.S. Geological Survey, and the Federal Emergency Management Administration to initiate a program to predict tsunamis more accurately. As a tsunami traverses the ocean, a network of sensitive recorders on the sea floor measures pressure changes in the overhead water, sending the information to sensors on buoys, which in turn relay the data to satellites for immediate transmission to warning centers.
• The Tsunami Warning System (TWS) in the Pacific, composed of 26 member countries, monitors seismological and tidal stations throughout the Pacific region. The system evaluates potentially tsunami-causing earthquakes and issues tsunami warnings. An international warning system for tsunamis in the Indian Ocean was launched in June 2006.
• Use your common sense. If you feel or hear of a strong earthquake do not wait for an official tsunami warning. Tell your family and friends to join you in leaving for high ground.
Safety first for tsunamis
(How are tsunamis formed)
If you are on land:
- 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 are at home and hear there is a tsunami warning, you should make sure your entire family is aware of the warning. Your family should evacuate your house if you live 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.
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.
How are tsunamis formed
HOW ARE TSUNAMIS FORMED 2
Tsunamis can be generated when the sea floor abruptly deforms and vertically displaces the overlying water. Tectonic earthquakes are a particular kind of earthquake that are associated with the earth's crustal deformation; when these earthquakes occur beneath the sea, the water above the deformed area is displaced from its equilibrium position. Waves are formed as the displaced water mass, which acts under the influence of gravity, attempts to regain its equilibrium. When large areas of the sea floor elevate or subside, a tsunami can be created.
Large vertical movements of the earth's crust can occur at plate boundaries. Plates interact along these boundaries called faults. Around the margins of the Pacific Ocean, for example, denser oceanic plates slip under continental plates in a process known as subduction. Subduction earthquakes are particularly effective in generating tsunamis.
A tsunami can be generated by any disturbance that displaces a large water mass from its equilibrium position. In the case of earthquake-generated tsunamis, the water column is disturbed by the uplift or subsidence of the sea floor. Submarine landslides, which often accompany large earthquakes, as well as collapses of volcanic edifices, can also disturb the overlying water column as sediment and rock slump downslope and are redistributed across the sea floor. Similarly, a violent submarine volcanic eruption can create an impulsive force that uplifts the water column and generates a tsunami. Conversely, supermarine landslides and cosmic-body impacts disturb the water from above, as momentum from falling debris is transferred to the water into which the debris falls. Generally speaking, tsunamis generated from these mechanisms, unlike the Pacific-wide tsunamis caused by some earthquakes, dissipate quickly and rarely affect coastlines distant from the source area.
How are tsunamis formed...
Physical Characteristics of Tsunamis
All types of waves, including tsunamis, have a wavelength, a wave height, an amplitude, a frequency or period, and a velocity.
Wavelength is defined as the distance between two identical points on a wave (i.e. between wave crests or wave troughs). Normal ocean waves have wavelengths of about 100 meters. Tsunamis have much longer wavelengths, usually measured in kilometers and up to 200 kilometers
* Wave height refers to the distance between the trough of the wave and the crest or peak of the wave.
* Wave amplitude- refers to the height of the wave above the still water line, usually this is equal to 1/2 the wave height. Tsunamis can have variable wave height and amplitude that depends on water depth as we shall see in a moment
* Wave frequency or period - is the amount of time it takes for one full wavelength to pass a stationary point.
* Wave velocity is the speed of the wave. Velocities of normal ocean waves are about 90 km/hr while tsunamis have velocities up to 950 km/hr (about as fast as jet airplanes), and thus move much more rapidly across ocean basins. The velocity of any wave is equal to the wavelength divided by the wave period.
V = l/P
Tsunamis are characterized as shallow-water waves. These are different from the waves most of us have observed on a the beach, which are caused by the wind blowing across the ocean's surface. Wind-generated waves usually have period (time between two successive waves) of five to twenty seconds and a wavelength of 100 to 200 meters. A tsunami can have a period in the range of ten minutes to two hours and wavelengths greater than 500 km. A wave is characterized as a shallow-water wave when the ratio of the water depth and wavelength is very small. The velocity of a shallow-water wave is also equal to the square root of the product of the
acceleration of gravity, g, (980cm/sec/sec) and the depth of the water, d.
V=Ög * d
The rate at which a wave loses its energy is inversely related to its wavelength. Since a tsunami has a very large wavelength, it will lose little energy as it propagates. Thus, in very deep water, a tsunami will travel at high speeds with little loss of energy. For example, when the ocean is 6100 m deep, a tsunami will travel about 890 km/hr, and thus can travel across the Pacific Ocean in less than one day.
As a tsunami leaves the deep water of the open sea and arrives at the shallow waters near the coast, it undergoes a transformation. Since the velocity of the tsunami is also related to the water depth, as the depth of the water decreases, the velocity of the tsunami decreases. The change of total energy of the tsunami, however, remains constant
Furthermore, the period of the wave remains the same, and thus more water is forced between the wave crests causing the height of the wave to increase.
Because of this "shoaling" effect, a tsunami that was imperceptible in deep water may grow to have wave heights of several meters or more.
If the trough of the tsunami wave reaches the coast first, this causes a phenomenon called drawdown, where it appears that sea level has dropped considerably.
Drawdown is followed immediately by the crest of the wave which can catch people observing the drawdown off guard. When the crest of the wave hits, sea level rises (called run-up ).
Run-up is usually expressed in meters above normal high tide.
Run-ups from the same tsunami can be variable because of the influence of the shapes of coastlines. One coastal area may see no damaging wave activity while in another area
destructive waves can be large and violent. The flooding of an area can extend inland by 300 m or more, covering large areas of land with water and debris. Flooding tsunami waves tend to carry loose objects and people out to sea when they retreat.
Tsunamis may reach a maximum vertical height onshore above sea level, called a run-up height, of 30 meters. A notable exception is the landslide generated tsunami in Lituya Bay, Alaska in 1958 which produced a 60 meter high wave because the wavelengths and velocities of tsunamis are so large, the period of such waves is also large, and larger than normal ocean waves. Thus it may take several hours for successive crests to reach the shore. (For a tsunami with a wavelength of 200 km traveling at 750 km/hr, the wave period is about 16 minutes). Thus people are not safe after the passage of the first large wave, but must wait several hours for all waves to pass. The first wave may not be the largest in the series of waves. For example, in several different recent tsunamis the first, third, and fifth waves were the largest.
How Tsunamis Are Formed.
What is a Tsunami?
What does "tsunami" mean?
Tsunami is a Japanese word with the English translation, "harbor wave." Represented by two characters, the top character, "tsu," means harbor, while the bottom character, "nami," means "wave." In the past, tsunamis were sometimes referred to as "tidal waves" by the general public, and as "seismic sea waves" by the scientific community. The term "tidal wave" is a misnomer; although a tsunami's impact upon a coastline is dependent upon the tidal level at the time a tsunami strikes, tsunamis are unrelated to the tides. Tides result from the imbalanced, extraterrestrial, gravitational influences of the moon, sun, and planets. The term "seismic sea wave" is also misleading. "Seismic" implies an earthquake-related generation mechanism, but a tsunami can also be caused by a nonseismic event, such as a landslide or meteorite impact.
A tsunami is a very long-wavelength wave of water that is generated by sudden displacement of the seafloor or disruption of any body of standing water. Tsunamis are sometimes called "seismic sea waves", although, as we will see, they can be generated by other mechanisms than
earthquakes. Tsunamis have also been called "tidal waves", but this term should not be used because they are not in any way related to the tides of the Earth. Because tsunamis occur suddenly, often without warning, they are extremely dangerous to coastal communities. How are tsunamis formed....
Life of a Tsunami
How are tsunamis formed...
Image of Tsunami Generation
Panel 1—Initiation: Earthquakes are commonly associated with ground shaking that is a result of elastic waves traveling through the solid earth.
However, near the source of submarine earthquakes, the seafloor is "permanently" uplifted and down-dropped, pushing the entire water column up and down. The potential energy that results from pushing water above mean sea level is then transferred to horizontal propagation of the tsunami wave (kinetic energy). For the case shown above, the earthquake rupture occurred at the base of the continental slope in relatively deep water. Situations can also arise where the earthquake rupture occurs beneath the continental shelf in much shallower water.
Note: In the figure, the waves are greatly exaggerated compared to water depth. In the open ocean, the waves are at most several meters high spread over many tens to hundreds of
kilometers in length.
Image of Tsunami Wave Split
Panel 2—Split: Within several minutes of the earthquake, the initial tsunami (Panel 1) is split into a tsunami that travels out to the deep ocean (distant tsunami) and another tsunami that travels towards the nearby coast (local tsunami). The height above mean sea level of the two
oppositely traveling tsunamis is approximately half that of the original tsunami (Panel 1). (This is somewhat modified in three dimensions, but the same idea holds.) The speed at which both tsunamis travel varies as the square root of the water depth. Therefore, the deep-ocean tsunami travels faster than the local tsunami near shore.
Panel 3—Amplification: Several things happen as the local tsunami travels over the continental slope. Most obvious is that the amplitude increases. In addition, the wavelength decreases. This results in steepening of the leading wave--an important control of wave runup at the coast (next panel).
Note that the first part of the wave reaching the local shore is a trough, which will appear as the sea receeding far from shore. This is a common natural warning sign for tsunamis.
Note also that the deep ocean tsunami has traveled much farther than the local tsunami because of the higher propagation speed. As the deep ocean tsunami approaches a distant shore, amplification and shortening of the wave will occur, just as with the local tsunami shown above.
Image of Tsunami Runup
Panel 4—Runup: Tsunami runup occurs when a peak in the tsunami wave travels from the near-shore region onto shore. Runup is a measurement of the height of the water onshore
observed above a reference sea level.
Except for the largest tsunamis, such as the 2004 Indian Ocean event, most tsunamis do not result in giant breaking waves (like normal surf waves at the beach that curl over as they approach shore). Rather, they come in much like very strong and fast-moving tides (i.e., strong surges and rapid changes in sea level). Much of the damage inflicted by tsunamis is caused by strong currents and floating debris.
The small number of tsunamis that do break often form vertical walls of turbulent water called bores. Tsunamis will often travel much farther inland than normal waves.
Do tsunamis stop once on land? No! After runup, part of the tsunami energy is reflected back to the open ocean and scattered by sharp variations in the coastline. In addition, a tsunami can generate a particular type of coastal trapped wave called edge waves that travel back-and forth, parallel to shore. These effects result in many arrivals of the tsunami at a particular point on the coast rather than a single wave as suggested by Panel 3. Because of the complicated behavior of tsunami waves near the coast, the
first runup of a tsunami is often not the largest,
emphasizing the importance of not returning to a beach many hours after a tsunami first hits. How are tsunamis formed..