Tsunamis affecting South Asia

There have been 7 Tsunamis recorded in the Indian Ocean in the last 200 years. Out of these the 1941 Tsunami was the most dramatic.

South Asian Tsunamis (Pronounced: Sue-Naa-Me) See the Amateur Seismic Network

The infamous offspring of undersea earthquakes, tsunamis conjour images of towering waves such as those depicted in this Japanese wood cut, whose name. when translated means "In the hollow of the Great Wave off the coast of Kanagawa". Part of a set of 36 views of Mount Fuji, renowned Edo-era painter, Katsushika Hokusai depicts a mammoth wave that dwarfs Fujiyama in the back. Though towering waves rarely accompany

tsunamis, they are an immediate secondary threat to coastal and low-lying communities following offshore or coastal earthquakes. The word "Tsunami" is Japanese for "Harbour Wave". They are often wrongly called "Tidal Waves" and have nothing to do with tides. Most are generated by shallow earthquakes in the sea or by temblors near the coast that can set off underwater landslides. Volcanic eruptions scan also cause tsunamis.

Causes of Tsunamis

Shallow undersea earthquakes are responsible for most tsunamis though at time landslides triggered by smaller seismic events can also generated potentially lethal waves. Strong earthquakes cause a displacement of the crust. When they occur underwater, this crustal movement disturbs a large volume of water like a giant paddle and ripples spread out in all directions at speeds of 600-800 kilometres per hour, comparable to commercial aircraft. In the open ocean, they go unnoticed but once they reach shallower waters they slow down and begin to crest. The waves thus given rise to are known as "Tsunamis". They are scientifically described as a series of very long wavelength ocean waves caused by the sudden displacement of water by earthquakes, landslides, or submarine slumps and are mostly caused by earthquakes of magnitude 7.5 or greater. The run-up of a tsunami is the maximum height above normal sea level up to which the water level rose during a tsunami.

Major Tsunamis in South Asia

Though rare and relatively unheard of, tsunamis have struck the shores of South Asia in the past. The deadliest was in 1945 which originated off the Mekran coast in Pakistan and caused deaths as far as Mumbai. In earliest known tsunami occurred in the Bay of Bengal in 1762, caused by an earthquake on Myanmar's, Arakan Coast.

Tsunamis affecting India

Tsunamis on thE Coast Lines of India

Although the majority of the reported tsunamis are from littoral countries of the Pacific Ocean, there are a few cases of tsunamis in the Indian Ocean. The approximate length of the Indian coast is about 6000 kilometers. The coasts run from north to south and have two arms in the east and west with a tapering end at Kanyakumari. The tsunamigenic earthquakes occur mostly at the following three locations;

1. The Andaman sea,
2. Area about 400-500 kilometers SSW of Sri Lanka (Ceylon)
3. The Arabian Sea about 70-100 kilometers south of Pakistan Coast -- off Karachi and Baluchistan

The oldest record of tsunami is available from November 326 BC earthquake near the Indus delta/Kutch region. Alexander the Great was returning to Greece after his conquest and wanted to go back by a sea route. But an earthquake of large magnitude destroyed the mighty Macedonian fleet as reported by Lietzin (1974)

The earliest record of tsunami is reported to be about 1.5 meters at Chennai (formerly Madras) which was created due to the August 8, 1883 Krakatoa volcanic explosion in Indonesia. An earthquake of magnitude 8.25 occurred about 70 kilometers south of Karachi (Pakistan) at 24.5 N and 63.0 E on November 27, 1945. This created a large tsunami of about 11.0 to 11.5 meters high on the coasts of India in the Kutchch region, as reported by Pendse (1945). An earthquake of magnitude 8.1 occurred in the Andaman Sea at 12.9 N and 92.5 E on June 26, 1941 and a tsunami hit the east coast of India. As per non-scientific/journalistic sources, the height of the tsunami was of the order of 0.75 to 1.25 meters. At the time no tide gauge was in operation. Mathematical calculations suggest that the height could be of the order of 1.0 meter. There are a few more cases of earthquakes of magnitude less than 8.0 which have given rise to some smaller tsunamis. Bapat, et al (1983) have reported a few more earthquakes on the coast of Myanmar (formerly Burma).

1941 Tsunami in the Bay of Bengal

The Tsunami of 1941 following an earthquake in the Bay of Bengal was detected in Sri Lanka and was reported on as follows

1883 - Reports from Ceylon on the Tsunami

Reports on the Tsunami of 1883 in Sri Lanka

A series of three explosions on the morning of the 27 th of August 1883 aboit 05.28 local time in Krakatoa led to the destruction of Kraktoa's mountain peak and led to a tsunami that propagated across the Indian Ocean. Thereafter at 6.36 500 m high peak at Danan exploded and collapsed and the thrid blask tore the remaining part of Krakatau Island (Rakata Island) apart. The total energy released by the explosion was equivaled to 200 megatons atomic bombs (8.4 x 10E7 joules). At least 36,000 people were killed particularly in Java and Sumatra; wave heights rearched 15 to 42 meters.

In Sri Lanka (3113 km from Krakatau) two descriptions of the tsunami are avaialable:

At Galle:

"An extraordinary occurence was witnessed at the wharf at about 01:30 local time (15:30 Krakatau time). The sea receded as far as the landing stage on the jetty. The boats and canoes moored along the shored were left high and dry for about three minutes. A great number of prowans and fishes were taken up by the coolies and stragglers abbout the place before the water returned. Since the above was written, the sea has receded twice throught the harhour".

At Negombo at 03:00:

" the rise of the tide was so much above teh usual water-mark that many of the low morasses lying in close proximity to the seaside were replete with water that flowed into them. However the water thus accumulated did not remain long, but, foring into a stream, wended its course in a southerly direction, through low lands, to a distance of nearly a quarter of a mile, and found a passage back to the sea; thus the the water that had so abruptly covered up such an extent of land did not take many days in draining off"

"The receding waters were not slow behind in their action, for they washed away a belt of land about 132-198 feet (40-60m) in extent, including the burial ground situated on the coast to the south-west of teh bay compelling the inhabitants to seek shelter in a neighbouring cocoa-nut garden."

Sixteen recessions were counted between noon and 03:000 LT on 27 and rushing water produced what was described as a hissing sound. The crest height seems to have been more than 1 m. The wave reached Sri Lanka 5-7 hours after the earthquake.

At Arugam Bay in the Southeast:

"Three moorwomen, three children and a man were crossing the bar about 03:00 LT. A big wave came up from the sea and washed them inland. Soon after the water returned to the sea. The man said that the water came up to his chest: he isa tall man. These people were tumbling abou in the water, but were rescued by people in the Kalapuwa (inland estuary). They lost the paddy these were carrying and one of the women died two days after of her injusries.

Extracts as reported by

Choi, Pe
linovsky, Kim and Lee, Simulation of the trans-oceanic tsunami propagation due to the 1883 Krakatau volcanic eruption. Natural Hazards and Earth System Sciences, 3:321-332, 2003.

Indian Ocean Currents in the Equatorial Indian Ocean

Oceanography in the Equatorial Indian Ocean

To understand, why such a short-circuit is likely, we need to know about the ocean currents in the Indian Ocean close to Sri Lanka. Here, currents that come travel along the equator, those that come from open seas and those that are driven along the coasts of India, merge, clash and set off eddies and waves. The ocean current is driven from the Bay of Bengal to the Arabian Sea during the North-East monsoon and from the Arabian sea to the Bay of Bengal during the South-West monsoon. During the NE monsoon, the East Indian Coastal Current (EICC) travels down the East coast impelled by the monsoon winds and fed by the discharge of the Ganga and other rivers along the West Coast. These currents are particularly rapid when there is heavy rains or cyclones. During the South-West monsoon, the West Indian Coastal Current (WICC) travels down the West Coast. To the South of Sri Lanka, the Equatorial Monsoon Current (EMC) flows West and East during the South-West and North-East monsoons respectively.

Figure caption: The ocean currents during the North-East and South-West monsoons.

The Monsoon System in the Indian Ocean:
Courtesy: Tomzack and Godfreys: Regional Oceanography
Basin Topography of the Indian Ocean

The Monsoon System in the Indian Ocean from Regional Oceanography by Tomzack and Godfrey
The Equatorial Currents from Sumatra, past Sri Lanka to Somalia is brought out in this figure.

Disaster Preparedness Forecasting and Warning for Tsunamis

see Pararas-Carayannis George. TSUNAMI: FORECASTING. PREPAREDNESS AND WARNING, Fifteenth Conference on Broadcast Meteorology of the American Meteorological Society, April 9-12, 1985, Honolulu, Hawaii and http://www.drgeorgepc.com/TsunamiForecasting.html

Deep-ocean Assessment and Reporting of Tsunamis (DART): Brief Overview and Status Report

F. I. Gonzalez, H.B. Milburn, E.N. Bernard1, J. Newman

Pacific Marine Environmental Laboratory / NOAA Seattle, WA 98115

Joint Institute for the Study of the Atmosphere and Ocean / U. Washington Seattle, WA 98195

Abstract As part of the U.S. National Tsunami Hazard Mitigation Program, the DART Project is an effort by the Pacific Marine Environmental Laboratory of the National Oceanic and Atmospheric Administration to develop a capability for real-time reporting of tsunami measurements in the deep ocean. The systems utilize bottom pressure recorders (BPRs) capable of detecting and measuring tsunamis with amplitude as small as 1 cm in 6000 m of water. The data are transmitted by acoustic modem to a surface buoy, which then relays the information to a ground station via satellite telecommunications. This concept has been proven through several deep ocean deployments of prototype systems that provided extended periods of excellent data return. Design improvements in the next generation of systems will reduce the high data losses experienced during other periods. A planned network of six buoys in the north Pacific and equatorial region focuses on the hazard to U.S. coastal communities. Once this technology matures, consideration should be given to a coordinated international effort to establish additional stations of direct benefit to other Pacific Rim countries.

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With No Plan for a Warning, Indian Ocean Was Exposed

By ANDREW C. REVKIN
http://www.nytimes.com/2004/12/27/science/27science.html
December 27, 2004

he earthquake that struck northwest of Sumatra, Indonesia, at dawn yesterday was a perfect wave-making machine, and the lack of a tsunami warning system in the Indian Ocean essentially guaranteed the devastation that swept coastal communities around southern Asia, experts said.

Although waves swamped parts of the Sumatran coast and nearby islands within minutes, there would have been time to alert more distant communities if the Indian Ocean had a warning network like that in the Pacific, said Dr. Tad Murty, an expert on the region's tsunamis who is affiliated with the University of Manitoba in Winnipeg.

Within 15 minutes of the earthquake, in fact, scientists running the existing tsunami warning system for the Pacific, where such waves are far more common, sent an alert from their Honolulu hub to 26 participating countries, including Thailand and Indonesia, that destructive waves might be generated by the Sumatra tremors.

But there was no way to convey that information speedily to countries or communities an ocean away, said Dr. Laura S. L. Kong, a Commerce Department seismologist and director of the International Tsunami Information Center, an office run under the auspices of the United Nations.

"Outside the Pacific these things don't occur very often at all so the challenge is how to make people and government officials aware," she said.

Tsunamis, sometimes referred to as tidal waves but having nothing to do with tidal forces, are generated when an earthquake, eruption or landslide abruptly moves the seabed, jolting the waters above.

Resulting waves can cross thousands of miles of deep ocean at near jetliner speeds as near-invisible disturbances before welling up in shallower coastal waters to heights of 30 feet or more.

But even at such speeds, prompt warnings can provide ample time to evacuate people, Dr. Murty and other experts said. The Pacific network of stations gauging wave and earthquake activity is able to alert potential targets within minutes.

Tsunamis have swept the Indian Ocean, as well, oceanographers said yesterday, noting one that killed several hundred people near Bombay in 1945 and another - one of the earliest tsunamis recorded in the region - that ravaged what is now Bangladesh and other parts of the Bay of Bengal in 1762.

With population densities enormously high in many parts of coastal southern Asia, the region should have started setting up such a network long ago, said Dr. Murty, who is originally from India.

Other scientists have voiced similar concerns. At a meeting in June of the Intergovernmental Oceanographic Commission, a United Nations body, experts concluded that the "Indian Ocean has a significant threat from both local and distant tsunamis" and should have a warning network.

But Dr. Murty said that India, Thailand, Malaysia and other countries in the region had "never shown the initiative to do anything."

"They see this as a Pacific problem," he said. "I have a feeling that after this tragedy that may change."

The earthquake near Sumatra was the fifth most potent in the world in the last 100 years and the worst in 40 years, registering a magnitude of 9.0. It was followed by more than a dozen aftershocks, but none of those was expected to produce dangerous waves, said federal geologists.

The quake occurred at one of the many seams in the ever-shifting crust of the Earth where one plate slips beneath another in an incessant, but spasmodic process. In this case, the quake was set off by an abrupt slippage along 700 miles of the seam where the plate beneath the Indian ocean slides under the Indonesian archipelago.

This caused a vast stretch of seabed to shift about 50 feet, said geologists at the National Earthquake Information Center in Golden, Colo.

The biggest danger from earthquakes on land tends to come when the earth heaves horizontally, as is the case along the San Andreas fault in California. But faults where earthquakes tend to cause abrupt vertical motion, like the ones along western Indonesia, pose the biggest risk of generating tsunamis because they can act like a giant piston, deforming the sea. In such submarine earthquakes, gravity and the incompressibility of water force the seas above to react immediately to the change thousands of feet below.

"You're taking hundreds of miles of ocean bottom and moving it dozens of feet," said David Wald, a seismologist at the center. "That displaces a huge amount of water. The water at the surface starts to shift downhill and that makes a tsunami."

Most research on such waves and efforts to create warning systems have focused on the Pacific, where the Ring of Fire, a great arcing seam of volcanic and tectonic activity, causes a significant tsunami about once a decade.

One of the first efforts to alert communities to impending tsunamis came in Hawaii in the 1920's, said Dr. George D. Curtis, a tsunami expert affiliated with the University of Hawaii at Hilo.

A geologist at Hawaii's volcano observatory, upon detecting telltale tremors, would call local harbor officials and tell them to have boats moved to safety, Dr. Curtis said. Efforts greatly intensified in 1946, after a powerful earthquake in the Aleutian island chain of Alaska unexpectedly sent waves smashing into Hawaii, killing more than 150 people.

In 1948, the United States created its Pacific Tsunami Warning Center, which has been linked to an international data and warning network since 1965.

Any underwater earthquake with a magnitude greater than 6.5 starts the process, Dr. Murty said, and if a single wave gauge signals that the ocean is reacting, an alert is issued. The system has helped prevent loss of life, but only in places where alerts can quickly be communicated.

The last catastrophic tsunami occurred in July 1998, when more than 2,500 villagers on the coast of Papua New Guinea died after a magnitude 7.0 earthquake caused an undersea landslide.

But there has been little work done outside the Pacific, other than informal discussions, to expand the tsunami monitoring network.

"There's no reason for a single individual to get killed in a tsunami," Dr. Murty said. "The waves are totally predictable. We have travel-time charts covering all of the Indian Ocean. From where this earthquake happened to hit, the travel time for waves to hit the tip of India was four hours. That's enough time for a warning."

Other experts agreed that the Indian Ocean region could reduce its vulnerability with a warning system, but said no one should expect a quick fix.