Scientific Background on the Indian Ocean Earthquake and Tsunami

An earth-quake that measured 8.9 on the Richter Scale of the West Coast on Northern Sumatra set off a series of other earthquakes lasting 12 hours on the 26th of December (from 00:58 to 11:05 UTC), 2004 led to widespread catastrophe particularly in Sri Lanka, India, Maldives, Indonesia and Thailand with damage also in Malaysia, Bangladesh, Somalia and Seychelles.

Information relating to the submarine earthquake in between Aceh, Indonesia and Sri Lanka of the 26th of December, 2004 has been compiled here. This compilation archives much of the readily available scientific information. Aspects that were not immediately brought out by news reports were:

• The 9.0 Earthquake at 6.58 hours at the epicenter (and in Sri Lanka) led to a sequence of 15 other quakes across the Andaman region.
• While earthquakes could not be predicted in advance, once the earthquake was detected it would have been possible to give about 3 hours of notice of a potential Tsunami. Such a system of warnings is in place across the Pacific Ocean. However, there was no warning system in the Indian Ocean. In addition, coastal dwellers are educated in the Pacific littoral to get to high ground quickly following waves. However, those in the Indian Ocean were quite unaware.
• Tsunamis are rarer in the Indian Ocean as the seismic activity is much less than in the Pacific. However, there have been 7 records of Tsunamis set off by Earthquakes near Indonesia, Pakistan and one at Bay of Bengal.
• Earthquakes occur when any of the 12 or 13 plate collide at their boundaries. The present collision is due to compression between the Indian and Burmese plates. Scientists now believe that one plate that comprised the landmass from India to Australia has broken up into two. The initial 8.9 eruption happened near the location of the meeting point of the Australian, Indian and Burmese plates. Scientists have shown that this is a region of compression as the Australian plate is rotating counterclockwise into the Indian plate. This also means that a region of seismic activity has become active in the South Eastern Indian Ocean.
• Tsunamis are not entirely unknown in Sri Lanka. For example, the Tsunami in 1883 generated by the Volcanoes at Krakatoa led to a surge of at least 1 m in Sri Lanka. The damage was much less then. However, one difference was that this particular episode happened in the month of August. In the month of December, under the North-East monsoon, the Equatorial Indian Ocean jet propagates along the equator from Sumatra (near the epicenter of the quake) slightly to the South of Sri Lanka and to Somalia. This may be why the impact of the quake led to severe impacts in Sri Lanka.
• Once the large amount of pent-up energy in the compression zones of the plate boundaries have been released, it takes another buildup of energy for another event of similar magnitude. This is unlikely in the short-term. However, in the future, Indian Ocean littoral regions should generate and pay attention to earthquake and tsunami warnings and be aware of the interplay of the seasonal oceanographic currents.

	Lamont Doherty Seismographs for 25th and 26th December 2004			Records from the US Geological Survey and Pacific Tsunami Detection Center
	USGS: Warning System Could have Saved Thousands			USGS: Background on the Earthquake
	World Seismic Activity from the Global Seismic Assessment Program			Compression near the Boundaries of the Indian, Burma and Australia Plates
	Tsunamis affecting South Asia			Tsunamis affecting Indian Coastlines
	1941 Tsunami in the Bay of Bengal			1883 - Reports from Ceylon on the Tsunami
	Indian Ocean Currents During January			Disaster Preparedness Forecasting and Warning for Tsunamis
	Surviving a Tsunami—Lessons from Chile, Hawaii, and Japan			Additional Links

The seismographs at Lamont Doherty Earth Observatory at Columbia University in New York, USA shows that at around 1.20 am GMT on the December 26th, there was extraordinary oscillations that subsided 12 hours later. The seismograph for the previous day too is shown showing regular activity. (Please click on the images to view a larger image) The LDEO seismogrpahs goes off the chart around 01:20 GMT and then subsides by 06:45 GMT. However, things do not become normal until 15:00 GMT.

Seismographs at Lamont Doherty Earth Observatory

The earth quake location and magnitude as recorded by the US Geological Services Earth Quake Network provided the sequence of Earthquakes with those in Red being above 6.0 in the Richter Scale.

USGS - Recent Earthquakes of Magnitude Greater Than 2.5

Tsunami Information Bulletins from Pacific Tsunami Warning Center

The Pacific Tsunami Warning Center Detected the Earthquake and after 15 minutes issued a communique to Pacific Islands saying that there shall be no threats to them and also advises that there is a Tsunami watch going on but not an alert. 65 minutes later a second communique repeats the same information and says that there may a possibility of a Tsunami at the epicenter.

The tragedy is that this information was not communicated to the Indian Ocean region. The NOAA officers says that they did not have the addresses of the appropriate officials.

................. TSUNAMI INFORMATION BULLETIN ..................

THIS MESSAGE IS FOR INFORMATION ONLY. THERE IS NO TSUNAMI WARNING
OR WATCH IN EFFECT.

AN EARTHQUAKE HAS OCCURRED WITH THESE PRELIMINARY PARAMETERS

ORIGIN TIME - 0059Z 26 DEC 2004
COORDINATES - 3.4 NORTH 95.7 EAST 
LOCATION - OFF W COAST OF NORTHERN SUMATERA
MAGNITUDE - 9.0

EVALUATION

SOME ENERGY FROM YESTERDAYS TSUNAMI IN THE INDIAN OCEAN HAS
LEAKED INTO THE PACIFIC BASIN... PROBABLY FROM SOUTH OF THE
AUSTRALIAN CONTINENT. THIS ENERGY HAS PRODUCED MINOR
SEA LEVEL FLUCTUATIONS AT MANY PLACES IN THE PACIFIC. FOR
EXAMPLE...

50 CM CREST-TO-TROUGH AT CALLAO CHILE
19 CM CREST-TO-TROUGH AT IQUIQUE CHILE
13 CM CREST-TO-TROUGH AT PAGO PAGO AMERICAN SAMOA
11 CM CREST-TO-TROUGH AT SUVA FIJI
50 CM CREST-TO-TROUGH AT WAITANGI CHATHAM IS NEW ZEALAND
65 CM CREST-TO-TROUGH AT JACKSON BAY NEW ZEALAND
18 CM CREST-TO-TROUGH AT PORT VILA VANUATU
06 CM CREST-TO-TROUGH AT HILO HAWAII USA
22 CM CREST-TO-TROUGH AT SAN DIEGO CALIFORNIA USA

HOWEVER... AT MANZANILLO MEXICO SEA LEVEL FLUCTUATIONS WERE
AS MUCH AS 2.6 METERS CREST-TO-TROUGH PROBABLY DUE TO FOCUSING
OF ENERGY BY THE EAST PACIFIC RISE AS WELL AS LOCAL RESONANCES.

THIS IS TO ADVISE THAT SMALL SEA LEVEL CHANGES COULD CONTINUE
TO BE OBSERVED ACROSS THE PACIFIC OVER THE NEXT DAY OR TWO
UNTIL ALL ENERGY FROM THIS EVENT IS EVENTUALLY DISSIPATED.

THIS WILL BE THE FINAL BULLETIN ISSUED FOR THIS EVENT UNLESS
ADDITIONAL INFORMATION BECOMES AVAILABLE.

THE WEST COAST/ALASKA TSUNAMI WARNING CENTER WILL ISSUE BULLETINS
FOR ALASKA - BRITISH COLUMBIA - WASHINGTON - OREGON - CALIFORNIA.


**************************************************************


TSUNAMI BULLETIN NUMBER 002
PACIFIC TSUNAMI WARNING CENTER/NOAA/NWS
ISSUED AT 0204Z 26 DEC 2004

THIS BULLETIN IS FOR ALL AREAS OF THE PACIFIC BASIN EXCEPT
ALASKA - BRITISH COLUMBIA - WASHINGTON - OREGON - CALIFORNIA.

.................. TSUNAMI INFORMATION BULLETIN ..................

ATTENTION: NOTE REVISED MAGNITUDE.

THIS MESSAGE IS FOR INFORMATION ONLY. THERE IS NO TSUNAMI WARNING
OR WATCH IN EFFECT.

AN EARTHQUAKE HAS OCCURRED WITH THESE PRELIMINARY PARAMETERS

ORIGIN TIME - 0059Z 26 DEC 2004
COORDINATES - 3.4 NORTH 95.7 EAST 
LOCATION - OFF W COAST OF NORTHERN SUMATERA
MAGNITUDE - 8.5

EVALUATION
REVISED MAGNITUDE BASED ON ANALYSIS OF MANTLE WAVES.
THIS EARTHQUAKE IS LOCATED OUTSIDE THE PACIFIC. NO DESTRUCTIVE 
TSUNAMI THREAT EXISTS FOR THE PACIFIC BASIN BASED ON HISTORICAL 
EARTHQUAKE AND TSUNAMI DATA. 

THERE IS THE POSSIBILITY OF A TSUNAMI NEAR THE EPICENTER. 

THIS WILL BE THE ONLY BULLETIN ISSUED FOR THIS EVENT UNLESS
ADDITIONAL INFORMATION BECOMES AVAILABLE.

THE WEST COAST/ALASKA TSUNAMI WARNING CENTER WILL ISSUE BULLETINS
FOR ALASKA - BRITISH COLUMBIA - WASHINGTON - OREGON - CALIFORNIA.




**************************************************************

Tsunami Bulletins by Pacific Tsunami Warning Center

WASHINGTON (Reuters) - A warning center such as those used around the Pacific could have saved most of the thousands of people who died in Asia's earthquake and tsunamis, a U.S. Geological Survey official said on Sunday.

None of the countries most severely affected -- including India, Thailand, Indonesia and Sri Lanka -- had a tsunami warning mechanism or tidal gauges to alert people to the wall of water that followed a massive earthquake, said Waverly Person of the USGS (news - web sites) National Earthquake Information Center.

"Most of those people could have been saved if they had had a tsunami warning system in place or tide gauges," he said.

"And I think this will be a lesson to them," he said, referring to the governments of the devastated countries.

Person also said that because large tsunamis, or seismic sea waves, are extremely rare in the Indian Ocean, people were never taught to flee inland after they felt the tremors of an earthquake.

Tsunami warning systems and tide gauges exist around the Pacific Ocean, for the Pacific Rim as well as South America. The United States has such warning centers in Hawaii and Alaska operated by the U.S. Geological Survey. But none of these monitors the Indian Ocean region. The 8.9-magnitude underwater quake -- one of the most powerful in history -- off the Indonesian island of Sumatra devastated southern Asia and triggered waves of up to 30 feet high.

DEVASTATING TSUNAMI

U.S. seismologists said it was unlikely the Indian Ocean region would be hit any time soon by a similarly devastating tsunami because it takes an enormously strong earthquake to generate one.

"That's really what has created all of these problems -- is that the earthquake is just so massive," said Dan Blakeman, a USGS earthquake analyst.

But Person said governments should instruct people living along the coast to move after a quake. Since a tsunami is generated at the source of an underwater earthquake, there is usually time -- from 20 minutes to two hours -- to get people away as it builds in the ocean. "People along the Japanese coasts, along the coasts of California -- people are taught to move away from the coasts. But a lot of these people in the area where this occurred -- they probably had no kind of lessons or any knowledge of tsunamis because they are so rare."

A major tsunami, a Japanese word meaning "harbor wave," occurs in the Pacific Ocean about once a decade. It is generated by vertical movement during an earthquake and sometimes incorrectly referred to as a tidal wave, according to the Web site of the U.S. National Geophysical Data Center.

Because of the lack of monitoring mechanisms, the U.S. Geological Survey had no access to government or scientific information in the areas affected by the latest tsunamis.
"I've been talking to our tsunami people and they have no contact with any of these nations on the tsunamis," said Person. "We don't have anyone there. We get it from the press."

26th of December, 2004
Today's shallow, thrust-type earthquake occurred off the west coast of northern Sumatra at the interface between the India and Burma plates. In this region, the Burma plate is characterized by significant strain partitioning due to oblique convergence of the India and Australia plates to the west and the Sunda and Eurasian plates to the east. Off the west coast of northern Sumatra, the India plate is moving in a northeastward direction at about 5 cm per year relative to the Burma plate. Preliminary locations of larger aftershocks following today's earthquake show that approximately 1000 km of the plate boundary slipped as a result of the earthquake. Aftershocks are distributed along much of the shallow plate boundary between northern Sumatra (approximately 3 degrees north) to near Andaman Island (at about 14 degrees north).

27th December, 2004
The devastating megathrust earthquake of December 26th, 2004 occurred on the interface of the India and Burma plates and was cause by the release of stresses that develop as the India plate subducts beneath the overriding Burma plate. The India plate begins its decent into the mantle at the Sunda trench which lies to the west of the earthquake's epicenter. The trench is the surface expression of the India-Burma plate interface.

The tectonics of the region is complex and involves the interaction of the Australian, Sunda and Eurasian plates in addition to the India and Burma plate. The India and Australia plates move northeastwards at a rate of about 6 cm/year relative to the Burma plate. This results in oblique convergence at the Sunda trench. Some of this oblique motion is accommodated on the right-lateral transform faults and rifts that separate the Burma and Sunda plates.

Preliminary locations of larger aftershocks following the megathrust earthquake show that approximately 1000 km of the plate boundary slipped as a result of the earthquake. Aftershocks are distributed along much of the shallow plate interface and primarily extend northwards of the epicenter to the Andaman Islands.

The worlds largest recorded earthquakes were all megathrust events and occur where one tectonic plate subducts beneath another. These include: the magnitude 9.5 1960 Chile earthquake, the magnitude 9.2 1964 Prince William Sound, Alaska earthquake, the magnitude 9.1 1957 Andreanof, Alaska earthquake, and the magnitude 9.0 1952 Kamchatka earthquake. As with the recent event, megathrust earthquakes often generate large tsunamis that can cause damage over a much wider area than is directly effected by ground shaking near the earthquake's rupture.

Information on the earthquake at USGS

The earthquake took place in a region with previous earthquake activity as shown in the map prepared by the The Global Seismic Hazard Assessment Program (GSHAP) in the framework of the United Nations International Decade for Natural Disaster Reduction (UN/IDNDR). The present earthquake took place in a seismically active region at the plate boundary separating the Indian-Australian and East-Asian Plates. There are 12 plates in the world and earthquakes occur when these collide. A 13th plate was created by the breakup of the Indo-Australian plate was documented in 1995. This breakup has set up compression zone near Northern Sumatra. 

The spatial density of strong earthquakes around the Earth

The density of earthquakes has been estimated by the USGS as 

USGS: Earthquake Density

James Van Orman, Jim Cochran, Jeff Weisel and Florence Jestin and others of Lamont Doherty Earth Observatory have argued lately that the Indo-Australian plate is breaking up into two due to extraordinary stresses that have been set up and stretching in the India. Their paper was published in Earth and Planetary Science Letters in 1995.  

The counter-clockwise rotation of the new Australian plate in relation to the Indian Plate has led to compression in the Eastern boundary near Indonesia and tension in teh Western part of the new plate. The series of earth quakes that took place were on the eastern boundary of the new plate.

Scientists at Columbia University's Lamont-Doherty Earth Observatory report direct evidence that one of the Earth's great crustal plates is cracking in two - 1995.

In a report published in the most recent issue of Earth and Planetary Science Letters (vol. 133), the scientists say they have confirmed that the Indo-Australian Plate--long identified as a single plate on which both India and Australia lie--appears to have broken apart just south of the Equator beneath the Indian Ocean. The break has been underway for the past several million years, and now the two continents are moving independently of one another in slightly different directions.

Scientists have known that for some 50 million years, the Indian subcontinent has been pushing northward into Eurasia, forcefully raising the Tibetan Plateau and the Himalayan Mountains. The new research suggests that starting about 8 million years ago, the accumulated mass became so great that the Indo-Australian Plate buckled and broke under the stress.
"The result of this critical stage in the collision between India and Asia is the breakup of the Indo-Australia Plate into separate Indian and Australian plates," Jeffrey Weissel, a scientist at Lamont-Doherty, Columbia's earth sciences research institute in Palisades, N.Y., said in an interview.

"In the Central Indian Ocean, Nature is conducting a large-scale laboratory experiment for us, showing us what happens to the oceanic lithosphere (Earth's outer layer) when force is applied," Dr. Weissel said in an interview. Essentially pushed into an immovable object, "it can buckle like a piece of tin," he said.

A fundamental tenet of plate tectonics theory is that the Earth's surface is divided into rigid plates that move together and apart like pieces of a jigsaw puzzle. Scientists have long recognized 12 major plates. Now there are 13. In the 1970's, scientists first discovered a broad zone, stretching more than 600 miles from east to west where the equatorial Indian Ocean floor was compressed and deformed. Drilled samples had shown that the zone had begun to buckle and crack about 8 million years ago at the same time that the Tibetan Plateau had reached its greatest height. Dr. Cochran was chief scientist of the drilling cruise that collected this data.

More recently, researchers at Northwestern University, led by Richard Gordon and Seth Stein, used data on how newly created seafloor had spread outward from mid-ocean ridges to the west and south of the deformed region in the Indian Ocean. They theorized that the movements of the newly created seafloor could be accommodated only if a distinct plate boundary existed between separate Indian and Australian plates across the equatorial Indian Ocean.

In relation to the Indian plate, the Australian Plate is moving counterclockwise, the Northwestern University scientists calculated. In the western part of the new plate boundary, the plates are moving away from each other. To the east, the Australian Plate is converging on the Indian Plate, they said. If the theory was correct, the ocean floor in the eastern part of the new plate boundary should be compressed, buckled, cracked and eventually thrust upward along the cracks. More critically, if a separate Australian Plate were rotating counterclockwise in relation to a separate Indian Plate, the amount of compression should increase rapidly and systematically from west to east across the Central Indian Ocean. <>To test the theory, the Lamont-Doherty team took actual measurements of how compressed the Indian Ocean floor has become in the region believed to be the new plate boundary. Using sound waves to probe oceanic rock layers, they created images of subseafloor structures.

The images were collected during two separate research voyages that each spanned the entire deformed zone from north to south. Dr. Weissel and Dr. Jestin were aboard the 1991 "Phedre" cruise of the French research vessel Marion Dufresne. In 1986, Lamont-Doherty's former research vessel, the Robert D. Conrad, obtained images along a north-to-south line 185 miles farther west. <>The images showed scores of systematically aligned cracks, or faults, in the oceanic lithosphere--created as the once-whole plate buckled and cracked. As the now-distinct plates continued to converge, slabs of ocean floor slid upward along the faults to alleviate the strain. The more the two plates converged, the farther the slabs slid upward. The measurements clearly showed that two separate plates were converging. More importantly, the thrusting observed on the "Phedre" seismic line was about twice that found along the Conrad's line. That proved that compression was more intense to the east--confirming the Northwestern group's prediction from the data on spreading rate and direction at the mid-ocean ridges.

South Asian Tsunamis (Pronounced: Sue-Naa-Me)
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. 

TSUNAMIS ON THE COAST LINES OF INDIA
T. S. Murty
Baird and Associates Coastal Engineers
Ottawa, Canada
A. Bapat
Sadashiv Peth, Puna, 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). 

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

The Tsunami of 1941 following an earthquake in the Bay of Bengal was detected in Sri Lanka 
from Amateur Seismic Network

1941 - Andaman Islands, India, Mw 7.7
Date
26th June 1941
Epicentre:
20.5 kilometres W of Flat Island, India
Latitude:
12.500o N (5)
Longitude:
92.570o E (5)
Origin Time:
11:52:03 UTC (5)
Magnitude:
Mw 7.71 (3), Mb 8.0 (4), Ms 7.7 (4)
Moment:
4.25*10*20 Nm (3)

The earthquake of June 26, 1941 is among the strongest earthquakes ever recorded in the Andaman & Nicobar Islands. It had a magnitude of 7.7 (Mw).

It was centred (5) in the Bay of Bengal, roughly, 20.5 kilometres W of Flat Island, India
or 23.6 kilometres WNW of Yadita (Middle Andaman Isl.), India,
or 96.7 kilometres NNW of Port Blair (South Andaman Island), India,
or 617 kilometres SW of Yangon, Myanmar,
or 834 kilometres NNW of Banda Aceh (Sumatera), Indonesia.

It was the last great earthquake in the Andaman and Nicobar Islands. The 1881 Nicobar Islands earthquake (M7.9) is the only other event of comparable magnitude.

This 1941 earthquake caused widespread damage on Middle and South Andaman Islands. Most masonry structures in and around Port Blair were badly affected. The Cellular Jail which was a 3 storey building with 696 solitary cells and infamous for the imprisonment, torture and murder or freedom fighters, including Vinayak D. Savarkar was destroyed as were all the elegant buildings and wide roads, on Ross Island, the administrative centre of the British. The maximum intensity (4) was experienced at Baratang Island, Shoal Bay creek, north of Port Blair and near Port Anson.

Tremors from the earthquake were felt (6) in cities along the Coromandel (eastern) coast of India and even in Colombo, Sri Lanka. In Madras (now Chennai), two tremors were felt, the first of 2 seconds and the second lasting 15 seconds. It was felt throughout the city, mostly by people in tall buildings. At some locations, doors and windows are reported to have "slammed with a bang" and "chairs rocked". Articles kept on shelves also fell onto the floor. The tremors in the city were reported to have been the strongest since 1899. At Vishakhapatnam, two shocks were experienced within two minutes. People went outdoors on feeling the tremors, as did employees at the Municipal buildings in the city, as they felt the buildings rocking. People outdoors are said to have had an "unusual experience". Tremors were also experienced at Calcutta (now Kolkata), Chandernagar and Cuttack. Shaking was felt for a duration of 4 minutes at Cuttack. Tremors were felt in Colombo, Sri Lanka for a few seconds and also at Syhlet, Bangladesh, where the Car Festival was suspended due to the quake. There are no reports of tremors being felt from Sumatera, Indonesia in June 1941.

The earthquake was followed by several powerful aftershocks (5). Two magnitude 6.0 events struck within 24 hours of the main shock on June 27th, 1941. The first occurred at 07:32:47 UTC and was followed by another at 08:32:19 UTC. These were then followed by 14 earthquakes of magnitude 6.0 upto January 1942.

A tsunami (1) was triggered by this earthquake in the Bay of Bengal. As per journalistic sources, the height of the tsunami was of the order of 0.75 to 1.25 meters. At the time no tidal gauge was in operation. Mathematical calculations suggest that the height could be of the order of 1.0 meter. This tsunami was witnessed along the eastern coast of India. It is believed that nearly 5,000 people were killed by the tsunami on the east coast of India. Local newspaper are believed to have mistaken the deaths and damage to a storm surge, however, a search of meteorological records (2) does not show any storm surge on that day on the Coromandel Coast. National dailies like the Times of India (6), which reported the quake's shaking effects did not mention any deaths, either as a result of a storm surge or a tsunami.

REFERENCES

1) International Tsunami Symposium.

2) Murty, T.S., "Storm surges - meteorological ocean tides", Bulletin of the Fisheries Research Board of Canada, Ottawa, 1984.

3) Pacheco, Javier F., and Sykes, Lynn R., "Seismic moment catalog of large shallow earthquakes, 1900 to 1989", Bulletin of the Seismological Society of America, v. 82, no. 3, p. 1306 - 1349, 1992.

4) Dasgupta, S., Pande, P., Ganguly, D., Iqbal, Z, Sanyal, K, Venkatraman, N.V., Dasgupta, S., Sural, B., Harendranath, L., Mazumdar, K., Sanyal, S., Roy, K., Das, L.K., Misra, P.S., Gupta, H., "Seismotectonic Atlas of India and its Environs", Geological Survey of India, 2000.

5) Tandon, A.N., and Srivastava, H.N., "Earthquake occurrence in India: Earthquake Engineering", Sarita Prakashan, Jai Krishna Vol., 1-48, Meerut, 1974.

6) Times of India newspaper archives (Mumbai), India 

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, Pelinovsky, 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.

Download the pdf

The seas aroung Sri Lanka serves as a choke point as the ocean is in effect bounded on the South by the monsoon induced equatorial jet. Around Sri Lanka, 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.

The equatorial jet travels at around 1 m/s (Tomzack and Godrey, p 201-203) or around 3-4 km / hour. Indeed, this is much slower than the speed of the Tsunami of 500 km/hour. Perhaps, the currents themselves may not have a serious effect on Tsunami. However, the basin topography (see below), in the way it focusses  the energy of the tsunami may indeed have effected. In particular the 94 degree N-S ridge seems to have served as "reflector" of the Tsunami wave.

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

THE MONSOON SYSTEM OF THE INDIAN OCEAN
Courtesy: Tomzack and Godfreys: Regional Oceanography

Basin Topography of the Indian Ocean

The Equatorial Currents from Sumatra, past Sri Lanka to Somalia is brought out in this figure.

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.

Tsunami Forecasting - George Pararas-Carayannis

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At Warning Center, Alert for the Quake, None for a Tsunami

New York Times

December 28, 2004

ONOLULU, Dec. 27 - When experts at the Pacific Tsunami Warning Center in Honolulu were first alerted that an earthquake had struck Sunday off Indonesia, they had no way of knowing that it had generated a devastating tsunami and no way to warn the people most likely to suffer.

Tsunamis are rare in the Indian Ocean, which has no system for detecting them and alerting those in danger, and scientists do not have the tools to tell when an earthquake has created one.

Not until the deadly wave hit Sri Lanka and the scientists in Honolulu saw news reports of the damage there did they recognize what was happening.

"Then we knew there was something moving across the Indian Ocean," said Dr. Charles McCreery, the center's director.

"We wanted to try to do something, but without a plan in place then, it was not an effective way to issue a warning, or to have it acted upon," Dr. McCreery said. "There would have still been some time - not a lot of time, but some time - if there was something that could be done in Madagascar, or on the coast of Africa."

"One of the things that was running through my mind is just that our international group has in many past meetings had discussions about what can be done for other ocean basins," he said. "And I guess I was just wishing in retrospect that more progress had been made in that area."

The sequence of events as knowledge of the earthquake, the tsunami and the destruction unfolded suggest the speed and precision of science and modern communication, as well as their limits. If there had been a warning system for tsunamis in the Indian Ocean, thousands of people might have had a chance to flee.

The first notice of the earthquake that anyone at the Pacific tsunami center received was a computer-generated page set off by seismic sensors at 2:59 p.m. on Saturday Honolulu time. The immediate message received by people like Laura S. L. Kong, a Department of Commerce expert who is the head of a United Nations tsunami education center in Hawaii, included the time of the quake, latitude, longitude and an initial estimate of magnitude, about 8.0.

Nobody was in the office of the Pacific tsunami center. But staff members who received the pages reached the office, took a closer look at available data and sent out a warning to a preset list of contacts around the Pacific.

The center was advising of sea level changes in Fiji, Chile and California measured in inches, the echo of a distant event that had sloshed through the straits that connect the oceans. The warning center continued to refine its estimate of the quake, eventually raising it to a magnitude of 9.0, which is 10 times more powerful than the initial estimate of 8.0, because the scale is logarithmic.

The Pacific center, operated by the National Oceanographic and Atmospheric Administration, faced two problems in recognizing what was occurring in the Indian Ocean and alerting potential victims. There is no direct connection between an earthquake magnitude and a resulting tsunami. Not all quakes under the ocean lift the ocean floor to displace the water needed to create a tsunami.

For the Pacific, there are computer models to analyze the consequences of an earthquake, based on years of observations of previous quakes and tsunamis. For the Indian Ocean, there are no such models, according to Vasily V. Titov, a research oceanographer with the Oceanographic and Atmospheric Administration, based in Seattle. "They assemble quite a bit of data to get the right information and the right warning message," he said of such models.

Another difficulty is that countries that have experienced tsunamis in recent memory are set up with warning systems. Hawaii, for example, has warning sirens, and the "weather radio" network of oceanographic administration can also carry tsunami warnings.

Nor is a tsunami obvious as it races across an ocean at hundreds of miles per hour. In the open ocean, the wave may be only inches high. Boats on the ocean would feel almost nothing. Only when it hits the shallow water of a continental shelf does the wave rise to its destructive height.

Dr. McCreery, the Honolulu center's director, said the initial estimate of the earthquake's magnitude, 8.0, would have been likely to generate a local tsunami.

"Based on it being an 8.0, we assumed the damage would be confined to Sumatra and would be a local tsunami event, one that strikes shore within minutes of the event," he said. "We weren't overly concerned at that point that it was something larger."

But using another, sometimes more accurate method of measuring, Dr. McCreery said, the staff quickly determined that the magnitude had been closer to 8.5, more intense, but still only borderline for generating more distant damage. The center issued a follow-up bulletin.

But it was not until they saw news reports of casualties in Sri Lanka that all that changed.

Dr. McCreery spoke to the American ambassador to Sri Lanka, who wanted to know whether there were more giant waves expected. Then he had a conference call with a State Department official and embassy staff in Madagascar and Mauritius to address potential threats headed their way, and how the local authorities might be notified. But with no system in place, they would basically be scrambling.

The Indian Ocean and some other ocean basins do not have tsunami warning systems. The Pacific basin has had a well-constructed warning system with instrumentation throughout the region and plans for communicating critical information since 1965, Dr. McCreery said.

He said there have been frequent international discussions about improving systems for other oceans. "I know for certain after this event the entire global tsunami community will be looking for ways to implement better preparedness," he said.

One of the few places in the Indian Ocean that got the message of the quake was Diego Garcia, a speck of an island with a United States Navy base, because the Pacific warning center's contact list includes the Navy. Finding the appropriate people in Sri Lanka or India was harder.

The experts knew they were set up for the wrong ocean, but over a holiday weekend, Dr. Kong said, "it's tough to find contact information."

"I think they made their best efforts to contact as many people they thought they could get to," she said. After pushing the button to send out the initial warning to contacts in the Pacific, where about 90 percent of the tsunamis are observed, "they're trying to think of personally who might they call, who might then know who to call."

Last year a book on another Indonesian cataclysm, "Krakatoa: The Day the World Exploded," by Simon Winchester, asserted that that volcano, which set off an even more deadly tsunami when it exploded in 1883, was the world's first international disaster, because new undersea telegraph cables spread the news around the world within a few minutes. With high-speed data links and the Internet, it takes fewer minutes now, but if the information spreads wide, it does not always go deep.

Michele Kayal reported from Honolulu for this article, and Matthew L. Wald from Washington.

New York Times Article (subscription required)

This 18-page booklet was originally published in paper by the U.S. Geological Survey and is now available here as a PDF document. It contains lessons on how to survive a tsunami based on accounts from people who survived the tsunami generated by the largest earthquake ever measured—the magnitude 9.5 earthquake in Chile on May 22, 1960. It also contains an excellent description of what tsunamis are.

This project was sponsored by The National Tsunami Hazard Mitigation Program. The booklet was prepared in cooperation with Universidad Austral de Chile, the University of Tokyo, the University of Washington, the Geological Survey of Japan, and the Pacific Tsunami Museum.

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