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EARTHQUAKE
AND TSUNAMI OF JUNE 26, 1941 IN THE ANDAMAN SEA AND THE BAY OF
BENGAL
George
Pararas-Carayannis
Introduction
The earthquake of
June 26, 1941 was one of the strongest in the Andaman & Nicobar
Islands of India. The quake generated a tsunami in the Andaman
Sea and the Bay of Bengal which was particularly destructive
along the east coast of India.
The
Earthquake of June 26, 1941
 Date and Origin Time: June 26, 1941, at 11:52:03 UTC (5)
Epicenter
:12.50 North, 92.57 East.
Earthquake
Location: About 20.5
kms W of Flat Island, 23.6 kms WNW of Yadita (Middle Andaman
Island Group), 96.7 kms NNW of Port Blair (South Andaman Islands),
617 kms SW of Yangon, Myanmar, and 834 kms NNW of Banda Aceh
(Sumatra), Indonesia.
Earthquake
Focal Depth -
Shallow.
Magnitude:
Originally the magnitude
was estimated as Moment Magnitude, Mw 7.7M. However, based on
the earthquake's long rupture, a Mw 8.1 was subsequently assigned
to this event.
Aftershocks:
In the hours following
the major earthquake, a series of powerful aftershocks struck
the region. Two magnitude 6.0 events struck within 24 hours of
the main shock on June 27th, 1941. The first major afteshock
occurred at 07:32:47 UTC and was followed by another at 08:32:19
UTC. There were 14 earthquakes of magnitude 6.0 - up to January
1942.
Felt Reports: Strong
ground motions were felt throughout the Andaman and Nicobar Island
groups. Highest intensities were experienced at Baratang Island,
Shoal Bay Creek - north of Port Blair - and near Port Anson.
Also, the quake was strongly felt over a large geographical region
in the Bay of Bengal - particularly from the eastern coast of
India (Coromandel ) to Colombo, Sri Lanka.
In Madras (Chennai),
two strong tremors were felt, mostly by people in tall buildings.
The first of these tremors lasted 2 seconds and the second 15
seconds. Two shocks were experienced also at Vishakhapatnam within
a two-minute period. The earthquake was strongly felt in Calcutta
("Kolkata"), Chandernagar and Cuttack. At Cuttack the
strong motions reportedly lasted for about four minutes. In Colombo,
Sri Lanka, and at Syhlet, Bangladesh, the strong ground motions
were felt for a few seconds.
 Earthquake and Tsunami
Damage and Death Toll:
No detailed records are available from the Andaman and Nicobar
islands - which were under Japanese occupation at that time.
However, the death toll on the islands is estimated at over 3,000
and there is no specific record on the death toll and damage
caused by the tsunami. Extensive eartquake damage occurred primarily
on the Middle and South Andaman Islands. Most of the masonry
structures in and around Port Blair were badly affected. The
earthquake destroyed Cellular Jail - a three-story building -
and all buildings on Ross Island, the administrative center of
the British government.
Although there is little and confusing information, it is believed
that the 1941 tsunami impacted the east coasts of India and Sri
Lanka. It is estimated that about 5,000 people on the east coast
of India were killed. Although there had been no storms in the
region during this period, the local newspapers incorrectly attributed
the deaths and damage to a storm surge. The possible effects
of the tsunami on Thailand, Bangladesh or Myanmar are not known.
Crustal
Displacements and Rupture
The distribution of
aftershocks indicates that the total length of the rupture was
about 800 km.
The Andaman
Sea Basin and the Andaman and Nicobar Group of Islands
The Andaman Sea is
a highly folded and spreading geosynclinal basin, about 650 km
wide from east to west and about 1200 km long from north to south.
Its total area is estimated to be 600,000 to 800,000 km2.
The Andamans and the
Nicobars are a group of 349 islands - summits of a submarine
mountain range situated on the western side of the basin, formed
by tectonic interactions. The present configuration resulted
about 26 million years ago. The islands are the boundary separating
the Andaman Sea basin from the Indian Ocean. The Andaman group
has a total of 325 islands, while the Nicobar group has 24 islands.
Only 38 of these islands are inhabited.
 Tectonic Setting and
Seismotectonic History of the Andaman Sea Basin.
Tectonic
Setting - The Andaman
Sea Basin, is a seismically active region at the southeastern
end of the Alpine-Himalayan belt,. For millions of years the
India tectonic plate has moved in a north/northeast direction,
colliding with the Eurasian tectonic plate. The Indian plate's
eastern boundary, along the Andaman and Nicobar islands and Northern
Sumatra, is a diffuse zone of seismicity and deformation, characterized
by extensive faulting and numerous large shallow and intermediate
earthquakes.
The Burma microplate
encompasses the northwest portion of the island of Sumatra, as
well as the Andaman and Nicobar Islands. Further to the east
of the Andaman and Nicobar islands, a divergent boundary separates
the Burma plate from the Sunda plate.
Seismotectonic
History - The seimotectonic
history of the region is extensively covered in the scientific
literature (Sinvhal et al.1978, Verma et al. 1978). More recent
research documents the following regional tectonic evolution.
Accoesingly, an extensional feature developed along a leaky transform
segment of the megashear zone - the Andaman fault - between the
Indo-Australian domain and the Sunda-Indochina block (Uyeda and
Kanamori, 1979; Taylor and Karner, 1983). This old shear zone
acted as a western strike slip guide for the extrusion of the
Indochina block about 50-20 My (Tapponnier et al., 1986) - and
in response to the indentation of the Indian tectonic plate into
Eurasian block.
 Collision of Indochina with the Sunda and Australian
blocks stopped this crustal extrusion process. Subsequently,
the Andaman fault system - recently prolonged through the Sumatra
zone (the Sumatra fault) - reactivated due to the lateral escape
of the Sumatra forearc sliver plate and as a result of the oblique
convergence and subduction with the Indo-Australian plate.
The Indian plate's
oblique subduction beneath the Burmese Microplate has created
the Andaman segment of the great Sunda Trench. The Andaman and
Nicobar Islands are located within the tectonic sliver near the
boundary of the Indian plate and the Burmese Microplate. Similarly
the oblique subduction has created the north-south trending West
Andaman fault - another strike-slip fault system in the Andaman
Sea to the east of the island chain.
The Volcanic
Arc - The subduction
process has also formed a volcanic arc. There are two known volcanoes
along this arc. The one in the North is known as the Barren Island
Volcano - considered active as it has erupted within recent times.
The other is known as the Narcondum volcano and is considered
dormant.
 Seismicity of the Region
- Shallow and occasional
intermediate-depth earthquakes delineate the subducted slab under
the Andaman-Nicobar islands joining the seismicity trend of the
Indo-Burman ranges. The active seismicity of the Andaman Sea
Basin, has caused many minor and intermediate earthquakes, a
few major events ,and only one known earthquake with magnitude
greater than 8. According to the literature (Bapat 1982) from
1900 to 1980, a total of 348 earthquakes were recorded in the
area bounded by 7.0 N to 22.0 N and 88.0 E to 100 E.
Seismicity of the
region The map shows all historically recorded earthquakes
in the Andaman & Nicobar Islands. Earthquakes having magnitudes
greater than 4.0 since 1973 are also shown.
The
Tsunami of June 26, 1941 in the Bay of Bengal
The earthquake of
June 26, 1941 generated a tsunami in the Andaman Sea and the
Bay of Bengal which impacted India's east coast and Sri Lanka.
The possible effects of the tsunami on Thailand, Bangladesh or
Myanmar are not known. It is estimated that about 5,000 people
on the east coast of India were killed. Local newspapers incorrectly
reported that the deaths and damage were caused by a storm surge.
Meteorological records do not support the occurrence of a storm
system on the Coromandel Coast during that period (Murty, 1984).
There is not much information other than some newspaper accounts
which indicate that the height of the tsunami may have been about
0.75 to 1.25 meters. At the time no tidal gauge was in operation.
Mathematical estimates indicate that the height of the tsunami
could have been about 1.0 meter.
 Historical Earthquakes
and Tsunamis in the Andaman Sea
During an eighty year
period, from 1900 to 1980, a total of 348 earthquakes were recorded
in the area bounded by 7.0 N to 22.0 N and 88.0 E to 100 E. These
earthquakes ranged in magnitude from 3.3 to 8.5 (Bapat, 1982),
but only five of these had magnitudes equal to or greater than
7.1 and generated tsunamis (Murty and Bapat, 1999). For the shorter
period from 1916 to 1975, only three of the earthquakes had magnitudes
greater than 7.2 and generated significant tsunamis. (Verma et
al., 1978).
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. The historical
record indicates that in April 1762, an earthquake along the
Araken Coast off Myanmar generated the earliest known tsunami
in the Bay of Bengal. On October 1847, an earthquake near the
Great Nicobar Island generated another tsunami, but no details
are available. On 31 December 1881 a magnitude 7.9 earthquake
near Car Nicobar, generated yet another tsunami in the Bay of
Bengal. Its height recorded at Chennai was one meter.
Major Faults in the
Andaman Sea Basin
Two other earthquakes
with magnitudes of 7.25 to 7.3 (which occurred on 17 May 1955,
and 23 August 1936) probably did not generate significant tsunamis
for the following reason. Tsunamis usually are associated with
dip-slip type earthquakes rather than with strike-slip type earthquakes.
According tto Pickering (1981) earthquakes in the Andaman region
are associated mainly with strike-slip type of faulting. It is
believed that these events probably occur along the north-south
trending West Andaman fault.
Up to the 26 December
2004, the earthquake of 26 June 1941 had been the strongest ever
recorded in the Andaman and Nicobar Islands, in generating a
destructive tsunami. The 1941 event was the last great earthquake
in the Andaman and Nicobar Islands. The 1881 Nicobar Islands
earthquake (M7.9) was the only other event of comparable magnitude.
Potential
for Large Earthquakes and Tsunamis in the Andaman Sea
Based on these statistical
and historical information, it can be concluded that most of
the earthquakes in the Andaman Sea Basin - even those with magnitudes
greater than 7.1 - do not usually generate significant tsunamis.
As already discussed, the possible reason for the low number
of tsunamis is that most of the earthquakes in the Andaman Basin
are mainly associated with strike-slip type of faulting that
involves lateral crustal movements. The exception was the 26
December 2004 earthquake, which, not only ruptured the Great
Sunda Arc along the northern Sumatra region but also ruptured
the same segment in the Andaman Sea as that in 1941. A possible
explanation for the extreme tsunami generated in the Andaman
segment in December 2004 is that this event had a different mechanism
which involved both thrust and bookshelf faulting within the
compacted sediments of the Andaman Sea segment of the Great Sunda
Arc and large ocean floor dispacements over a long rupture (Pararas-Carayannis,
2005).
It can be reasonably
concluded that large earthquakes along the northern end of the
Great Sunda subduction boundary in the Andaman Sea do not occur
frequently. However, events with magnitudes greater than 7.1
- when they occur - have the potential of generating local destructive
tsunamis. Finally, earthquakes with magnitude 8.0 or greater
(such at the 1941 and 2004 events) - associated with "dip-slip"
types of vertical crustal displacements along thrust faults -
have the potential of generating very destructive tsunamis in
the entire Bay of Bengal Region the Andaman Sea and the Indian
Ocean.
REFERENCES
Bapat, A. (1882).
"Tsunamis and
earthquakes in the Bay of Bengal", Tsunami Newsletter, International Tsunami Information
Center, Honolulu, Volume XV, No. 3, 36-37.
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.
Murty. T. and A. Bapat,
"Tsunamis on
the coastlines of India",
International Tsunami Symposium, May 1999 (Abstract).
Murty , T.S. 1984,
"Tsunamis in
the Andaman Sea",
Institute of Ocean Sciences, Dept. of Fisheries and Oceans Sidney,
Canada
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.
Sinvhal, H., Khattri,
K.N., Rai, K. and V.K. Gaur. (1978), "Neotectonics and time-space seismicity
of the Andaman Nicobar region", Bulletin of the Seismological Society of America,
Volume 68, No. 2, 399-409.
Srivastava V. K. and
Ranjana Ghosh, Report of the Dept. of Applied Geophysics, Indian
School of Mines.
Tandon, A.N., and
Srivastava, H.N., "Earthquake
occurrence in India: Earthquake Engineering", Sarita Prakashan, Jai Krishna
Vol., 1-48, Meerut, 1974.
Tapponnier, P., Peltzkr,
G. and R. Armijo, 1986, "On
the mechanism of collison between India and Asia". In: COWARD, M.P. & RIES,
A.C. (eds) Collision Tectonics. Geological Society, London, Special
Publications, 19, 115-157.
Taylor B. and Karner
G.D., 1983, "On
the evolution of marginal basins". Rev.Geophys., 21, 1727-1741
Uyeda S. and Kanamori
H., 1979, "Back-arc
opening and mode of subduction". J.Geophys.Res., 84, 1049-1061
Verma, R.K., Mukhopadhyay,
M. and N.C. Bhuin. (1978), "Seismicity,
gravity and tectonics in the Andaman Sea", in "Geodynamics
of the Western Pacific",
Proceedings of the International Conference on Geodynamics of
the Western Pacific - Indonesian Region, March 1978, Tokyo, Advances
'in Earth and Planetary Science 6, Supplement Issue of Journal
of Physics of the Earth, edited by S. Uyeda, R.W. Murphy and
K. Kobayashi, Center for Academic Publications, Japan Scientific
Societies Press, Tokyo, 233-248.
SEE ALSO
Pararas-Carayannis,
G., 1992. "The
Earthquake and Tsunami of 2 September 1992 in Nicaragua"
Pararas-Carayannis,
G. 2001. "The
Great Tsunami of August 26, 1883 from the Explosion of the Krakatau
Volcano ("Krakatoa") in Indonesia"
Pararas-Carayannis,
G., 2003. "Near
and far-field effects of tsunamis generated by the paroxysmal
eruptions, explosions, caldera collapses and massive slope failures
of the Krakatau volcano in Indonesia on August 26-27, 1883". Presentation for the International
Seminar/Workshop on Tsunami "In Memoriam 120 Years of Krakatau
Eruption _ Tsunami And Lesson Learned From Large Tsunami",
August 26th _ 29th 2003, Jakarta and Anyer, Indonesia; Journal
of Tsunami Hazards, Volume 21, Number 4. 2003.
Pararas-Carayannis,
G., 2005. "The
Great Earthquake and Tsunami of 26 December 2004 in Southeast
Asia and the Indian Ocean"
Pararas-Carayannis,
G., 2005. "The
Great Earthquake and Tsunami of 1833 off the coast of Central
Sumatra in Indonesia"
Pararas-Carayannis,
G., 2005. "The
Great Earthquake and Tsunami of 28 March 2005 in Sumatra, Indonesia"
Pararas-Carayannis,
G. 2007, "The
Earthquakes and Tsunami of September 12, 2007 in Indonesia"
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