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Selected portion from GSI publication on the 2004 earthquake
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‚È◊ÊòÊÊ-•¢«U◊ÊŸ ÷Í∑§ê¬ ∞fl¢ ‚ÈŸÊ◊Ëw{ ÁŒ‚¢’⁄U wÆÆy
SUMATRA - ANDAMAN
EARTHQUAKE AND TSUNAMI
26 DECEMBER 2004
÷Ê⁄UÃËÿ ÷ÍflÒôÊÊÁŸ∑§ ‚fl¸ˇÊáÊÁfl‡Ê· ¬˝∑§Ê‡ÊŸ ‚¢ÅÿÊ }~
GEOLOGICAL SURVEY OF INDIA
SPECIAL PUBLICATION NO. 89
‚ê¬ÊŒ∑§— ‚ÈÁ¡Ã ŒÊ‚ªÈ#Editor : Sujit Dasgupta
÷Ê⁄Uà ‚⁄U∑§Ê⁄U ∑§ •ÊŒ‡Ê ‚ ¬˝∑§ÊÁ‡ÊÃPUBLISHED BY ORDER OF THE GOVERNMENT OF INDIA
2007
PGSI-275
500-2005 (DSK-II)
© India, Geological Survey (2007)
Manuscript Processing for Printing and Copy-editing by: Satyabrata Guha, Geologist (Sr.)
Nabanita Nandy, Asstt. Geologist
Under the Guidance of: Goutam Sarkar, Director, P&I Division
Overall Supervision by: Ajit Kumar
Deputy Director General
Operation: Map and Publication
Geological Survey of India
Central Headquarters
29, Jawaharlal Nehru Road
Kolkata - 700 016, India
Published by: Director General
Geological Survey of India
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Kolkata - 700 016, India
Printed by: The Radiant Process Pvt. Ltd.
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Price: Inland : Rs. 390.00; Foreign : $ 22.00 or £ 14.00
Cover: A panoramic view of Marina beach, Chennai, India seized by tsunami, 26 December 2004
Foreword
The festivities of Christmas turned into a horrifying nightmare for thousands of people at nature’s fury
on 26 December 2004. On that fateful day, the whole world witnessed an unprecedented unleash of the killer
tsunami following a great undersea earthquake that took away more than 10,000 human lives, rendered
thousands homeless and destroyed property worth crores in India alone. It was a national tragedy. Rising to
the occasion, GSI mobilized all its resources to investigate and analyse the hazard and its aftermath from a
multi-pronged scientific angle.
Geological Survey of India has a legacy of earthquake studies since the days of Sir R.D. Oldham who
laid the foundation of modern seismology with the survey of the Great Assam Earthquake of 1897. Powered
by the legacy of earthquake studies, scientists of GSI explored different aspects of the event, faithfully
documented the records and critically examined the effects of the calamity of 26 December 2004. The
outcome of that scientific investigation was a report released in September 2005.
The report is now being published to disseminate the findings to a larger community of scientists with
the hope that it will help further scientific study of earthquakes.
It is hoped that this publication will proffer knowledge and help secure a safer society in future and to
accomplish a complete understanding of the science of earthquake and tsunami.
(P.M TEJALE)
Kolkata, Director General
Dated 2 January 2007 Geological Survey of India
Contents
Introduction Sujit Dasgupta 1
Earthquake
K. N. Mathur, S. K. Ray, 5
S. Sengupta, Prabhas Pande
and Sujit Dasgupta
A. K. Ghosh Roy, S. Bardhan, 17
P. Jana and S. R. Basir
D. P. Das, S. S. Ghosh, 51
D. Chakraborty and K. Pramanik
Sumit Kumar Ray and 63
Anshuman Acharyya
R. Sengupta and 12 others 82
Sujit Dasgupta, 95
Basab Mukhopadhyay and
A. Acharyya
O. P. Mishra, G. K. Chakraborty 105
and O. P. Singh
T. Ghosh, P. Jana, T. S. Giritharan, 153
S. Bardhan, S. R. Basir and
A. K. Ghosh Roy
M. Raju, B. K. Bhandaru, 171
V. Singaraju and B. M. Shah
R. Srinivasan and K. Nagarajan 183
B. Kanishkan and 204
B. Lakshminarayanan
K. Jayabalan and U. Durairaj 223
235
1. A Preliminary Report on Investigation of Effects of the
Sumatra - Andaman Earthquake of 26 December 2004 in
Andaman and Nicobar Islands
2. Macroseismic Survey in Andaman and Nicobar Island in
the Aftermath of the Great Earthquake of 26 December
2004
3. Analysis of Satellite Data for Detection of Changes
in Coastal Geomorphology of Andaman - Nicobar Islands
due to 26 December 2004 Earthquake
4. 26 December 2004 Earthquake: Coseismic Vertical
Ground Movement in the Andaman Islands
5. Bathymetry and Magnetic Observations along Andaman
Arc-Trench Gap in the Post-earthquake Scenario of
26 December 2004
6. Seismotectonics of the Andaman - Nicobar Region:
Constraints from Aftershocks within 24 Hours of the Great
26 December 2004 Earthquake
7. Aftershock Investigation of the 26 December 2004
Sumatra-Andaman Islands Earthquake
Tsunami
8. Tsunami Survey in the Andaman-Nicobar Group of
Islands
9. Tsunami Survey in the Srikakulam-Pulicat Segment,
Andhra Pradesh
10. Tsunami Survey in the Chennai - Nagapattinam Segment,
Tamil Nadu Coast
11. Tsunami Survey in the Nagapattinam - Kanyakumari
Segment, Tamil Nadu Coast
12. Tsunami Survey in the Kanyakumari - Cochin Segment in
Parts of Tamil Nadu and Kerala
APPENDIX I: Locality Index
INTRODUCTION
SUJIT DASGUPTA
Geological Survey of India, Kolkata
The Sumatra-Andaman subduction zone has been
a known potential tectonic candidate for earthquakes.
Tsunamis are rare but not totally absent. Yet there
was insufficient knowledge on its capability for
developing such an incomprehensible tsunami. The
impression that tsunami is largely a Pacific Ocean
phenomenon has been drastically confuted by the
Indian Ocean earthquake and tsunami of 26
December 2004. This was one of the largest
interplate shallow-thrust earthquakes that occurred
at the interface of the subducting Indian lithosphere
and the overriding Burma plate. The event happened
to be the second largest earthquake in the recorded
history after the Chile Earthquake of 1960 (Mw 9.5).
The mainshock of the great earthquake of 26
December measured Ms 8.6 (IMD), Ms 8.8 (GSI,
Nagpur), Ms 8.6 (GSI, Jabalpur), Mw 9.0 (USGS),
(revised Mw 9.3) and occurred off the west coast of
northern Sumatra (Indonesia) at 00:58:53 hrs (UTC).
[06:28:51.1hrs IST (IMD)]. This undersea
earthquake triggered giant tsunamis that devastated
the coastal regions of the countries rimming the
Indian Ocean travelling as far as the coast of east
Africa. In India, damage from the tremor of the
earthquake itself was moderate to high in the
Andaman and Nicobar Islands. Above and beyond,
the high tsunami waves unleashed by the earthquake
wreaked havoc on life and property in the coastal
regions of Andhra Pradesh, Tamil Nadu, Kerala and
Andaman & Nicobar Islands. Besides death of
10,479 people, a total of 2,39,024 dwelling units
were affected, 35,605 cattle lost, 22,750 hectares of
cropped area and 83,788 boats damaged in the
calamity in India alone. This estimate may change
with time but the frightening memory and potential
threat will haunt the nation and the earthscientists in
particular for years to come. GSI made a
conscientious attempt to study and analyze the event
for immediate planning and for posterity.
The Andaman - Nicobar Archipelago is located
in a unique and complicated tectonic regime. It has
components of trench, volcanic arc, fault systems,
spreading ridge, sea-rises, transform faults and
obducted suites of rocks. In a broader view, tectonic
features bordering the Indian subcontinent in the west
(Suleiman-Kirthar fold belt), north (the Himalayas)
and east (Indo-Burmese arc) are thought to have
resulted from the northward drift of India since
Cretaceous and its collision with the Tibetan landmass
by Early-Mid Eocene. The Indo-Burmese range and
the Andaman island arc together describe tectonically
continuous belt displaying various geologic elements
of an arc-trench system, though the northern part of
the belt, i.e., the Indo-Burmese range, emerged above
sea level as early as Oligocene. The Burmese-
Andaman Arc System (BAAS) presents nearly 3500-
km-long subducting margin in northeastern part of
the Indian plate where varying degrees of seismic
activity, volcanism and active tectonism are
evidenced. The region is of particular interest due to
several interesting features, namely (1) it serves as
an important tectonic link between the Eastern
Himalayas (a typical collisional margin) and the
Sunda arc (which is a part of the Western Pacific Arc
System); (2) an initial collisional phase has already
set in the northernmost segment of BAAS (in the
Naga Hills) within an overall subducting regime; (3)
Burma is one of the few regions in the world where
a subduction zone up to about 200 km depth is clearly
discernible in a land environment; (4) coastal Burma
and northern part of the Andaman Sea are largely
aseismic, suggesting that subduction of the Indian
plate in this regions has stopped recently or occurs
aseismically, and the hanging lithospheric slab is
being dragged northward through the surrounding
lithosphere; (5) The Andaman backarc Spreading
Ridge (ASR) underlying the Andaman Sea relates to
the oblique convergence of the Indian plate at the
Asian continental margin; actual spreading occurred
through several short leaky-transforms, producing the
‘pull-apart’ Andaman basin in southern half of the
BAAS, and (6) further south is the intense seismic
zone of the West Sunda arc with its attendant
volcanism.
The Indo-Burmese range and the Andaman-
Mentawai arc form the outer arc ridge of the arc-
trench system developed during the Tertiary in
consequence of subduction of the Indian plate below
the Burma-Sumatra segment. The various
morphotectonic units recognized along the
convergent margin of the Indian plate may be
described as follows :
(i) The overriding Southeast Asian continental
block including the west Kachin unit of
northeast Burma, Shan-Tenasserim highland
and Sumatra.
(ii) A narrow linear faulted backarc basin between
the magmatic arc and the west Kachin unit
involving the Indawgyi and also Bhamo-
Myitkyina valley that extends southward up
to the Andaman Sea through Shwebo, Sittang
basins and Gulf of Martaban.
(iii) The magmatic arc extending from the Jade
Mines in north Burma to Narcondam-Barren
volcanic islands through Monywa, Mt. Popa
and Irrawaddy delta; this intra-basinal arc
continues to the continental margin arc in north
Sumatra.
(iv) A well-developed forearc basin that extends
from the Chindwin valley in north Burma to
the Mentawai trough, off Sumatra; in the
Andaman sector the forearc basin is
represented by the ‘Nicobar deep’.
(v) The subduction-accretion complex at the
leading edge of the Indian plate is represented
by sediments of the Burmese-Andaman outer
arc, where several dismembered ophiolite
bodies occur along the seaward flank of the
forearc trough.
As already mentioned the Andaman-Nicobar arc-
trench region is a highly seismic tectonic domain.
Earthquakes occur along the plate margin with a well-
defined seismic Benioff zone. Large-magnitude
shallow-foci thrust earthquakes are known to occur
in and around the outer-arc ridge including the events
of 1847, 1881 and 1941. Besides, the Andaman
spreading ridge yields earthquakes mainly with
normal fault mechanism whereas earthquakes along
the West Andaman fault display strike-slip geometry.
Following the mainshock of 26 December 2004,
thousands of aftershocks have been recorded. It is
noteworthy that these aftershocks mainly occur north
of the mainshock till the 28 March 2005 event of
magnitude 8.7 (USGS), located on a fault segment
160 kilometres to the southeast of the rupture zone
of the 26 December 2004 earthquake. Previous events
in the vicinity occurred in 1833 and 1861.
Interestingly the earthquake of 28 March 2005 did
not generate tsunami.
Geological Survey of India took up immediate
assignments deploying scientists from various
streams. Dr. K. N. Mathur, Director General, led a
team of senior officers of the Survey to South
Andaman and Baratang Islands during 7-12 January
2005 to take stock of the situation and to implement
the work plan. Macroseismic and tsunami survey in
the Andaman-Nicobar Islands and in the coastal tract
of Andhra Pradesh, Tamil Nadu and Kerala were
launched. Deployment of seismometers to record and
analyse aftershocks and installation of geodetic
instruments to study nature of deformation caused
by the earthquake in the Andaman- Nicobar Islands
were taken up. This was followed by cruise of GSI
vessel in parts of Indian Ocean across the Andaman
Islands for first-hand assessment on changes of
bathymetry and magnetics. Apart from the analyses
of satellite digital data for pre- and post-earthquake
scenes in Andaman-Nicobar Islands, estimation of
vertical ground movement and study of rupture
propagation characteristics have been attempted to
explain the overall seismotectonics of the region
around the archipelago.
This report embodies the outcome of major work
accomplished by the geoscientists of GSI. There are
twelve contributions in total including different
aspects of earthquake and tsunami. Earthquake-
related studies are dealt in seven chapters.
2 SUJIT DASGUPTA
This begins with the preliminary report that was
submitted on 17 January 2005 after the visit of the
Director General, GSI in Andaman Islands. This was
a first-hand account of intensity assessment in South
Andaman as well as mud volcano eruptions and
surface rupture at Baratang Island from sympathetic
faults. This is followed by the contribution by Ghosh
Roy et al. on the macroseismic survey of the A&N
Islands. Results indicated Nicobar Islands had higher
intensity of VIII (revised MSK scale) than Andaman
Islands where the general intensity was VII with few
local highs of VIII in western part of the island. The
Havelock Island showed a lower intensity of VI.
Intensity in the mainland varied from III to IV. Strong
seismic seiches have been recorded from West
Bengal, Andhra Pradesh and Tamil Nadu. In the third
chapter, Das et al. used pre- and post-earthquake
digital satellite data to detect morphological changes
in A&N Islands. The study revealed emergence of
islands of varying magnitude along east and west
coasts in the North Andaman Island, and
submergence of islands in the south, in Nicobar. Ray
and Acharyya estimated coseismic vertical movement
distribution in the Andaman group of islands (north
of 11°N latitude) showing uplift in some parts and
subsidence on others in a domain of thrust faulting.
There are locales where there is no perceptible ground
movement, designated as ‘neutral line’, west of that
there is land emergence while submergence is
recorded in the east. Sengupta et al. outlined the
marine survey carried out by GSI marine vessel R.V.
Samudra Manthan in Andaman arc-trench gap. The
cruise includes 4678 line-km for bathymetric as well
as magnetic studies along 24 transects. Bathymetric
profile showed perceptible structural and
morphological changes in the sea-floor particularly
in the areas south of 10°N latitude. Dasgupta et al.
explained the aftershock propagation characteristics
within 24 hours of the earthquake and illustrated
rupture segments, aftershock propagation rate and
differential seismic loading. Mishra et al. analysed
1177 aftershocks (M ³ 3.0) recorded from 6.1.2005
to 31.1.2005 from their total database of about 18,000
aftershocks up to 16 March 2005. The epicentre map
indicated a N-S-trending aftershock cluster in an area
of about 750 x 300 km2. The aftershocks occurred
mostly at the depth range of 5-55 km, except a few
beyond that depth range.
There are five contributions on tsunami survey.
Results of tsunami survey in Andaman-Nicobar
Islands have been documented by Ghosh et al. A
stronger impact in the Nicobar group of islands is
evident. While the run-up distance is more than
1 km in Car Nicobar, South Andaman witnessed about
150 m of run-up. Notwithstanding tsunami wave
heights of nearly 10 m in few locales, the run-up
height is generally restricted within 1.5 m and 5.5 m.
The impact of tsunami in coastal mainland has been
extensive. A coastal stretch of about 2050 km from
Srikakulam in the East Coast to Cochin in the West
Coast has been affected. GSI has covered the entire
coast in four segments to record the details. Raju
et al. described the distribution of tsunami in Andhra
Pradesh coast where run-up varied from 200 m to
1km with a maximum run-up elevation of 2 m.
Srinivasan and Nagarajan demonstrated the
characteristics of tsunami in Chennai-Nagapattinam
of Tamil Nadu coast. The run-up elevation in the
Chennai-Nagapattinam segment varied between 1 m
and 3 m while the run-up distance ranged from
150 m to 1 km. Kanishkan and Lakshminarayanan
recorded the outcome of tsunami studies between
Nagapattinam and Kanyakumari, Tamil Nadu. The
coastal stretch lying between Nagapattinam and Point
Calimere showed a maximum run-up elevation of
3 m with run-up length (inundation zone) varied
between 200 m and 1.25 km. In terms of life, property
and landscape loss, the stretch of coast in Karaikkal-
Nagapattinam-Velankanni had highest damage in the
East Coast. Jayabalan and Durairaj documented
impact of tsunami in parts of Tamil Nadu and Kerala
coast from Kanyakumari to Cochin. In the West
Coast, the run-up distance varied from 200 to
500 m and run-up elevation ranges from 3 to 4 m. In
Kerala, the worst-affected area was the stretch
between Cheriyazhikkal (Kollam district) and
Tharailkadavu (Alappuzha district).
GSI SPL. PUB. 89 : SUMATRA-ANDAMAN EARTHQUAKE & TSUNAMI, 26 DEC.’04 3
The collation of data on earthquake and tsunami
survey of one of the largest recorded seismic event is
indeed an assignment for the sake of better
understanding of a lesser-known phenomenon in this
part of the world. The attempt will be rewarding if
new frontiers of science open up for the safety and
existence of all living milieu in this fragile tectonic
regime.
This ‘Introduction’ will remain incomplete
without acknowledging the cooperation received
from all the contributors of this volume. The support
rendered by S/Shri K. Nagarajan, B. Kanishkan and
O. P. Mishra is gratefully acknowledged. Shri
Saudipta Chattopadhyay has been instrumental in
final knitting of this volume with all odd DTP jobs.
The task of editorial assistance was entrusted with
Shri Anshuman Acharyya who carried out the work
meticulously. In spite of our efforts there will still be
flaws that may be ignored.
4 SUJIT DASGUPTA
CHAPTER 1
A PRELIMINARY REPORT ON INVESTIGATION OF EFFECTS OF THE
SUMATRA - ANDAMAN EARTHQUAKE OF 26 DECEMBER 2004 IN
ANDAMAN AND NICOBAR ISLANDS#
K. N. MATHUR*, S. K. RAY, S. SENGUPTA, PRABHAS PANDE AND SUJIT DASGUPTA
Geological Survey of India, Kolkata
INTRODUCTION
A great earthquake measuring Ms 8.6 (IMD), Mw
9.0 (USGS) occurred off the west coast of North
Sumatra (Indonesia) at 00:58:53 hrs. [06:28:51.1hrs
IST (IMD)] on 26.12.2004. This is one of the largest
interplate shallow-thrust earthquakes that occurred
at the interface of the subducting Indian lithosphere
and the overriding Burma plate. This mega-seismic
event from the Sumatra subduction zone in the Indian
Ocean triggered giant tsunamis that devastated the
coastal regions of Indonesia, Malaysia, Thailand, Sri
Lanka, India and Maldives and affected even the coast
of east Africa. The loss of human lives in the
catastrophe has been put at 1.5 lakh. The impact of
the tsunami was quite severe in the coasts of
Andaman and Nicobar group of islands, Tamil Nadu,
Andhra Pradesh, Pondicherry and Kerala where over
10,000 people lost their lives, thousands were injured
and property worth several hundreds of crores
destroyed.
The Geological Survey of India has taken up the
investigation of this earthquake and resultant tsunami
in the Andaman-Nicobar region and other parts of
the country. In this connection, a team of senior
scientists of the Department reached Port Blair on
7 January 2005. After discussions with the officials
of the Andaman & Nicobar Administration, the GSI
team visited different parts of South Andaman and
Baratang Islands to study the effects of the earthquake
and tsunami (Fig. 1.1). This team after initial surveys
# This report was submitted on 17.01.2005
* Director General, GSI
returned to Kolkata on 12.01.05. Another team of
officers from Eastern Region, GSI also reached Port
Blair on 07.01.05 and took up detailed investigations
to document the effects of the earthquake and tsunami
and install GPS in campaign mode. Another group
of GSI scientists who reached Port Blair on 06.01.05
is installing an array of five digital short-period
seismometers in different islands to record the
aftershocks.
This report contains the observations from the
studies carried out between 7 and 12 January 2005.
A detailed report with analysis on the effects of the
earthquake and tsunami and other related issues will
be submitted on completion of the work.
Earthquake Parameters
Earthquake parameters for this great earthquake
are continuously being revised and refined by USGS
since the first estimate released on 26.12.2004. As
on 13.01.2005 the parameters as per USGS are as
follows :
Date: 26 December 2004
Origin Time: 00:58:53 (UTC) [local time at the
epicentre 07:58:53]
Location: 3.316°N, 95. 854°E [± 5.6 km (3.5 miles)
horizontally]
Region: Off west coast of North Sumatra
Magnitude: Mw 9.0
Depth: 30 km (18.6 miles)
Harvard Best Double-Couple Solution:
NP1: Strike 329, Dip 08, slip 110
NP2: Strike 129, Dip 83, slip 87
Principal Axes: T: Val 4.01, Plg 52, Az 36; N: Val
0.12, Plg 3, Az 130, P: Val 3.98, Plg 38, Az 222
TECTONIC SETTING
The Andaman-Nicobar-Nias (off Sumatra)
sedimentary arc in the northeastern Indian Ocean
defines a nearly 2200-km-long trench slope break and
outer arc ridge between the Indian plate and the SE
Asia/Burma plate. This convergent margin joins the
Burmese arc to the north and the Sunda arc towards
the south. The entire 3500-km-long Burmese-
Andaman arc constitutes an important transitional
link between the Himalaya and the Western Pacific
arc system characterized by varying degree of seismic
activity and volcanism. Active subduction of the
Indian plate below the Burma plate is documented
by the presence of the Barren-Narcondam active
volcanic arc that continues to the continental margin
arc in Sumatra and an east-dipping Benioff zone
defined by earthquakes up to 250-km focal depth.
The geologic and tectonic history of the region is
complex due to the presence of active faults/tectonic
features such as the West Andaman fault in the
Andaman arc, the Semangko fault in Sumatra, the
Sagaing fault in Burma and the Neogene Andaman
backarc spreading ridge.
Seismicity Pattern of the Region between
01.01.2004 and 25.12.2004
A total of 260 earthquake events occurred in
the region during 2004 up to 25.12.2004 (USGS
Catalogue). Out of these, 241 events are of magnitude
less than 5.0, 18 shocks between magnitudes 5.0 and
6.0 while there is only one event of magnitude 6.2.
Majority (162) of these earthquakes are of shallow-
focus (» 20 km) origin. There is an apparent seismic
quiescence between 28 November and 25 December
2004 with the last event registered on 27 November
2004 (M 5.3, depth 41 km, 1.97°N : 97.89°E).
MACROSEISMIC SURVEY
The 26 December 2004 earthquake was strongly
felt in the entire Andaman group of islands and the
seismic intensity was enough to cause low- order
damage to many civil constructions. A cross- section
of people belonging to different parts of the islands
was interviewed to get first-hand information on the
nature of seismic shaking. The general human
perceptions are as follows: At 6.35 AM (local time)
feeble tremors were felt that made many feel giddy.
This was followed by strong to and fro shaking which
lasted for almost 40 seconds. The time gap between
the initial feeble shocks and the strong shocks that
followed was reported to be sufficient for most of
the people to come out of their buildings even from
second floor. No sound, however, accompanied the
tremors. People ran outdoors in great panic; most
people lost balance, fell or sat down and crawled out
of their buildings. Those who were riding bicycles
or motorbikes felt strong wobbling effect and stopped
immediately. Parked cycles and a scooter fell down
during strong shaking. A parked bus was visibly
vibrating. Objects and utensils were thrown off the
shelves. At few places even heavy objects like steel
almirah and racks overturned. The total duration of
shaking have been reported by many to be of the order
of 3 minutes.
Different grades of damage to buildings have been
recorded from different parts of South Andaman. In
Port Blair area, places like Marine Park, Aberdeen
jetty, Chatham, Nayagaon, Bamboo Flat, etc, were
visited. Buildings like the Secretariat, Haddo Circuit
House, Blair Hotel which are type C structures
suffered damage of grades 1 and 2. Most buildings
of B/C type in and around Port Blair suffered similar
damage. In a single case at Nayagaon, a newly
constructed 3-storey building over stilt with RCC
columns and beams suffered grade-5 damage. The
entire soft-storey ground floor caved in due to failure
of load-bearing base column (Figs. 1.2 & 1.3). The
upper two floors though tilted, were much less
damaged. In the Bamboo Flat area many of the
6 K. N. MATHUR & OTHERS
buildings showed grade-2 cracks. In a newly
constructed house belonging to C. Mahammad Arif,
which at the time of the earthquake was not even
occupied, much higher damage was seen in
comparison to rest of the area. This 2-storey structure
with RCC columns, beams and RCC roof caved in
such a manner that the ground floor got completely
crushed and the first floor came to the ground-floor
level (Fig. 1.4). This was also a stilted structure where
the base columns were not tied with shear walls. It
appears that under condition of prolonged lateral
loading the base columns supporting a heavy load
sheared off resulting in grade-5 damage.
In the Kanyapuram locality one newly constructed
house belonging to Mr. Hamid was reduced to a heap
of rubble. The two-storeyed RCC structure with GI
roof completely caved in and a car parked in the
ground floor got completely crushed (Fig. 1.5).
In the Ograbraij locality, a godown of Malabar
Society was heavily damaged. This was a structure
with approximate dimensions of 8 m (w) x 15 m (l) x
6 m (h). The walls were of hollow concrete blocks
with RCC columns at the corners and at the central
parts of N-S aligned long walls. The slanting roof
with GI sheets was supported by heavy wooden
beams and rafters. The three walls and the roof
suffered total collapse. The quality of construction
was very poor, where steel used was found to be
rusted and concrete of low strength (Fig. 1.6). The
area was subsequently inundated by the tsunami back
flow (Fig. 1.7). In the Collinpur locality, almost all
the buildings suffered grade-2 or 3 damages. In a
single case, a single-storey restaurant suffered
damage of grade 5 in the form of total collapse of the
structure. The long wall of the shack of GI sheet
roofing seems to have thrusted in N45°E direction.
In this area, the foundation comprised clayey soil with
shallow groundwater table.
In the Baratang Island, similar seismic intensity
was recorded. The Forest Range Office suffered
grade-2 or 3 damage in the form of shear cracks in
walls and gaping settlement cracks in the floor. A
number of steel almirah and racks containing office
records and the hanging tubelights fell down during
the earthquake.
Water Supply Schemes for Port Blair
Dhanikhari Dam
This water retention structure on Dhanikhari River
was constructed during 1970-1973 for supply of
water to Port Blair town. The dam is a 132-m-long
and 32.23-m-high straight gravity-concrete structure
with a central gated spillway having a capacity of
26,000 cusecs. The reservoir extends to an area of
0.49 x 10 sq miles and the storage capacity is of the
order of 9000 lakh litres. On 26 December 2004, the
reservoir stood at R.L. 60.60 m. Inspection of the
dam revealed some minor distress to the main
structure due to the earthquake. Development of fine
cracks and plaster chipping off along two of the right
abutment block joints were visible (Fig. 1.8).
Inspection of foundation gallery showed cracking of
the RCC along the fifth block joint, through which
considerable amount of seepage was taking place.
Some minor seepage was also coming from the right
abutment slopes of the gallery. It was reported that
prior to the earthquake, the water collecting in the
foundation gallery used to be pumped after every six
hours. After the earthquake and due to the increased
seepage, it now requires hourly pumping. The
reservoir water was also considerably agitated due
to the passage of the shock waves as manifested by
the seiche (standing water waves), which rose by as
much as 3 to 4 m. After the earthquake, the main
supply pipes were dislodged and therefore, in the
initial days, the water supply to the town remained
disrupted. It has later been restored.
Chauldari Dam
Chauldari water supply scheme is a 19-m-high
and 95-m-long earthen dam structure with a 10-m-
wide and 80.58-m-long left bank ungated RCC chute
spillway. The earthen section has pitching of basalt
blocks, both in the upstream and downstream sides.
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A concrete apron has been placed over the entire
length of the crest. The distress to the dam on account
of the earthquake is seen at the junction of the earthen
section and spillway concrete. Here, the concrete
apron has buckled by as much as 8 cm along the block
joint (Fig. 1.9). The profile of the earth section
otherwise does not show any deformation or distress.
It is reported that on 26 December morning the
reservoir level (reservoir area 15 ha) was quite low.
But due to the tremors the waves in the reservoir rose
so high that they splashed on to the crest portion that
was about 5.6 m above the reservoir level.
Ground Fissures and Liquefaction
Ground fissures, slumping and subsidence were
witnessed at several places in the coastal belt of
Andaman Islands. At Collinpur locality, the fissures
are arcuate in disposition, trends N-S and appeared
to be a product of liquefaction and consequent lateral
spreading (Fig. 1.10). Here, the water table is barely
a metre or less deep and the topsoil clayey silt. During
the tremors, fissures were formed through which the
ground water spouted. At a few places, cream-
coloured fine sand/silt also ejected out. As reported
by the locals this zone of ground fissure continues
intermittently for 6-7 km between Tirur in the north
and Khurma beach in the south. A few buildings,
which were founded over such a spreading zone
suffered conspicuous damage. In a stray case, rolling
down of a boulder from a hill has been reported at
Collinpur. In the Baratang area the ground fissures
are so pronounced that they damaged considerable
stretches of the metalled road. These fissures are
described below.
Mud Volcano Eruptions and Surface Rupture at
Baratang
In the Baratang Island, two major and some minor
mud volcanoes among the chain of dormant ones
erupted during the earthquake of 26 December 2004.
Of the two major sites of eruption, the one near Jarwa
creek was examined in some detail. It is reported
that soon after the major tremors, this volcano erupted
with great violence. A series of explosions that lasted
for several minutes accompanying the eruption could
be heard from distances as far as 2-3 km from the
site. A resident of Rajatgarh village narrated that he
saw the mud splashing to above the forest tree height.
At the eruption site on the following day he witnessed
flames coming out of one vent. During the present
study the site was visited after 17 days of the eruption.
The mudflow of 26 December 2004 has spread to an
area of around 10,000 sq m and had a very distinct
bulged contact with the older mudflow (Fig. 1.11).
The shape of the mudflow can be described as that
of a flattened bun. The main crater, located at the
centre of the mud deposits was no more active. Gases
along with small quantity of sticky and viscous mud
was still coming out by fits and starts through another
newly formed vent located about 10m away from the
previous one (Fig. 1.12). Blowouts with an average
frequency of 2 minutes accompanied by low blurring
and hissing sound, was audible from a distance of
10 m or so. This crater is about 0.75 m in diameter
with a vent of about 20 cm at the centre.
The erupted mud consisted of very fine clay
particles containing small angular fragments of rocks.
The wet spouted clay dried and hardened almost
immediately after coming into contact with air and
was getting deposited at the rim of the new crater.
Gas emanated is odourless and inflammable at
ambient temperature (probably methane). On the
whole, a feeble sulphurous smell pervaded over the
mud volcano. The maximum height of the recent
deposits is estimated to be around 3 m. The total
volume of the erupted mud is calculated to be 1600
cu m. However, estimate by the Forest authorities
places this figure at 2400 cu m. It is quite certain
that this entire mud was ejected within a very short
time span after the earthquake. The mudflow in the
rim portion has partially flooded some of the tree
plantations.
A stretch of about 500 to 700-m-long metalled
road from the mud volcano site towards Baratang
Divisional Forest Office is highly fractured. These
8 K. N. MATHUR & OTHERS
open fractures trending N25°E cut across the road
(Fig. 1.13) and continue on either side on ground as
irregular fractures. Both right- and left-lateral slip of
5 to 10 cm were observed on the edge of the road. In
this stretch at one place the black-topped road surface
has buckled up to develop as an antiform with height
of the hinge part around 25-30 cm. Almost parallel
to the road a 90-cm- to 1.5-m-wide surface fracture
trending N55°W continues for about 50 m and joins
with one of the fractures that cut across the road. In
this stretch, a healthy long tree with its roots was
found neatly split vertically into two parts and shifted
horizontally (Fig. 1.14). While the left-hand portion
of the tree remained almost in situ the right hand part
was displaced about 1.20 m diagonally towards north,
thus showing left-lateral shear. The horizontal
component of slip along the fault plane is about
85 cm. Close to the intersection of this fault and the
fractures cutting the road a newly formed small mud
volcano with a distinct crater was seen. Through the
pulsating vent, a small amount of odourless gas and
wet clay with a film of black-coloured odourless
substance were spouting at regular interval. In two
nearby sites minor quantities of gas were found
continuously escaping from a pool of water.
Effect of Tsunami in Andaman Islands
The tsunami or the sea waves generated by the
Sumatra Earthquake of 26 December 2004 was very
profound in the low-lying coastal regions of Andaman
& Nicobar group of islands. The sea that rose as much
as 2.5 m above the high tide line (HTL) entered
inhabited areas with great velocity. The waves
flattened a number of dwellings and constructions,
breached the shore protection walls and certain
sections of the low-level roads, impaired some bridge
and harbour structures and inundated vast stretches
of shore land (Figs. 1.15, 1.16, 1.17 & 1.18). Influence
of the waves was greatly accentuated due to run-up
and ingress of the seawater (Fig. 1.19) into the low-
lying cultivated fields and human settlements through
the various creeks. Many such areas are still
inundated under the saline water and there is fear of
the soil becoming unfit for cultivation in future. The
tsunamis also flooded many of the dug wells thereby
contaminating the fresh-water sources.
A number of residents who witnessed the
catastrophe were interviewed to reconstruct the
scenario. The general observation was that after about
15 to 20 minutes of the mainshock the first influx of
sea waves approached the shores. The water level
rose to above the HTL. After some time the second
influx came in during which the water level increased
still further and then receded. The recession in water
level was so much that the seabed became visible for
quite a distance. The residents never witnessed such
a phenomenon earlier. The velocity of waves in the
two influxes was slightly above normal. At around
8.30 AM the third influx came to the shore with such
a velocity that everybody was caught unaware. The
water level rose to the maximum, in some cases to
over 2.5 m above the HTL.
The velocity of the ingushing water was such
that even those who were running away from the
water front, were soon overtaken. After the 26
December 2004 tsunami, the sea has remained
at a higher level than normal and the difference
between the HTL and LTL seems to have
reduced. Now, during the high tide, some areas
are still getting flooded and on an average the
HTL is about 1m higher than the pre-earthquake
situation. This was never the situation before the
tsunamis struck. However, it is observed that
with the passage of time sea-level is slowly
tending to recede.
AFTERSHOCK MONITORING
An intense aftershock activity has been recorded
following the Great Sumatra-Andaman Earthquake
(Fig. 1.20). The IMD seismic observatories have
recorded a total of 124 aftershocks in excess of M
5.0 from 26 December 2004 to 11 January 2005. The
largest aftershock was of M 7.0 that occurred on
26.12.2004, 120 km west of Nicobar Island. Eleven
aftershocks are of magnitude ³ 6.0 while the
GSI SPL. PUB. 89 : SUMATRA-ANDAMAN EARTHQUAKE & TSUNAMI, 26 DEC.’04 9
Fig. 1.1 : GSI team led by the Director General at Port Blair investigating
the tsunami effectsFig. 1.2 : Shearing of load-bearing base columns leading to caving in of
G+2-storeyed RCC structure, Nayagaon, Port Blair
Fig. 1.3 : Sheared base column and caved-in stilt, Nayagaon, Port Blair Fig. 1.4 : Complete crushing of ground floor due to failure of base column
with first floor collapsing to ground level, Bamboo Flat
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Fig. 1.5 : Complete caving in of stilt and heavy damage of first floor (now
at ground floor level). A car parked in the stilt got completely
crushed; newly constructed building, Kanyapuram
Fig. 1.6 : Failure of walls and roof of Malabar Society godown, Ograbraij
Fig. 1.7 : Inundation of the structure in Fig. 1. 6 and surrounding buildings
by the tsunami
Fig. 1.8 : Fine cracks and chipping
off plaster at the block
joints of Dhanikhari
concrete dam
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Fig. 1.9 : Buckling by 8cm of concrete apron at the interface of concrete
spillway and earth section on the crest portion, Chauldari damFig. 1.10 : A ground fissure cutting across the road through Anganbari
community centre, Collinpur
Fig. 1.11: Mud volcano that erupted after the 26 December 2004
earthquake near Jarwa Creek, Baratang. The recent mudflow
has a distinct contact with an older flow
Fig. 1.12 : An active crater within the recent mud volcano
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Fig. 1.13 : Fissure developed on the road at Baratang
Fig. 1.14 : A tree trunk with
its roots separated into two
halves along a wide open
(90cm) crack showing left
lateral shear
Fig. 1.15 : Structure flattened by the tsunami in Wandoor Fig. 1.16 : Breach in a section of the shore protection wall at Mazar, Port Blair
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Fig. 1.17 : Bridge washed off by the tsunami at
Corbyn’s Cove, Port Blair
Fig. 1.18 : Tsunami water mark (brown-green
interface) about 2 m above the high-tide
level at Chidiatapu
Fig. 1.19 : Inundation of paddy field through the
backwaters in Ograbraij
14 K. N. MATHUR & OTHERS
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remaining 112 events are in the magnitude range of
5.0-6.0. USGS has recorded 223 aftershocks up to
09.01.05 of magnitude ³ 4.4. While 52 events are of
magnitude £ 5.0 the remaining recorded above 5.0.
Aftershock sequence from the IMD list gives a b-
value of 1.08 while those from the USGS catalogue
gives 1.20. Predicted Mmax is 7.1 from both the
catalogue, which has already struck on 26 December
2004 itself. The p-value calculated from IMD list is
0.97 while that from USGS is 1.27 suggesting that
aftershocks of magnitude ³ 5.0 will possibly decay
within 40 days.
The Geological Survey of India has dispatched 5
short-period digital seismometers to monitor the
aftershocks. The first station was operational in the
Naval Base Defence Colony, Vijaybagh, Port Blair
(a) (b)
Fig. 1.20 : (a) Aftershock seismicity map up to 09.01.05 as recorded by USGS. (b) Frequency-magnitude plot of
aftershocks as recorded by IMD (top) and USGS (bottom)
from 6.1.2005. The second station was established
in Car Nicobar Air Force Base on 8.1.2005. A third
station was installed in Little Andaman (Hut Bay)
on 10.1.2005. Two more stations are planned to be
deployed in Rangat and Diglipur, thus covering a
length of 470 km between the northern parts of
Andaman and Nicobar Islands. It is proposed to run
the seismic network for about a month.
GPS STUDIES
GSI has planned to install several GPS and operate
in campaign mode at different islands from Diglipur
in North Andaman to Car Nicobar in the south
covering a distance of about 470 km. The GPS
stations are proposed to be re-occupied 2-3 times
annually. The first station has been installed over rock
exposure near GSI drilling camp site at Beadonabad
GSI SPL. PUB. 89 : SUMATRA-ANDAMAN EARTHQUAKE & TSUNAMI, 26 DEC.’04 15
1. © Government of India, copyright 2006.
2. The responsibility for the correctness of internal details rests with the publisher.
3. The territorial waters of India extend into the sea to a distance of twelve nautical miles measured from the appropriate base line.
4. The coast line of India agree with the Record/Master Copy certified by Survey of India.
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on 10.01.05. Another station will be at Chidiatapu
Forest Rest House. Diglipur in North Andaman and
Baratang Island will be occupied soon. Installation
at other sites will depend on the availability of
logistics for going to places like Little Andaman and
Car Nicobar.
CONCLUSIONS
1. The Sumatra Earthquake of 26 December 2004
is the largest recorded seismic event along the
Andaman-Sunda subduction zone. The giant
tsunamis generated by this offshore fault rupture have
been unprecedented in the Indian Ocean and therefore
call for inclusion of tsunami hazard in the disaster
management plans of the country.
2. To locate earthquakes precisely from this highly
seismic belt, the 800-km-long Andaman-Nicobar
Islands have to be covered by adequate seismograph
stations.
3. The entire belt of Andaman & Nicobar group
of islands is an area of intense seismic activity and
therefore has been included in the highest hazard
Class V of the Seismic Zoning Map of India. It is,
therefore, of great importance that for any
construction activity the BIS code on “Earthquake
Resistant Designs” should be strictly followed. This
applies more to any lifeline and structures of
importance like schools, hospitals, elevated water
retention structures and defence installations, etc.
4. The earthquake has demonstrated in very clear
terms that stilted structures without provision of any
shear resistant walls behave very poorly under lateral
seismic loading of even lower seismic intensity of
VII of MSK-64 scale. The results are similar to what
was observed in case of Ahmedabad and Surat cities
during the Kutch Earthquake of 26 January 2001. It
is, therefore, essential that design of RCC structures
particularly G+2 and taller buildings, should be
examined by competent structural engineers so that
earthquake resistant elements are properly
incorporated.
5. Prima facie, quality of RCC in case of the three
collapsed structures in and around Port Blair was
found to be inferior. It is, therefore, necessary to
carry out proper geotechnical tests to determine the
strength and durability of the concrete made out of
locally available construction material.
6. Future development plans and activities in the
tsunami run-up zones in coastal tracts and in areas
delineated by the HTL and maximum possible
tsunami run-up elevation needs to be regulated.
Existing structures and human settlements are to be
relocated accordingly (ports, jetties, harbours,
research stations, data collection centres, etc.
excluded). Regulatory measures and practices being
followed in other countries which are frequently
visited by tsunami, may be consulted for this purpose
and codal provisions made.
7. As earthquake waves travel faster, the tsunami
waves arrive later than the P-waves. The time lag
depends on distance of the source area. So in islands
and coastal areas of India, all felt earthquakes may
be considered as natural tsunami alert signals and
local residents as well as the administration should
respond accordingly. As all earthquakes do not
generate tsunami this response may be considered as
a watch alert only and not a forecast or warning.
ACKNOWLEDGEMENTS
Support provided by the Andaman and Nicobar
Administration during the investigation particularly
by Mr Rishikesh and Mr Bhadra of the Department
of Science & Technology, A & N Administration is
gratefully acknowledged. Officers of Geodata and
Database Division and Shri Anshuman Acharyya,
Geologist, Monitoring Division, Kolkata are some
among others without whose active support this
document could not have been released just in 3 days.
16 K. N. MATHUR & OTHERS
CHAPTER 6
SEISMOTECTONICS OF THE ANDAMAN - NICOBAR REGION: CONSTRAINTS
FROM AFTERSHOCKS WITHIN 24 HOURS OF THE GREAT
26 DECEMBER 2004 EARTHQUAKE
SUJIT DASGUPTA, BASAB MUKHOPADHYAY AND A. ACHARYYA
Geological Survey of India, 27 J. L. Nehru Road, Kolkata 700 016, India
INTRODUCTION
The Andaman arc together with the Burmese
Arakan Yoma hill ranges present a nearly 3500-km-
long subducting margin in the northeastern part of
the Indian plate where varying degrees of seismic
activity, volcanism and active deformation are
evidenced. The region serves as an important
transitional link between the Eastern Himalaya
collision margin and the Sunda arc (a part of the West
Pacific Arc System). Seismicity and tectonics of this
convergent margin though studied in detail (see
among others Curray, 2005; Dasgupta et al., 2003)
nevertheless is insufficient to propose any medium-
to short-term predictive model for the occurrence of
such great interplate earthquake like the one that
struck on 26 December 2004. Notwithstanding a few
soft claims in the media on the forecast of this major
earthquake that created havoc via tsunami all along
the Indian Ocean rim countries, the event could not
have been predicted within a reasonable space, time
and size window with the present knowledge of
earthquake physics, statistics and tectonics.
Basic seismological data, on which our
understanding of this mega-event is derived, is largely
provided by the USGS web site. From the study of
NEIC earthquake catalogue both in the pre- and post-
26 December 2004 scenario we had demonstrated
(see http//:www.gsi.gov.in/suma_eq.html) spatio-
temporal variation of seismicity pattern; between
1 January and 26 November 2004 there are records
of 260 events from the region, while in the period
since 27 November 2004 till the great earthquake of
26 December 2004 there was a clear seismic
quiescence of one month. We had further shown that
all aftershocks that struck on 26 December 2004 (193
as listed on 08.02.2005) form three distinct linear
clusters along the subduction mega-thrust and two
more clusters along the West Andaman fault (WAF).
We have revisited the NEIC catalogue (as on
09.06.2005) to find that 283 aftershocks are recorded
on 26 December 2004 itself and in this note we
discuss the seismotectonic setup of the Andaman-
Nicobar region based on spatio-temporal behaviour
of 174 aftershocks (M ³ 4.8) that occurred on 26
December 2004 following the mainshock.
ANALYSIS
A total of 112 mega-thrust plane aftershocks
(M ³ 4.8) are plotted in solid circles on a simplified
tectonic map (after Curray, 2005) superimposed with
40-km contour (red line) on top of the Benioff zone
and nine (f 8 to f 16) lithospheric hinge faults within
the subducting Indian plate ( Fig. 6.1; after Dasgupta
et al., 2003). The mainshock locates on the Benioff
zone where it is segmented by the fault f 16. Except
one event (No.25; Table 6.1) that locates south of
Nias the rest occur north of the mainshock. The
northernmost aftershock (No. 147; Table 6.3)
recorded locates close to 14°N latitude. The fault
rupture due to the earthquake thus propagated
around 11° (> 1200 km) from 3° (epicentre) to 14°N
latitude. Another 62 aftershocks occur within the
overriding Andaman-Sumatra lithospheric plate,
loading primarily the West Andaman Fault
(WAF) system up to western part of the Andaman
Spreading Ridge (ASR). Except 11 slightly deeper
(max. depth : 61 km) shocks all are shallow-foci
events (< 40 km). Ten aftershocks are of magnitude
(mb, Ms or Mw) greater than 6.0 including one event
of Mw 7.2 (No. 53; Table 6.1).
From the spatial distribution pattern of aftershocks
the entire rupture plane can be divided into three
segments. The southernmost Segment I containing
the mainshock, is about 570 km long, trends N40°W
and extends up to the hinge fault f 13 (revised from
our earlier study; see Dasgupta et al., 2005). Fifty-
four aftershocks originating from this segment define
the rupture plane. The strongest aftershock (No.53)
locates close to the northern margin of Segment I.
Best double-couple solutions (HRVD) for the
mainshock and 6 aftershocks (No. 53, 133, 141, 150,
164 and 165; in bold font, Table 6.1) are schematically
shown (Fig. 6.1) and parameters listed (Table 6.5).
While the mainshock, aftershocks 53 and 133 show
thrust mechanism, No.141 is a downdip compression
reverse fault with moderate right-lateral slip and No.
150 and 164 display normal faulting. The central or
Segment II is between the hinge faults f 13 and f 11.
This segment trends N15°-20°E and is about 400-
km long with 31 recorded aftershocks (Table 6.2).
The northern sector of this segment (between No. 63
and fault f 11) has apparently remained unbroken on
26 December 2004. The largest aftershock (No. 100,
Table 6.2) is of magnitude (Mw) 6.6 that gives thrust-
fault solution (HRVD). Fault Segment III, between
f 11 and f 8, trends N15°E with a fault rupture length
of 500 km. The rupture plane is defined by 26
aftershocks (Table 6.3) with the strongest shock
(No.110) of magnitude (Mw) 6.3. While this event
shows normal fault mechanism, 3 more closeby
shocks (No. 120, 140 and 162) give strike-slip
solutions with the NW nodal plane indicating activity
along fault f 9 (see also Dasgupta et al., 2003). Due
to the presence of lithospheric hinge faults within
the subducting Indian plate, the Benioff zone is
segmented resulting several shallow- and steeper-dip
segments; this is clearly brought out by the swerving
nature of the 40-km contour on top of the Benioff
zone (Dasgupta et al., 2003). Though aftershocks are
distributed throughout the entire megathrust plane
they appear to be more concentrated in the steeper
segments.
We further shortlist 7 events that occurred in
temporal succession along the unilateral direction of
rupture propagation. These 7 aftershocks (red solid
circle in Fig. 6.1) are No.1, 2 (in Rupture Segment I,
bold font, Table 6.1) and 9, 13, 14, 24 and 30 (in
Rupture Segment III, bold font, Table 6.3) that define
the entire fault rupture from the mainshock in the
south to shock No. 30 in the north. It took 2 hours 9
minutes 50.76 seconds since the mainshock to break
up to the northernmost point of the megathrust
traversing a total of about 1300-km fault length with
an average rate of about 167 m/sec. We are inclined
to believe that these 7 sequentially propagating
shocks are triggered events rather than aftershocks
(sensu stricto) that usually strike via residual stress
to break small asperities left by the mainshock
rupture. The unilateral propagating rates for the inter-
events given by the respective length/time are: 242
m/sec (from mainshock to aftershock 1); 730 m/sec
(from aftershocks 1 to 2); 250 m/sec (from 2 to 9);
158 m/sec (from 9 to 14) and 147 m/sec (from 24 to
30). These unilateral afterslip or triggered slip rates
are, however, less than the modelled rupture velocity
of 2.0 to 2.5 km/ sec in the mainshock rupture segment
[Yagi, 2005; Chen Ji, 2005; Yamanaki, 2005].
Average down-dip width of fault rupture is 150 km
in Fault Segment I, and 130 km in Segments II and
III. Total rupture area is around 2.0 ́ 105 sq km. With
an average slip of 15 m [and rigidity (m) as 4´
1011dyne/cm2], seismic moment (Mo) calculates to
the order of 1.2 ´ 1030 dyne-cm, a value very close
to that given by Stein and Okal (2005).
The main earthquake of 26 December 2004 has
loaded the entire fault system in the region both in
the subducting and overriding plates and transferred
stress particularly to the WAF. Several large
aftershocks locate along this fault system and
continue up to 10.5°N latitude close to the junction
96 SUJIT DASGUPTA & OTHERS
Fig. 6.1: Tectonic map of Sumatra-Andaman region (after Curray, 2005) with 112 aftershocks (· focal depth £ 40 km;
+ > 40 km) that occurred on 26 December 2004 following the mainshock (star); f 8 - f 16 are lithospheric
hinge faults and red line is the 40-km contour on top of the subducting Indian lithosphere (both after Dasgupta
et al., 2003). The fault rupture plane is shown in shades of yellow and green for the 3 segments (I-III).
· 7aftershocks that occurred sequentially from south to north along the megathrust; for numbers refer to
Tables 6.1-6.4. Beach-ball diagrams are HRVD best double-couple solutions; for parameters see Table 6.5.
Red star: volcano; N- Narcondam, B- Barren. ASR- Andaman spreading ridge; MPF- Mae Ping fault; TPF-
Three Pagodas fault; SSF- Shan Scrap fault; WAF- West Andaman fault; RF- Ranong fault; KMF- Khlong
Marui fault
1. © Government of India, copyright 2006.
2. The responsibility for the correctness of
internal details rests with the publisher.
3. The territorial waters of India extend into the
sea to a distance of twelve nautical miles
measured from the appropriate base line.
4. The coast line of India agree with the Record/
Master Copy certified by Survey of India.
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TABLE 6.1
CHRONOLOGICAL LISTING OF AFTERSHOCKS ON 26 DECEMBER 2004
FROM FAULT RUPTURE SEGMENT I
No YEAR MO DA Hr Mn Sec LAT LONG DEPTH MAGNITUDE
mb Ms Mw Mo
0 2004 12 26 00 58 53.45 3.30 95.98 30 8.9 8.9 9.0 3.95E+29
1 2004 12 26 01 17 10.33 4.94 94.27 30 5.5
2 2004 12 26 01 21 20.66 6.34 93.36 30 6.1
7 2004 12 26 01 40 07.13 5.84 93.15 30 5.3
8 2004 12 26 01 48 52.07 5.43 94.46 51 5.7
12 2004 12 26 02 15 23.57 6.17 93.47 30 5.6
16 2004 12 26 02 30 28.94 6.72 93.08 15 5.1
17 2004 12 26 02 34 52.15 3.99 94.14 30 5.7
23 2004 12 26 02 46 20.74 4.24 93.61 30 5.7
25 2004 12 26 02 53 13.04 0.06 97.04 30 5.4
27 2004 12 26 02 59 14.39 3.18 94.38 30 5.7
31 2004 12 26 03 09 34.08 4.05 93.53 30 5.4
34 2004 12 26 03 19 13.05 3.55 94.29 30 5.5
36 2004 12 26 03 24 54.94 4.47 94.07 26 5.8
38 2004 12 26 03 30 01.38 4.64 94.00 25 5.2
41 2004 12 26 03 46 42.04 6.72 93.33 46 5.0
42 2004 12 26 03 50 22.18 5.51 94.25 48 5.3
44 2004 12 26 03 54 44.77 6.48 92.89 30 5.1
45 2004 12 26 04 00 42.83 4.76 93.79 16 5.2
47 2004 12 26 04 02 12.52 3.04 95.89 30 5.4
50 2004 12 26 04 10 12.71 5.48 92.92 36 5.4
51 2004 12 26 04 12 35.65 6.44 93.23 3 4.8
53 2004 12 26 04 21 29.81 6.91 92.96 39 6.1 7.5 7.2 7.23E+26
55 2004 12 26 04 31 29.06 6.99 93.18 36 5.0
69 2004 12 26 05 23 50.8 3.35 94.09 18 5.2
71 2004 12 26 05 51 40.01 6.45 93.43 29 5.2
72 2004 12 26 05 55 49.4 3.17 93.94 23 5.1
74 2004 12 26 06 09 30.84 6.34 93.20 29 4.8
76 2004 12 26 06 16 14.68 5.84 93.36 26 4.9
78 2004 12 26 06 22 35.25 5.34 93.07 23 5.1
80 2004 12 26 06 38 36.05 6.65 92.96 16 5.4
90 2004 12 26 07 59 37.72 3.23 93.91 31 5.3
97 2004 12 26 09 07 38.95 3.42 94.34 25 4.9
105 2004 12 26 09 43 19.38 5.53 93.14 30 5.1
106 2004 12 26 09 44 20.36 5.73 93.10 36 5.2
111 2004 12 26 10 29 49.0 5.17 93.48 46 5.3
113 2004 12 26 10 43 29.95 6.53 92.83 36 5.4
121 2004 12 26 11 17 08.52 3.25 93.75 30 5.2
123 2004 12 26 11 50 28.09 6.39 93.25 61 5.2
126 2004 12 26 12 30 59.49 3.89 94.44 30 4.9
127 2004 12 26 12 46 06.37 5.40 93.28 25 4.9
132 2004 12 26 13 44 08.07 3.97 94.39 31 5.1
133 2004 12 26 13 56 40.17 2.78 94.47 30 5.5 5.9 5.9 8.62E+24
135 2004 12 26 14 11 28.31 3.67 94.02 30 5.2
141 2004 12 26 15 06 33.24 3.65 94.09 17 5.6 6.1 6.0 1.07E+25
Table 6.1 (contd.)...
98 SUJIT DASGUPTA & OTHERS
No YEAR MO DA Hr Mn Sec LAT LONG DEPTH MAGNITUDE
mb Ms Mw Mo
142 2004 12 26 15 12 21.55 6.73 92.98 18 5.3
143 2004 12 26 15 13 20.84 5.37 93.43 30 5.4
146 2004 12 26 15 36 54.02 4.12 93.85 39 4.9
148 2004 12 26 16 21 27.41 5.15 94.32 41 5.4
150 2004 12 26 16 55 17.27 3.86 94.50 30 5.3 5.4 1.70E+24
156 2004 12 26 181449.54 4.80 94.09 30 4.8
157 2004 12 26 181655.97 3.37 94.10 28 4.8
159 2004 12 26 183143.48 6.32 93.32 30 5.3
160 2004 12 26 183207.92 3.84 93.32 26 5.1
164 2004 12 26 19 03 49.21 4.09 94.22 30 5.5 5.5 2.36E+24
165 2004 12 26 191955.57 2.79 94.16 30 5.5 6.2 6.1 1.76E+25
TABLE 6.2
CHRONOLOGICAL LISTING OF AFTERSHOCKS ON 26 DECEMBER 2004
FROM FAULT RUPTURE SEGMENT II
No. YEAR MO DA Hr Mn Sec LAT LONG DEPTH MAGNITUDE
mb Ms Mw Mo
10 2004 12 26 01 59 13.99 8.39 92.45 30 5.3
15 2004 12 26 02 22 01.84 8.87 92.47 15 5.7
19 2004 12 26 02 38 09.35 8.49 92.35 33 5.6
20 2004 12 26 02 40 59.85 7.48 92.43 30 5.4
22 2004 12 26 02 45 17.65 8.46 92.61 30 5.2
26 2004 12 26 02 56 40.37 8.61 92.29 30 4.9
28 2004 12 26 03 02 38.08 8.61 92.33 30 5.5
29 2004 12 26 03 06 13.05 8.19 92.46 27 5.1
33 2004 12 26 03 17 52.38 7.21 92.92 30 5.6
59 2004 12 26 04 53 09.1 8.19 92.93 30 4.9
62 2004 12 26 05 01 10.56 9.30 92.21 30 5.3
63 2004 12 26 05 01 21.37 9.46 92.18 30 5.4
64 2004 12 26 05 08 04.83 9.03 92.46 30 5.0
66 2004 12 26 05 12 34.14 8.46 92.28 36 5.1
86 2004 12 26 07 24 53.05 7.42 92.64 34 5.1
89 2004 12 26 07 55 27.13 7.48 92.36 30 5.3
98 2004 12 26 09 13 54.71 7.31 92.19 33 5.2
100 2004 12 26 09 20 01.61 8.88 92.38 16 6.0 6.6 6.6 9.77E+25
103 2004 12 26 09 36 39.27 9.35 91.86 30 4.6
104 2004 12 26 09 38 39.35 8.96 92.33 30 4.9
107 2004 12 26 10 02 07.76 7.65 92.79 31 4.8
112 2004 12 26 10 33 05.16 8.70 92.62 39 5.4
114 2004 12 26 10 51 19.82 7.63 92.31 30 5.5
137 2004 12 26 14 39 07.37 8.30 92.36 30 5.1
149 2004 12 26 16 48 24.11 7.22 93.03 49 4.9
158 2004 12 26 182931.78 8.06 92.20 30 5.0
168 2004 12 26 21 20 42.31 8.58 92.14 30 4.9
171 2004 12 26 21 44 38.22 7.03 92.56 30 4.8
172 2004 12 26 22 46 11.06 8.99 92.51 36 4.9
173 2004 12 26 23 04 26.65 9.29 91.97 30 5.1
174 2004 12 26 23 31 45.58 9.02 92.38 30 4.8
Table 6.1 (contd.)...
GSI SPL. PUB. 89 : SUMATRA-ANDAMAN EARTHQUAKE & TSUNAMI, 26 DEC.’04 99
TABLE 6.3
CHRONOLOGICAL LISTING OF AFTERSHOCKS ON 26 DECEMBER 2004
FROM FAULT RUPTURE SEGMENT III
No YEAR MO DA Hr Mn Sec LAT LONG DEPTH MAGNITUDE
mb Ms Mw Mo
9 2004 12 26 01 52 43.0 10.38 92.12 12 5.2
13 2004 12 26 02 15 49.5 12.26 92.28 20 5.3
14 2004 12 26 02 15 59.78 12.32 92.50 26 5.7
18 2004 12 26 02 36 10.09 12.18 92.94 38 5.8
24 2004 12 26 02 52 01.83 12.50 92.60 30 5.8
30 2004 12 26 03 08 44.21 13.74 93.01 30 5.9
40 2004 12 26 03 44 08.34 13.47 92.74 22 5.2
60 2004 12 26 04 58 04.02 11.07 92.00 29 5.3
68 2004 12 26 05 20 27.92 12.16 92.40 31 5.3
77 2004 12 26 06 22 00.42 10.68 92.32 26 5.4
81 2004 12 26 06 56 47.4 10.98 92.28 23 5.5
87 2004 12 26 07 38 27.0 13.13 93.04 30 5.7
110 2004 12 26 10 19 31.73 13.46 92.74 26 6.1 6.0 6.3 3.22E+25
118 2004 12 26 10 57 38.36 12.45 92.44 5 5.4
119 2004 12 26 11 03 53.29 11.10 93.95 30 4.8
120 2004 12 26 11 05 00.72 13.53 92.84 13 6.3 6.3 6.2 2.37E+25
124 2004 12 26 12 09 42.46 12.19 92.60 20 5.4
125 2004 12 26 12 11 57.66 11.57 92.41 25 5.4
136 2004 12 26 14 14 18.03 13.50 92.92 17 5.0
138 2004 12 26 14 40 30.41 11.47 92.18 30 5.3
140 2004 12 26 14 48 44.26 13.59 92.91 30 5.8 5.7 5.7 4.40E+24
147 2004 12 26 16 12 53.01 13.94 93.31 4 4.8
152 2004 12 26 17 50 12.59 13.60 92.85 26 5.0
153 2004 12 26 17 56 35.84 12.86 92.48 45 5.1
162 2004 12 26 18 42 43.89 13.71 92.95 26 5.3 4.7 5.4 1.69E+24
163 2004 12 26 18 55 46.1 11.98 91.97 30 4.9
of WAF transform and ASR (Fig. 6.2). Sixty-two
aftershocks of magnitude ³ 4.8 (Table 6.3) are
recorded on 26 December 2004 that display two
distinct linear clusters. The southern cluster locates
west of northern Sumatra while the other occurs east
of Nicobar group of islands, both due to activity along
different strands of WAF. Four shocks are of
magnitude (mb or Mw) ³ 6.0 and the largest (No.
109) gives a reverse fault solution (HRVD) but both
the nodal planes trend ENE almost normal to the
WAF. In this segment of WAF there are also 7
unilaterally propagating aftershocks from No. 3 in
the south to No.128 close to ASR through Nos. 5,
21, 67, 82 and 83 (bold font in Table 6.3 & green
solid circle in Fig. 6.2). Northward propagation rate
from shock Nos. 3 to 5 is 335 m/sec and 255 m/sec
from 82 to 83, while it is very slow (» 5 m/sec) in a
patch between shock Nos. 5 and 82 through 21 and
67 involving junction of two strands of WAF.
CONCLUSION
The 26 December 2004 Sumatra-Andaman
Mw-9.3 earthquake is the largest recorded event from
this part of Indo- Southeast Asia convergent margin.
Aftershock distribution pattern on the day the
earthquake struck indicates that rupture propagated
unilaterally northwards from the mainshock epicentre
to break around 1300 km of plate interface. Though
this part of the subducting Indian plate is fragmented
by a number of lithospheric hinge faults, some of
100 SUJIT DASGUPTA & OTHERS
Table 6.4 (Contd.)...
TABLE 6.4
CHRONOLOGICAL LISTING OF AFTERSHOCKS ON 26 DECEMBER 2004
FROM THE ANDAMAN- SUMATRA UPPER PLATE
No. YEAR MO DA Hr Mn Sec LAT LONG DEPTH MAGNITUDE
mb Ms Mw Mo
3 2004 12 26 01 22 25.59 7.42 93.99 30 6.0
4 2004 12 26 01 25 48.76 5.50 94.21 30 6.1
5 2004 12 26 01 30 15.74 8.83 93.71 30 5.5
6 2004 12 26 01 33 22.38 7.76 93.71 25 5.5
11 2004 12 26 02 00 40.03 6.85 94.67 30 6.0
21 2004 12 26 02 43 05.26 9.22 94.00 30 4.9
32 2004 12 26 03 14 13.84 7.44 94.26 30 5.4
35 2004 12 26 03 22 57.48 5.82 95.09 20 5.4
37 2004 12 26 03 26 45.79 4.91 96.40 30 5.3
39 2004 12 26 03 40 15.64 5.53 94.33 30 5.6
43 2004 12 26 03 51 12.36 5.05 94.77 30 5.7
46 2004 12 26 04 00 58.43 6.79 94.08 29 5.5
48 2004 12 26 04 02 55.73 4.98 94.72 47 5.8
49 2004 12 26 04 09 08.4 8.16 93.82 30 4.9
52 2004 12 26 04 17 56.81 8.96 93.72 30 5.3
54 2004 12 26 04 26 03.63 7.89 93.99 30 5.2
56 2004 12 26 04 40 11.46 9.12 93.84 38 5.2
57 2004 12 26 04 46 23.44 8.53 93.88 32 5.4
58 2004 12 26 04 48 56.49 8.87 93.75 27 5.2
61 2004 12 26 04 59 15.4 8.97 93.43 25 5.2
65 2004 12 26 05 09 32.5 9.16 93.89 26 5.2
67 2004 12 26 05 16 10.98 9.32 94.04 22 5.4
70 2004 12 26 05 42 49.27 5.49 94.29 30 5.1
73 2004 12 26 06 02 28.38 8.27 94.06 23 5.7
75 2004 12 26 06 11 04.6 9.31 93.91 23 5.1
79 2004 12 26 06 28 48.4 4.96 94.79 30 5.4
82 2004 12 26 06 59 57.26 9.36 93.70 30 5.4
83 2004 12 26 07 07 10.27 10.36 93.75 19 5.6
84 2004 12 26 07 11 40.39 4.81 94.97 35 5.2
85 2004 12 26 07 23 38.81 5.44 94.41 30 4.7
88 2004 12 26 07 52 28.8 8.13 94.07 17 5.5
91 2004 12 26 08 02 34.62 5.34 94.48 34 5.1
92 2004 12 26 08 12 38.7 9.26 93.84 36 4.8
93 2004 12 26 08 14 59.09 6.79 94.54 30 4.8
94 2004 12 26 08 41 48.85 8.90 93.48 25 5.2
95 2004 12 26 08 47 46.72 4.86 95.10 50 5.3
96 2004 12 26 09 02 42.55 8.29 93.98 26 4.9
99 2004 12 26 09 17 51.19 7.06 94.39 21 5.0
101 2004 12 26 09 30 29.54 7.39 93.99 13 4.9
102 2004 12 26 09 30 55.8 7.18 93.76 30 5.4
108 2004 12 26 10 12 10.15 10.25 94.31 30 5.1
109 2004 12 26 10 18 13.79 8.86 93.74 30 5.5 6.3 3.87E+25
115 2004 12 26 105358.42 10.19 93.68 30 5.3
116 2004 12 26 10 55 07.5 4.26 95.13 30 5.2
117 2004 12 26 10 56 02.59 10.07 93.83 30 5.5
GSI SPL. PUB. 89 : SUMATRA-ANDAMAN EARTHQUAKE & TSUNAMI, 26 DEC.’04 101
No. YEAR MO DA Hr Mn Sec LAT LONG DEPTH MAGNITUDE
mb
122 2004 12 26 11 34 20.02 5.28 94.36 30 4.8
128 2004 12 26 12 52 45.76 10.43 93.91 30 5.1
129 2004 12 26 13 10 42.5 7.59 94.24 30 4.8
130 2004 12 26 13 13 27.14 6.14 95.43 30 4.9
131 2004 12 26 13 28 56.52 7.72 94.03 19 5.2
134 2004 12 26 14 02 05.02 4.80 94.78 30 4.8
139 2004 12 26 14 47 17.44 4.63 95.10 30 4.9
144 2004 12 26 15 23 05.33 7.44 94.22 17 5.0
145 2004 12 26 15 24 08.86 7.56 94.23 17 4.9
151 2004 12 26 17 44 52.77 8.93 93.97 22 5.2
154 2004 12 26 17 59 00.46 8.31 93.95 0 4.8
155 2004 12 26 18 10 43.16 8.95 94.04 25 5.0
161 2004 12 26 18 33 55.6 9.43 93.66 47 5.1
166 2004 12 26 21 06 48.8 4.47 96.34 30 5.5
167 2004 12 26 21 19 30.79 4.23 97.81 30 4.9
169 2004 12 26 21 25 33.15 4.75 94.85 30 5.0
170 2004 12 26 21 25 43.24 4.33 95.07 30 4.8
TABLE 6.5
PARAMETERS FOR BEST DOUBLE-COUPLE SOLUTION (HRVD)
OF MAINSHOCK AND 12 AFTERSHOCKS
No.* Nodal plane 1 Nodal plane 2 T- axis P- axis B-axis
st dip slip st dip slip Pl Az Pl Az Pl Az
0 329 8 110 129 83 87 53 35 37 221 3 130
53 351 27 121 137 67 75 64 25 22 239 13 144
100 333 38 82 163 53 96 80 102 7 248 4 340
109 272 40 115 61 54 70 73 283 7 164 15 72
110 1 41 -116 215 54 -69 9 291 71 181 17 23
120 29 56 158 132 72 36 38 355 11 257 49 155
133 307 35 83 136 56 95 78 58 11 221 4 314
140 137 56 15 38 78 145 33 350 13 91 53 200
141 96 48 33 343 67 133 47 299 12 43 39 144
150 289 37 -73 88 55 -102 10 190 76 317 10 96
162 144 69 174 236 84 21 19 102 11 09 68 251
164 304 38 -91 126 52 -89 9 216 82 38 16 23
165 342 34 139 108 68 63 56 342 19 220 24 120
Table 6.4 (Contd.)...
102 SUJIT DASGUPTA & OTHERS
Fig. 6.2: Tectonic map of Sumatra-Andaman region (after Curray, 2005) with 62 overriding plate aftershocks (· focal
depth £ 40 km; + > 40 km) that occurred on 26 December 2004 following the mainshock (star). · Sequential
aftershocks along the West Andaman fault. Other legends same as in Fig. 6.1
1. © Government of India, copyright 2006.
2. The responsibility for the correctness of
internal details rests with the publisher.
3. The territorial waters of India extend into the
sea to a distance of twelve nautical miles
measured from the appropriate base line.
4. The coast line of India agree with the Record/
Master Copy certified by Survey of India.
GSI SPL. PUB. 89 : SUMATRA-ANDAMAN EARTHQUAKE & TSUNAMI, 26 DEC.’04 103
CMYK
them acted as barriers for smooth propagation of
rupture resulting three well-defined fault segments.
Seven unilaterally northward propagating shocks
from the mainshock to the distal part of the rupture
occurred within 130 minutes at a rate of 167 m/sec
and these events are likely to be ‘triggered
earthquakes’ rather than usual aftershocks. This great
shallow-foci interplate thrust earthquake has also
seismically loaded the overriding plate to activate two
different strands of the WAF. HRVD best double-
couple solutions indicate that though thrust faulting
is the main mode of rupture, normal and strike-slip
mechanisms are also operative. Detail study of
aftershocks in relation to seismo-geological depth
sections across and along the arc is necessary to
decipher the details of seismotectonics.
REFERENCES
Curray, J. R. (2005). Tectonics and History of the
Andaman Sea Region. Jr. Asian Earth Sc.,
25 : 187-232.
Dasgupta, S., Mukhopadhyay, M., Bhattacharya, A.
and Jana, T.K. (2003). The geometry of the
Burmese-Andaman subducting lithosphere.
Jour. Seism., 7 : 155-174.
Dasgupta, S., Mukhopadhyay, B. and Acharyya, A.
(2005). Aftershock propagation characteristics
during first 3 hours following the 26 December
2004 Sumatra- Andaman Earthquake. Gond.
Res. (GNL section), 8/4 : 585-588.
HRVD (2005). http://www.seismology.harvard.edu/
CMTsearch.html.
Ji Chen (2005). http://www.gps.caltech.edu/~jichen/
Earthquake/2004/aceh/aceh.html.
Mukhopadhyay, B., Acharyya, A. and Dasgupta, S.
(2005). Aftershock investigation of 26
December 2004 earthquake. http://www. gsi.
gov.in/suma_eq.html.
Stein, S. and Okal, E. A. (2005). Speed and Size of
the Sumatra earthquake. Nature, 434 : 581-
582.
Yagi, Y. (2005). http://iisee.kenken.go.jp/staff/yagi/
eq/Sumatra2004/Sumatra2004.html.
Yamanaki, Y. (2005). http://www.eri.u-tokyo.ac.jp/
sanchu/Seismo_Note/2004/EIC161e.html.
104 SUJIT DASGUPTA & OTHERS