26 december 2004 earthquake and tsunami

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SUMATRA - ANDAMAN

EARTHQUAKE AND TSUNAMI

26 DECEMBER 2004

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GEOLOGICAL SURVEY OF INDIA

SPECIAL PUBLICATION NO. 89

‚ê¬ÊŒ∑§— ‚ÈÁ¡Ã ŒÊ‚ªÈ#Editor : Sujit Dasgupta

÷Ê⁄UÃ ‚⁄U∑§Ê⁄U ∑§ •ÊŒ‡Ê ‚ ¬˝∑§ÊÁ‡ÊÃPUBLISHED BY ORDER OF THE GOVERNMENT OF INDIA

2007

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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.


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


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


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

10

K. N

. MA

TH

UR

& O

TH

ER

S

CM

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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

GS

I SP

L. P

UB

. 89

: SU

MA

TR

A-A

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AM

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& T

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MI, 2

6 D

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.’04

11

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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

12

K. N

. MA

TH

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& O

TH

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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

GS

I SP

L. P

UB

. 89

: SU

MA

TR

A-A

ND

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& T

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.’04

13

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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


CMYK

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


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.

CMYK

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.


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


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.


CMYK

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.)...



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


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.)...


TABLE 6.3


FROM FAULT RUPTURE SEGMENT III


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


Table 6.4 (Contd.)...

TABLE 6.4


FROM THE ANDAMAN- SUMATRA UPPER PLATE


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



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.)...


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


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.


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.


26 december 2004 earthquake and tsunami

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