Journal Code Article ID Dispatch: 20.02.13 CE: J G R B 50122No.ofPages: 18 ME: – 1 JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 118, 1 18, doi:10.1029/2012JB009541, 2013 61 2 62 3 63 4 64 5 65 6 Ages and relative sizes of pre-2004 tsunamis in the Bay of Bengal 66 7 67 8 inferred from geologic evidence in the Andaman and Nicobar Islands 68 9 1 1 1 2 69 10 C. P. Rajendran, Kusala Rajendran, Vanessa Andrade, and S. Srinivasalu 70 11 Received 19 June 2012; revised 28 January 2013; accepted 2 May 2013. 71 12 72 [1] Geologic evidence along the northern part of the 2004 Aceh-Andaman rupture suggests 13 that this region generated as many as five tsunamis in the prior 2000 years. We identify this 73 14 evidence by drawing analogy with geologic records of land-level change and the tsunami in 74 15 2004 from Andaman and Nicobar Islands (A&N). These analogs include subsided mangrove 75 16 swamps, uplifted coral terraces, liquefaction, and organic soils coated by sand and coral 76 17 rubble. The pre-2004 evidence varies in potency, and materials dated provide limiting ages on 77 18 inferred tsunamis. The earliest tsunamis occurred between the second and sixth centuries A. 78 19 D., evidenced by coral debris of the southern Car Nicobar Island. A subsequent tsunami, 79 20 probably in the range A.D. 770–1040, is inferred from deposits both in A&N and on the 80 21 Indian subcontinent. It is the strongest candidate for a 2004-caliber earthquake in the past 81 22 2000 years. A&N also contain tsunami deposits from A.D. 1250 to 1450 that probably match 82 23 those previously reported from Sumatra and Thailand, and which likely date to the 1390s or 83 24 1450s if correlated with well-dated coral uplift offshore Sumatra. Thus, age data from A&N 84 25 suggest that within the uncertainties in estimating relative sizes of paleo-earthquakes and 85 26 tsunamis, the 1000 year interval can be divided in half by the earthquake or earthquakes of A. 86 27 D. 1250–1450 of magnitude >8.0 and consequent tsunamis. Unlike the transoceanic tsunamis 87 28 generated by full or partial rupture of the subduction interface, the A&N geology further 88 29 provides evidence for the smaller-sized historical tsunamis of 1762 and 1881, which may 89 30 have been damaging locally. 90 31 91 Citation: 32 Rajendran C. P., K. Rajendran, V. Andrade and S. Srinivasalu (2013), Ages and relative sizes of pre-2004 92 J. Geophys. Res. 118 33 tsunamis in the Bay of Bengal inferred from geologic evidence in the Andaman and Nicobar Islands, , , 93 doi:10.1029/2012JB009541. 34 94 35 95 36 1. Introduction et al., 2008; Monecke et al., 2008]. An older tsunami 96 (A.D. 780–990) has also been reported from Meulaboh 37 97 [2] The Mw 9.2 2004 earthquake and its accompanying [Monecke et al., 2008]. Reports from Sri Lanka concur with 38 tsunami were unprecedented, as there was no recognized an approximately 1000 year old tsunami [Ranasinghage 98 39 evidence that the Aceh-Andaman subduction zone plate et al., 2010]. Studies on the sediment cores from a lagoon 99 40 boundary could generate a megathrust earthquake and a on the southeastern coast of Sri Lanka suggest three 100 41 transoceanic tsunami. How often is a 2004-type earthquake tsunamis in the Indian Ocean between ~4000 and 5500 years 101 42 likely to occur? Initial results from the east coast of India ago and one event around 1600 years ago [Jackson, 2008]. 102 suggest that a predecessor of the 2004 tsunami occurred 43 Improving recurrence estimates of 2004-type events requires 103 ~1000 years ago on the east coast of India [Rajendran identification of synchronous evidence from other parts of 44 et al., 2006, 2011]. Studies elsewhere along the Indian 104 the 2004 rupture zone. The Andaman and Nicobar Islands 45 Ocean coasts have led to evidence of older tsunamis at Phra 105 (referred to as A&N) are key to constraining tsunami recur- 46 Thong (Thailand), Meulaboh (northern Sumatra, Indonesia), 106 rence because they together span and adjoin two thirds of the 47 F1 and Sri Lanka (see Figure 1 for locations). The penultimate 107 length of the 2004 rupture area (Figure 1). In A&N, the 2004 48 tsunami at Phra Thong is dated at A.D. 1300–1450, with 108 – event is recorded by evidence for coseismic uplift and subsi- 49 matching ages from Meulaboh (A.D. 1290 1400) [Jankaew dence, by liquefaction features and by tsunami deposits. 109 50 Some of these have been described previously [Rajendran 110 51 et al., 2007, Malik et al., 2011]. 111 Q1 All Supporting Information may be found in the online version of 52 this article. [3] Historical documents for the last 500 years that seem 112 53 1Centre for Earth Sciences, Indian Institute of Science, Bangalore, India. reliable do not point to any coastal flooding leading to major 113 54 2Department of Geology, Anna University, Chennai, India. loss of life on the east coast of India [Rajendran, 2012]. The 114 55 Corresponding author: C. P. Rajendran, Centre for Earth Sciences, earliest historically documented earthquake from this region 115 56 Indian Institute of Science, Bangalore 560012, India. (cprajendran@ is the 2 April 1762 earthquake (M ≥7.6) off Myanmar, which 116 57 ceas.iisc.ernet.in) caused a tsunami that affected the coasts of West Bengal and 117 58 ©2013. American Geophysical Union. All Rights Reserved. Bangladesh (Figure 1, inset) [Chhibber, 1934; Cummins, 118 2007]. This earthquake left sedimentary records near Port 59 2169-9356/13/2012JB009541 119 60 120 1 1 RAJENDRAN ET AL.: AGES AND SIZES OF PRE-2004 TSUNAMIS 61 2 62 3 attempted to obtain multiple lines of evidence collated from 63 4 coseismic features, establish their chronology and regional 64 distribution, and use these to estimate the timing and size 5 65 of previous tsunamigenic earthquakes. Even though a 6 combination of multiple coseismic effects (like subsidence, 66 7 uplift, and liquefaction) and occurrence of varied types of 67 8 tsunami deposits make the A&N potentially well suited to 68 9 search for geological clues of pre-2004 events, the logistical 69 10 and administrative problems in reaching remote areas and 70 11 restrictions to visit tribal reserves prevented us from 71 12 accessing some potential investigative sites. Nevertheless, 72 13 our effort provides a starting point in documenting the 73 14 effects of a great earthquake in an important part of the 74 15 Andaman-Sumatra plate boundary. Based on the intermittent 75 observations made during the last 6 years, along the A&N, 16 76 covering regions from 7 to 14N latitude, this study presents 17 the first comprehensive regional database on the sedimentary 77 18 records of potential tsunamis or earthquakes of the past. 78 19 79 20 2. Tsunami Geology: Background and Methods 80 21 81 22 2.1. Evidence of Tsunamis Along Subduction-Zone 82 23 Coasts 83 24 [5] Great subduction-zone earthquakes and tsunamis 84 25 invariably leave a trail of telltale geomorphic features and 85 26 deposits on the shores of island arcs. Coseismic and 86 interseismic bending and unbending of the upper crust lead 27 87 to characteristic geomorphic responses. A great thrust earth- 28 quake occurs when ridge-push and slab-pull forces exceed 88 29 the strength of the locked interface thrust zone (Figure 2, F2 89 30 top panel). Thus, prior to such an earthquake, ridge-push 90 31 forces dominate, leading to upwarping of the crust, while 91 32 slab-pull forces overtake all other components of plate driv- 92 33 ing forces during coseismic and postseismic periods 93 34 [Spence, 1987; Lay et al., 1989]. For instance, interseismic 94 35 uplift in the A&N segment has been inferred from pre- 95 36 2004 emergence of coral platforms as well as dominance 96 37 of thrust faulting earthquakes on the overriding plate 97 [Rajendran et al., 2007; Andrade and Rajendran, 2011]. 38 Figure 1. Regional map of the northern Indian Ocean 98 [6] Tsunami-deposited sands in conjunction with subsided 39 countries showing the 2004 and 2005 ruptures [Chlieh coastal lowlands have been inferred as evidence for local 99 40 et al., 2007] and the 1679, 1881, and 1941 ruptures [Ortiz tsunamigenic earthquakes in Alaska, Cascadia (United 100 41 and Bilham, 2003; Rajendran et al., 2007]. The broken States), Chile, and Hokkaido (Japan) [Ovenshine et al., 101 42 black hinge line separates areas of 2004 coseismic uplift 1976; Atwater, 1987; Minoura et al., 1987; Nelson et al., 102 43 from subsidence. Filled circles are sites discussed in this 103 fi 1996; Nanayama et al., 2003; Cisternas et al., 2005; 44 study; lled squares are sites from other published work. Shennan and Hamilton, 2006]. Coseismic emergence of 104 Selected earthquake mechanisms are from Global CMT 45 – land along subduction-zone coasts has also been recorded 105 (gray: 2000 2004; red: 2004-present). Inset: index map of [Ota and Yamaguchi, 2004]. 46 the region showing the sources of known great earthquakes. 106 [7] Geological evidence of tectonism can often be 47 Abbreviations: BD: Bangladesh, WB: West Bengal, Kp: 107 48 deduced from the remains of subsided vegetation buried in 108 Kaveripattinam, Mp: Mamallapuram, Me: Meulaboh, MY: the regional stratigraphy. Where tsunami deposits are syn- 49 109 Myanmar, PT: Phra Thong, Sim: Simeulue, SL: Sri Lanka, chronous with subsidence, there is more certainty in ascrib- 50 TH: Thailand, Va: Vakarai.