Tectonophysics 455 (2008) 53–60

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Tectonophysics

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Age estimates of coastal terraces in the Andaman and and their tectonic 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, b Centre for Earth Science Studies, Trivandrum, 695031, India c Institute of Physics, Sachivalaya Marg, Bhubaneswar, 751005, India

ARTICLE INFO ABSTRACT

Article history: The great earthquake of December 26, 2004 caused significant vertical changes in its rupture Received 4 July 2007 zone. About 800 km of the rupture is along the Andaman and Nicobar Islands, which forms the outer arc Received in revised form 30 April 2008 ridge of the subduction zone. Coseismic deformation along the exposed land could be observed as uplift/ Accepted 6 May 2008 subsidence. Here we analyze the morphological features along the coast of the Andaman and Nicobar Islands, Available online 13 May 2008 in an effort to reconstruct the past tectonics, taking cues from the coseismic effects. We obtained radiocarbon dates from coastal terraces of the island belt and used them to compute uplift rates, which vary from Keywords: − 1 − 1 −1 Seismicity and tectonics 1.33 mm yr in the to 2.80 mm yr in South Andaman and 2.45 mm yr in the North Subduction zones Andaman. Our radiocarbon dates converge on ∼600 yr and ∼1000 yr old coastal uplifts, which we attribute Tsunami to the level changes due to two major previous subduction earthquakes in the region. Coastal terrace © 2008 Elsevier B.V. All rights reserved. Earthquake Age dating

1. Introduction finding discernible fossil records. This paper presents results of our preliminary studies on coral terraces along the Andaman and Nicobar Defining the intervening processes during the successive stages of Islands, (here after abbreviated as A&N Islands), within the rupture earthquake cycles is a useful means of characterizing interseismic zone of the 2004 great Sumatra earthquake. In combination with behavior of active fault zones, thereby enabling long-term forecast. other geologic evidence, they serve as proxies of past tectonics. Where faults and geomorphic features are exposed on land, these Coseismically uplifted or subsided shorelines are known to serve as studies are relatively easy, but due to the general lack of exposure of paleogeodetic markers of Holocene coastal tectonism (Taylor et al., fault traces, they are difficult for subduction zones (see Sieh, 1981, for a 1980; Vita-Finzi, 1981; Lajoie, 1986). It should, however, be noted that review). Great earthquakes generate tsunamis that travel across isolating the influence of eustatic sea level processes on the oceans, affecting global communities, and historical records often development of coastal terraces may not be easily done in many document inundations by the sea caused by such events. In countries areas. But, we place a higher confidence on our results because of the such as Greece, Italy and Japan, the historical tsunami records stretch predominant role of tectonism in the A&N Islands in shaping the back to a few thousand years and these contain information useful to constituent coastal landforms. Similar attempts on the records of reconstruct earthquake/tsunami histories (Atwater et al., 2005; coastal uplift in other emergent shorelines to determine the long-term Cisternas et al., 2005, for example). In regions like the United States spatial and temporal characteristics of large subduction zone earth- where interseismic intervals are much longer than the documented quakes provide further rationale for our studies (e.g. Plafker and history, the reconstruction of tsunami history has to be based Rubin, 1978; Matsuda et al., 1978; Berryman et al., 1989; Ota and primarily on geological records (e.g. Atwater, 1987, 1992). Settings in Yamaguchi, 2004; Natawidjaja et al., 2004). Further, where precise the tropics, characterized by higher level of human activities, tropical measurements are possible, coastal subsidence and uplift of the storm surges and poor preservation potential add to the challenges of tectonically active zones are also used to infer fault slip parameters and model coseismic rupture (e.g. ten Brink et al., 2006; Shennan and Hamilton, 2006) and tsunami generation (e.g. Cummins, 2007). The ⁎ Corresponding author. Tel.: +91 80 2293 2633. A&N group of islands accommodated about 800-km-length of the E-mail address: [email protected] (K. Rajendran). rupture (Lay et al., 2005) and we believe that as the outer arc of the 1 Formerly at Centre for Earth Science Studies, Trivandrum, India, 695031. 2 Currently at Centre for Mathematical Modelling and Computer Simulation, subduction zone, this region is likely to preserve evidence of past Bangalore, India 560037. tectonic movements. The coseismic elevation changes caused by the

0040-1951/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tecto.2008.05.004 54 K. Rajendran et al. / Tectonophysics 455 (2008) 53–60

2004 earthquake provide cues in identifying older features formed the outer ridge lies along the trace of the West Andaman Fault. Our through similar events. In this paper we present data on elevated investigations in this paper are restricted to some of the accessible terraces on the A&N group of islands and use them to compute the islands along the western margins of the outer arc ridge, where step- long-term uplift rates. like coral terraces are observable along the coast. The A&N region has experienced large earthquakes in the past; of 2. Morphology, tectonic features and major earthquakes those 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 of The A&N group of islands form part of the outer arc ridge of the these produced significant tsunamis (Ortiz and Bilham, 2003; northern segment of the Andaman–Sunda subduction zone (Fig. 1A). Rajendran et al., 2007). These earthquakes are inferred to have Tectonic evolution and salient morphological features are discussed involved smaller ruptures (Fig. 2A). In a recent review based on by Curray (2005). Curray describes the basic structure of the historical records, Dominey-Howes et al. (2006) have collated details Andaman–Nicobar ridge, the accretionary prism of the subduction of Indian Ocean tsunamis since B.C. 326; their study does not provide zone, as an imbricate stack of eastward dipping fault slices and folds. any information to suggest the occurrence of any past events The younger series of rocks, termed as the Archipelago Series, cap comparable to the 2004 tsunami. After the 2004 event, there have these imbricate stacks and they consist typically of limestone, formed been more efforts to search for evidence of past events. In a recent from coral and shell sands. study, Cummins (2007) modeled the rupture parameters and the A section along 11°N parallel (Fig. 1A, B) shows the morphological tsunami generation, based on observations of landform changes. features typical of subduction zones—the outer arc (Andaman and Earthquakes of magnitude ∼8 or smaller are also likely to cause Nicobar Ridge), forearc basin (West Basin, Invisible Bank), volcanic arc morphological changes, but they will be spatially restricted. The ability (Barren, Narcondum) and the backarc basin (Central Andaman basin, to distinguish morphologic changes caused by such earthquakes from East basin). Curray (2005) mentions two normal faults (Diligent Fault, those affecting larger portions of the subduction zone depends largely DF and Eastern Margin Fault, EMF), with the west side up and running on the multiplicity and spatial distribution of evidence. parallel to the outer arc. The southward continuity of EMF to the History of great earthquakes in the Andaman–Sunda subduction zone Nicobar Islands is ambiguous. Further south, off Northern Sumatra, is poorly understood. Rajendran et al. (2006 and 2007) used historical and archeological data from the east coast of India, together with geologic evidence including paleoliquefaction features and subsided vegetation in the marshes on the east coast of , to suggest that the pre-2004 event may have occurred around 1000 yr B.P. Ongoing paleotsunami investigations in the A&N Islands have indicated the possibility of anyounger event around 600 yr B.P. (Rajendran et al., 2008). Recent work by Jankaew et al. (2008) reports presence of sandsheets dating to about 14th to 15th Century A.D., based on paleotsunami evidence from the Phra Thong Island, on the coastal plains of Thailand. Obtaining chronological constraints of great earthquakes generated by the Andaman–Sunda subduction zone is an effort that is continuing; only with more data from the Indian Ocean littoral countries, the history of great events along this plate boundary can be better understood. The 2004 earthquake caused vertical displacements all along its rupture 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 these features and defined a pivot line separating the zones of uplift and subsidence (Fig. 2B and C). We also observed many geomorphological features, such as stepped coral terraces, that suggest episodes of past movements. Similarly, coseismic subsidence of coastal marshes in 2004 is potentially analogous to land-level changes that occurred in the past. We observed buried layers of peat in core sections at sites of some mangrove swamps that subsided in the 2004 earthquake, which we consider as evidence of past events (Rajendran et al., 2007). These features serve as proxies of tectonism and, if interpreted carefully, they can be used to reconstruct the regional tectonic history. The exposures of land within the Indian political boundary are limited; the study was also constrained by the logistical problems to work in the isolated and uninhabited 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 accompanying large earthquakes (e.g., Lajoie, 1986; Berryman et al., 1989; Vita-Finzi and Hidayat, 1991; Ota and Yamaguchi, 2004). Elastic dislocation models for subduction megathrust earthquake predict coseismic uplift in the up-dip portions of the rupture and subsidence in the Fig. 1. A. Map of the Andaman and Nicobar region showing major tectonic units. B. down-dip part (e.g., Plafker and Savage, 1970). Post-seismic relaxation 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: occurs soon after the earthquake, followed by long-term visco-elastic Barren; N: Narcondum; EMF is Eastern Margin Fault. relaxation (Hu et al., 2004) and the nature of recovery may vary from K. Rajendran et al. / Tectonophysics 455 (2008) 53–60 55

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 from Ortiz 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 rectangular boxes in Fig. 2A. Line in Fig. 2B separates regions of uplift and subsidence, based on field observations (modified from Rajendran et al., 2007). one region to another (Wells et al., 2003). Signals of residual effects in 1.5 m at . While the South Andaman group of islands generally terms of long-term vertical deformation are complex, especially in the submerged, subsidence as well as emergence was observed on Middle forearc regions. In this paper we are primarily dealing with the land- and North Andaman (see Fig. 2B and C and Rajendran et al., 2007 for level changes observed along the coastal margins of the accretionary details). On Middle Andaman, the eastern margins submerged by wedge within the outer arc, where the reversals of signals may not be about 1 m, whereas the western margins were uplifted by the same significant and the net effect is that of uplift. amount. In the northern part of the rupture zone, coseismic ground The 2004 earthquake produced maximum uplift in the Andaman deformation was characterized by uplift of the land, both along the Islands, in the northern part of the rupture zone. The coseismic elevation islands' western and eastern margins. Morphologic changes were changes helped to estimate the magnitude of uplift along the margins of manifested mostly in the form of elevated shorelines and coral beds, these islands. During our post-earthquake investigations we observed emerged mangrove swamps, and recessed water marks exposing the pre- several coseismically elevated coral terraces along the coast of the A&N earthquake survival levels of mussels and barnacles, above the present- Islands. Although the existence of terraces has been observed in the past, day high-tide level. Along the western margins of the islands, raised they have not been used in the context of estimating tectonic uplift. beaches were a common sight. For example, the beaches of Avis Island, Along the southern Sumatra portion of the Sunda megathrust, however, located on the eastern margin of the North Andaman, were uplifted estimates of short-term vertical movements and the uplift histories are by N1 m; along the western margin of , microatolls better documented. For example, Zachariasen et al. (2000) have used remain exposed 1–1.5 m above the present high-tide level (see Fig. 2B coral microatolls in the Mentawai Islands, off Sumatra, to suggest and C for general areas of uplift and subsidence). Uplift has been submergence of 4–10 mm yr−1 over the last four or five decades. For the reported also from in the north, but during the surveys A&N region, only limited data are available on the past uplift/subsidence reported in this work, we could not extend our studies to this island. history. The limited extent of land exposure is a major problem in obtaining a spatially continuous pattern of tectonic deformation along 4. Age estimates of Holocene marine terraces this ∼800-km stretch of the island arc (Fig. 2A). The 2004 earthquake resulted in elevation changes ranging from Since the 2004 earthquake caused significant uplift along the subsidence of 3 m at Indira Point (Great Nicobar) and uplift of 1.0 to western margins of the North Andaman, it is reasonable to assume 56 K. Rajendran et al. / Tectonophysics 455 (2008) 53–60 that such episodes in the past may have left similar signatures. The the marine terraces formed from the corals which grow close to the regions where coseismic uplift occurred in 2004 are generally surface, Z0 is taken as zero), SL is sea level at the time and t is the age of associated with receded marine terraces, showing step-like morphol- the feature (Pinter and Gardner, 1989). ogy and subhorizontal platforms. Although erosion and other The global sea levels have generally remained stable from about intervening processes such as sea level fluctuations may substantially 7000 yrs before to the present; the rise in sea level has been marginal modify these features, some signatures are likely to be preserved. (see for example, Fleming et al., 1998). However, a recent study by Elevation profiles of some of the accessible terraces were obtained Horton et al. (2005) for the Malay–Thai Peninsula revealed an upward with a digital theodolite, using the present-day high-tide level as the trend of the sea level from −22 m at 9670–9250 cal yr B.P. to a mid- base line. The reference line for our survey is the highest high-water Holocene high-stand of 5 m at 4850–4450 cal yr B.P. Thus there may level. In the absence of established bench marks in these remote areas be some spatial variability in the sea level history and in the absence of to determine absolute high-tide, we have to rely on the level inferred a sea level curve for the A&N region we are unable to account for any from the coastal strand line. Even though the high-tide varies with the such changes. For periods prior to 10 K yrs, we have used the curves for day, the signatures of debris and growth of vegetation are easily Barbados, which also compares with that of the Sunda shelf of recognizable. These signatures of the highest high-water level, southern Asia (Lambeck and Chapell, 2001). For periods between 10 K together with tidal charts published by the Survey of India, which and present we have used the global sea level changes by Fleming considers the tidal variability, were used to define the high-water et al. (1998). Sea levels reported by Horton et al. (2005) are almost level. Elevations of the terraces are computed with reference to this similar to the curve by Fleming et al. (1998) for the period between baseline. 7.5 K and 9 K yrs, but vary for the later periods of the Holocene. We used the coral fragments collected from the terraces for dating. Because of the possible region-specific nature of the observations by At some locations we dug shallow pits (10–15 cm deep) and collected Horton et al. (2005) we have not used them in this study. samples of shells and coral fragments. 14C dating of samples wascarriedoutatthreedifferentlaboratories(seeTable 1). 4.1. Interview Island 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 modern The 2004 earthquake caused notable uplift along both the western elevation of the feature, Z0 is the elevation at which the feature and eastern margins of Interview Island (see Fig. 2B for location). formed relative to the sea level at the time (depths are negative; for Along the western margins where the changes in elevations were

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

Sample number Sample namesa Laboratory codeb 14C-age (BP)±1σ Calendar agec (BP)±2σ Sea leveld change (m) Elevation (m) Remarks Interview Island (Fig. 3) 1 IN/TOP/A R-29033/2 30,880±300 36,200±600 50 4 km inland 2 IN/TOP/B IP-352 25,021±280 30,000±1000 −140a 50 Duplicate sample from the above site 3 T2/IN/D/P1/A R-29033/3 22,890±120 27,500±400 −140a 26 ∼265 m from shore 4 T3/IN/D/P1/A R-29033/4 19,894±100 23,100±500 −130a 18 ∼220 m from shore 5 T3/IN/D/P1/C IP-349 19,304 ±220 22,600±600 −130a 18 ∼215 m from the shore 6 T3/IN/D/P1/D IP-322 16,849 ±130 19,600±300 −120a 13 ∼180 m from shore 7 IN/St/D IP-320 6977±85 7500±200 −6.0 7 ∼150 m from shore

Hut Bay (Little Andaman) (Fig. 4) 8 HB/L2/CP NA 3286±100 3100±300 −1.0 4.0 ∼100 m shore 9 T2(B)/HUT/D/P4 IP-323 6775±135 7300±300 −6.0 4.5 120 m from shore 10 T2/HUT/D/P1 IP-353 3661±133 3600±300 −1.0 2.8 100 m from shore 11 T2(A)/HUT/D/P3 IP-342 3123±47 2900±200 −1.0 2.8 100 m from shore 12 T2/HUT/D/P5 IP-329 2295±62 1900±200 0.0 2 50 m from shore 13 T2(A)/HUT/D/P1 IP-328 1904±61 1400±200 −1.0 2 50 m from shore 14 T3/HAR/HUT IP-330 1843±77 1400±200 0.0 Harmandir Bay (within 250 m of #13) 15 HB/L1/CP NA 1042±90 620±160 0.0 1.7 ∼10 m from shore

Car Nicobar (Fig. 6) 16 CN/TC/1 BS-2258 5420±80 5800±200 −3.0 13 ∼5 m from shore 17 CN/SW/1 NA 2750±130 2400±300 0.0 Keating Point 18 CN/SA/1 NA 35,910±1040 41,200±1000 Sawai,

Avis Island (Fig. 5) 19 AV/T2/E IP-337 2450±84 2100±200 0.0 2.5 Coral fragments; 15 cm below surface 20 AV/T2/B IP-341 1942±114 1500±300 0.0 2.5 Gastropods; 15 cm below surface 21 AV/T2/C IP-346 1678±89 1200±200 0.0 2.5 Shells; 10–15 cm below surface 22 AV/T2/A IP-351 1628±94 1200±200 0.0 2.5 Coral fragments 23 AV/T2/D2 IP-376 1364±126 910±260 0.0 2.2 Coral fragments 24 AV/T2/E IP-375 1319±91 860±190 0.0 2.0 Coral fragments; 15 cm below surface 25 AV/T2/D IP-338 1172±42 730±110 0.0 1.8 Coral fragments 26 AV/T1/PB IP-326 967±104 540±190 0.0 1.5 Coral fragments 27 AV/T1/A IP-314 906±108 480±190 0.0 1.5 Shells, 15 cm below surface 28 AV/PD/LT/1 IP-316 623±78 240±190 0.0 0.75 Present-day high-tide, coral fragments

a All samples are coral fragments unless otherwise specified. b 14C measurements were done at the Institute of Physics, Bhubaneswar, India (Lab code “IP”), Rafter Radiocarbon Laboratory, New Zealand (Lab code “R”), and Birbal Sahni Institute of Palaeobotany, Lucknow, India (Lab code “BS”). Half-life for 14C taken as 5570±30 yrs at the Birbal Sahni Institute of Palaeobotany, 5568 yrs at the Institute of Physics, and 5730 yrs at Rafter Radiocarbon Laboratory. c 14C ages (b22 ka BP) were calibrated using the calibration program Calib5.1, using MARINE04 dataset (Hughen et al., 2004). Samples older than 22 ka BP were calibrated using “Fairbanks0107” curve, as described in Fairbanks et al. (2005). ΔR correction value for all samples were taken as 11±35 yrs, as reported for the eastern near the (Dutta et al., 2001). d Sea level changes are based on the calibration curves by Fleming et al. (1998); Horton et al., 2005; Lambeck and Chapell (2001). K. Rajendran et al. / Tectonophysics 455 (2008) 53–60 57 more perceptible, freshly exposed, recently dead, flat-topped micro- atolls extend for about 100 m inland, from the present-day high-tide level. Based on multiple measurements of individual coral heads exposed along the west coast of the island, we estimated the coseismic elevation change as 1.5 m±5 cm. (Rajendran et al., 2007). Kayanne et al. (2007) estimated coseismic uplift of 1.3 m at Interview Island, based on the pre- and post-2004 earthquake HLS (Highest Level of Survival). Beyond this bed of recently dead microatolls occurring about 1 m above the present-day high-tide level, we noted eroded faces of older terraces, which we attribute to previous episodes of tectonism. We took two elevation profiles of varying length (up to 170 m) on the western margin of Interview Island. Due to inclement weather 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 100 m stretch of the coseismically raised flat bed closer to the ocean. Thus, the profiles start from about 100 m from inland from the present-day high-tide shoreline and the elevation changes are relative to that level. Elevation changes of nearly 25 m were observed over distances of 100–150 m, and here we present one profile for which we have obtained age data from coral fragments. The farthest terrace we mapped along this profile (about 265 m inland) yielded a calibrated age of 27,500±400 cal yr B.P. (Table 1; Fig. 3). The terraces below have yielded calibrated ages ranging from 19,600±300 to 23,100±500 cal yr B.P. The youngest terrace that we could date occurs about 150 m inland, at an elevation of 7 m above the present shore line, and this was dated at 7500±200 cal yr B.P. A coral sample from a terrace in Hut Bay, Little Andaman, has yielded a similar age, which is discussed later. Based on studies of elevated microatolls exposed in the Interview Fig. 3. Coral terrace from the west coast of Interview Island (top); elevation profile with Island, Kayane et al. (2008) have reported existence of marine terraces dates (bottom), modified from Rajendran et al. (2007). Note that the profile starts from 100 m inland. dating to ages of 6600, 6250 and 6000 yr B.P., which they attribute to tectonism. This period around 7 K yr B.P. is regarded as the last phase of melting of ice (Fleming et al., 1998; Lambeck and Chapell, 2001). from what appears to be an eroded, intervening platform yielded an These terraces formed at a time when the sea level was about 6 m age of 1400±200 cal yr B.P. The oldest terrace located 120 m from the below the sea level are now located about 7 m above. present sea level and at an elevation of 4.5 m, was dated at 7300 Terraces younger than ∼7500 yr B.P. at this site are poorly ±300 cal yr B.P. Representing a period when the sea level changes preserved; erosion and wave action have substantially modified subsequent to the glacial melting were minimal, this terrace (as in the them. The youngest terrace on the west coast of the Interview Island case of the contemporaneous terrace at Interview Island), may be − occurs about 120 m inland of the present shoreline, for which we do more useful in estimating the uplift rate. An uplift rate of 1.44 mm yr 1 not have data. However, occurring at an elevation of 2.5 m above the was obtained based on the age and elevation of this terrace; the long- − present-day high-tide level, and with a well-developed platform term uplift rates averaged at 1.33 mm yr 1. This is significantly lower (Fig. 3), this terrace is a significant feature that needs to be studied than the long-term uplift rates observed for the Interview Island. The further. We used the calibrated ages, present elevation of the terrace 2004 coseismic elevation change at Hut Bay was also marginal, and the sea levels at the time the terraces formed, to obtain the compared with that at Interview Island. average uplift rates. We obtained an uplift rate of 1.73 mm yr− 1 for the terrace dating to 7500±200 cal yr B.P. However, a much higher uplift 4.3. Avis Island rate of 6.44 mm yr− 1 was obtained for the older terraces. Uplift rates are comparable for the older terraces and an average based on the Avis Island, located on the east coast of North Andaman, registered maximum and minimum ages was used to calculate the average long- general uplift during the 2004 earthquake (Fig. 5). We took four term uplift (serial numbers 3–6inTable 1). Possible implications of profiles to map the morphology of the coast and collected samples for this observation are discussed later. dating; here we show one representative profile for which we have obtained dates. The profile shown in Fig. 5 shows an elevation change 4.2. Hut Bay of 2.5 m over a distance of less than 30 m. Samples for dating were collected from various levels, starting with the present-day high-tide Four profiles of varying length (70 to 260 m in length) were taken level and the samples included coral fragments from the terraces as across the east coast of Hut Bay (the west coast is poorly accessible); a well as shells from shallow pits (10–15 cm) below the surface. Corals representative profile for which we have age data is shown in Fig. 4. from the present-day high-tide level were dated at 240±190 cal yr B.P. The youngest terrace, stripped considerably by the 2004 tsunami, is The youngest terrace, occurring about 15 m inland of the present-day located 10 m from the shoreline. Occurring at an elevation of 1.75 m shoreline yielded ages in the range of 480±190 and 540±190 cal yr B.P. from the present sea level, coral fragments from this terrace yielded an based on coral fragments and shells collected from 10 cm below the age of 620±160 cal yr B.P. Rajendran et al. (2007) reported the age of surface (see Table 1). Samples collected from the slope of the this terrace as 1042±90 yr B.P., which was not a calibrated age. An intervening platforms have yielded progressively older ages older terrace, 100 m inland and currently occurring at about 4 m above (Table 1). An older terrace, about 25 m inland, yielded an age ranging the datum, was dated at 3100±300 cal yr B.P. Another sample from from 730±110 to 910±260 cal yr B.P. Based on these age estimates, an nearby location yielded an age of 3600±300 cal yr B.P. A coral sample uplift rate of 2.45 mm yr− 1 was estimated for Avis Island. 58 K. Rajendran et al. / Tectonophysics 455 (2008) 53–60

coseismic subsidence of N1 m (based on GPS estimates, see Rajendran et al., 2007). Rajendran et al. (2007) have reported a pre-earthquake emergence of ∼20 cm based on observations of coral microatolls raised above the HSL from various locations. While the other regions discussed here showed pre-seismic and coseismic uplift, the Car Nicobar region, located at similar distance from the subduction front showed 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 net elevation change is attributable to a combination of pre-earthquake emergence, coseismic subsidence and post-seismic relaxation. The uplift/subsidence rate computed from terrace data should be considered as a result of these processes and their relative influences are likely to vary with respect to the distance from the subduction front. 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. This is the only data available for the Car Nicobar region and we have used this to compute the uplift rate as 2.77 mm yr− 1, a higher estimate than what is observed for the northern part of the subduction front. The oldest date we have is 41,200±1000 cal yr B.P., from Sawai, from a location about 20 m above the present-day sea level (not shown in the figure). Terraces of comparable age (36,200±300 yr B.P.) in the Interview Island occur about 50 m above the preset-day sea level.

Fig. 4. Coral terrace from the east coast of Hut Bay, Little Andaman (top); elevation 5. Evidence for previous subsidence events profile with dates (bottom).

Coseismic subsidence is known to occur in association with great 4.4. Car Nicobar subduction zone earthquakes; stratigraphic sequences characterized by peat-mud couplets in such environments result from cycles of Part of the southern island of Car Nicobar was surveyed in 2003, coseismic subsidence and interseismic shoaling. Coastal wetlands in but this site could not be occupied after the earthquake due to the the Pacific Northwest of Canada and the U.S. and in Chile have been massive destruction by the 2004 tsunami. This region showed a proven to preserve such evidence from repetitive cycles of past earthquakes (Atwater, 1987, 1992; Cisternas et al., 2005; Shennan and Hamilton, 2006). Identification of such successions may prove useful in constraining tsunami history in the A&N Islands. Regions close to Port Blair experienced coseismic subsidence of ∼1 m, and these sites are good candidates to preserve evidence of sudden submergence. Rajendran et al. (2007) used cores collected from 30–60 m deep bore holes drilled in various locations at Port Blair during September 2005. They reported intervening layers (1–2m) 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 from which the coral sample for dating was collected. Keating Point is identified in the figure. Sawai is located Fig. 5. Coral terrace from Avis Island (top); elevation profile with dates (bottom). about 500 m east of the coast. K. Rajendran et al. / Tectonophysics 455 (2008) 53–60 59 the present MSL, suggesting submergence of vegetated land. Samples The uplift rates for the terrace dating to about 7500 yr B.P. at Hut collected from 4–5 m depth in one of the wells yielded radiocarbon Bay (1.44 mm yr− 1) and Interview Island (1.73 mm yr− 1)are dates of 6643±107 cal yr B.P. (plant material) and 6739±85 cal yr B.P. comparable, with a slightly higher rate at the Interview Island. Data (shell) possibly suggesting an event around that time. Comparable on older terraces at Interview Island 19 K to 27 K yrs B.P. however ages are observed at Hut Bay (7300±300 cal yr B.P.) and Interview yielded a higher uplift rate of 6.45 mm yr− 1. The influence of sea level Island (7500±200 cal yr B.P.), at elevations of 2.8 m and 7 m changes subsequent to the last glacial period and the choice of sea respectively. Thus, evidence from Port Blair located on the eastern level calibration curves are factors that could influence the calcula- margins of the outer arc suggests repeated events of subsidence where tions. Contribution from the upper plate deformation at this northern as observations from Interview Island, located on its western margin terminus of the 2004 rupture also cannot be ignored. Differentiating suggests continued uplift. the contributions from upper plate and elastic deformations is a Evidence for past submergence was reported also from mangrove difficult issue in many subduction fronts (e.g. Scholz and Kato, 1978). swamps at Rangachanga, about 8 km Port Blair. This region subsided Lack of coseismic observations associated with great earthquakes is a during the 2004 earthquake as evidenced by mangrove forests limitation in quantifying these processes and from that perspective, presently underwater by about 1 m. Rajendran et al. (2007) reported the coseismic land-level changes observed in the outer arc region of a line of dead tree trunks at 1 m depth, and dated one trunk at 656 the Andaman–Sunda subduction zone provides useful data for future ±141 cal yr B.P., (uncalibrated age of 740±100 yr B.P. as reported studies. The large historical earthquakes (1881 and 1941-type events) earlier) which they attributed to a previous subsidence event. that ruptured smaller segments of the arc are likely to contribute to Corroborative evidence of a paleoliquefaction feature from Diglipur land-level changes. We believe that such changes would be spatially dated at 910±186 cal yr B.P. (uncalibrated age 1050±100 yr B.P.) was restricted, whereas those from mega events will be observable in used as supporting evidence to suggest the occurrence of an older larger areas. At this point we have tentative evidence for two past event. Further, based on far field evidence from the east coast of India, events, one around 500–600 yr B.P. and another ∼900 yr B.P. Rajendran et al. (2007) suggested this event to have occurred around Continuing studies at various locations along the A&N arc and 900 yr B.P. elsewhere will bring out more evidence to refine these estimates.

5.1. Discussion Acknowledgments

Coral reef terraces in the A&N region provide age estimates and This work is funded by the Department of Science and Technology, terrace history for some locations along the island arc. Because of the Government of India, New Delhi and the Indian National Centre for limited availability of land in the southern part of the arc, most of our Ocean Information Services, Government of India, Hyderabad. We observations are confined to the regions around South, Middle and thank Terry Machado, P.M. Mohan and D. Raju for help during the North Andaman. The age data presented in this study are spatially fieldwork. The Andaman administration and the Department of Forest restricted; these estimates can be refined only through future studies. and Environment provided us logistics on Interview Island. We are An uplift event during 620±160 cal yr B.P. denotes the youngest grateful to Aron Meltzner for his critical comments and helpful terrace at Hut Bay. A terrace of comparable age (540±190 cal yr B.P.) is suggestions that contributed to greatly improving this manuscript. observed on the Avis Island. These terraces occur at about 10 m and 15 m, respectively, inland of the present strand line, both at Hut Bay References and Avis Island. A subsidence event reported from Port Blair, dated at 656±141 cal yr B.P. gives credence to the argument that a tectonic event Atwater, B.F., 1987. Evidence for great Holocene earthquakes along the outer coast of Washington State. Science 236, 942–944. may have occurred around this time. The ongoing investigations in Phra Atwater, B.F., 1992. Geologic evidence for earthquakes during the past 2000 years along Thong Island, Thailand have suggested occurrence of an Indian Ocean the Copalis River, southern coastal Washington. J. Geophys. Res. 97, 1901–1919. tsunami in the 14th or 15th century A.D. (Jankaew et al., 2008). 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