TECTONIC DEFORMATION DUE TO 26 DECEMBER 2004EARTHQUAKE — REVISITED IN ANDAMAN

ANSHUMAN ACHARYYA * AND SUJIT DASGUPTA

Geological Survey of India27 Jawaharlal Nehru Road, Kolkata 700 016

*e-mail: anshuman_cal@yahoo.com

ABSTRACT

Tectonic deformation produced by the fault rupture of the great Sumatra-Andaman earthquake of 26December 2004 shows zones of uplift and subsidence separated by neutral line (locale of zero verticaldisplacement) in the Andaman Islands. The ground deformation pattern estimated during May 2005 has beenre-constrained on the basis of additional data collected in January 2006. A maximum uplift of 1.5 m to the westof the neutral line is recorded at the west coast of North Andaman near Temple Sound while the westernmostlandmass of Middle Andaman exhibits uplift of 1.42 m and 1.3 m at Interview Island and South Reef Islandrespectively. Areas of subsidence east of the neutral line register 0.70 m and 1.20 m in Chidiatapu and Chathamareas of South Andaman respectively. In either side of the neutral line, signatures of post-seismic adjustmentare witnessed at places. As a consequence of post-seismic incremental diurnal rise of tidal water in an otherwisedomain of emergence, residual uplift of 0.50 m is estimated in Kalighat Jetty (North Andaman) in comparisonto +0.95 m estimated in May 2005. Likewise, diurnal fall in tide level is observed in the submergence domain atChidiatapu as a likely fallout of post-seismic relaxation. Inferred maximum slip at the trench in North Andaman(~5.3 m) is comparable with that in South Andaman (~4.2 m), both being much less in comparison to 10-15mestimated slip in Sumatra generating catastrophic tsunami waves. Two simple slip dislocation cartoons illustrate160 km of locked interface of the rupture in North Andaman while in South Andaman it is 100 km from thetrench boundary. Geometry of Benioff zone in South Andaman demonstrates marginally higher dip with wideraccretionary prism than that in North Andaman.

INTRODUCTION

One of the largest shallow (» 30 km) interplatethrust earthquakes occurred on 26 December 2004 atthe interface of the subducting Indian lithosphere(Indian plate) and the overriding Burma plate. Theearthquake was so powerful that it altered the Earth’srotation and the energy radiated by seismic waveswas estimated to be 1.1 x 1018 J from P waves at 11stations over a distance range of 45°-95° (Lay et al.,2005). This megaseismic event triggeredunprecedented tsunamis that devastated coastalregions of Indonesia, Malaysia, Thailand, Sri Lanka,India and Maldives. There was immediate groundresponse of the earthquake resulting subsidencefollowed by the surge of tsunami. Within 30 minutesof the earthquake low-lying areas around Port Blairbecame inundated/submerged. With restoration ofdiurnal tidal cycle and removal of seiches, it became

clear in South Andaman (initially in and around PortBlair) that post-earthquake low-tide levels almostmatch pre-quake high-tide levels. While considerablearea in the South Andaman (and also Katchal, Trincat,Great Nicobar, etc. in Nicobar Islands) remainedsubmerged, sea level fell permanently in North- andparts of Middle Andaman. The observed quasi-permanent submergence (apparent rise in local sealevel) is a direct reflection of ground subsidence whilethe ground emergence (apparent fall in local sea level)is related to uplift. To assess the extent of emergenceand submergence, Geological Survey of India (GSI)took up detail investigations in Andaman both byremote sensing (Das et al., 2005, 2007) and by groundsurvey during May 2005 (Ray and Acharyya, 2005,2007). Subsequently several groups have workedalong Andaman – Sumatra arc (Subarya et al., 2006;Meltzner et al., 2006; Tobita et al., 2006; Malik and

Indian Minerals, Volume 60, No. 3 & 4(July-December, 2006); pp. 119-136

120 INDIAN MINERALS

Murty, 2006; Rajendran et al., 2007; Kayanne et al.,2007) and record the spatial distribution of upliftand subsidence.

One year after the mega-earthquake, additionalfield surveys were undertaken in January 2006 to re-assess the status of ground elevation in differentlocations of Andaman Islands as well as to collect datafrom locations not visited during May 2005. Thispaper attempts to provide estimates of static verticaluplift and subsidence from additional locations andalso to record the on going changes in inter- to post-seismic period.

TECTONIC SETTING

The geologic and tectonic history of the region iscomplex with the presence of various tectonic features(Fig. 1a). The Andaman-Nicobar-Simeulue-NiasIslands in the northeastern Indian Ocean are in themidst of a unique tectonic setting constituting a nearly2200-km-long trench slope break (marked bycurvilinear Sunda-Andaman trench) between theIndian plate and the Burma / SE Asia plate, fore arc /outer arc ridges(with accretionary prisms) and basin,active volcanic inner arc of Barren-Narcondam andback-arc basin with spreading ridge in the Andamansea (Fig.1b, after Dasgupta and Mukhopadhyay, 1993).The packet of accretionary prism is partially exposedin the Andaman-Nicobar-Nias Islands comprisingophiolites, ultramafics and sediments. There are anumber of discrete and dismembered thrust-boundedophiolitic/sediment slabs. The dip of these easterlydipping thrusts vary between 8º-10º in westernmostpart of Andaman Islands while it increases to 65º-70ºin the easternmost part of the Island (Pal et al., 2003).Several fault system traverses the entire setting suchas the West Andaman Fault (WAF) in the Andamanarc, the Semangko fault in Sumatra, the Sagaing faultin Myanmar and the back-arc Andaman SpreadingRidge (ASR) of Neogene age in the Andaman Sea.Development of ASR relates to oblique convergenceof Indian plate at the SE Asian continental margin.The effect of oblique plate convergence includes strike-slip faulting parallel to trench formation of sliver plate,back-arc extension, etc. (for detail coverage on thetopic see Curray, 2005; Dasgupta et al., 2003; KameshRaju et al., 2004). Near Sumatra, subduction of Indianplate below Sunda plate occurs at 40-50 mm/year while

oblique convergence near Andaman takes place atabout 14 mm/year.

The Burmese-Andaman arc constitutes animportant transitional link between the Himalayas andthe Western Pacific arc system characterised by varyingdegree of seismic activity and volcanism. Activesubduction of the Indian lithosphere below the Burmaplate along the Sunda-Andaman trench is documentedby the presence of an east-dipping Benioff zone definedby earthquakes up to focal depth of 250 km coupledwith characteristic volcanoes of Barren-Narcondamhaving continuity in the continental-margin arc inSumatra.

PAST RECORDS OF STATIC VERTICALDISPLACEMENTS

Recorded for more than 100 years, thrust-relatedtectonics in the convergent margin has a directmanifestation in ground deformation in Andaman-Nicobar Islands. Geological evidence galore showingvertical ground movement around Andaman fromhistorical period. The earliest record could be of the“very destructive and violent earthquake felt all overBengal, Arracan &c., chiefly or most severely in thenorth part of the east coast of the Bay of Bengal” on2nd April 1762 (Oldham, 1883). Both the features ofsubmergence and emergence were recorded in thedescriptions. (In Chittagong) “earth continued to sinkday by day little and little. Sixty squire miles said tohave been permanently submerged” whereas elevationof the coast of Aracan was stated to have extended“more than 100 miles in length”. Describing staticuplift from Cheduba (west coast of Myanmar) and FlatIsland (west coast off Middle Andaman, Fig.1a), therecord recounts, “The elevation was greatest aboutthe centre. At the Terribles it was 13 feet; at variouspoints of the north-west of Cheduba 22 feet,diminishing to 9 feet at Foul Island at south. Men wereliving at the time of Halsted’s visit who had fished overthe then dry land. A third elevated beach was tracedalso on the west coast of Cheduba half way down onFlat Island. Oysters were found adhering to a pinnacleof rock, about 40 ft high, on a line about 13 ft abovethe 2nd line of beach (that produced in 1766) whichwas itself marked in a similar way”. Notwithstandingthe fact that Captain Halsted visited the area in 1841,80 years after the event and chance of meeting

TECTONIC DEFORMATION DUE TO 26 DECEMBER 2004 EARTHQUAKE — REVISITED IN ANDAMAN 121

Fig 1. (a) Tectonic map of the Andaman arc region (after Curray, 2005). Main shock epicentre, 26 December2004 earthquake, shown by star in the right-hand diagram. B: Barren Volcanic Island; N: NarcondamVolcanic Island; WAF: West Andaman Fault; ASR: Andaman Spreading Ridge; SFS: Sumatra FaultSystem; SSF: Shan Scrap Fault; MPF: Mae Ping Fault; TPF: Three Pagodas Fault; RF: Ranong Fault;KMF: Khlong Marui Fault. LA: Little Andaman, K: Katchal, Na: Nancowry, CN: Car Nicobar, LN: LittleNicobar, GN: Great Nicobar, P: Preparis Island, SI: Simeulue, NI: Nias. Enlarged part of the archipelagoshows study area; asterisks with numbers are data sample sites (see also Table 1).

Fig 1. (b) Schematic section between X and Y (see Fig. 1a) across North Andaman Island illustratingmorphotectonic elements (after Dasgupta and Mukhopadhyay, 1993). Ocean depth and distance oflocations from trench are shown. The westernmost bathymetry low (also gravity low) defines probabletrench location east of which the Benioff zone starts developing. Outer arc ridge with accretionaryprism has the aerial exposure in the Andaman Island. X-Y line passes in between Barren and Narcondamvolcanoes of the inner arc.

122 INDIAN MINERALS

eyewitness was remote, the message the above noteconveys seems significant. The near- shore coastalareas used for fishing was uplifted and became dryland due to the 1762 event — a very similarphenomenon was experienced in the 26 December2004 Sumatra-Andaman mega-earthquake.

A few more historical records have been presentedby Bilham et al. (2005). Ancient expeditions notedthe shallowness of corals along the western islandscompared to the deep-water varieties farther west,providing an early hint of the tectonic origin of theAndaman Islands. The earthquake of Car Nicobar on31 December 1881 (Oldham, 1884) with a slip of 2.7m equivalent to Mw=7.9 earthquake (Ortiz and Bilham,2003) gives a reliable record of ground movement.The location of the 1881 rupture was sufficiently closeto Car Nicobar to have tilted the island, raising itswestern edge 50 cm relative to its eastern shore. Thetsunami resulted in the flooding of stilt houses to thebase of their floors. Oldham (1884) describedwidespread presence of old uplifted marine terraces, araised beach not more than 2-2.6 m above sea level,throughout the coast of South Andaman. The beach,the description narrated, “can be seen forming aterrace, from a few yards to over a mile in width inalmost every bay”. At places Oldham noted a higherterrace, 10-13 m above the lower one. The presenceof contemporaneous shells on those marine terraceswas mentioned by Gee (1926). Oldham also confirmedsimultaneous subsidence identifying the presence of adrowned forest on the NE coast of Havelock Island,40 km northeast of Port Blair.

Magnitude Mw 7.7 was assigned to the 26 June1941 Andaman earthquake (Jhingran, 1953). Reportof subsidence (particularly forest) near (and north of)Port Blair was consistent with the rupture terminatingnear the western shoreline of the Andaman Islands.Bilham et al. (2005) inferred a slip less than 3m on a<50-km-wide and 150-km-long down-dip rupture forthe 1941 earthquake.

THE 26 DECEMBER 2004 EARTHQUAKE

The earthquake commenced on 00:58:53 GMT(about 06-29 AM IST) near 3.3° N, 96.0° E at 30 kmdepth. The Harvard CMT solution indicated that it waspredominantly a thrust faulting in a shallow NE-

dipping (8º) plane with a strike of 329º producing arupture length of 1300 km from northwestern Sumatrato Andaman. The moment magnitude of Mw 9.1-9.3is widely quoted and average slip of 7-10 m withvarying width of 240-160 km was modelled (Stein andOkal, 2005; Lay et al., 2005). Study of the aftershockzone defines the rupture area that extends fromNorthern Sumatra to the Andaman Islands withdowndip width of fault rupture varying from 90 to 173km. Focal mechanisms of the aftershocks suggestpredominant thrusting in the frontal arc and strike-slipto normal faulting in the back-arc region (Dasguptaet al., 2005).

Slow slip north of 9°N advocated by Bilham et al.(2005) is consistent with the absence of strongrecognised tsunami phase at these latitudes. Accordingto their study aftershocks were not recorded in northernregions for more than 80 minutes after the mainshock,again suggestive of delayed slip in the neighbourhoodof Andaman Islands (10-14°N). Dasgupta et al. (2005)calculated time lag of reaching first aftershockfollowing the mainshock at Little Andaman as 53min49.55sec. The largest slip, about 13 to 25 m, waslocated off Sumatra Island and the second largest slip,up to 7 m, was shown near the Nicobar Islands (Fujiand Satake, 2007). Based on ground deformationestimates, Ray and Acharyya (2005) modelled amaximum vertical slip of 6.3 m at the trench margin inNorth Andaman. Dasgupta et al. (2005) described threedistinct rupture segments of 444, 469 and 386 kmlength. It was suggested that seven unilaterallypropagating shocks along the mega-thrust wereprobably not aftershocks sensu stricto, rathersequentially triggered shocks each rupturing a smallsegment of the fault. Kanamori (2006) also opined thatthe rupture in the southernmost segment triggered therupture in the north. The rupture had the longestinstrumentally recorded duration (ca. 500 s). Thissuccessive trigger and interaction of different parts ofthe fault segment could be one of the causes for such agreat earthquake.

CO-SEISMIC TO POST-SEISMIC TECTONICDEFORMATIONS

Field survey was carried out (Ray and Acharyya,2005, 2007) to estimate and map vertical groundmovement independent of seismological and GPS data

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in North, Middle and South Andaman Islands duringMay 2005 combined with observations made inJanuary 2005. Estimate of ground movement wasworked out using pre- and post-earthquake sea levelat jetties, harbours, landing sites, tidal inlets, mangroveforest, etc. as reliable reference data. Pattern ofcoseismic vertical ground movement distribution wasbrought out along with locus of a zone of no movement(designated ‘neutral line’) which roughly trends N-S.The area to the west of the neutral line displayed upliftwhere the rise increases from the neutral line towardsthe trench. From zero displacement at the neutral lineto a maximum of +1.2 m on land was estimated atPaschimsagar in North Andaman. The ground to theeast of neutral line showed tectonic subsidenceincreasing towards east. A maximum subsidence of –1.2 m was estimated at Chatham in South Andaman.Thus in the post-earthquake scenario, large areas ofmangrove swamps were found uplifted above the hightide level with many tidal creeks in the inland thrownabove zone of tidal wave play. On the other handsubsided areas including residential and agriculturalland, roads, jetties, etc. became zones of permanenttidal play. The differential tectonic uplift in NorthAndaman (0.3 m at the eastern coast of Kalipurincreasing to 1.2 m along same latitude atPaschimsagar) was utilised to obtain maximum slip of6.3 m further west at the trench margin. Similarly, widthof the rupture surface from trench margin to neutralline (downward edge of the rupture surface) wasestimated at 143 km assuming 15º dip of the rupturesurface. Model for such simultaneous uplift andsubsidence in a convergent tectonic setting was alsopresented (for details see Ray and Acharyya, 2005,2007).

From measurements of coral microatolls andGlobal Positioning System (GPS) stations, Briggset al. (2005) documented trench-parallel uplift on theouter-arc islands above the rupture and subsidencetrough farther from the trench in the Sumatra Islands.Uplift as high as 1.45 m has been recorded in thenorthwestern flank of Simeulue Island that taperedtoward the southeast to zero. From measurement ofheads of coral microatolls around Simeulue Island,Subarya et al. (2006) noted that pre-quake highest levelof survival (HLS) of corals to be systematically 0.2 to1.5 m higher than the post-quake level of HLS, with

values rising towards the northwest. Using satelliteimages (ASTER, SPOT, QUICKBIRD, etc.) theyfurther showed uplift of variable magnitude fromSimeulue to Preparis Island (Myanmar) over a distanceof 1,600 km along the trench and also described nodisplacement ‘hinge line points’ in Andaman andSimeulue Island. Tobita et al. (2006) advocated thatthe method of using SAR data was more efficient forinvestigating vertical displacements. A line 145 kmeast of the trench was shown to separate westernuplifted zone from eastern subsided zone.

Based on satellite imagery and field measurementsof emerged coral microatolls, regions of uplift andsubsidence separated by a “pivot line” weredemonstrated by Meltzner et al. (2006). Uplift wasdetected from middle of Simeulue Island (Sumatra) toPreparis Island (Myanmar) while in Nicobar Islandsand west coast of Aceh province in Sumatra,subsidence was recorded. Similar to the observationof Ray and Acharyya (op cit.) in North Andaman, asharp uplift gradient was shown across Simeulue wherewestern tip emerged to the tune of 1.05 m andsoutheastern part subsided. From the study ofmangrove forests, coral microatolls, mussels, etc.Rajendran et al. (2007) estimated uplift of 1.0 m atAvis Island (east of North Andaman), 0.5 m atMayabandar and 1.5 m at Interview Island. They alsorecognised five older terraces in Interview Island anddated corals (C

14) from each terrace demonstrating

uplift rate. Kayanne et al. (2007) showed bothcoseismic uplift and subsequent post-seismicsubsidence mainly from North Reef Island andInterview Island. They explained biological signatures(using Porites microatoll) presenting estimated upliftof 1.3 m in North Reef Island. Post-seismic temporalchanges at Mayabandar estimated as 0.3 m ofsubsidence from an initial uplift of 1.0 m within 2months after the 26 December 2004 quake wassuggested. In a similar note Subarya et al. (2006)suggested that postseismic slip took place within first1.5 months of the mega-event.

FURTHER FINDS ON TECTONICDEFORMATION

Raised Marine Terraces

During our field campaign we identified markedrise of marine terraces both from the west coast and

124 INDIAN MINERALS

east coast of North Andaman. Along the west coastoff Radhanagar (at Temple Sound, Location 2, Fig.1)raised marine terraces, both old and new, areconspicuous (Fig.2). There is a marked fall of high-tide level (HTL) consequent with newly risen marineterrace. The retreat of HTL results in newly accreted25-m-wide beach and exposure of underlying old mudflats (Fig.3). Near the southern part of debouchmentmouth of Radhanagar creek (Location 3, Fig.1a), sub-aerial exposure of approximately 3 sq km of newlyemerged beach has taken place. The area, presentlyconverted to an extensive beach/sandbar, was easilynavigable before the earthquake. The difference of 1.5m between pre- and post-earthquake HTL in the raisedterrace suggests an uplift of 1.5 m in the eastern partof Temple Sound. The disposition of coral microatolland oyster bed at this location also providesquantitative estimates of uplift (see next section).

Towards south of Casuarina bay (west ofPaschimsagar, Location 4, Fig.1a), there is notableaddition of emerged sandbar (Fig.4) on whichneodunes have stabilised. Remnant of wave rippleson the sandbar carry telltale suggestion of wave actionregime before uplift. The area was totally navigablebefore earthquake. Uplift to the tune of 1.3 m isestimated similar to what estimated in May 2005 atPaschimsagar. There is marked difference in theperipheral outline of Rowe Island, off west coast ofNorth Andaman. Extended periphery of Rowe Islandis conspicuous due to uplift of land (Fig.5).

There are different levels of uplifted terraces in thewest coast of the Interview Island (Location 13, Fig.1a). Two marine terraces having sharp break in slopeare discernible within a stretch of 50 m. The seawardlower terrace was developed consequent upon 26December 2004 earthquake. The pre-mega quake HTLused to reach the top of the old marine terrace (asinformed by Forest officials) while post-quake HTLtouches base of the older terrace/top of the recentterrace. The difference in elevation is 1.42 m, whichmay be taken as an estimate for uplift in the west coastof Interview Island.

Nature and type of ground movement along theeastern coasts of North and Middle Andaman Islandswas discussed at length by Ray and Acharyya (op cit).Successive terraces are observed in Karmatang beach

in the east coast of Middle Andaman (Location 18,Fig.1a). There are two old terraces and one recentterrace developed across the profile of the beachcomplex at Karmatang. The old terraces are stablewith casuarinas and had a height of about 40 cm fromthe level of intertidal flat. Pre-quake HTL used to reachthe old terrace. A new terrace/ berm of height 35 cm israised at a distance of 10-12 m from the old terracetowards sea (Fig.6). According to local Forest officials,there was no such terrace, berm or break-in-slope inthe beach before the 2004 earthquake and the beachhad a continuous profile in that part. The terrace in allpossibility was generated either by sudden uplift ordue to combination of erosion and uplift. As a result,in the post-earthquake scenario the HTL never overtopthe newly risen terrace and remain at its base at springtides. In the nearby Rampur beach (Location 19,Fig.1a) there is also a berm of 0.60 m height, whichexists since pre-2004 earthquake. During lowest tideat Rampur, a beach of 11.5 m gets exposed along withlarge expanse of old coral banks. We assign an upliftof 0.20 m both for Karmatang and Rampur based ondifference in HTL.

Uplift of Coral Banks and Oyster beds

Coral survives only in clear seawater with typicalsunlight conditions and may thrive in shallow waterto a maximum depth of 48 m with optimal temperaturerange of 23º-25ºC required for growth. Coral cantolerate only a narrow range of salinity between 30and 40 ppt. Shallow-water colonies are also known aspatch-reef community that survive in the depth rangeof 3-6 m.

Daytime lowest low-tide level (LLTL) is theoptimum level of sea water above which no coral cangrow upward and survive. This optimum LLTL isknown as highest level of survival (HLS) of coral.Upward growth of coral is thus limited by HLS (Tayloret al., 1987; Zachariasen et al., 2000; Natawidjaja etal., 2004 and references therein). Fluctuation in sealevel due to static ground movement has a directbearing in growth of coral. The HLS of coral istherefore a guiding tool for measuring groundmovement. Coral provides natural records of sea-levelchanges with sensitivity of 1cm. Coral microatollsrecord magnitude of vertical deformation ininterseismic and post-seismic periods via the height

TECTONIC DEFORMATION DUE TO 26 DECEMBER 2004 EARTHQUAKE — REVISITED IN ANDAMAN 125

Fig 2. Raised marine terrace in the beach at TempleSound, west coast off Radhanagar, NorthAndaman. An exposed coral bank is seen inthe horizon (circled).

Fig 3. Marine terrace emerged due to 26 December2004 earthquake as well as old terrace (dashedline) at the background, at Temple Sound. Noteexposed mud flat due to emergence of thebeach.

Fig 4. Tectonically uplifted sandbar above tidal playdomain near erstwhile confluence ofKishorinagar creek and Casuarina Bay, westcoast of North Andaman. Neodunes developedover the sandbar due to wind action.

Fig 5. Rowe Island at Casuarina Bay off west coast ofNorth Andaman; notable increase in the periph-ery of the island as a result of uplift.

difference of dead head / top of the coral microatollrepresenting pre-earthquake HLS /LLTL andprevailing HLS at the top of living coral (Fig.7; referBriggs et al., 2006 for details).

Uplift of fringing coral reefs accompanied bysiltation due to turbulent tsunami water causedwidespread death of coral colonies in Andamanarchipelago. Ramachandran et al. (2005) whileassessing tsunami-inflicted damage via satellite images

calculated about 41 to 100% damage of coralecosystem in coastal areas of Andaman-Nicobar.Surges of tsunami ripped off sediments releasing silts,pollutants, etc. and coral in shallow areas becameshrouded in debris. Following tsunami, extensive silt-laden turbid waters for 10 days resulted deposition ofsilt and mud on the reef area that led to choking anddeath of the live coral reefs.

California-based Reef Check Foundation

126 INDIAN MINERALS

Fig 6. Raised terrace at Karmatang, east coast, Mid-dle Andaman, displaying break-in-slope due to2004 earthquake.

Fig 7. A simple sketch showing LLTL /HLS control forupward limit of coral growth. Difference of deadheads (top) and living heads of coral gives ameasure of ground movement in the simplestsituation.

(www.reefcheck.org) on the basis of their survey inAceh Province in Sumatra, observed “the earthquakedamage to coral reefs was more severe than that causedby the tsunami. Damage included uplifted reefs,shattered beds of coral, and overturned coral colonies.Several islands such as Simeulue were tilted, with oneend rising as much as 2 m while the other enddescending by a similar amount. This caused tens ofhectares of living coral reef to be raised above thehigh-tide level and killed, while other reefs descendedinto deeper water, altering the ecological zonation.On land, the earthquakes and tsunami caused slopefailures and removed vegetation facilitating increasederosion, sediment transport, and discharge duringrainy periods. A longer-term and more insidious typeof reef damage could occur if the observed turbidityand sedimentation continue. In addition to inhibitingcoral settlement, sedimentation can directly injure andkill adult corals” (Foster et al., 2006). We haverecorded similar trend of destruction of coral colonyin Interview and South Reef Islands.

In the Temple Sound coast west of North Andaman,there is a clear retreat of sea from sandy coast (withpatches of rocky outcrop) exposing uplifted marineterraces (already described in the earlier section) andremarkable uplift of coral banks (Fig.8). The exposedwidth of coral bank is about 400 m with <1º beachslope. The colony comprises a variety of membersincluding lettuce coral (leaf-like plates), tube coral,plate coral and brain coral. Since the HTL had retreatedfor 300-400 m after the quake, high-tide water seldom

reaches base of the corals (vertical height of 60-70cm). There is an elevation difference of 1.0 m betweenthe dead coral head (must have been at HLS/ LTL inpre-earthquake scenario) and post-quake HTL. Theaverage post-quake LTL is about 0.5 m below the post-quake HTL. As such a conservative estimate wouldput a total elevation difference of 1.5 m (1 + 0.5 m)between the pre- and post-quake LTL suggesting netuplift of 1.5 m.

Habitat tracking of tidewater-sensitive marinebiotic species has also been used for understandingsea-level change in this sector. Shells of oyster needstable substrate for growth at intertidal to subtidaldepths and may be used as sea-level indicators. Oysterbeds have usefully been used to demarcate oldstrandlines, even in Indian mainland coast (Juyal etal., 1995; Purnachandra Rao et al., 2003) marking highseastrands. Ancient oyster beds preserved at elevationsabove modern equivalents are commonly taken asaccurate indicators of the mid-Holocene sea-level highstrand (e.g. Beaman et al.,1994). In different parts ofAndaman Islands oyster barnacles are common bothover rock outcrops and on coral substrate. Oyster bedsare present on many abandoned fishing vessels,mangrove stems and coastal structures. In the coast ofTemple Sound (Location 2, Fig.1a) we have noted twovarieties of oyster beds — on the rock substrate andon pre-existing coral reef. This would give anadditional lead on the status of land uplift. Minimumsea-level lifeline requirement of oysters is at higher

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elevation than coral. We found a distinctive sectionover a rock outcrop of red chert in the coast. This chertacted as substrate for overlying coral colony (thickness35 cm), which is again overlain by oyster bed (70 cmthickness) (Fig. 9). The pre-earthquake HTL isconsidered at the upper level of oyster bed whereasthe post-earthquake HTL evidently touches the baseof chert outcrop. Total height between pre- and post-earthquake HTL is ~1.5 m. Thus taking recourse tothree-fold data from marine terrace uplift, uplift of coralmicroatoll and oyster bed, a net uplift to the tune of1.5 m may be assigned to the west coast of NorthAndaman, at Temple Sound.

We also examined the corals exposed on InterviewIsland, South Reef Island, Anderson Island and BennettIsland. The estimates are recorded on a spring tide (newmoon, 29 January 2006) day. In the eastern coast ofInterview Island there is a landing jetty of ForestDepartment (Location 11, Fig.1a). Difference of pre-and post-quake HTL (confirmed by Mr. Sawbonny,In-charge, Forest Camp) assign an uplift of 0.95 m atthe location. In the Interview Passage linear emergedbanks of corals in the fringing reefs as well as raisedmangrove colony above tidal lifeline is conspicuousin the east coast of Interview Island. HTL used toreach the sandy beach and mangrove forest during pre-earthquake tides whereas HTL in post-earthquakescenario is unable to overtop the emerged fringing coralreef (Fig. 10). The newly emerged coral bank increasedthe beach width by at least 30 m in the eastern coast.

Southernmost tip of Interview Island (known as NancyPoint) is also rimmed by raised fringing coral reef. Inthe west of Interview Island there is a spectacular riseof ‘underwater rain forest’ of coral (Fig.11). Shoalshave been stabilised above and near the sea surface.Extensive area (>300 m of intertidal width) of coralcolony is aerially exposed and remained so abovespring tide resulting in a colossal loss of coral colony.Heads of the dead coral microatolls (Fig.12)representing HLS (thus LTL) are at 20-35 cm abovethe base of the atolls. In post-2004 scenario even theHTL does not touch the base of the atoll. We recordedthe elevation difference between the coral’s pre-earthquake HLS and average sea-water level at the siteduring time of measurement that gives an estimate ofminimum uplift of ground. The sea-water retreated for> 300m. This gives a difference of at least 1m betweenthe present-day sea front and top of dead coral heads.In addition to that, Fig.13 shows dead head ofsubmerged corals 2 hours after high-tide time. Sensedthrough the oar of the country boat and also visuallyno living part of submerged corals is visible for at least1m below sea level. Top of the coral heads aremarkedly covered with silts and sediments; at someplaces recast bivalves remained intact and embeddedover the coral. Death of coral colony with siltationcovering the top surface is presumably due to slurry

Fig 8. Uplift of coral bank above tidal level of survival,west coast of North Andaman (at TempleSound).

Fig 9. Rock outcrop of chert, west of North Andamanat Temple Sound. Coral had grown over the topof chert substrate. Oyster beds in turn developedabove the coral bed. Top of the oyster bed markspre-earthquake HTL. Post-quake spring HTLtouches base of the chert bed.

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brought about by the tsunami. Coral colony even at orbelow the present LTL (thus likely HLS) are largelyfound to be dead. Since there is no visible living headof coral in the southwestern flank of Interview Island,it is felt unwise to estimate the vertical grounddisplacement via HLS of coral microatolls. Hence anestimate of 1.42 m of uplift based on the position of

Fig 10. Exposed fringing reef along the east of coastof Interview Island. Note retreat of sea from beachshowing water percolation caves in the limestone.

Fig 11. Vast areas of uplifted coral reef, southwesterncoast of Interview Island. Sea front retreatedfor >300m after 26 December 2004.

Fig.12 Exposed and uplifted coral microatolls over alarge stretch of Interview Island.

Fig 13. Uplifted shoals of fringing coral reef, InterviewIsland. Note dead heads of submerged coralreef. The oar (red, measures 1.5 m) of the boatwas dipped into the sea to estimate depth oflive part of coral.

spring HTL at the uplifted marine terrace seems moreappropriate at the southwestern flank (Location 13,Fig.1a) of the Interview Island.

In the South Reef Island located south of InterviewIsland (Location 14, Fig.1a), newly exposed beach with

fringing coral reef is noted (Fig.14). A raised marineterrace comprising beach sand deposit of ~0.60 m isuplifted possibly due to the quake. Here also fringingcoral reefs remain exposed even in spring tides. A totaluplift (raised terrace plus permanently upthrown coral

bank) of 1.30 m is estimated at South Reef Island.Peripheral coastal boundaries in both Anderson Islandand Bennette Island also show raised coral reefs. Theeastern coast of Bennette Island in the InterviewPassage (Location 12, Fig.1a) demonstrates rise offringing reefs with an estimated uplift of 0.90 m(Fig.15). Interestingly, on rising above sea level, shoals

TECTONIC DEFORMATION DUE TO 26 DECEMBER 2004 EARTHQUAKE — REVISITED IN ANDAMAN 129

are transformed into exclusive coral islands withoutany connection to land part of any earlier island nearby.There are also features showing enlargement of originallysmaller islands now expanded with stabilised coral banks.At the Tugapur creek landing site (Location 16, Fig.1a) thedifference of LTL is estimated to be 0.80 m.

In addition to rise of coral banks due to the mega-earthquake of 2004, old coral colonies are found indifferent locations both in the east and west of theAndaman Islands. Uplifted coral banks of past eventsare observed in the west coast of Radhanagar andKishorinagar (Fig.16) as well as in the Interview Island.In the east coast also near Aerial Bay, Mayabandar(Pokadera) and Rampur, such pre-existing ancientcoral beds are common. In the Rampur coast, ancientcorals are characterised by peneplained white top,without remains of corallite structure where tubes arecompletely filled with cement material (Fig.17). Theseancient coral banks might have been uplifted eitherduring long interseismic periods or due to majorearthquake events prior to 2004.

Estimates of emergence and subsidence from other areas

During revisit to locations studied in May 2005(Ray and Acharyya, 2005, 2007) some changes arenoted in January 2006. At a few locations previousestimates have been duly revised incorporating post-seismic relaxations. The areas where revisions aremade include Mayabandar, Kalighat jetty, Kalipurbeach, Aerial Bay, Austin Bridge, Chidiatapu, etc.Most of the other previously visited areas do not showany major changes (except minor ones of ± 5 cm; see

Table 1). Around Radhanagar and Kishorinagar areas(North Andaman) many smaller creeks dried up dueto co-seismic emergence and remained so even afterone year. There is remarkable uplift of mangrovecolony above tidal lifeline, culminating in dried upmangrove forest, erosion of creek banks and completeexposure of mangrove roots. Growth of land plants inerstwhile mangrove forest is another indicator for lossof salinity in the mangrove domain.

The uplift at Austin Bridge over Austin strait(Location 15, Fig. 1a) was examined (Fig.18). Theexposed part of the iron piers of the bridge used to getinundated in pre-2004 high tides. Dead barnaclesaffixed to the pier provide revealing evidence for that.The iron pillars had become completely rusted due toaerial exposure after the earthquake as a consequenceof lowering of HTL for 60 cm. The uplift at thislocation is thus estimated to be 0.60 m. In the eastcoast of North Andaman at the Aerial Bay jetty,Andaman Harbour Works maintain a tide pole gauge.This provides an estimate of difference in berth levelof ships at the jetty (uplift with a difference from+5.000 to +4.137 mark in the pole) indicating 0.863m of uplift. However, this revised data is comparativelyrather high than the previous estimate during May2005. Uplift at Mayabandar (Middle Andaman,Location 17, Fig.1a), has been revised as 0.50 m incomparison to the previous estimate of 0.15 m. It isalso revised in North Andaman at Kalighat jetty(Location 8, Fig.1a) as 0.50 m (instead of previouslyestimated 0.95 m), and at Kalipur beach (Location 6,Fig.1) as 0.40 m against 0.20-0.40 m in May 2005.

Fig 14. Uplifted marine terrace and raised fringing reefin South Reef Island.

Fig 15. Bund of emerged coral reef at the fringe ofBennette Island, in Interview passage.

130 INDIAN MINERALS

Temporal changes in sea level in South AndamanIsland (near Chidiatapu) as well as from MiddleAndaman (Karmatang) and North Andaman (Kalighat)may offer some insight into the changing tectonicdynamics of the region in the post-seismic to earlyinterseismic period. In Kalighat jetty (emergencedomain in North Andaman), HTL has risen by 0.4-0.5m since November 2005. Thus in comparison toprevious record of 0.95 m of uplift, residual uplift hasbecome 0.50 m. Similar trend in change in tidal levelis also obtained from the Forest Department officials

at Karmatang (in otherwise emergence domain due to2004 earthquake). The Forest Officials report that theHTL that receded on 26 December 2004 was gettingraised from November 2005. A revised uplift of 0.05m is estimated in the area. On the contrary at Chidiatapuin the southernmost tip of South Andaman(submergence domain due to the quake, Location 30,Fig.1a), the lowest tide (LTL) is reportedly recedingin the post-seismic period. After the 2004 earthquake,a new tide gauge is installed at Chidiatapu on23.2.2005. The ‘0’ level of the gauge was fixed at thelowest low tide that time. The highest HTL of 1.45mis recorded at the gauge on 23.7.2005 (New Moon).Since July 2005 lowest low water started reachinglower than ‘0’ level of tide gauge and the HTL also

Fig 16. Part of old coral bank west coast of NorthAndaman presumably uplifted in tectonicmovement much before 2004 event. Inset (lefthand top corner) shows coral turned white dueto aerial exposure with tubes completely filledby secondary material.

Fig 17. Old, peneplained, structureless coral bank inRampur beach, east coast of North Andaman.Uplift of the bank took place before 2004 event.

Fig 18. Pier of the Austin bridge on Austin strait,between North and Middle Andaman Islands.Left and right hands point at pre-earthquakeLTL and HTL respectively. Post-earthquakeHTL touches level of pre-earthquake LTL.

started receding. Residual submergence at Chidiatapuis estimated to be 0.70m. In the subsidence domain inRutland Island (Location 31, Fig.1a) there is also achange. Locals of Rutland Island report submergenceof 0.80 m in the area around Kalapahar and Aram Pointin comparison to 1m immediately after the quake. Thisbears testimony to the ongoing crustal adjustmentseven after one year in the Andaman archipelago. Thisrecord is in contrast to what has been suggested bySubarya et al. (2006) and Kayanne et al. (2007)describing a quick crustal readjustment within first twomonths (? by February 2005) of the mega-event.

TECTONIC DEFORMATION DUE TO 26 DECEMBER 2004 EARTHQUAKE — REVISITED IN ANDAMAN 131

DISCUSSIONS

Tectonic deformation in terms of verticalcomponent of slip has been re-assessed from differentlocations of Andaman Islands during repeat survey inJanuary 2006. A contour map showing distribution ofstatic ground displacement based on revised data (seeTable 1) is presented (Fig.19; modified after Ray andAcharyya, op cit). It may be mentioned that the exerciseadopted to estimate the uplift and subsidence atdifferent waterfront was not by precision geodeticsurvey. Consecutive field records, berth levels of boats/ships at jetties or landing sites, mangrove/ oyster/barnacles-lifeline, top of dead coral heads in coralmicroatolls, net rise in raised beaches, etc. provide

telltale estimates for uplift or subsidence. However, inthe absence of pre-/post-earthquake instrumentalmeasurements, there are uncertainties in the estimatesfrom ±5 cm to ±20 cm. Possible uncertainties for eachlocation are shown in Table 1. The differenceswhatsoever between the 2005 and 2006 estimates arealso clear from the table. Since scope of taking datawas very insufficient and confined only in theaccessible part of the islands, the control of the contourlines are based on limited data. The extrapolatedcontours illustrate the broad regimes of uplift andsubsidence (Fig. 19). The zero contour line representszone of no displacement. While land subsidence tookplace east of zero contour (contours with ‘-’ sign),ground was uplifted west of it (contours with ‘+’ sign).

Fig 19. Static displacement contours in Andaman Islands (modified after Ray and Acharyya, 2005, 2007).Contours are extrapolated on limited observation data in the Islands. ‘0’ contour denotes locale of zerouplift whereas contours with plus signs in the west indicate uplift and contours carrying minus signs inthe east indicate subsidence. Numbers with asterisks correspond to locations described in Table 1. AA/

and BB/ are lines originating from trench margin to east of the archipelago along which depth sectionand static slip dislocation is shown in Fig. 20.

132 INDIAN MINERALS

TABLE 1UPLIFT (+) AND SUBSIDENCE (-) IN THE ANDAMAN ISLANDS MEASURED IN 2005 AND 2006

Location Location Longitud Latitude Uplift/ Uplift/ ChangeIndex Subsidence Subsidence

(m) in (m) in2005 2006*

1 Landfall Island* 93.01 13.64 - +0.6 -

2 East of Temple Sound 92.88 13.43 - +1.5±0.2 -

3 Radhanagar 92.92 13.38 +1±0.2 +1± -

4 Paschimsagar near Casuariana bay 92.85 13.26 +1.2±0.2 +1.1±0.2 0.1±0.4

5 Aerial bay 93.02 13.27 +0.20±.05 +0.86±.1 0.66±.15

6 Kalipur 93.04 13.2 +0.30±0.05 +0.40±0.05 0.1±0.1

7 Kishorinagar 92.88 13.18 +1.2±0.5 +1±.05 0.1±.10

8 Kalighat jetty 92.96 13.12 +0.90±0.10 +0.5±0.1 0.4±0.20

9 Ramnagar 93.02 13.06 +0.40±0.05 +0.4±0.05 -

10 North Reef Island* 92.71 13.09 - +1.3 -

11 Forest Check Post, Interview Island 92.71 12.89 - +0.95± -

12 Bennette Island 92.71 12.83 - +0.9± -

13 Interview Island 92.65 12.83 - +1.42± -

14 South Reef Island 92.66 12.75 - 1.3± -

15 Austin bridge 92.81 12.88 +0.4±0.05 +0.6±0.05 0.2±0.10

16 Tugapur creek 92.79 12.81 - +0.8± -

17 Mayabandar 92.89 12.88 +0.10±0.05 +0.5±0.05 0.4±0.10

18 Karmatang 92.93 12.83 +0.15±0.05 +0.05±0.05 0.10±0.10

19 Rampur 92.94 12.79 +0.15±0.05 +0.05±0.05 0.10±0.10

20 Nimbudera, South of Cuthbert Bay 92.95 12.64 0 0± -

21 Panchawati 92.96 12.56 0 0± -

22 Nimbutala (Rangat) jetty 92.95 12.49 -0.2±0.10 -0.1±0.10 0.1±0.20

23 Bakultala 92.83 12.49 -0.05±0.05 -0.05±0.05 -

24 Uttara jetty 92.78 12.33 -0.2±0.10 -0.2±0.10 -

25 Nilambur jetty 92.75 12.17 -0.5±0.10 -0.5±0.10 -

26 Radhanagar (Havelock Island) 92.95 11.97 -0.2± - -

27 Ograbraj 92.66 11.66 -0.8±0.20 -0.7±0.10 0.1±0.30

28 Chatham 92.71 11.67 -1.2±0.10 -1.1±.05 0.1±0.15

29 Corbyn’s cove 92.73 11.62 -1.0±0.05 -1.0±0.05 -

30 Chidiyatapu 92.7 11.52 -0.75±0.05 -0.70±0.02 0.05±0.07

31 Wandoor -0.30±0.05 -0.10±0.05 0.20±0.10

32 Aram Point, Rutland Island 92.59 11.49 - -0.8±0.10 -

33 North Sentinel Island* 92.19 11.57 - +1.5 -

* Except for Locations 1,10,33.Locations 5,6,8,15,17,18 are revisited in January 2006 and revised from that of Ray and Acharyya (2005).Locations 2,11,12,13,14,16,32 not visited during May 2005 field campaign. Data for location 1 (Rajendran etal., 2006), 10 (Kayanne et al., 2007) and 33 (Bilham et al., 2005) taken from published material.

TECTONIC DEFORMATION DUE TO 26 DECEMBER 2004 EARTHQUAKE — REVISITED IN ANDAMAN 133

Estimates for Landfall, North Reef and NorthSentinel Islands are supplemented from publishedmaterial (reference at Table 1). Area west of neutralline records highest uplift of 1.5 m at the west coast ofNorth Andaman near Temple Sound, whilewesternmost landmass of Middle Andaman enduresuplift of 1.42 m and 1.30 m at Interview Island andSouth Reef Island respectively. The contour line +1.5m is construed in this study based on field observations.Contours denoting +0.5 m and +1.0 m are revised basedmainly on data at Austin Bridge, Aerial Bay, Kalighatand Mayabandar. Areas of subsidence occupy east ofthe neutral line, registering 0.70 –1.20 m in Chidiatapu-Chatham area of South Andaman respectively. Theneutral line represents downward edge of the lockedinterface along the zone of subduction. Post-seismicrelaxation and interseismic readjustment are witnessedfrom a few locations in Andaman. The sense of post-seismic changes demonstrated as gradual rise of HTLin the uplifted domains is recorded from places likeKalighat (North Andaman) or Karmatang (MiddleAndaman). On the contrary, at Chidiatapu (SouthAndaman) there was retreat of HTL from the erstwhilesubsidence domain. From the eyewitness accounts itappears that the adjustment continued during July 2005and December 2005. Thus the opinion of completecrustal relaxation within first two months of the mega-earthquake (as proposed by Subarya et al., 2006 andKayanne et al., 2007) does not match with the field data.

Simultaneous uplift and subsidence due tocoseismic fault rupture are known from subduction-zone earthquakes. Uplift takes place above the rupturezone up to the downdip edge of fault rupture (whendowndip edge projected in the surface coincides withneutral line) while subsidence occurs between thedowndip edge of fault rupture and the volcanic arc(Carver and McCalpin, 1996). The model forcoexisting uplift and subsidence in Andaman Islandswas already presented by Ray and Acharyya (op cit.).In the present study we have refined the displacementdata with two important field features— raised marineterrace and raised coral banks/microatolls.

In the tectonically emerged coastal terrains,identification of pre- and post-earthquake HLS of coralreef is a guiding tool for estimation of uplift (see Fig.7 for a simple explanation). It appears difficult toascertain HLS of post-2004 earthquake everywhere

of the fringing reef colony unless live coral having thesame species of the dead and uplifted colony isunequivocally spotted in the retreated seafront. Wehave noted that coral colony in the fringing reefsparticularly in the Interview Island and South ReefIsland is dead even at or below present-day LTL orHLS. This is largely because major part of coral colonyeven below the depth of required post-quake HLS dieddue to tsunami. Death of coral colony is caused bysiltation from the turbid sea. Tsunami surge, silt slurryand uplift together played havoc to the coral colony. Itwas difficult to get live coral even in otherwiseavailable depth of HLS. Estimates of uplift in InterviewIsland is thus mainly deduced by height of HTL at oldand new marine terraces and further seconded fromthe position of coral microatolls. Raised marine terracesin Andaman are a reliable resource of estimation ofuplift. We tried to document the raised marine terracesdue to 2004 event and also recorded ancient terraces.This suggests repeated ground movement in the areaincluding aseismic slip and interseismic movement.Marine terraces documented by Rajendran et al. (2007)in the Interview Island are indicative of total gamut oftectonic movement in the plate interface but not theepisodic event of 26 December 2004 earthquake.

While the static displacement pattern has beendisplayed through the contours (Fig.19) twolithospheric sections across North and South Andamanwith superposition of vertical displacement areattempted as cartoon-slip dislocation model (Fig.20)along line AA/ and BB/ (Fig.19). Two depth sectionsare prepared for AA/ and BB/ based on teleseismicdatabase (mb ³ 4.0) from January 1964 to December2005. The database comprises ISC data up to 2002and NEIC, USGS data from 2003 to 2005. The zoneof consideration of each depth section coversearthquake events in a block of 1° latitude (about 100km). The AA/ depth section contains 311 events havingdepth range 0-219 km. There are 181 events before 26December 2004 (few representative events shown assquares in Fig. 20a) while the rest 130 events are frompost-26 December 2004 main shock (but includesaftershocks) up to December 2005 (a fewrepresentative events shown as triangles in Fig.20a).Similarly in BB/ depth section (Fig. 20b) a total of 343events are located within a depth range of 0-194 km.This includes 263 events prior to 26 December 2004earthquake and 80 events after the great earthquake.

134 INDIAN MINERALS

The events corresponding to lower/subducting plate(Indian plate) and upper/overriding plate (Burma plate)along the Benioff zone are separated manually toconstruct the geometry of the Benioff zone. In AA/

depth section most of the mega-earthquake relatedevents (post-2004 events) are confined in thesubducting lower plate while also in BB/ majority ofpost-2004 events are localised in the lower plate. Thedatabase has clearly redefined the outline of the Benioffzone. The dip of the ‘shallow thrust zone’ (Savage,1983) in the Benioff is higher in BB/ than in AA/.

In the slip dislocation model along AA/ (upper partof Fig.20a) we have ground displacement data (uplift)at locations (4) and (5) with zero displacement (traceof neutral line) at A/. The downward edge of the lockedinterface ends at A/ (160 km from the trench). Similarlyfor BB/, uplift for location 33 and subsidence forlocations 31 and 29 gives a good approximation of thedisplacement (uplift in the west and subsidence in theeast) scenario. For BB/ the zero displacement is placed100 km from the trench.

The dislocation curve (upper part of Fig. 20a & b)if extended to the position of trench margin (vertical

Fig 20. Simple cartoons of static-slip dislocation along (a) AA/ in North Andaman and (b) BB/ in South Andamanas shown in Fig.19. Lower part of figures illustrate depth section and geometry of Benioff zone based onrelocated aftershocks in a block of 1° latitude. Squares indicate events from 1964 till pre-26 December2004 earthquake, while triangles indicate post-earthquake events (including aftershocks) up to the endof 2005. Dots in the dislocation curve denote data points, solid lines refer to well-constrained segment,dashed line stands for segments not supported by ground data. Position (0,0) of the graph representstrench margin; numbers 4,5,31,33,29 are locations described in Table 1 (and also shown in Fig. 1(a) and19). Location of ‘0’ is extrapolated from the “0” contour in Fig.19.

(a) (b)

axis) gives an estimate of likely maximum uplift ofthe rupture along that line. Inferred maximum slip atthe trench margin in North Andaman (~5.3 m) is littlehigher along AA/ than that in South Andaman alongline BB/ (~4.2 m). However, both the values areobviously lower than previous estimates in Andaman.It indicates an overall low range of slip in Andaman incomparison to slip inferred near Sumatra (10-15m) byseveral workers. Comparatively shorter tsunami run-up height in Andaman is further seconded by low andslow slip in the Andaman Islands.

ACKNOWLEDGEMENTDrs. S. Sengupta and B. Chattopadhyay

encouraged us in the field. A. Bhattacharya and B.Mukhopadhyay, Geodata & Database Division, CHQare thanked for their help. Prof. Roger Bilham offereduseful comments on an earlier draft of the manuscript.Constructive comments of an erudite reviewer andcomments from the Editor’s desk are gratefullyacknowledged. Authors are indebted to the ForestDepartment, Andaman-Nicobar Administration for thesupport extended during visit to Interview Island andSouth Reef Island. DST (A&N) is also thanked forhelp whenever required.

TECTONIC DEFORMATION DUE TO 26 DECEMBER 2004 EARTHQUAKE — REVISITED IN ANDAMAN 135

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Manuscript received on 21.08.2007