GEOPHYSICAL RESEARCH LETTERS, VOL. 34, L01310, doi:10.1029/2006GL028200, 2007 Click Here for Full Article

Coseismic and postseismic creep in the associated with the 2004 Sumatra-Andaman earthquake Hajime Kayanne,1 Yasutaka Ikeda,1 Tomoo Echigo,1,2 Masanobu Shishikura,3 Takanobu Kamataki,3,4 Kenji Satake,3 Javed N. Malik,5 Shaikh R. Basir,6 Gautam K. Chakrabortty,6 and Ashish K. Ghosh Roy6,7 Received 18 September 2006; revised 16 November 2006; accepted 1 December 2006; published 13 January 2007.

[1] Two field campaigns (March 2005 and March 2006) remote sensing and global positioning system (GPS) data for measurements of biological indicators and eyewitness indicate that the fault slip also occurred in this region accounts confirm that large coseismic and postseismic [Meltzner et al., 2006; Tobita et al., 2006]. Moreover, surface deformation occurred over the Andaman Islands in GPS data suggest that postseismic slip occurred within association with the 2004 Sumatra-Andaman earthquake. 1.5 months after the main shock [Subarya et al., 2006]. Amount of uplift was as large as 1.3 m in the islands off However, no on-site observations have previously been NW coast, and decreased to ESE with a zone of subsidence reported. in the SE Andaman. The coseismic deformation did not [3] We conducted field investigation in the Andaman generate large seismic waves or tsunamis, indicating that Islands to collect ground-truth data related to coseismic slip was much slower than that off Sumatra. The coseismic and postseismic surface deformation. To estimate the uplift was followed by postseismic subsidence, which was amount of vertical deformation, we mainly focused on evidenced by local residents to have occurred within two relative sea level changes associated with the earthquake months after the earthquake. A simple dislocation model as shown by biological indicators and described in the explains the deformation pattern by a coseismic rupture eyewitness accounts of local residents. Coseismic and (50 km wide with its down-dip limit 130 km east of the postseismic deformation was explained by a simple dislo- trench axis) followed by an up-dip propagation of cation model. the rupture front for about 10 km. Citation: Kayanne, H., Y. Ikeda, T. Echigo, M. Shishikura, T. Kamataki, K. Satake, J. N. 2. Methods Malik, S. R. Basir, G. K. Chakrabortty, and A. K. G. Roy (2007), Coseismic and postseismic creep in the Andaman Islands [4] The elevations of uplifted biological signatures associated with the 2004 Sumatra-Andaman earthquake, (Porites microatolls and oyster beds) and shoreline posi- Geophys. Res. Lett., 34, L01310, doi:10.1029/2006GL028200. tions indicated by local residents were measured during two field campaigns in March 2005 and March 2006. Elevations were measured using a total station or an auto 1. Introduction level with respect to the sea level at the time of the survey [2] The rupture area of the Sumatra-Andaman earthquake and then converted into elevations above/below the present of 26 December 2004 (Mw = 9.1–9.3) extended approxi- mean sea level (MSL) using the NAOTIDE tide prediction mately 1500 km. Analyses of seismic waves [Ammon et al., program [Matsumoto et al., 2000]. The amount of uplift 2005; Lay et al., 2005], tsunamis [Lay et al., 2005; Neetu et was estimated by the elevation above the reference level al., 2005], and crustal deformation [Subarya et al., 2006; for each signature. Meltzner et al., 2006; Tobita et al., 2006] all indicate that [5] Living Porites microatolls have flat upper surfaces the fault slip was as large as 30 m on the southern segment located at the low-water level [Subarya et al., 2006]. near the epicenter off Sumatra Island (Figure 1). However, Consequently, these levels were used to estimate the amount little is known of how the rupture terminated at the northern of uplift caused by the 2004 earthquake. The reference end. A paradox is that little slip was estimated on the fault elevation was the highest level of survival (HLS) of living beneath the Andaman Islands from seismic waves [Ammon Porites. The HLS was determined by measuring the top of et al., 2005] and tsunami [Fujii and Satake, 2007], but living Porites at eleven sites in March 2005 and 2006, and was 83 ± 4 cm below mean sea level. This level is 24 cm above the lowest water level occurring near midday 1 Department of Earth and Planetary Science, University of Tokyo, (À107 cm) and mirrors relationships documented in Tokyo, Japan. 2Now at Geo-Research Institute, Osaka, Japan. Sumatra [Zachariasen, 1998]. Differences in the elevations 3Active Fault Research Center, National Institute of Advanced of the upper levels of dead uplifted and living oyster beds Industrial Science and Technology, Tsukuba, Japan. were also measured. This is another biological indicator of 4Now at Tono Geoscience Center, Japan Atomic Energy Agency, Toki, the amount of uplift, but as the upper level of the oyster Japan. beds fluctuated by ±10 cm, the error using this indicator is 5Department of Civil Engineering, Indian Institute of Technology, Kanpur, . greater than that for the microatolls. 6Geological Survey of India, Kolkata, India. [6] Local residents were interviewed regarding time- 7Deceased 12 July 2006. series changes in the shoreline position during the spring high tide (highest high water level: HHWL) at new/full Copyright 2007 by the American Geophysical Union. moons on the survey date, and before and just after the 0094-8276/07/2006GL028200$05.00

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(< 50 km) is quite good although the deeper geometry is not well constrained.

3. Coseismic and Postseismic Deformation 3.1. Distribution of Vertical Deformation [8] Biological signatures of coseismic uplift are widely observable in the North and Middle Andaman Islands. Three months after the earthquake, we found recently dead, flat-topped corals (Porites microatolls) on the islands of North Reef and Interview. These corals were exposed above the low-water level (Figure 2), with their upper surfaces at the same elevation (47 ± 3 cm above mean sea level, Table S1 in the auxiliary material1). These corals were most likely emerged at the time of the 2004 earthquake. Emerged microatolls were also observed at the same level (48 ± 2 cm) at the northern coast of , 9 km south of North Reef Island. From the level of the microatoll surface above the present highest level of survival (HLS = 83 cm below mean sea level) of the same coral species, we estimated that an uplift of 1.3 m occurred (Figure 3). The amount of uplift decreased to 1.1 m on the eastern side of Interview Island. [9] Differences in the elevations of the upper levels of Figure 1. Inferred areas of fault slip (red) and tsunami living and dead oyster beds were also measured. The (blue) in the 2004 earthquake. The fault beneath the estimated amount of uplift decreased to 0.8 m along the Andaman Islands did not participate in tsunami generation, Austen Strait, 0.7 m at on the east coast of but a large slip appears to have occurred. northern Middle Andaman, and 0.6 m at . Our estimates based on biological indicators were checked using on-site GPS measurements at Diglipur. The amount of uplift earthquake. Only individuals who lived at the shore and between 2004 and April 2005 was measured by GPS as knew and checked the shoreline every day were inter- 63 cm [Earnestetal., 2005] and then recalculated as viewed. The variation among interviewees were ±10 cm 60.1 cm (K. Rajendran, personal communication, 2005). at each point. A high-tide marker placed by residents on the This value matches well with our estimate of 63 cm based survey date was used as the reference level. The earthquake on the oyster beds measurement. Farther to the southeast is (26 December 2004) took place during a period of compar- a broad zone of subsidence with its axis near atively high tides, but the tide level on this date was 25 cm [Singh et al., 2006; Malik and Murty, 2005]. lower than the highest annual level; therefore, the reference [10] Figure 3 illustrates the distribution of vertical defor- level just after the earthquake was 25 cm lower than the mation in the Andaman Islands based on our observations general reference. with reference to a general survey by the Geological Survey [7] A simple dislocation fault model [Okada, 1985] in a of India [Ray and Acharyya, 2005]. The contour lines of homogeneous, elastic half space was used to calculate coseismic uplift extend in the NNE–SSW direction, passing vertical displacements due to both co- and postseismic slip obliquely through the Andaman Islands. This direction on the plate interface. Amount of slip is assumed to be represents the strike of the rupture surface that extends uniform over both co- and postseismic ruptures. Secular beneath the Andaman Islands. The area of large uplift is vertical movements due to interplate coupling during inter- localized along the northwest coast of the Andaman Islands, seismic periods were calculated by using the conventional suggesting that the leading edge of the rupture was not far ‘‘back-slip’’ model [Savage, 1983] and were found to be from the coast. This coseismic surface deformation indicates negligible (<10 mm/yr) for modelling postseismic deforma- that the fault beneath the Andaman Islands did slip at the tion in the Andaman Islands. The calculated pattern of time of earthquake. Because seismic wave or tsunami vertical displacements is sensitive to the geometry of plate analyses indicate little slip here [Ammon et al., 2005; Fujii interface. However, the geometry of the Wadati-Benioff and Satake, 2007], the rupture process must be too slow to zone beneath the Andaman Islands has been poorly con- generate these waves. strained because of the lack of local seismic networks. We therefore used aftershock data observed with an ocean- 3.2. Postseismic Deformation bottom seismographic (OBS) network in the southern part [11] Eyewitness accounts of local residents living on the of the 2004 source area [Araki et al., 2006] to determine the shores of indicate postseismic geometry of plate interface. Although the area of the OBS subsidence following coseismic uplift. We interviewed local observation is far from the Andaman region, sea-floor residents at three sites in Mayabunder and one site on topography of these two areas is quite similar. We employed a simple elastic plate model [Watts, 2001] and fitted it to the aftershock distribution data. The model fit at shallow depths 1Auxiliary material data sets are available at ftp://ftp.agu.org/apend/gl/ 2006gl028200. Other auxiliary material files are in the HTML.

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Figure 2. A microatoll that was uplifted during the 2004 earthquake at North Reef Island off the western coast of the Andaman Islands. This photo was taken in March 2005.

Interview Island regarding temporal changes in the shore- line position. Some noted that the shoreline receded at the time of the earthquake and subsequently returned gradually without reaching its pre-earthquake position. All informants reported the same general trend, i.e. large oceanward retreat of the shoreline immediately after the earthquake followed by landward transgression (Figure S1). [12] At Mayabunder, we estimated a coseismic uplift of 1.0 m and postseismic subsidence of 0.3 m with an error of 0.1 m (Figure 4a and Table S2). We observed an uplift of 0.7 m in March 2005, which showed the residual amount of

Figure 4. (a) Changes in relative sea level after the 2004 earthquake based on interviews of Mayabunder residents. (b) Dislocation fault model of temporal changes in vertical displacement. The rupture areas and corresponding vertical displacement are shown with blue (coseismic), red (post- seismic) and black (co- and postseismic) lines, respectively. NR, North Reef; II, Interview Island; MB, Mayabunder; and PB, Port Blair.

uplift. The subsidence at Mayabunder ceased by March 2005, judging from a benchmark that was established in March 2005 and resurveyed in March 2006. The postseis- mic subsidence at Mayabunder must have occurred within 2 months after the earthquake and then stabilized. The uplift amount estimated from microatolls and oyster beds in March 2005 or 2006 (Figure 3) also shows the residual Figure 3. Distribution of vertical displacement in the one after the postseismic subsidence. Further eyewitness Andaman Islands. The amount of residual (coseismic + accounts from southeastern Interview Island indicate coseis- postseismic) uplift remaining after March 2005 (in meters) mic uplift of 1.8 m and postseismic subsidence of 0.7 m is based on assessment of emerged coral microatolls and (Table S2). oyster beds. The subsidence of À0.95 m at Port Blair is from Singh et al. [2006]. The uplift and subsidence contour 4. Up-Dip Propagation of the Rupture Front lines (red) are based on our observations with reference to a general survey by the Geological Survey of India [Ray and [13] We fitted the dislocation model to the observed data Acharyya, 2005]. to infer the rupture extent and slip amount. The coseismic

3of4 L01310 KAYANNE ET AL.: POSTSEISMIC CREEP IN THE ANDAMAN ISLANDS L01310 rupture is about 50 km wide with its down-dip limit 130 km Briggs, R. W., et al. (2006), Deformation and slip along the Sunda mega- thrust in the great 2005 Nias-Simeulue earthquake, Science, 311, 1897– east of the trench axis at a depth of 23 km. The amount of 1901. dip slip on it is 6.2 ± 1.2 m (Figure 4b). It is noteworthy that Byrne, D. E., D. M. Davis, and L. R. Sykes (1988), Loci and maximum size the rupture surface beneath the Andaman Islands is nearly of thrust earthquakes and the mechanics of the shallow region of sub- duction zones, Tectonics, 7, 833–857. identical in extent and depth to the locked plate interface off Earnest, A., C. P. Rajendran, K. Rajendran, R. Anu, G. M. Arun, and P. M. central Sumatra [Sieh et al., 1999]. Mohan (2005), Near-field observations on the co-seismic deformation [14] The coseismic rupture ended up-dip beneath the associated with the 26 December 2004 Andaman-Sumatra earthquake, western coast of Middle Andaman, which generated the Curr. Sci., 89, 1237–1244. Fujii, Y., and K. Satake (2007), Tsunami source of the 2004 Sumatra- large surface uplift in a zone from North Reef Island (NR) Andaman earthquake inferred from tide gauge and satellite data, Bull. to Mayabunder (MB). The observed postseismic deforma- Seismol. Soc. Am, 97, S192–S207. tion is best explained by up-dip propagation of the rupture Jo´nsson, S., P. Segall, R. Pedersen, and G. Bjornsson (2003), Post- earthquake ground movements correlated to pore-pressure transients, front for about 10 km as illustrated by Figure 4b. The Nature, 424, 179–183. dislocation model predicts that as the rupture front prop- Lay, T., et al. (2005), The great Sumatra-Andaman earthquake of agates up-dip, the large uplift area shifts toward the trench, 26 December 2004, Science, 308, 1127–1133. Malik, J. N., and C. V. R. Murty (2005), Landscape changes in Andaman and postseismic subsidence occurs in the zone from NR to and Nicobar Islands (India) due to Mw 9.3 tsunamigenic Sumatra earth- MB, while Port Blair (PB) remains almost stable. The quake of 26 December 2004, Curr. Sci., 88, 1384–1386. observed large uplift followed by subsidence on Interview Matsumoto, K., T. Takanezawa, and M. Ooe (2000), Ocean tide models Island (II) and in MB, and the coseismic subsidence in PB is developed by assimilating TOPEX/POSEIDON altimeter data into hydrodynamical model: A global and a regional model around Japan, explained by this up-dip propagation of the rupture front. J. Oceanogr., 56, 567–581. [15] Alternative mechanisms for such rapid subsidence Meltzner, A. J., K. Sieh, M. Abrams, D. C. Agnew, K. W. Hudnut, J. P. include viscoelastic relaxation in the asthenosphere and Avouac, and D. H. Natawidjaja (2006), Uplift and subsidence associated with the great Aceh-Andaman earthquake of 2004, J. Geophys. Res., 111, poroelastic relaxation due to pore-fluid flow in the upper B09101, doi:10.1029/2005JB003897. crust [Jo´nsson et al., 2003]. Both mechanisms predict Moore, G. E., J. R. Curry, and E. J. Emmel (1982), Sedimentation in the postseismic uplift in areas that have subsided coseismically. Sunda Trench and forearc region, in Trench-Forearc Geology: Sedimen- However, the tide gauge record at Port Blair did not tation and Tectonics on Modern and Ancient Active Plate Margins, edited by J. K. Leggett, Geol. Soc. Spec. Publ., 10, 245–258. indicate such postseismic uplift. Comparison of far-field Moore, J. C., and P. Vrolijk (1992), Fluids in accretionary prisms, Rev. (continuous) and near-field (campaign) GPS data also Geophys., 30, 113–135. suggests postseismic up-dip propagation, but cannot con- Neetu, S., I. Suresh, R. Shankar, D. Shankar, S. S. C. Shenoi, S. R. Shetye, D. Sundar, and B. Nagarajan (2005), Comment on ‘‘The great Sumatra- firm it due to the lack of data close to the trench [Subarya et Andaman earthquake of 26 December 2004’’, Science, 310, 1431. al., 2006]. Since the 2005 Nias-Simeulue earthquake, Okada, Y. (1985), Surface deformation due to shear and tensile faults in a similar postseismic deformation with a decay time of half-space, Bull. Seismol. Soc. Am., 75, 1135–1154. Ray, S. K., and A. Acharyya (2005), 26 December 2004 earthquake: 1.5 months has been detected by near-field GPS observa- Coseismic vertical ground movements in the Andaman, Spec. Publ. tions and has been attributed to up-dip propagation of the Ser. Geol. Surv. India, 89, 71–90. rupture front [Briggs et al., 2006]. Savage, J. C. (1983), A dislocation model of strain accumulation and release at a subduction zone, J. Geophys. Res., 88, 4984–4996. [16] The up-dip limit of the 2004 coseismic rupture Sieh, K., S. N. Ward, D. Natawidjaja, and B. W. Suwargadi (1999), Crustal seems to be controlled by the presence of a shallow, deformation at the Sumatran subduction zone revealed by coral rings, unconsolidated, accretionary wedge [Moore et al., 1982] Geophys. Res. Lett., 26, 3141–3144. (Figure 4b). Shallow accretionary wedges are generally Singh, S. K., M. Ortiz, H. K. Gupta, and D. G. A. Ramadass (2006), Slow slip below Port Blair, Andaman, during the great Sumatra-Andaman aseismic [Byrne et al., 1988] and are floored by detachment earthquake of 26 December 2004, Geophys. Res. Lett., 33, L03313, faults, the behaviour of which is not fully understood doi:10.1029/2005GL025025. [Moore and Vrolijk, 1992]. Creep slip on a shallow detach- Subarya, C., M. Chlieh, L. Prawirodirdjo, J. P. Avouac, Y. Bock, K. Sieh, A. J. Meltzner, D. H. Natawidjaja, and R. McCaffrey (2006), Plate- ment in the Sunda subduction zone off central Sumatra was boundary deformation associated with the great Sumatra-Andaman suggested by Sieh et al. [1999], but has not been fully earthquake, Nature, 440, 46–51. evidenced by observations. Our observations, however, Tobita, M., H. Suito, T. Imakiire, M. Kato, S. Fujiwara, and M. Murakami suggest that massive faulting at seismogenic depths triggers (2006), Outline of vertical displacement of the 2004 and 2005 Sumatra earthquakes revealed by satellite radar imagery, Earth Planets Space, 58, slow postseismic slip on a detachment fault. e1–e4. Watts, A. B. (2001), Isostasy and Flexure of the Lithosphere,458pp., Cambridge Univ. Press, New York. [17] Acknowledgments. The field survey was funded by the Ministry Zachariasen, J. (1998), Paleoseismology and paleogeodesy of the Sumatran of Education, Culture, Sports, Science and Technology, Japan; and by the subduction zone: A study of vertical deformation using coral microatolls, Indian Institute of Technology, Kanpur, India. We thank D. Balaji, Ph.D. thesis, 418 pp., Calif. Inst. of Technol., Pasadena. Andaman Public Works Division, for overall arrangements during our field survey; the Andaman administration and police for providing us with ÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀÀ necessary help and permissions; M. Tobita, Geographical Survey Institute, S. R. Basir and G. K. Chakrabortty, Geological Survey of India, Eastern Japan, B. L. N. Kenett, Australian National University, and B. F. Atwater, Region, DK-6 Block, Sector-I, Salt Lake, Kolkata, 700-091, India. U.S. Geological Survey, for valuable discussion and comments; and T. Kato, T. Echigo, Geo-Research Institute, 4-3-2 Itachibori, Nishi-ku, Osaka Earthquake Research Institute, University of Tokyo, for encouragement and 550-0012, Japan. support. Y. Ikeda and H. Kayanne, Department of Earth and Planetary Science, University of Tokyo, Hongo 7-3-1, Bunkyo-ku, Tokyo 113-0033, Japan. ([email protected]) References T. Kamataki, Tono Geoscience Center, Japan Atomic Energy Agency, Ammon, C. J., et al. (2005), Rupture process of the 2004 Sumatra- 959-31 Jorinji, Izumicho, Toki 509-5102, Japan. Andaman earthquake, Science, 308, 1133–1139. J. N. Malik, Department of Civil Engineering, Indian Institute of Araki, E., M. Shinohara, K. Obana, T. Yamada, Y. Kaneda, T. Kanazawa, Technology, Kanpur 208-016 UP, 208-016, India. and K. Suyehiro (2006), Aftershock distribution of the 26 December K. Satake and M. Shishikura, Active Fault Research Center, AIST, 1-1-1 2004 Sumatra-Andaman earthquake from ocean bottom seismographic Higashi, Tsukuba 305-8567, Japan. observation, Earth Planets Space, 58, 113–119.

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