Study Material | Test Series
Drishti IAS
call1800-121-6260 / 011-47532596
Drishti The Vision Foundation
(A unit of VDK Eduventures Pvt. Ltd.)
International Collaboration to prevent disaster of TSUNAMI
May 18, 2015

“Tsunami” (soo-NAH-mee) is a series of traveling 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], and a wave height of only a few tens of centimeters or less. Tsunami waves are distinguished from ordinary ocean waves by their great length between wave crests, often exceeding 100 km 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 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 Even a tsunami 3–6 meters  high can be very destructive and cause many deaths and injuries.

Earth quake and Tsunami

An earthquake can be caused by volcanic activity, but most are generated by movements along fault zones associated with the plate boundaries. Most strong earthquakes, representing 80% of the total energy released worldwide by earthquakes, occur in subduction zones where an oceanic plate slides under a continental plate or another younger oceanic plate.

Not all earthquakes generate tsunamis. To generate a tsunami, the fault where the earthquake occurs must be underneath or near the ocean, and cause vertical movement of the seafloor (up to several meters) over a large area (up to a hundred thousand square kilometers). Shallow focus earthquakes (depth less 70 km or 42 mi) along subduction zones are responsible for most destructive tsunamis. The amount of vertical and horizontal motion of the sea floor, the area over which it occurs, the simultaneous occurrence of slumping of underwater sediments due to the shaking, and the efficiency with which energy is transferred from the earth’s crust to the ocean water are all part of the tsunami generation mechanism.

Underwater landslides associated with smaller earthquakes are also capable of generating destructive tsunamis. The tsunami that devastated the northwestern coast of Papua New Guinea on July 17, 1998, was generated by an earthquake that registered 7.0 on the Richter scale that apparently triggered a large underwater landslide. Three waves measuring more than 7 meter high struck a 10-kilometer stretch of coastline within ten minutes of the earthquake/slump. Three coastal villages were swept completely clean by the deadly attack leaving nothing but sand and 2,200 people dead. Other large-scale disturbances of the sea -surface that can generate tsunamis are explosive volcanoes and asteroid impacts. The eruption of the volcano Krakatoa in the East Indies on Aug. 27, 1883 produced a 30-meter tsunami that killed over 36,000 people. In 1997, scientists discovered evidence of a 4km diameter asteroid that landed offshore of Chile approximately 2 million years ago that produced a huge tsunami that swept over portions of South America and Antarctica.

Based on the distance from the source and the travel time tsunamis are classified in to three types by UNESCO-IOC. They are:

1. Local Tsunami originates from a nearby source for which its destructive effects are confined to coasts within 100 km or less than 1 hour tsunami travel time from its source. These can often be the most dangerous because there is often little warning between the triggering event and the arrival of the tsunami.

2. Regional Tsunami is capable of creating destruction in a particular geographic region, generally within 1,000 km or 1-3 hours tsunami travel time from its source. Regional tsunamis occasionally have very limited and localized effects outside the region.

3. Distant Tsunami (also called an ocean-wide, distant, tele or far-field tsunami) is a tsunami that originates from a far away source, which is generally more than 1,000 km away from the area of interest or more than 3 hours tsunami travel time from its source. A distant tsunami is capable of causing widespread destruction, not only in the immediate region of its generation, but across an entire ocean. All ocean-wide tsunamis have been generated by major earthquakes in the subduction regions.

Speed of tsunami

Where the ocean is over 6,000 m deep, unnoticed tsunami waves can travel at the speed of a commercial jet plane, over 800 km per hour. They can move from one side of the Pacific Ocean to the other in a day and across the Indian Ocean in 12 hrs. This great speed makes it important to be aware of the tsunami as soon as it is generated. Scientists can predict when a tsunami will arrive at various places by knowing the source characteristics of the earthquake that generated the tsunami and the characteristics of the seafloor along the paths to those places. Tsunamis travel much slower in shallower coastal waters where their wave heights begin to increase dramatically.

Height of Tsunami

Offshore and coastal features can determine the size and impact of tsunami waves. Reefs, bays, entrances to rivers, undersea features and the slope of the b each all help to modify the tsunami as it attacks the coastline. When the tsunami hits the coast, often as a wall of water, sea levels can rise many meters. In extreme cases, water level has risen to more than 15 m  for tsunamis of distant origin and over 30 m  for tsunami waves generated near the epicenter. The first wave may not be the largest in the series of waves. One coastal community may see no damaging wave activity while in another nearby community destructive waves can be large and violent. The flooding can extend kilometers inland or more, flooding large expanses of land with water and debris.



Frequency of Tsunami

Since scientists cannot predict when earthquakes will occur, they cannot determine exactly when a tsunami will be generated. However, by looking at past historical tsunamis, scientists know where tsunamis are most likely to be generated. Past tsunami height measurements are useful in predicting future tsunami impact and flooding limits at specific coastal locations and communities.

Paleotsunami research, in which scientists look for sediments deposited by giant tsunamis, is helping to extend the documented historical tsunami record further back in time. As more events are found, better estimates of the frequency of occurrence of tsunamis in a region are obtained. During each of the last five centuries, there were three to four Pacific-wide tsunamis, most of them generated off the Chilean coasts. The tsunami on December 26, 2004 claimed 227,000 lives and caused damage throughout the Indian Ocean, making it the worst tsunami catastrophe in history. It was also the first known basin-wide destructive tsunami in the Indian Ocean.

Tsunamigenic zones

Thy are major subduction zones of world and cause of Tsunami.

The Pacific Rim countries are affected frequently by tsunamis (about 900 events in the 20th century) due to the high seismic activity (75 per cent of global activity) in the circum-Pacific belt. The Indian Ocean rim countries are likely to be affected by tsunamis generated in the two known tsunamigenic zones, viz., the Andaman-Nicobar-Sumatra island arc and the Makran subduction zone north of Arabian Sea (about 5 per cent or less). These zones have been identified by considering the historical tsunamis and earthquakes, their magnitudes, location of the area relative to a fault, and also by tsunami modelling. The east and west coasts of India and the island regions are likely to be affected by Tsunamis generated mainly by subduction zone related earthquakes from the two potential source regions, viz., the Andaman-Nicobar-Sumatra Island Arc and the Makran subduction zone north of Arabian Sea. These two possible locations with major plate boundaries are shown in the blue ellipses in the figure below. However, in spite of infrequent occurrence of tsunamis (about 6 events reported in the 20th century) in the Indian Ocean, they could occur at any time and could be very devastating.

Historic Tsunamis in India

The Indian coastal belt has not recorded many Tsunamis in the past. Waves accompanying earthquake activity have been reported over the North Bay of Bengal. During an earthquake in 1881 which had its epicenter near the centre of the Bay of Bengal, Tsunamis were reported. The earthquake of 1941 in Bay of Bengal caused some damage in Andaman region. This was unusual because most Tsunamis are generated by shocks which occur at or near the flanks of continental slopes. During the earthquakes of 1819 and 1845 near the Rann of Kutch, there were rapid movements of water into the sea. There is no mention of waves resulting from these earthquakes along the coast adjacent to the Arabian sea, and it is unlikely that Tsunamis were generated. Further west, in the Persian Gulf, the 1945 Makran earthquake (magnitude 8.1) generated Tsunami of 12 to 15 meters height. This caused a huge deluge, with considerable loss of life and property at Ormara and Pasi. The estimated height of Tsunami at Gulf of Cambay was 15m but no report ofdamage is available. The estimated height of waves was about 2 meters at Mumbai, where boats were taken away from their moorings and casualties occurred.

A list showing the Tsunami that affected Indian coast prior to Sumatra Earthquake of December 26, 2004, is given in the below table.

Date Cause Impact 
12th April, 1762 Earthquake in Bay of Bengal. Tsunami wave of 1.8m at Bangladesh coast 
31st December, 1881 Magnitude 7.8 earthquake beneath the Car Nicobar Entire East coast of India including Andaman & Nicobar coast was affected by tsunami
27th August, 1883 Eruption of karkatoa volcano (Sunda Strait) Indonesia East coast of India was affected and 2 m Tsunami was reported at Chennai.
26th June, 1941 A 8.1 Magnitude earthquake in Andaman East Coast of India was affected by tsunami.
27th November,1945 27th November, 1945 (21 Hour 56 Min. 50 Sec. UTC) in the Makran subduction zone (Baluchistan, Pakistan) (24.5N & 63.0E) with Mw of 8.1 West coast of India was affected by Tsunami.

TSUNAMI WARNING CENTRES

The Intergovernmental Oceanographic Commission of UNESCO coordinates the global tsunami warning and mitigation system that covers the Pacific Ocean, Indian Ocean, Caribbean, Northeastern Atlantic Ocean, and the Mediterranean. The Pacific system, established in 1965 in response the 1960 M9.5 earthquake and deadly tsunami, is the oldest. The ICG/PTWS, encompassing 46 Pacific countries, oversees warning system operations and facilitates coordination and cooperation of international tsunami mitigation activities.Warning centres then monitor incoming sea level data to determine whether a tsunami has occurred. If a significant tsunami with destructive potential is detected, warning areas are extended. PTWC

The Intergovernmental Oceanographic Commission (IOC)

It is  a body with functional autonomy within the United Nations Educational, Scientific and Cultural Organization (UNESCO), provides Member States with an essential mechanism for global co-operation in the study of the ocean. The IOC assists governments to address their individual and collective ocean and coastal problems through the sharing of knowledge, information and technology and through the coordination of national and regional programs. The functions of the IOC are to develop, recommend and coordinate international programs for scientific study of the oceans and related ocean services; to promote and make recommendations for the exchange of oceanographic data and the publication and dissemination of scientific investigation results; to promote and coordinate the development and transfer of marine science and its technology; to make recommendations to strengthen education and training and to promote scientific investigation of the oceans and application of the results thereof for the benefit of all mankind. In 2014, 147 countries were Member States of the IOC. The Assembly meets every two years at the UNESCO headquarters in Paris, France. The IOC consists of an Assembly, an Executive Council, a Secretariat and such subsidiary bodies as it may establish. Under this concept, the Commission has created Global and Regional Programmes which examine and execute specific projects, or consist of committees composed of Member States interested in such projects. This is the case for the Intergovernmental Tsunami Warning and Mitigation Coordination Groups in the Pacific (ICG/PTWS), Indian Ocean (ICG/IOTWS), Caribbean (ICG/CARIBE-EWS), and Northeastern Atlantic and Mediterranean (ICG/NEAMTWS)

receives sea level data from more than 600 coastal stations through cooperative data sharing between US agencies, research centres, countries, and international networks such as the Global Sea Level Observing System (GLOSS). It also receives data from more than 60 deepocean or DART sensors around the world. These data enable accurate forecasts to be done. In Japan, GPS wave gauges provide offshore sea surface heights of oncoming tsunami. Warnings are then disseminated to designated emergency agencies and the general public by a variety of communication methods. Countries may operate National or Sub-regional Centres to provide more rapid or detailed warnings for regional or local tsunamis. In the Pacific, the JMA provides local warnings to Japan, and its NWPTAC provides forecasts to countries in the Northwest Pacific, South China, and North Pacific island nations. The Centre Polynesien de Prevention des Tsunamis in French Polynesia, Chile, and Russia have operated national warning systems for decades. Australia, Canada, Colombia, Ecuador, El Salvador, Indonesia, Korea, Malaysia, Mexico, New Zealand, Nicaragua, Peru, Philippines, Samoa, Solomon Islands, Thailand, Tonga, and Vietnam have improved their systems since 2004. In the US, the US National Tsunami Warning Center (NTWC) warns North America, including Canada, and the PTWC warns Hawaii, American Samoa, Guam and the Northern Mariana Islands. Through the open and timely sharing of data, Warning Centres can provide backup and supplemental analyses of events should a Centre become disabled. The Centres can also serve as focal points for regional tsunami awareness, education, and other mitigation activities.The PTWC serves as the primary Tsunami Service Provider (TSP), with the Japan Meteorological Agency (JMA) Northwest Pacific Tsunami Advisory Center (NWPTAC) acting since 2006 as a regional TSP, and interim South China Sea TSP with PTWC. In the Indian Ocean, the PTWC and the JMA provided interim services from 2005-2013 until Australia, India, and Indonesia assumed roles as permanent TSPs. In the Caribbean, PTWC has been the interim TSP since 2006. The initial objective of a tsunami warning or advisory centre is to detect, locate and determine the seismic parameters of potentially tsunamigenic earthquakes occurring in its area of responsibility. To accomplish this, PTWC continuously receives seismographic data from more than 500 stations around the world through cooperative data exchanges with the U.S. Geological Survey, Incorporated Research Institutions for Seismology Global Seismic Network, International Deployment of Accelerometers, GEOSCOPE, and other national and international agencies running seismic networks. If an earthquake has the potential to generate a destructive tsunami based on its location, depth, and magnitude, a tsunami warning is immediately issued to advise of an imminent tsunami threat. Initial warnings may apply only to the nearest areas the tsunami could reach and messages include the predicted tsunami arrival times and/or wave heights. Coastal forecasts exceeding 0.3 m in height with strong water currents can cause significant damage and death as waves crush structures and floating debris become battering rams.

Warning centres then monitor incoming sea level data to determine whether a tsunami has occurred. If a significant tsunami with destructive potential is detected, warning areas are extended. PTWC receives sea level data from more than 600 coastal stations through cooperative data sharing between US agencies, research centres, countries, and international networks such as the Global Sea Level Observing System (GLOSS). It also receives data from more than 60 deepocean or DART sensors around the world. These data enable accurate forecasts to be done. In Japan, GPS wave gauges provide offshore sea surface heights of oncoming tsunami. Warnings are then disseminated to designated emergency agencies and the general public by a variety of communication methods. Countries may operate National or Sub-regional Centres to provide more rapid or detailed warnings for regional or local tsunamis. In the Pacific, the JMA provides local warnings to Japan, and its NWPTAC provides forecasts to countries in the Northwest Pacific, South China, and North Pacific island nations. The Centre Polynesien de Prevention des Tsunamis in French Polynesia, Chile, and Russia have operated national warning systems for decades. Australia, Canada, Colombia, Ecuador, El Salvador, Indonesia, Korea, Malaysia, Mexico, New Zealand, Nicaragua, Peru, Philippines, Samoa, Solomon Islands, Thailand, Tonga, and Vietnam have improved their systems since 2004. In the US, the US National Tsunami Warning Center (NTWC) warns North America, including Canada, and the PTWC warns Hawaii, American Samoa, Guam and the Northern Mariana Islands. Through the open and timely sharing of data, Warning Centres can provide backup and supplemental analyses of events should a Centre become disabled. The Centres can also serve as focal points for regional tsunami awareness, education, and other mitigation activities.

Source: http://www.kevinuhrmacher.com/ 


Ecological Impact of the tsunami

The consequences vary from loss of livelihood for fishermen to unknown damages to coral reefs and flora and fauna where the waves came a few miles inland. In some fragile areas near the Indonesian coast, it may take years for the coral reefs to get back the balance and mangrove stands and coastal tree plantations may have been destroyed or severely affected.

With so much seawater coming inland, salination is another effect that not only makes the soil less fertile to support vegetation but also increases vulnerability to erosion, the impacts of climate change and food insecurity.

For humans, on the other hand, fisheries, housing and infrastructure were the worst affected. For an in-depth account of the ecological and economic consequences of the tsunami, read here. It throws light on how reclamation of land for agriculture and settlement, setting up of resorts in high-risk beaches and the drastic loss of mangrove forests by man increased the damage caused by the tsunami.

Case study: Tsunami 2004

On Sunday 26 December 2004, a magnitude 9 earthquake occurred off the West Coast of Northern Sumatra in the Indian Ocean. This caused the Indian Ocean tsunami that affected 13 countries and killed approximately 230,000 people.

This tsunami was particularly devastating because:

♦ The earthquake which caused the tsunami was magnitude 9.

♦ The epicentre was very close to some densely populated coastal communities, eg Indonesia. They had little or no warning. The only sign came just before the tsunami struck when the waterline suddenly retreated, exposing hundreds of metres of beach and seabed.

♦ There was no Indian Ocean tsunami warning system in place. This could have saved more people in other countries further away from the epicentre.

♦ Many of the countries surrounding the Indian Ocean are LEDCs so they could not afford to spend much on preparation and prevention.

♦ In some coastal areas, mangrove forests had been removed to make way for tourist developments and therefore there was less natural protection

Social impacts of the tsunami (effects on people)

♦ 230 000 deaths.

♦ 1.7 million homeless.

♦ 5-6 million needing emergency aid, eg food and water.

♦ Threat of disease from mixing of fresh water, sewage and salt water.

♦ 1,500 villages destroyed in northern Sumatra

Economic impacts of the tsunami (effects on money and jobs)

♦ Ports ruined.

♦ Fishing industry devastated – boats, nets and equipment destroyed. An estimated 60% of Sri Lanka’s fishing fleet destroyed.

♦ Reconstruction cost billions of dollars.

♦ Loss of earnings from tourism - foreign visitors to Phuket dropped 80% in 2005.

♦ Communications damaged, eg roads, bridges and rail networks.

Environmental impacts of the tsunami

♦ Crops destroyed.

♦ Farm land ruined by salt water.

♦ 8 million litres of oil escaped from oil plants in Indonesia.

♦ Mangrove forests along the coast were destroyed.

♦ Coral reefs and coastal wetlands damaged

Responses to the tsunami

Non-Governmental Organisations (NGOs) and local authorities typically have immediate and secondary responses to devastation of this kind.

♦  Immediate responses

♦ Search and rescue

♦ Emergency food and water.

♦ Medical care.

♦ Temporary shelter.

♦ Re-establishing infrastructure and communications.

Secondary responses

♦ Re-building and improving infrastructure and housing.

♦ Providing jobs and supporting small businesses.

♦ Giving advice and technical assistance.  


Helpline Number : 87501 87501
To Subscribe Newsletter and Get Updates.