Part 1.2: Tsunamis

This chapter of the module is designed to:

enhance your knowledge of the causes and characteristics of tsunamis

contribute to your understanding of the threat to human settlements

expand your awareness of the predictability of tsunamis and the importance of warning systems

provide options for reducing the impact of tsunamis on humans, structures and infrastructure.

Introduction

Tsunami is a Japanese word meaning "harbor wave." Tsunamis are popularly called tidal waves but they actually have nothing to do with the tides. The waves, which often affect distant shores, originate from undersea or coastal seismic activity, landslides, and volcanic eruptions. Whatever the cause, sea water is displaced with a violent motion and swells up, ultimately breaking over land with great destructive power.

TSUNAMI HAZARD DATA SHEET
Casualty and damage data for selected tsunamis

Year	Location	Number of Deaths	Damage
1933	Japan	3,000
1939	Chile	30,000 (also from earthquake)
1946	Japan	1,400
1946	Hawaii	173	488 buildings destroyed
1960	Hawaii	61	537 buildings destroyed
1964	Alaska	9	158 houses, fishing fleet destroyed

Between 1900 and 1983, 20 tsunamis caused casualties and significant damage on the Pacific coasts of Mexico, Guatemala, El Salvador, Costa Rica, Panama, Colombia, Ecuador, Peru and Chile.

Sources: Land Management Guidelines in Tsunami Hazard Zones, 1982; and Primer on Natural Hazard Management, 1991.

Causal phenomena

The geological movements that cause tsunamis are produced in three major ways. The most common of these is fault movement on the sea floor, accompanied by an earthquake. A fault is defined as a planar zone of weakness passing through the earth's crust. To say that an earthquake causes a tsunami is not completely correct. Rather, both earthquakes and tsunamis result from fault movements.

Probably the second most common cause of tsunamis is a landslide either occurring underwater or originating above the sea and then plunging into the water. The highest tsunamis ever reported were produced by a landslide at Lituya Bay, Alaska in 1958. A massive rock slide produced a wave that reached a high water mark of 1,740 feet above the shoreline!

The third major cause of tsunamis is volcanic activity. The flank of a volcano, located near the shore or underwater, may be uplifted or depressed similar to the action of a fault. Or, the volcano may actually explode. In 1883, the violent explosion of the famous volcano, Krakatoa in Indonesia, produced tsunamis measuring 130 feet high which crashed upon Java and Sumatra. Over 36,000 people lost their lives as a result of tsunamis from Krakatoa.

Although tsunamis caused by landslides and volcanic activity may be very destructive near their sources, they have relatively little energy, decreasing rapidly in size and becoming almost unnoticeable at great distances. The giant tsunamis that are capable of crossing oceans are nearly always created by submarine faulting associated with earthquakes.

Figure 1.2.1 - Genesis of tsunamis

Q. What are the three main causes of tsunamis?

A. ____________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________ ______________________________________________________________
______________________________________________________________ ______________________________________________________________ ______________________________________________________________

ANSWER The three main causes of tsunamis are: 1. Fault movement on the sea floor 2. Landslides, either above or below water 3. Volcanic activity

General characteristics

Tsunamis differ from ordinary deep ocean waves, which are produced by wind blowing over water. Normal waves are rarely longer than 300 m from crest to crest. Tsunamis, however, may measure 150 km between successive wave crests. Tsunamis travel much faster than ordinary waves. Compared to normal wave speed of around 100 km per hour, tsunamis in the deep water of the ocean may travel the speed of a jet airplane - 800 km per hour! And yet, in spite of their speed, tsunamis increase the water height only 30-45 cm and often pass unnoticed by ships at sea. In 1946, a ship's captain on a vessel lying offshore near Hilo claimed he could feel no unusual waves beneath him although he saw them crashing on the shore.

Figure 1.2.2 - Tsunami formation

Contrary to popular belief, the tsunami is not a single giant wave. It is possible for a tsunami to consist of ten or more waves which is then termed a "tsunami wave train." The waves follow each other between 5 and 90 minutes apart.

As they approach the shore, tsunamis begin to change. The shape of the nearshore seafloor influences how tsunamis will behave. Where the shore drops off quickly into deep water, the waves will be smaller. Areas with long shallow shelves, such as the major Hawaiian islands, allow formation of very high waves. In the bays and estuaries, the water may slosh back and forth (these phenomena are called seiches) and can amplify waves to some of the greatest heights ever observed. As the waves approach shore, they travel progressively slower, finally decreasing to about 48 km per hour.

The initial onshore sign of a tsunami depends on what part of the wave first reaches land: a wave crest causes a rise in the water level and a wave trough will cause a recession. The rise may not be significant enough to be noticed by the general public. Observers are more likely to notice the withdrawal of water which may leave fish floundering on the seafloor. A tsunami does not always appear as a vertical wall of water, known as a bore, as typically portrayed in drawings. More often the effect is that of a incoming tide that floods the land. Normal waves and swells may ride on top of the tsunami or the tsunami may roll across relatively calm inland waters.

The flooding produced by a tsunami may vary greatly from place to place over a short distance due to a number of variables. These include submarine topography, shape of the shoreline, reflected waves, and modification of waves by seiches and tides. The Hilo tsunami of 1946, originating in the Aleutian trench, produced 30 foot waves in one location and only half that height a few miles away. The sequence of the largest wave in the tsunami wave train also varies and the destructiveness is not always predictable. In 1960 in Hilo, many people returned to their homes after two waves had passed only to be swallowed up in a giant bore which, in this case, was the third wave.

Predictability

Tsunamis have occurred in all oceans and in the Mediterranean Sea, but the great majority of them occur in the Pacific Ocean simply because the rim of the Pacific Ocean Basin is the most geologically active region in the world.

Tsunamis have occurred in all oceans and in the Mediterranean Sea, but the great majority of them occur in the Pacific Ocean. The zones stretching from New Zealand through East Asia, the Aleutians and the western coasts of the Americas all the way to the South Shetland Islands are characterized by deep ocean trenches, explosive volcanic islands and dynamic mountain ranges.

About 180 tsunamis were recorded between the years 1900 and 1970 in the Pacific. Of these, 35 caused casualties and damage only locally, while nine struck areas throughout the Pacific. A Tsunami Warning System (TWS) was developed in Hawaii shortly after the 1946 Hilo tsunami and is headquartered in the Pacific Warning Center in Honolulu, Hawaii. It has been improved and expanded and now consists of 62 tide stations, 77 seismic stations and hundreds of points for of information. There are 24 member countries in the Pacific Basin.

Figure 1.2.3 - Tsunami origins and vulnerable shorelines.

NOAA, Munich Re, 1988.

Figure 1.2.4 - National Tsunami Warning System Communications of Chile

The Tsunami Warning System works by monitoring seismic activity from a network of seismic stations. A tsunami is almost always generated by an undersea earthquake of magnitude 7 or greater. Therefore, special warning alarms sound when a quake measuring 6.5 or over occurs anywhere near the Pacific. A Tsunami Watch is declared if the epicenter is located dose enough to the ocean to be of concern. Government and voluntary agencies are then alerted and local media are activated to broadcast information. The five nearest tide stations monitor their gauges and trained observers watch the waves. If there are positive indicators, a Tsunami Warning is issued.

The TWS had met with general success in saving lives during the tsunamis of 1952 and 1957 in Hawaii. In 1960, however, two major earthquakes, occurring a day apart, rocked the coast of Chile in South America. The first registered 7.5 on the Richter scale and produced a small but noticeable wave in Hilo bay. The second registered a stunning 8.5, more than 30 times the energy of the first, and authorities predicted generation of a large, destructive tsunami. When the waves hit Hilo, fifteen hours after the earthquake, not all of the public had taken the warnings seriously and 61 people were killed. About 7 hours later, the tsunami struck Japan killing 180. When information of conditions in Chile reached TWS, it was learned that three giant waves had destroyed villages along a 500 mile stretch of coastal South America, arriving only 15 minutes after the earthquake.

The Chilean Government in recent years has experimented with use of satellite technology to provide nearly immediate warnings of potentially tsunamigenic earthquakes. Project THRUST (Tsunami Hazards Reduction Utilizing Systems Technology) can provide lifesaving tsunami hazard information, in an average elapsed time of two minutes within its communication radius. In conjunction with this satellite communications network, historical data, model simulations and emergency operations plans are used (more details are provided in the preparedness section of this chapter).

Hilo, Hawaii parking meters bent parallel to ground by force of 1960 tsunami.

Dudley and Lee, Tsunami!

Factors contributing to vulnerability

The major factors contributing to vulnerability to tsunamis are:

Growing world population, increasing urban concentration, and larger investments in infrastructure, particularly on the coastal regions. Some of these settlements and economic assets sit on low-lying coastal areas likely to be affected by tsunamis.

Lack of tsunami-resistant buildings and site planning.

Lack of a warning system or lack of sufficient education for the public to create awareness of the effects of a tsunami and unpredictable intensity. For example, having observed relatively moderate tsunamis in 1952 and 1957, citizens at Hilo in 1960 actually converged on the coast to watch the waves come in with catastrophic results.

Typical adverse effects

Physical damage

The majority of tsunami damage to human communities occurs within 30 minutes and 400 km of its source." The force of water in a bore (a steep fronted wave which moves inland at high speed) can raze everything in its path with pressures of up to 10,000 kg per square meter. It is the flooding effect of a tsunami, however, that most greatly effects human settlements by water damage to homes and businesses, roads and infrastructure.

Withdrawal of tsunamis also causes significant damage. As the waves are dragged back toward the sea, bottom sediments are scoured out collapsing piers and port facilities and sweeping out foundations of buildings. Entire beaches have disappeared and houses carried out to sea. Water levels and currents may change unpredictably and boats of all sizes may be swamped, sunk or battered.

Casualties and public health

Deaths occur principally from drowning as water inundates homes or neighborhoods. Many people may be washed out to sea or crushed by the giant waves. There may be some injuries from battering by debris. There is little evidence of tsunami flooding directly causing large scale health problems. In some cases malarial mosquitoes may increase breeding due to collection of water in trapped pools.

Water supply

Open wells and other groundwater may be contaminated by salt water and debris or sewage. Normal water supplies may be inaccessible for days due to broken water mains.

Crops and food supplies

Flooding and damage by tsunamis may result in the following:

loss of entire harvest, depending on time of year

land may be rendered infertile due to salt water incursion from the sea

food stocks not moved to high ground will be damaged

animals not moved to high ground may perish

farm implements may be lost hindering tillage

boats and fishing nets may be lost.

Possible risk reduction measures

Some systematic measures to protect coastlines against tsunamis include:

1) Site planning and land management for development of coastal areas.

2) Establishment of building codes or guidelines such as construction of houses on stilts to survive the waves, or use of reinforced concrete structure. Buildings, such as the hotels in Hilo bay are specially constructed with first floor living area elevated above potential wave height. Ground floor and basement will be inundated. Structural columns resist the impact while other walls are expendable.

3) Building barriers or buffers such as special breakwaters or seawalls. Bayfront areas may be designated as a park or sports area.

Tsunami evacuation route sign.

Dudley and Lee, Tsunami!

Specific preparedness measures

Hazard mapping and evacuation routes and procedures

Historical incidence may be studied to determine the areas most vulnerable to tsunamis. A hazard map should be created designating areas expected to be damaged by flooding or waves. Evacuation routes should constructed if necessary and mapped. Detailed plans should be made for actual evacuation procedures.

Early warning systems

As described earlier, the Tsunami Warning System is able to alert countries several hours before tsunamis strike. The weakness in the warning system is dissemination of information to the public where modern communication networks do not exist. In addition, sometimes tsunamis follow earthquakes in less than 15 minutes. There exists sufficient knowledge and technical expertise to develop early 'real-time' tsunami warning systems. A real-time seismic network permits accurate and almost instantaneous determination of the source parameters of all damaging earthquakes all over the world. Mar difficulties arise, however, in transferring scientific results to operational procedures.

It has been of great concern to experts that tsunamis occurring in areas of the globe other than the Pacific have not been focused on. Some of these tsunamis, such as those striking Greece and surrounding areas were disastrous and resulted in loss of lives. The Tsunami Warning Center of the Pacific encourages establishment of similar organizations and warning systems in other tsunami prone areas.

Community education

In areas where modern communication networks do not exist, the local population must be educated to recognize the signs of an approaching tsunami and what action to take. The following information should be disseminated:

Ground shaking signals the occurrence of an earthquake. Move away from low lying coastal areas since a tsunami may accompany the earthquake.

The coastal water may rise and fall, a sign of an approaching tsunami. The waves at one beach may be much smaller than at adjacent beaches.

A tsunami may have several waves. Stay away from the area for at least two hours. Do not stay to watch the waves, or you may not escape them.

Have respect for tsunami warnings issued and follow emergency evacuation plans and procedures.

Typical post disaster needs

The initial response by local authorities includes:

Implement warning and evacuation procedures

Perform search and rescue in the disaster area

Provide medical assistance

Conduct disaster assessment and epidemiological surveillance

Provide short term food, water, shelter.

Secondary responses include:

Repair and reconstruct buildings

Reestablish employment

Provide assistance for agricultural areas.

CASE STUDY

Project THRUST

Earthquakes in Chile result from subduction of the Nazca plate beneath the South American plate. The seismic potential in the Chilean trench is not completely known. In the past, tsunamis generated by local seismic activity, have struck the coast of Chile within 10 minutes. The National Tsunami Warning System in Chile could not be activated in less than 30 minutes. This situation led to an experimental installation of Project THRUST (Tsunami Hazards Reduction Utilizing Systems Technology) to upgrade the warning and response capacity. The benefits resulted from a systems approach to tsunami hazard mitigation and included:

1. Preparedness measures, including historical base studies, numerical model simulations and emergency operations plan development.

2. Instant local hazard assessment by using seismic triggers which activate a satellite to transmit signals to a ground station processor. (The average cost of hardware for the most basic system configuration consisting of a seismic station and a tsunami warning station was US$ 15,000.)

3. Rapid dissemination of information to local officials. The processor alerts the station manager and can also activate lights, alarms, telephone dialers and other emergency responses, thus providing rapid dissemination of information to local officials.

Using tsunami hazard maps of probable innundation areas combined with street maps to identify security areas, hospitals and evacuation routes, a THRUST Project Tsunami Emergency Operations Plan for Chile was devised. The plan listed measures to be taken upon issuance of the tsunami warning and long term relief efforts to be taken after the tsunami had receded, including responsibilities and functions of every disaster agency involved in a tsunami emergency.

This plan was tested by means of an exercise scenario and a control team which generated news or problems to the participants. A lack of coordination between several agencies was revealed by this exercise and necessitated a detailed revision of the plan which was later adopted. Emergency operations in Chile are organized on a regional, provincial and community basis and each administrative level has an Emergency Operating Center. It was found to be more advantageous to move the coordinating responses from the regional to the community level. Other weaknesses in the plan, such as lack of baseline innundation studies in some communities, were discovered.

What strong points does this project, as described, have in preparing for tsunamis?

1. Improved technology was incorporated into an already existing system which might have resulted in lower costs and more local acceptance than a completely new system.

2. The plan touched on the entire emergency system, not just the technological areas.

3. Representatives of every concerned government and non-government agency were consulted about the plan and had an opportunity to test it in the simulation.

4. The simulation identifed weaknesses in the emergency management system which may eventually save lives.

What are the weak points?

1. The project lacked a research component to work on further defining the seismic zones in Chile and conducting innundation studies on all of the villages.

2. The plan did not address issues of future planning and development in the innundation zones, or methods to lower risks to buildings and infrastructure.

3. No means of educating the general public about the tsunami hazard and emergency plan were mentioned.

References

Bernard, Eddie N., "Assessment of Project THRUST: Past, Present, Future", Natural Hazards, 4: 285-292,1991.

Disaster Management Center, Natural Hazards: Causes and Effects, University of Wisconsin Board of Regents, 1986.

Dudley, Walter C, and Min Lee, Tsunami!, University of Hawaii Press, 1988.

Erickson, Jon, Volcanoes and Earthquakes, Tab Books, Blue Ridge Summit, PA, 1988.

Gere, James M., and Haresh C. Shah, Terra Non Firma, W.H. Freeman and Company, New York, 1984.

Land Management Guidelines in Tsunami Hazard Zones, Urban Regional Research for the National Science Foundation, 1982.

Lockridge, Patricia, Tsunamis: The Scourge of the Pacific", in UNDRO NEWS, Jan/Feb. 1985, p. 15-16.

Lorca, E., Integration of the THRUST Project into the Chile Tsunami Warning System", Natural Hazards, 4:293-300,1991.

Tsunami Hazard: A practical guide for tsunami hazard reduction, edited by E.N. Bernard, Kluwer Academic Publishers, The Netherlands, 1991.

Verney, Peter, The Earthquake Handbook, Paddington Press, New York and London, 1979.