Tectonophysics 455 (2008) 53–60

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Tectonophysics

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Age estimates of coastal terraces in the Andaman and Nicobar Islands and theirtectonic implications

Kusala Rajendran a,⁎,1, C.P. Rajendran b, Anil Earnest b,2, G.V. Ravi Prasad c, K. Dutta c, D.K. Ray c, R. Anu b

a Centre for Earth Sciences, Indian Institute of Science, Bangalore 560 012, Indiab Centre for Earth Science Studies, Trivandrum, 695031, Indiac Institute of Physics, Sachivalaya Marg, Bhubaneswar, 751005, India

⁎ Corresponding author. Tel.: +91 80 2293 2633.E-mail address: kusala@civil.iisc.ernet.in (K. Rajendr

1 Formerly at Centre for Earth Science Studies, Trivan2 Currently at Centre for Mathematical Modelling

Bangalore, India 560037.

0040-1951/$ – see front matter © 2008 Elsevier B.V. Aldoi:10.1016/j.tecto.2008.05.004

A B S T R A C T

A R T I C L E I N F O

Article history:

The great Indian Ocean ear Received 4 July 2007Received in revised form 30 April 2008Accepted 6 May 2008Available online 13 May 2008

Keywords:Seismicity and tectonicsSubduction zonesTsunamiCoastal terraceEarthquakeAge dating

thquake of December 26, 2004 caused significant vertical changes in its rupturezone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arcridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands,in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbondates from coastal terraces of the island belt and used them to compute uplift rates, which vary from1.33 mm yr−1 in the Little Andaman to 2.80 mm yr−1 in South Andaman and 2.45 mm yr−1 in the NorthAndaman. Our radiocarbon dates converge on ∼600 yr and ∼1000 yr old coastal uplifts, which we attributeto the level changes due to two major previous subduction earthquakes in the region.

© 2008 Elsevier B.V. All rights reserved.

1. Introduction

Defining the intervening processes during the successive stages ofearthquake cycles is a useful means of characterizing interseismicbehavior of active fault zones, thereby enabling long-term forecast.Where faults and geomorphic features are exposed on land, thesestudies are relatively easy, but due to the general lack of exposure offault traces, they are difficult for subduction zones (see Sieh,1981, for areview). Great earthquakes generate tsunamis that travel acrossoceans, affecting global communities, and historical records oftendocument inundations by the sea caused by such events. In countriessuch as Greece, Italy and Japan, the historical tsunami records stretchback to a few thousand years and these contain information useful toreconstruct earthquake/tsunami histories (Atwater et al., 2005;Cisternas et al., 2005, for example). In regions like the United Stateswhere interseismic intervals are much longer than the documentedhistory, the reconstruction of tsunami history has to be basedprimarily on geological records (e.g. Atwater, 1987, 1992). Settings inthe tropics, characterized by higher level of human activities, tropicalstorm surges and poor preservation potential add to the challenges of

an).drum, India, 695031.

and Computer Simulation,

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finding discernible fossil records. This paper presents results of ourpreliminary studies on coral terraces along the Andaman and NicobarIslands, (here after abbreviated as A&N Islands), within the rupturezone of the 2004 great Sumatra earthquake. In combination withother geologic evidence, they serve as proxies of past tectonics.

Coseismically uplifted or subsided shorelines are known to serve aspaleogeodetic markers of Holocene coastal tectonism (Taylor et al.,1980; Vita-Finzi, 1981; Lajoie, 1986). It should, however, be noted thatisolating the influence of eustatic sea level processes on thedevelopment of coastal terraces may not be easily done in manyareas. But, we place a higher confidence on our results because of thepredominant role of tectonism in the A&N Islands in shaping theconstituent coastal landforms. Similar attempts on the records ofcoastal uplift in other emergent shorelines to determine the long-termspatial and temporal characteristics of large subduction zone earth-quakes provide further rationale for our studies (e.g. Plafker andRubin, 1978; Matsuda et al., 1978; Berryman et al., 1989; Ota andYamaguchi, 2004; Natawidjaja et al., 2004). Further, where precisemeasurements are possible, coastal subsidence and uplift of thetectonically active zones are also used to infer fault slip parametersand model coseismic rupture (e.g. ten Brink et al., 2006; Shennan andHamilton, 2006) and tsunami generation (e.g. Cummins, 2007). TheA&N group of islands accommodated about 800-km-length of therupture (Lay et al., 2005) and we believe that as the outer arc of thesubduction zone, this region is likely to preserve evidence of pasttectonic movements. The coseismic elevation changes caused by the

54 K. Rajendran et al. / Tectonophysics 455 (2008) 53–60

2004 earthquake provide cues in identifying older features formedthrough similar events. In this paper we present data on elevatedterraces on the A&N group of islands and use them to compute thelong-term uplift rates.

2. Morphology, tectonic features and major earthquakes

The A&N group of islands form part of the outer arc ridge of thenorthern segment of the Andaman–Sunda subduction zone (Fig. 1A).Tectonic evolution and salient morphological features are discussedby Curray (2005). Curray describes the basic structure of theAndaman–Nicobar ridge, the accretionary prism of the subductionzone, as an imbricate stack of eastward dipping fault slices and folds.The younger series of rocks, termed as the Archipelago Series, capthese imbricate stacks and they consist typically of limestone, formedfrom coral and shell sands.

A section along 11°N parallel (Fig. 1A, B) shows the morphologicalfeatures typical of subduction zones—the outer arc (Andaman andNicobar Ridge), forearc basin (West Basin, Invisible Bank), volcanic arc(Barren, Narcondum) and the backarc basin (Central Andaman basin,East basin). Curray (2005) mentions two normal faults (Diligent Fault,DF and Eastern Margin Fault, EMF), with the west side up and runningparallel to the outer arc. The southward continuity of EMF to theNicobar Islands is ambiguous. Further south, off Northern Sumatra,

Fig. 1. A. Map of the Andaman and Nicobar region showing major tectonic units. B.Schematic cross section along 11°N parallel (shown by arrows). OA: outer arc; FA:forearc basin; VA: volcanic arc, BA: back arc; WB: West Basin; IB: Invisible Bank; B:Barren; N: Narcondum; EMF is Eastern Margin Fault.

the outer ridge lies along the trace of the West Andaman Fault. Ourinvestigations in this paper are restricted to some of the accessibleislands along the western margins of the outer arc ridge, where step-like coral terraces are observable along the coast.

The A&N region has experienced large earthquakes in the past; ofthose in the historical record, only the 1881 (Mw 7.9) and 1941(Mw 7.7) events are important in terms of their sizes, but neither ofthese produced significant tsunamis (Ortiz and Bilham, 2003;Rajendran et al., 2007). These earthquakes are inferred to haveinvolved smaller ruptures (Fig. 2A). In a recent review based onhistorical records, Dominey-Howes et al. (2006) have collated detailsof Indian Ocean tsunamis since B.C. 326; their study does not provideany information to suggest the occurrence of any past eventscomparable to the 2004 tsunami. After the 2004 event, there havebeen more efforts to search for evidence of past events. In a recentstudy, Cummins (2007) modeled the rupture parameters and thetsunami generation, based on observations of landform changes.Earthquakes of magnitude ∼8 or smaller are also likely to causemorphological changes, but theywill be spatially restricted. The abilityto distinguish morphologic changes caused by such earthquakes fromthose affecting larger portions of the subduction zone depends largelyon the multiplicity and spatial distribution of evidence.

Historyof great earthquakes in theAndaman–Sunda subduction zoneis poorly understood. Rajendran et al. (2006 and 2007) used historicaland archeological data from the east coast of India, together withgeologic evidence including paleoliquefaction features and subsidedvegetation in the marshes on the east coast of Port Blair, to suggest thatthe pre-2004 event may have occurred around 1000 yr B.P. Ongoingpaleotsunami investigations in the A&N Islands have indicated thepossibilityof anyoungereventaround600yrB.P. (Rajendranet al., 2008).Recent work by Jankaew et al. (2008) reports presence of sandsheetsdating to about 14th to 15th Century A.D., based on paleotsunamievidence from the Phra Thong Island, on the coastal plains of Thailand.Obtaining chronological constraints of great earthquakes generated bytheAndaman–Sunda subduction zone is an effort that is continuing; onlywith more data from the Indian Ocean littoral countries, the history ofgreat events along this plate boundary can be better understood.

The 2004 earthquake caused vertical displacements all along itsrupture zone, with significant along-strike variations (Meltzner et al.,2006; Rajendran et al., 2007; Kayanne et al., 2007). During our post-earthquake surveys in the A&N region, we mapped many of thesefeatures and defined a pivot line separating the zones of uplift andsubsidence (Fig. 2B and C). We also observed many geomorphologicalfeatures, such as stepped coral terraces, that suggest episodes of pastmovements. Similarly, coseismic subsidence of coastal marshes in2004 is potentially analogous to land-level changes that occurred inthe past. We observed buried layers of peat in core sections at sites ofsomemangrove swamps that subsided in the 2004 earthquake, whichwe consider as evidence of past events (Rajendran et al., 2007). Thesefeatures serve as proxies of tectonism and, if interpreted carefully, theycan be used to reconstruct the regional tectonic history. The exposuresof land within the Indian political boundary are limited; the study wasalso constrained by the logistical problems to work in the isolated anduninhabited islands and entry restrictions to tribal preserves.

3. Nature of coseismic uplift and subsidence

Discontinuity in coastal landforms exhibiting step-like morphol-ogy is generally interpreted as resulting from uplift accompanyinglarge earthquakes (e.g., Lajoie, 1986; Berryman et al., 1989; Vita-Finziand Hidayat, 1991; Ota and Yamaguchi, 2004). Elastic dislocationmodels for subduction megathrust earthquake predict coseismicuplift in the up-dip portions of the rupture and subsidence in thedown-dip part (e.g., Plafker and Savage, 1970). Post-seismic relaxationoccurs soon after the earthquake, followed by long-term visco-elasticrelaxation (Hu et al., 2004) and the nature of recovery may vary from

Fig. 2. A. Rupture extent of the 2004 earthquake (Lay et al., 2005) and other known large earthquakes in the A& N region. Rupture areas of the 1881 and 1941 earthquakes are fromOrtiz and Bilham (2003); rupture area for 1679 is inferred from felt reports, in comparison with the 1941 event. The rupture zone of the 1762 earthquake, shown by an arrow(Cummins, 2007) is outside the area covered in the figure. Plate motions (shown by arrows) are from DeMets et al. (1994). B and C. Details of the regions identified in the rectangularboxes in Fig. 2A. Line in Fig. 2B separates regions of uplift and subsidence, based on field observations (modified from Rajendran et al., 2007).

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one region to another (Wells et al., 2003). Signals of residual effects interms of long-term vertical deformation are complex, especially in theforearc regions. In this paper we are primarily dealing with the land-level changes observed along the coastal margins of the accretionarywedge within the outer arc, where the reversals of signals may not besignificant and the net effect is that of uplift.

The 2004 earthquake produced maximum uplift in the AndamanIslands, in the northernpart of the rupture zone. The coseismic elevationchanges helped to estimate themagnitude of uplift along themargins ofthese islands. During our post-earthquake investigations we observedseveral coseismically elevated coral terraces along the coast of the A&NIslands. Although theexistence of terraceshas been observed in thepast,they have not been used in the context of estimating tectonic uplift.Along the southern Sumatra portion of the Sundamegathrust, however,estimates of short-term vertical movements and the uplift histories arebetter documented. For example, Zachariasen et al. (2000) have usedcoral microatolls in the Mentawai Islands, off Sumatra, to suggestsubmergence of 4–10mmyr−1 over the last four or five decades. For theA&N region, only limited data are available on the past uplift/subsidencehistory. The limited extent of land exposure is a major problem inobtaining a spatially continuous pattern of tectonic deformation alongthis ∼800-km stretch of the island arc (Fig. 2A).

The 2004 earthquake resulted in elevation changes ranging fromsubsidence of 3 m at Indira Point (Great Nicobar) and uplift of 1.0 to

1.5m at Diglipur. While the South Andaman group of islands generallysubmerged, subsidence as well as emergence was observed on Middleand North Andaman (see Fig. 2B and C and Rajendran et al., 2007 fordetails). On Middle Andaman, the eastern margins submerged byabout 1 m, whereas the western margins were uplifted by the sameamount. In the northern part of the rupture zone, coseismic grounddeformation was characterized by uplift of the land, both along theislands' western and eastern margins. Morphologic changes weremanifested mostly in the form of elevated shorelines and coral beds,emergedmangrove swamps, and recessedwatermarksexposing thepre-earthquake survival levels of mussels and barnacles, above the present-day high-tide level. Along the western margins of the islands, raisedbeaches were a common sight. For example, the beaches of Avis Island,located on the eastern margin of the North Andaman, were upliftedby N1 m; along the western margin of Interview Island, microatollsremain exposed 1–1.5m above the present high-tide level (see Fig. 2Band C for general areas of uplift and subsidence). Uplift has beenreported also from Landfall Island in the north, but during the surveysreported in this work, we could not extend our studies to this island.

4. Age estimates of Holocene marine terraces

Since the 2004 earthquake caused significant uplift along thewestern margins of the North Andaman, it is reasonable to assume

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that such episodes in the past may have left similar signatures. Theregions where coseismic uplift occurred in 2004 are generallyassociated with receded marine terraces, showing step-like morphol-ogy and subhorizontal platforms. Although erosion and otherintervening processes such as sea level fluctuations may substantiallymodify these features, some signatures are likely to be preserved.Elevation profiles of some of the accessible terraces were obtainedwith a digital theodolite, using the present-day high-tide level as thebase line. The reference line for our survey is the highest high-waterlevel. In the absence of established bench marks in these remote areasto determine absolute high-tide, we have to rely on the level inferredfrom the coastal strand line. Even though the high-tide varies with theday, the signatures of debris and growth of vegetation are easilyrecognizable. These signatures of the highest high-water level,together with tidal charts published by the Survey of India, whichconsiders the tidal variability, were used to define the high-waterlevel. Elevations of the terraces are computed with reference to thisbaseline.

We used the coral fragments collected from the terraces for dating.At some locations we dug shallow pits (10–15 cm deep) and collectedsamples of shells and coral fragments. 14C dating of sampleswas carried out at three different laboratories (see Table 1).The average uplift rate of terraces was calculated using the equation,U=(Z−Z0−SL)/t, where U is the average uplift rate, Z is the modernelevation of the feature, Z0 is the elevation at which the featureformed relative to the sea level at the time (depths are negative; for

Table 1Radiocarbon ages of fossil coral terraces of the Andaman & Nicobar Islands

Sample number Sample namesa Laboratory codeb 14C-age (BP)±1σ Calendar agec (B

Interview Island (Fig. 3)1 IN/TOP/A R-29033/2 30,880±300 36,200±6002 IN/TOP/B IP-352 25,021±280 30,000±10003 T2/IN/D/P1/A R-29033/3 22,890±120 27,500±4004 T3/IN/D/P1/A R-29033/4 19,894±100 23,100±5005 T3/IN/D/P1/C IP-349 19,304±220 22,600±6006 T3/IN/D/P1/D IP-322 16,849±130 19,600±3007 IN/St/D IP-320 6977±85 7500±200

Hut Bay (Little Andaman) (Fig. 4)8 HB/L2/CP NA 3286±100 3100±3009 T2(B)/HUT/D/P4 IP-323 6775±135 7300±30010 T2/HUT/D/P1 IP-353 3661±133 3600±30011 T2(A)/HUT/D/P3 IP-342 3123±47 2900±20012 T2/HUT/D/P5 IP-329 2295±62 1900±20013 T2(A)/HUT/D/P1 IP-328 1904±61 1400±20014 T3/HAR/HUT IP-330 1843±77 1400±20015 HB/L1/CP NA 1042±90 620±160

Car Nicobar (Fig. 6)16 CN/TC/1 BS-2258 5420±80 5800±20017 CN/SW/1 NA 2750±130 2400±30018 CN/SA/1 NA 35,910±1040 41,200±1000

Avis Island (Fig. 5)19 AV/T2/E IP-337 2450±84 2100±20020 AV/T2/B IP-341 1942±114 1500±30021 AV/T2/C IP-346 1678±89 1200±20022 AV/T2/A IP-351 1628±94 1200±20023 AV/T2/D2 IP-376 1364±126 910±26024 AV/T2/E IP-375 1319±91 860±19025 AV/T2/D IP-338 1172±42 730±11026 AV/T1/PB IP-326 967±104 540±19027 AV/T1/A IP-314 906±108 480±19028 AV/PD/LT/1 IP-316 623±78 240±190

a All samples are coral fragments unless otherwise specified.b 14C measurements were done at the Institute of Physics, Bhubaneswar, India (Lab cod

Institute of Palaeobotany, Lucknow, India (Lab code “BS”). Half-life for 14C taken as 5570±305730 yrs at Rafter Radiocarbon Laboratory.

c 14C ages (b22 ka BP) were calibrated using the calibration program Calib5.1, using MARI“Fairbanks0107” curve, as described in Fairbanks et al. (2005).ΔR correction value for all sampIslands (Dutta et al., 2001).

d Sea level changes are based on the calibration curves by Fleming et al. (1998); Horton e

the marine terraces formed from the corals which grow close to thesurface, Z0 is taken as zero), SL is sea level at the time and t is the age ofthe feature (Pinter and Gardner, 1989).

The global sea levels have generally remained stable from about7000 yrs before to the present; the rise in sea level has been marginal(see for example, Fleming et al., 1998). However, a recent study byHorton et al. (2005) for the Malay–Thai Peninsula revealed an upwardtrend of the sea level from −22 m at 9670–9250 cal yr B.P. to a mid-Holocene high-stand of 5 m at 4850–4450 cal yr B.P. Thus there maybe some spatial variability in the sea level history and in the absence ofa sea level curve for the A&N region we are unable to account for anysuch changes. For periods prior to 10 Kyrs, we have used the curves forBarbados, which also compares with that of the Sunda shelf ofsouthern Asia (Lambeck and Chapell, 2001). For periods between 10 Kand present we have used the global sea level changes by Fleminget al. (1998). Sea levels reported by Horton et al. (2005) are almostsimilar to the curve by Fleming et al. (1998) for the period between7.5 K and 9 K yrs, but vary for the later periods of the Holocene.Because of the possible region-specific nature of the observations byHorton et al. (2005) we have not used them in this study.

4.1. Interview Island

The 2004 earthquake caused notable uplift along both the westernand eastern margins of Interview Island (see Fig. 2B for location).Along the western margins where the changes in elevations were

P)±2σ Sea leveld change (m) Elevation (m) Remarks

50 4 km inland−140a 50 Duplicate sample from the above site−140a 26 ∼265 m from shore−130a 18 ∼220 m from shore−130a 18 ∼215 m from the shore−120a 13 ∼180 m from shore

−6.0 7 ∼150 m from shore

−1.0 4.0 ∼100 m shore−6.0 4.5 120 m from shore−1.0 2.8 100 m from shore−1.0 2.8 100 m from shore0.0 2 50 m from shore

−1.0 2 50 m from shore0.0 Harmandir Bay (within 250 m of #13)0.0 1.7 ∼10 m from shore

−3.0 13 ∼5 m from shore0.0 Keating Point

Sawai, Car Nicobar

0.0 2.5 Coral fragments; 15 cm below surface0.0 2.5 Gastropods; 15 cm below surface0.0 2.5 Shells; 10–15 cm below surface0.0 2.5 Coral fragments0.0 2.2 Coral fragments0.0 2.0 Coral fragments; 15 cm below surface0.0 1.8 Coral fragments0.0 1.5 Coral fragments0.0 1.5 Shells, 15 cm below surface0.0 0.75 Present-day high-tide, coral fragments

e “IP”), Rafter Radiocarbon Laboratory, New Zealand (Lab code “R”), and Birbal Sahniyrs at the Birbal Sahni Institute of Palaeobotany, 5568 yrs at the Institute of Physics, and

NE04 dataset (Hughen et al., 2004). Samples older than 22 ka BP were calibrated usingles were taken as 11±35 yrs, as reported for the eastern Bay of Bengal near the Andaman

t al., 2005; Lambeck and Chapell (2001).

Fig. 3. Coral terrace from the west coast of Interview Island (top); elevation profile withdates (bottom), modified from Rajendran et al. (2007). Note that the profile starts from100 m inland.

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more perceptible, freshly exposed, recently dead, flat-topped micro-atolls extend for about 100 m inland, from the present-day high-tidelevel. Based on multiple measurements of individual coral headsexposed along thewest coast of the island, we estimated the coseismicelevation change as 1.5 m±5 cm. (Rajendran et al., 2007). Kayanneet al. (2007) estimated coseismic uplift of 1.3 m at Interview Island,based on the pre- and post-2004 earthquake HLS (Highest Level ofSurvival). Beyond this bed of recently dead microatolls occurringabout 1 m above the present-day high-tide level, we noted erodedfaces of older terraces, which we attribute to previous episodes oftectonism. We took two elevation profiles of varying length (up to170 m) on the western margin of Interview Island. Due to inclementweather and time constraints of working on this uninhabited island,we were unable to profile the entire section during our reconnais-sance. We restricted the profile lengths to about 100 to 200 m,avoiding the 100m stretch of the coseismically raised flat bed closer tothe ocean. Thus, the profiles start from about 100 m from inland fromthe present-day high-tide shoreline and the elevation changes arerelative to that level.

Elevation changes of nearly 25 m were observed over distances of100–150 m, and here we present one profile for which we haveobtained age data from coral fragments. The farthest terrace wemapped along this profile (about 265 m inland) yielded a calibratedage of 27,500±400 cal yr B.P. (Table 1; Fig. 3). The terraces below haveyielded calibrated ages ranging from 19,600±300 to 23,100±500 calyr B.P. The youngest terrace that we could date occurs about 150 minland, at an elevation of 7 m above the present shore line, and thiswas dated at 7500±200 cal yr B.P. A coral sample from a terrace in HutBay, Little Andaman, has yielded a similar age, which is discussed later.Based on studies of elevated microatolls exposed in the InterviewIsland, Kayane et al. (2008) have reported existence of marine terracesdating to ages of 6600, 6250 and 6000 yr B.P., which they attribute totectonism. This period around 7 K yr B.P. is regarded as the last phaseof melting of ice (Fleming et al., 1998; Lambeck and Chapell, 2001).These terraces formed at a time when the sea level was about 6 mbelow the sea level are now located about 7 m above.

Terraces younger than ∼7500 yr B.P. at this site are poorlypreserved; erosion and wave action have substantially modifiedthem. The youngest terrace on the west coast of the Interview Islandoccurs about 120 m inland of the present shoreline, for which we donot have data. However, occurring at an elevation of 2.5 m above thepresent-day high-tide level, and with a well-developed platform(Fig. 3), this terrace is a significant feature that needs to be studiedfurther. We used the calibrated ages, present elevation of the terraceand the sea levels at the time the terraces formed, to obtain theaverage uplift rates. We obtained an uplift rate of 1.73 mmyr−1 for theterrace dating to 7500±200 cal yr B.P. However, a much higher upliftrate of 6.44 mm yr−1 was obtained for the older terraces. Uplift ratesare comparable for the older terraces and an average based on themaximum and minimum ages was used to calculate the average long-term uplift (serial numbers 3–6 in Table 1). Possible implications ofthis observation are discussed later.

4.2. Hut Bay

Four profiles of varying length (70 to 260 m in length) were takenacross the east coast of Hut Bay (the west coast is poorly accessible); arepresentative profile for which we have age data is shown in Fig. 4.The youngest terrace, stripped considerably by the 2004 tsunami, islocated 10 m from the shoreline. Occurring at an elevation of 1.75 mfrom the present sea level, coral fragments from this terrace yielded anage of 620±160 cal yr B.P. Rajendran et al. (2007) reported the age ofthis terrace as 1042±90 yr B.P., which was not a calibrated age. Anolder terrace,100m inland and currently occurring at about 4m abovethe datum, was dated at 3100±300 cal yr B.P. Another sample fromnearby location yielded an age of 3600±300 cal yr B.P. A coral sample

from what appears to be an eroded, intervening platform yielded anage of 1400±200 cal yr B.P. The oldest terrace located 120 m from thepresent sea level and at an elevation of 4.5 m, was dated at 7300±300 cal yr B.P. Representing a period when the sea level changessubsequent to the glacial melting were minimal, this terrace (as in thecase of the contemporaneous terrace at Interview Island), may bemore useful in estimating the uplift rate. An uplift rate of 1.44mmyr−1

was obtained based on the age and elevation of this terrace; the long-term uplift rates averaged at 1.33 mm yr−1. This is significantly lowerthan the long-term uplift rates observed for the Interview Island. The2004 coseismic elevation change at Hut Bay was also marginal,compared with that at Interview Island.

4.3. Avis Island

Avis Island, located on the east coast of North Andaman, registeredgeneral uplift during the 2004 earthquake (Fig. 5). We took fourprofiles to map the morphology of the coast and collected samples fordating; here we show one representative profile for which we haveobtained dates. The profile shown in Fig. 5 shows an elevation changeof 2.5 m over a distance of less than 30 m. Samples for dating werecollected from various levels, starting with the present-day high-tidelevel and the samples included coral fragments from the terraces aswell as shells from shallow pits (10–15 cm) below the surface. Coralsfrom the present-day high-tide level were dated at 240±190 cal yr B.P.The youngest terrace, occurring about 15 m inland of the present-dayshoreline yielded ages in the range of 480±190 and 540±190 cal yr B.P.based on coral fragments and shells collected from 10 cm below thesurface (see Table 1). Samples collected from the slope of theintervening platforms have yielded progressively older ages(Table 1). An older terrace, about 25 m inland, yielded an age rangingfrom 730±110 to 910±260 cal yr B.P. Based on these age estimates, anuplift rate of 2.45 mm yr−1 was estimated for Avis Island.

Fig. 4. Coral terrace from the east coast of Hut Bay, Little Andaman (top); elevationprofile with dates (bottom).

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4.4. Car Nicobar

Part of the southern island of Car Nicobar was surveyed in 2003,but this site could not be occupied after the earthquake due to themassive destruction by the 2004 tsunami. This region showed a

Fig. 5. Coral terrace from Avis Island (top); elevation profile with dates (bottom).

coseismic subsidence of N1 m (based on GPS estimates, see Rajendranet al., 2007). Rajendran et al. (2007) have reported a pre-earthquakeemergence of ∼20 cm based on observations of coral microatollsraised above the HSL from various locations. While the other regionsdiscussed here showed pre-seismic and coseismic uplift, the CarNicobar region, located at similar distance from the subduction frontshowed pre-seismic emergence and coseismic subsidence. Similarly,the post-earthquake relaxation is also relatively higher here, com-pared to other parts for which we have GPS data (Rajendran et al.,2007). Thus, the observation at Car Nicobar suggests that the netelevation change is attributable to a combination of pre-earthquakeemergence, coseismic subsidence and post-seismic relaxation. Theuplift/subsidence rate computed from terrace data should beconsidered as a result of these processes and their relative influencesare likely to vary with respect to the distance from the subductionfront.

The coral sample was collected from the top of the eroded cliff face(Fig. 6) rising to 12 m above an older surface, 1 m above the present-day sea level. The upper terrace was dated at 5800±200 cal yr B.P. Thisis the only data available for the Car Nicobar region and we have usedthis to compute the uplift rate as 2.77 mmyr−1, a higher estimate thanwhat is observed for the northern part of the subduction front. Theoldest date we have is 41,200±1000 cal yr B.P., from Sawai, from alocation about 20 m above the present-day sea level (not shown in thefigure). Terraces of comparable age (36,200±300 yr B.P.) in theInterview Island occur about 50 m above the preset-day sea level.

5. Evidence for previous subsidence events

Coseismic subsidence is known to occur in association with greatsubduction zone earthquakes; stratigraphic sequences characterizedby peat-mud couplets in such environments result from cycles ofcoseismic subsidence and interseismic shoaling. Coastal wetlands inthe Pacific Northwest of Canada and the U.S. and in Chile have beenproven to preserve such evidence from repetitive cycles of pastearthquakes (Atwater, 1987, 1992; Cisternas et al., 2005; Shennan andHamilton, 2006). Identification of such successions may prove usefulin constraining tsunami history in the A&N Islands.

Regions close to Port Blair experienced coseismic subsidenceof ∼1 m, and these sites are good candidates to preserve evidence ofsudden submergence. Rajendran et al. (2007) used cores collectedfrom 30–60 m deep bore holes drilled in various locations at Port Blairduring September 2005. They reported intervening layers (1–2 m)containing decayed vegetation at depths ranging from 2–26 m from

Fig. 6. Photograph of the 12 m high eroded cliff face at Car Nicobar fromwhich the coralsample for dating was collected. Keating Point is identified in the figure. Sawai is locatedabout 500 m east of the coast.

59K. Rajendran et al. / Tectonophysics 455 (2008) 53–60

the present MSL, suggesting submergence of vegetated land. Samplescollected from 4–5 m depth in one of the wells yielded radiocarbondates of 6643±107 cal yr B.P. (plant material) and 6739±85 cal yr B.P.(shell) possibly suggesting an event around that time. Comparableages are observed at Hut Bay (7300±300 cal yr B.P.) and InterviewIsland (7500±200 cal yr B.P.), at elevations of 2.8 m and 7 mrespectively. Thus, evidence from Port Blair located on the easternmargins of the outer arc suggests repeated events of subsidencewhereas observations from Interview Island, located on its western marginsuggests continued uplift.

Evidence for past submergence was reported also from mangroveswamps at Rangachanga, about 8 km Port Blair. This region subsidedduring the 2004 earthquake as evidenced by mangrove forestspresently underwater by about 1 m. Rajendran et al. (2007) reporteda line of dead tree trunks at 1 m depth, and dated one trunk at 656±141 cal yr B.P., (uncalibrated age of 740±100 yr B.P. as reportedearlier) which they attributed to a previous subsidence event.Corroborative evidence of a paleoliquefaction feature from Diglipurdated at 910±186 cal yr B.P. (uncalibrated age 1050±100 yr B.P.) wasused as supporting evidence to suggest the occurrence of an olderevent. Further, based on far field evidence from the east coast of India,Rajendran et al. (2007) suggested this event to have occurred around900 yr B.P.

5.1. Discussion

Coral reef terraces in the A&N region provide age estimates andterrace history for some locations along the island arc. Because of thelimited availability of land in the southern part of the arc, most of ourobservations are confined to the regions around South, Middle andNorth Andaman. The age data presented in this study are spatiallyrestricted; these estimates can be refined only through future studies.An uplift event during 620±160 cal yr B.P. denotes the youngestterrace at Hut Bay. A terrace of comparable age (540±190 cal yr B.P.) isobserved on the Avis Island. These terraces occur at about 10 m and15 m, respectively, inland of the present strand line, both at Hut Bayand Avis Island. A subsidence event reported from Port Blair, dated at656±141 cal yr B.P. gives credence to the argument that a tectonic eventmay have occurred around this time. The ongoing investigations in PhraThong Island, Thailand have suggested occurrence of an Indian Oceantsunami in the 14th or 15th century A.D. (Jankaewet al., 2008). Evidencefor the olderuplift event is obtained from theAvis Islandwhere theolderterrace yield an age ranging from 730±110 to 1200±200 cal yr B.P.Complementary evidence including paleoliquefaction features (dated at910±186 cal yr B.P.) have been found at Diglipur.

Our observations suggest variations in uplift rates from the south(1.33 mm yr−1) to the North Andaman (2.45 mm yr−1). Coseismicelevation changes observed along these islands are in line with thisinference. For example, parts of North Andaman registered largercoseismic emergence, compared to some locations in the south (HutBay, Little Andaman was raised by 0.3 m, whereas the uplift in theInterview Island was ∼1.5 m.

Among the factors that affect the nature and extent of coseismicground deformation are: (a) orientation of the arc with respect tothe direction of plate motion, (b) distance from the subduction front,and (c) contributions from upper plate deformation. The direction ofplate motion is generally consistent along the arc (Subarya et al.,2006), and the distance to the islands with respect to the subductionfront is comparable along its strike for the Andaman and Nicobarpart (Fig. 2A). Thus, the third factor, upper plate deformation seemsto be an important factor. While the southern islands like Car Nicobarand Great Nicobar subsided coseismically by 1 to 3 m, the northernpart of the island arc emerged. The northern islands are also noted fortheir more evolved step-like terraces with intervening platforms,whereas the southern islands preserve fewer terraces, which are lessdistinctive.

The uplift rates for the terrace dating to about 7500 yr B.P. at HutBay (1.44 mm yr−1) and Interview Island (1.73 mm yr−1) arecomparable, with a slightly higher rate at the Interview Island. Dataon older terraces at Interview Island 19 K to 27 K yrs B.P. howeveryielded a higher uplift rate of 6.45 mm yr−1. The influence of sea levelchanges subsequent to the last glacial period and the choice of sealevel calibration curves are factors that could influence the calcula-tions. Contribution from the upper plate deformation at this northernterminus of the 2004 rupture also cannot be ignored. Differentiatingthe contributions from upper plate and elastic deformations is adifficult issue in many subduction fronts (e.g. Scholz and Kato, 1978).Lack of coseismic observations associated with great earthquakes is alimitation in quantifying these processes and from that perspective,the coseismic land-level changes observed in the outer arc region ofthe Andaman–Sunda subduction zone provides useful data for futurestudies. The large historical earthquakes (1881 and 1941-type events)that ruptured smaller segments of the arc are likely to contribute toland-level changes. We believe that such changes would be spatiallyrestricted, whereas those from mega events will be observable inlarger areas. At this point we have tentative evidence for two pastevents, one around 500–600 yr B.P. and another ∼900 yr B.P.Continuing studies at various locations along the A&N arc andelsewhere will bring out more evidence to refine these estimates.

Acknowledgments

This work is funded by the Department of Science and Technology,Government of India, New Delhi and the Indian National Centre forOcean Information Services, Government of India, Hyderabad. Wethank Terry Machado, P.M. Mohan and D. Raju for help during thefieldwork. The Andaman administration and the Department of Forestand Environment provided us logistics on Interview Island. We aregrateful to Aron Meltzner for his critical comments and helpfulsuggestions that contributed to greatly improving this manuscript.

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