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International Strategy for Disaster Reduction
Platform for the Promotion of Early Warning


What is tsunami?
In brief - Recent and historical tsunamis - Tsunami early warning system technique - Tsunami warning systems
- Key tsunami actors & organisations - Research projects - Further reading

In brief

Compared to other natural hazards (tropical storms, floods, droughts, etc.) destructive tsunamis occur relatively rare. But the recent tsunami in the Indian Ocean dramatically showed what could happen if a tsunami waves triggered by a major earthquake reaches coastal areas without any early warning. The highly energetic tsunami waves struck the costal areas devastating everything on their path. The costal population lost everything, most of the poorly built houses could not stand the massive flood or they were destroyed by flooding material. As floods reached in some areas several kilometers inland wide districts are affected by salinity. Crops, soil and wells for drinking water are contaminated with salt water; it will take years until they could be used again.

What is a tsunami?
How tsunamis are triggered
Where tsunamis occur
How tsunami waves travel across the ocean
How tsunamis behave as approaching land
What is a Tsunami Early Warning System?

What is a tsunami?
Tsunamis (Japanese for “harbour wave”) are a series of very large waves with extremely long wavelength, in the deep ocean, the length from crest to crest may be 100 km and more. Its height will be only a few decimetres or less. That is why tsunamis can not be felt aboard ships nor can they be seen from the air in the open ocean.

They are generated by any rapid, large-scale disturbance of the sea. The waves could travel away from the triggering source with speeds exceeding 800 km/h over very long distances. They could be extremely dangerous and damaging when they reach the coast because when the tsunami enters shallower water of the coastal areas the velocity of its waves will decrease and therefore the wave height increase. In shallow waters a large tsunami can crest to heights exceeding 30 m or the water level could rise in a very short time for several tens of meters.

How tsunamis are triggered
Most tsunamis, including the most destructive ones are generated by large and shallow earthquakes which usually occur near geological plate boundaries, or fault-lines, where geological plates collide. When the seafloor abruptly deforms the sudden vertical displacements over large areas disturb the ocean's surface, displace water, and generate destructive tsunami waves.
Animation of an earthquake triggered tsunami

The main factor determining the initial size of a tsunami is the amount of vertical sea floor deformation which, in turn, is controlled by the earthquake's magnitude, depth, and fault characteristics. Parameters which influence the size of a tsunami along the coast are the shoreline and bathymetric configuration, the velocity of the sea floor deformation, the water depth near the earthquake source, and, the efficiency with which energy is transferred from the earth's crust to the water column. Usually, it takes an earthquake with a Richter magnitude exceeding 7.5 to produce a destructive tsunami.

Volcanic eruptions, landslides or asteroid impacts could also trigger a tsunami but much less frequently. Even so, one of the largest and most destructive tsunamis was generated in August 26, 1883 after the Krakatoa (Indonesia) eruption. Major earthquakes are suspected to cause underwater slides or slumps of sediment. It is interesting to know that the largest tsunami wave ever observed was triggered by a rock fall l in Alaska on July 9, 1958. A huge block (40 million cubic meter) fall into the sea generating a huge wave but the tsunami energy diminished rapidly away from the source and was hardly recognised by tide gauge stations.

Source: Tsunami Community

Where tsunamis occur
Tsunamis can be generated in all parts of the world’s oceans and inland seas. Because the majority of tsunamis are triggered by submarine earthquakes, most occur in the Pacific Ocean. The Pacific Ocean is mainly bounded by subducting geological plates which is also called the “ring of fire”. Even if not very frequent, destructive tsunamis have been also been generated in the Atlantic Ocean (Portugal 1883) and the Indian Ocean (Sumatra 2004).

For further information see also the global seismic hazard map.

Source: USGS

How tsunami waves travel across the ocean
Once a disruption of the ocean floor has generated a tsunami the waves will travel outward from the source – similar to the ripples caused by throwing a rock into a pond. The wavelength and the period of the tsunami waves depend on the generating source. A high magnitude earthquake of a long fault line will cause greater initial wavelength (~ hundred km) and period (from 5 to 90 minutes), similar to what a huge landslide could generate. The deeper the water, the faster the tsunami wave will travel. In deep oceans, waves can travel with a high speed of up to 800 km/h and lose very little energy while travelling. The great tsunami waves in 1960 in Chile reached Japan which is 16800 km away in less than 24 hours. In many cases, despite tsunami waves travelling very fast, people living in high-risk coastal areas can be warned if there are adequate communication structures established and the people are aware of the risks they face.

How tsunamis behave as they approach land
The speed of the tsunami is related to the depth of the water. As the water depth decreases, the speed of the tsunami declines. The transformation of total energy of the tsunami leads to the growth of the tsunami waves. Tsunami waves will, however, normally not reach the coast as a huge wall of water. It may appear as a rapidly rising or falling tide, a series of breaking waves, or even a bore. Reefs, bays, entrances to rivers, undersea features and the gradient of the slope of the beach all help to modify the tsunami as it approaches the shore. The rise of the water level on shore varies in extreme cases; the water level can rise to more than 15 metres for tsunamis of distant origin and over 30 metres for a tsunami generated near the earthquake's epicentre. Tsunamis may reach a maximum vertical height, called a run-up height, onshore above sea level of 30 meters. Tsunamis consist of a series of waves, the first of which may not be the largest. Tsunamis have great erosional potential. The flooding of an area can extend inland by many hundreds of metres, covering large expanses of land with water and debris. Flooding tsunami waves tend to carry loose objects as well as people out to sea when they retreat.

What is a Tsunami Early Warning System?
Early warning is much more than just a prediction. PPEW defines a complete and effective early warning system as a package of four elements, spanning knowledge of the risks faced through to preparedness to act on early warning. Strong linkages between the four elements are essential. Therefore the major players concerned with the different elements need to meet regularly to ensure they understand all of the other components and what other parties need from them, and to agree on specific responsibilities throughout all four elements.

Key activities of all types of early warning systems include:
(i) construction of risk scenarios, (ii) improvements to the early warning system itself by adjusting it according to data and analysis from studies of past events (iii) development and publishing of manuals, (iv) dissemination of information, (v) practicing and testing of operational procedures such as evacuations. All these activities need to have a solid base of political support, institutional responsibility, availability of trained people as well as necessary laws and regulations. Early warning systems are most effective when established and supported as a matter of policy and when preparedness to respond is engrained in society.

The same basic factors are also valid for tsunami early warning systems. When designing an effective early warning system, the following four elements have to be considered because failure in any one part can mean failure of the whole system.

Prior knowledge of the risks faced by communities
Risks arise from both the hazards and the vulnerabilities that are present; therefore, we need to ask what the patterns and trends in these factors are. Several activities have to be undertaken to gather knowledge of the communities and localities at risk.

  • Hazard assessment:
    • Risk, exposure and vulnerability maps
    • Vulnerability and hazard databases
    • Source location
    • Tsunami scenario databases
    • Simulation, modelling
  • Periodically reevaluate community vulnerability and exposure
  • Land use planning and strategies (no further development, redevelopment, open space uses such as parks and agriculture, keep development at a minimum level in hazard-prone areas)
  • Construction work such as reinforcement of buildings, dams, walls, drainage, channels).
    • Slowing techniques involves creating friction that reduces the destructive power of waves. (Forests, ditches, slopes)
    • Steering techniques guide the force of tsunamis away from vulnerable structures and people by using angled walls and ditches
    • Blocking walls include compacted terraces and berms.

Tsunami protection wall, Japan.
  • Special concentration on critical facilities (fire stations, power substations, hospitals, sewage treatment plants) is needed. These key facilities should not be located in inundation zones. Relocation of these types of facilities out of inundation areas is necessary, however, if the location within the hazardous area is unavoidable than the buildings have to be designed or retrofitted to survive tsunami damage.
  • Special building codes for buildings constructed in exposed/hazardous areas – ensure that these codes and standards address the full range of potential hazards (multi hazard approach).
  • Evacuation plan:
    • Evacuation lines and planes have to be in place
    • Vertical and horizontal evacuation have to be considered
  • Incorporation of hazard information into planning processes
  • Adoption of comprehensive risk management policies

Technical monitoring and warning service for these risks
It is essential to determine whether the right factors are being monitored and if accurate warnings could be, in fact, generated in a timely fashion.

  • Around the clock operational capability and well trained rapid response unites
  • Hazard and location assessment studies
  • Exchange of experiences when establishing a new early warning system
  • Highly reliable measurements
  • Technical equipment appropriate to local circumstances and well maintained
  • Trained staff to handle the instruments in a proper way
  • Maintenance of measurement instruments and facilities
  • Continuing education for the staff to keep up-to-date
  • Coordination and cooperation among relevant monitoring centres
  • Cooperation and communication channels/lines with the global seismic network
  • Cooperation and communication channels/lines with tide gauge stations network

Dissemination of understandable warnings to those at risk
A critical issue regarding early warning systems is that the warnings reach those at risk and that the people at risk understand them. The warnings need to deliver proper information in order to enable proper responses. An additional requirement for an effective evacuation/warning system is continuous public education. Without education a system cannot be totally effective no matter how expensive or sophisticated it is. To design an effective and efficient system, all groups that take part in the notification process (emergency managers, media, etc) should be involved in the planning and implementation of the system. This is to ensure that all aspects of the systems are considered.

Tsunami siren, Hawaii
  • The dissemination system has to be adjusted to incorporate factors such as the size of the area at risk (compact/spread out), its location (harbour/beach), people at risk (retirees/transit population/tourists), financial resources of the community and existing notification systems.
  • Communities at risk need to be equipped with the necessary instruments to receive and further disseminate the warnings. Some of these are:
    • Sirens: good coverage including isolated areas, controllable from a central point for rapid notification, low maintenance but high cost, vulnerable to sand and salt. The siren’s meaning may not be recognized or ignored.
    • Telephones: good coverage, audio component (better understanding of the purpose of the warning), tailored messaging but proximity to the instrument necessary, expensive if rarely used, cell phones beyond coverage area (??), direct dial only (motels, offices excluded) problems in serving large population.
    • Radio: easy to use, widespread, mobile, inexpensive, audio component, rapidly transmitted message, but coverage limited to those with radios, not all coastal areas are covered, high rate of false alarms, radio must be on at all times.
    • Emergency Alert System (EAS): (Example from the US) wide coverage, can broadcast evacuation notice, inexpensive, consistent and ability to rapidly send message, but radio/television must be switched on, not all radio/TV stations are staffed 24 hours, does not work on satellite TV, power dependent (some area no coverage)
    • Pagers: inexpensive if used for specific audience but coverage limited
    • Billboards: simple, always available, multicultural but often a victim of vandalism
    • Aircraft: reaches remote areas, reach accessible areas which are not covered by other means, complements other systems but limited in number and coverage, slow response.
  • Dissemination systems in place for local and distant tsunamis.

Knowledge and preparedness to act
Communities have to understand their risks, they need to respect the warning service, and they have to know how to react if a warning is issued.

  • Implement effective information and education programmesMaintain the programmes over a long term
  • Awareness building/ training within the communities
  • Curriculum for schools located in districts at risk
  • Training and regular drill of emergency situation
  • Plan for evacuation: horizontal evacuation (moving people to more distant locations or higher ground) vertical evacuation (moving people to higher floors in buildings)
  • Inventory of buildings (potential for vertical evacuation)
  • Identify specific buildings to serve as vertical shelters
  • Agreements with house owners
  • Maintain communities interest in natural hazards, annual events (tsunami week, integration into social life)

For further reading please see:

Designing for Tsunamis: Seven Principles for Planning and Designing for Tsunami Hazards, Richard Eisner and others, 2001. US National Tsunami Hazard Mitigation Program.

Tsunami Warning Systems and Procedures: Guidance for Local Officials
Oregon Emergency Management and Oregon Department of Geology and Mineral Industries, 2001.