Prof. Stephen A. Nelson
Up until December of 2004, the phenomena of tsunami was not on the minds of most of the world's population. That changed on the morning of December 24, 2004 when an earthquake of moment magnitude 9.1 occurred along the oceanic trench off the coast of Sumatra in Indonesia. This large earthquake resulted in vertical displacement of the sea floor and generated a tsunami that eventually killed about 230,000 people and affected the lives of several million people. Although people living on the coastline near the epicenter of the earthquake had little time or warning of the approaching tsunami, those living farther away along the coasts of Thailand, Sri Lanka, India, and East Africa had plenty of time to move higher ground to escape. But, there was no tsunami warning system in place in the Indian Ocean, and although other tsunami warning centers attempted to provide a warning, there was no effective communication system in place. Unfortunately, it has taken a disaster of great magnitude to point out the failings of the world's scientific community and to educate almost every person on the planet about tsunami.
Even with heightened world awareness of tsunami, disasters still occur. On September 29, 2009, earthquakes in the Samoa region of the southwest Pacific Ocean killed nearly 200 people, and as a result of the Chilean earthquake of February, 2010, at least 50 casualties resulted from a tsunami triggered by a moment magnitude 8.8 earthquake.
On March 11, 2011 a Moment Magnitude 9.0 earthquake struck off the northern Coast of Japan. The Earthquake generated a tsunami that rose up to 135 feet above sea level and killed over 20,000 people. Because of Japan’s familiarity with earthquakes and enforcement of earthquake resistant building codes, there was only minor destruction from the earthquake itself. But, despite that fact that a tsunami warning system was in place, the earthquake was so close to the coast, that little time was available for people to react.
Besides that high death toll, the tsunami caused one of the worst nuclear disasters in history. The Fukushima nuclear power plant, located on the coast was hit by a 49 ft. tsunami wave that overtopped the tsunami protection walls that were only 19 feet high, and flooded the backup generators for the plant that were somehow placed on the first floor in a known tsunami zone!
We will first exam videos of the Japanese tsunami, (see also - http://www.pbs.org/wgbh/nova/earth/japan-killer-quake.html) then discuss some important points about tsunami, followed by a PBS video concerning the 2004 Indonesian Tsunami which killed over 230,000 people (see http://www.pbs.org/wgbh/nova/tsunami/).
The lecture notes below cover the essential points discussed in class and provide more details.
What is a Tsunami
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. Tsunami are sometimes called "seismic sea waves", although they can be generated by mechanisms other than earthquakes. Tsunami 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 tsunami occur suddenly, often without warning, they are extremely dangerous to coastal communities.
Physical Characteristics of Tsunami
All types of waves, including tsunami, have a wavelength, a wave height, an amplitude,
a frequency or period, and a velocity.
V = λ/P
|Tsunami 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, (10m/sec2)
and the depth of the water, 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. Tsunami 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 tsunami 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 tsunami the first, third, and fifth waves were
How Tsunami are Generated
There is an average of two destructive tsunami per year in the Pacific basin. Pacific wide tsunami are a rare phenomenon, occurring every 10 - 12 years on the average. Most of these tsunami are generated by earthquakes that cause displacement of the seafloor, but, as we shall see, tsunami can be generated by volcanic eruptions, landslides, underwater explosions, and meteorite impacts.
Earthquakes cause tsunami by causing a disturbance of the seafloor. Thus, earthquakes that occur along coastlines or anywhere beneath the oceans can generate tsunami. The size of the tsunami is usually related to the size of the earthquake, with larger tsunami generated by larger earthquakes. But the sense of displacement is also important. Tsunami are generally only formed when an earthquake causes vertical displacement of the seafloor. The 1906 earthquake near San Francisco California had a Richter Magnitude of about 7.1, yet no tsunami was generated because the motion on the fault was strike-slip motion with no vertical displacement. Thus, tsunami only occur if the fault generating the earthquake has normal or reverse displacement. Because of this, most tsunami are generated by earthquakes that occur along the subduction boundaries of plates, along the oceanic trenches. Since the Pacific Ocean is surrounded by plate boundaries of this type, tsunami are frequently generated by earthquakes around the margins of the Pacific Ocean.
Examples of Tsunami generated by Earthquakes
Although the December 2004 Indian Ocean tsunami is by far the best well known and most deadly (and will be featured in a video in class), we here discuss other disastrous tsunami generated by earthquakes.
Volcanoes that occur along coastal zones, like in Japan and island arcs
throughout the world, can cause several effects that might generate a tsunami. Explosive eruptions can rapidly emplace pyroclastic flows into the water, landslides and
debris avalanches produced by eruptions can rapidly move into water, and collapse of
volcanoes to form calderas can suddenly displace the water.
The eruption of Krakatau in the Straights of Sunda, between Java and Sumatra, in 1883 generated at least three tsunami that killed 36,417 people. It is still uncertain exactly what caused the tsunami, but it is known that several events that occurred during the eruption could have caused such tsunami.
One of the tsunami had a run-up of about 40 m above normal sea level. A large block of coral weighing about 600 tons was ripped off the seafloor and deposited 100 m inland. One ship was carried 2.5 km inland and was left 24 meters above sea level, with all of its crew swept into the ocean.
Landslides moving into oceans, bays, or lakes can also generate tsunami. Most such landslides are generated by earthquakes or volcanic eruptions. As previously mentioned, a large landslide or debris avalanche fell into Lituya Bay, Alaska in 1958 causing a wave with a run-up of about 60 m as measured by a zone completely stripped of vegetation.
Nuclear testing by the United States in the Marshall Islands in the 1940s and 1950s generated tsunami.
While no historic examples of meteorite impacts are known to have produced a tsunami, the apparent impact of a meteorite at the end of the Cretaceous Period, about 65 million years ago near the tip of what is now the Yucatan Peninsula of Mexico, produced tsunami that left deposits all along the Gulf coast of Mexico and the United States.
Mitigation of Risks and Hazards
The main damage from tsunami comes from the destructive nature of the waves themselves. Secondary effects include the debris acting as projectiles which then run into other objects, erosion that can undermine the foundations of structures built along coastlines, and fires that result from disruption of gas and electrical lines. Tertiary effects include loss of crops and water and electrical systems which can lead to famine and disease.
Within the last century, up until the December 2004 tsunami, there were 94 destructive tsunami which resulted in 51,000 deaths. Despite the fact that tsunami warning systems have been in place in the Pacific Ocean basin since 1950, deaths still result from tsunami, especially when the source of the earthquake is so close to a coast that there is little time for a warning, or when people do not heed the warning or follow instructions associated with the warning. These factors point out the inadequacy of the world in not having a tsunami warning system in place in the Indian Ocean, where in one event, the death toll from tsunami was increased by a factor of 5 over all previous events.
Prediction and Early Warning
For areas located at great distances from earthquakes that could potentially generate a tsunami there is usually plenty of time for warnings to be sent and coastal areas evacuated, even though tsunami travel at high velocities across the oceans. Hawaii is good example of an area located far from most of the sources of tsunami, where early warning is possible and has saved lives. For earthquakes occurring anywhere on the subduction margins of the Pacific Ocean there is a minimum of 4 hours of warning before a tsunami would strike any of the Hawaiian Islands.
The National Oceanic and Atmospheric Administration (NOAA) has set up a Pacific warning system for areas in the Pacific Ocean, called the Pacific Tsunami Warning Center. It consists of an international network of seismographic stations, and tidal stations around the Pacific basin that can all send information via satellite to the Center located in Hawaii. When an earthquake occurs somewhere in the region, the Center immediately begins to analyze the data looking for signs that the earthquake could have generated a tsunami. The tidal stations are also monitored, and if a tsunami is detected, a warning is sent out to all areas on the Pacific coast. It takes at least 1 hour to assimilate all of the information and issue a warning. Thus for an average velocity of a tsunami of 750 km/hr, the regional system can provide a warning sufficient for adequate evacuation of coastal areas within 750 km of the earthquake.
In order to be able to issue warnings about tsunami generated within 100 to 750 km of an earthquake, several regional warning centers have been set up in areas prone to tsunami generating earthquakes. These include centers in Japan, Kamchatka, Alaska, Hawaii, French Polynesia, and Chile.
These systems have been very successful at saving lives. For example, before the Japanese warning system was established, 14 tsunami killed over 6000 people in Japan. Since the establishment of the warning system, up until March 2011, 20 tsunami have killed 215 people in Japan.
Like all warning systems, the effectiveness of tsunami early warning
depends strongly on local authority's ability to determine that their is a danger, their
ability to disseminate the information to those potentially affected, and on the education
of the public to heed the warnings and remove themselves from the area.
Tsunami Safety Rules
In case you are ever in an area where there is a threat of tsunami, I have downloaded the following tsunami safety rules from the West Coast & Alaska Tsunami Warning Center Home Page: http://wcatwc.arh.noaa.gov/safety.htm
Questions on this material that could be asked on an exam
Note that answers to some of these questions will come from the video