Tectonophysics 455 (2008) 53–60 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto Age estimates of coastal terraces in the Andaman and Nicobar Islands 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, India 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 Indian Ocean 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 Little Andaman 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 Port Blair, 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.