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Constraints on rupture of the December 26, 2004, Sumatra earthquake from far-field GPS observations
Earth and Planetary Science Letters 237 (2005) 673 – 679 www.elsevier.com/locate/epsl
Constraints on rupture of the December 26, 2004, Sumatra earthquake from far-field GPS observations Joshi K. Catherine, Vineet K. Gahalaut *, Vipul K. Sahu National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India Received 20 April 2005; received in revised form 23 June 2005; accepted 5 July 2005 Available online 10 August 2005 Editor: R.D. van der Hilst
Abstract We use Global Positioning System (GPS) data from nine permanent GPS sites surrounding the epicentre of the December 26, 2004, Sumatra earthquake to infer coseismic displacements at these sites. The results suggest that GPS site at SAMP, Medan in Sumatra Island, the nearest site from the epicentre, experienced a westward coseismic horizontal displacement of about 14 cm, while sites in southern India, namely, HYDE and IISC experienced predominantly eastward coseismic horizontal displacement of about 6–11 mm. We estimated the slip on the mainshock rupture and rupture extent in a uniform half space by analysing these coseismic displacements. We estimate an average reverse slip of about 11 m on the southern part of the rupture and an oblique slip of about 10 m on the northern part of the 1200 100–175-km2 rupture. Our results of estimated coseismic horizontal displacement in the Andaman and Nicobar region using the above rupture parameters are also consistent with the near-field GPS measured coseismic displacements in the region. D 2005 Elsevier B.V. All rights reserved. Keywords: GPS; coseismic displacement; Sumatra earthquake; coseismic slip
1. Introduction The December 26, 2004, Indian Ocean earthquake (Mw ~ 9.3), the greatest earthquake in past 40 years [1–4], nucleated just north of Simeulue island, off the western coast of northern Sumatra, Indonesia. The region lies at the western end of the earthquake belt along the subduction zone that accounts for 81% of
* Corresponding author. E-mail address: [email protected] (V.K. Gahalaut). 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.07.012
the world’s largest earthquakes. In the region of the earthquake, the Indian plate moves in the NNE direction at a rate of about 5 cm/year [3,5]. This results in oblique convergence at the Sunda and Andaman trench. The oblique motion is partitioned into thrustfaulting, which occurs on the frontal part of the subduction zone, and strike–slip faulting, which occurs a few hundred kilometres to the east of the trench in the back-arc region on Sumatra Fault System (SFS) [6,7]. The focal mechanism of the mainshock suggests thrust motion on a gently NE-dipping plane. Available models of the earthquake rupture suggest that during
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this earthquake, about 1200 km of subduction plate boundary in the Sumatra and Andaman–Nicobar region slipped up to 20 m [1,8,9]. High slip occurred in the northern Sumatra region, which decreased in the NNE and N direction. The slip did not occur instantaneously but took place in two phases over a period of several minutes [3,8,10–12]. The earthquake has prompted several investigators to analyse and estimate the size of the earthquake, slip on rupture, its duration, speed, spatial rupture extent, possible cause of its nucleation and Tsunami generation [1–12]. In this article, we analyse GPS data from far-field sites to see the effect of the earthquake and to provide constraints on the rupture and slip on it.
2. GPS data and analysis We used the GPS data from the permanent sites, which are located around the epicentre. The nearest site, SAMP, at Medan on the eastern coast of Sumatra, lies at about 330 km east of the epicentre. We used data from permanent GPS sites at SAMP, NTUS, BAKO, COCO, KUNM, PIMO, IISC, HYDE, LHAS, DGAR, MALD, SEY1 and KIT3 (Fig. 1). We processed daily observation files from these sites using GAMIT, GLOBK/GLORG [13,14] to estimate the coordinates of these sites in ITRF2000. We assumed that site KIT3, at Kitab in Uzbekistan, which lies very far away from the epi-
Fig. 1. Permanent GPS sites, identified with four letter words, around the December 26, 2004, Sumatra earthquake, shown with a star. Black arrows at the sites, tipped with 95% confidence ellipse, indicate the coseismic displacement derived from the GPS data. White arrows denote the computed coseismic horizontal displacement at that site. The scale for these arrows is given at the right bottom of the figure. Contours of computed horizontal displacement (in meters) due to slip on rupture are also shown. Note that the contour interval is not uniform. The black and white arrows in the Andaman–Nicobar region show the observed [21,22] and computed displacements at sites. The scale for these arrows is shown at the bottom left of the figure. Red dots denote the aftershocks of the mainshock.
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Table 1 Comparison of the observed and calculated coseismic displacement Site Name SAMP NTUS PIMO KUNM IISC HYDE COCO BAKO DGAR
Observed displacement Latitude 3.62 1.34 14.63 25.03 13.02 17.41 12.18 6.49 7.27
Longitude 98.71 103.68 121.08 102.79 77.57 78.55 96.83 106.85 72.37
Eoffset (mm)
Noffset (mm)
140 20 10 6 10 6 – – –
centre, is unaffected by the earthquake, and hence, this site was tightly constrained in our analysis. We estimated coseismic horizontal displacements at all these sites by estimating the change in the north and east coordinates averaged over 4 days before and 4 days after the earthquake (Table 1). Fig. 2 shows the change in east and north components of coordinates of GPS sites. For sites SEY1, LHAS and MALD, sufficient data before and after the earthquake were not available; hence, their time series are not shown here. Freymueller [15], Hashimoto [16] and Khan and Gudmundsson [17] reported similar changes at SAMP, NTUS and BAKO. Banerjee et al. [10] reported changes at several sites and our results are consistent with them.
3. Modelling of coseismic horizontal displacement derived from GPS data We used elastic dislocation formulation [18] to compute the displacement due to uniform slip on a rectangular rupture. The analytical expressions have been derived assuming the earth to be a flat homogenous elastic half space. From the observed coseismic horizontal displacement, we estimated the best fit model for slip on the rupture and rupture extent in the north and south direction by testing the range of models. The width of rupture in Andaman region is assumed to be 100 km which is justified from the observed subsidence and uplift in the Andaman region. After the earthquake, subsidence of about 1 m has been observed on the eastern coast of Andaman and Nicobar region which we noticed in the
13 0 4 6 0 3 – – –
Computed displacement Ej (mm)
Nj (mm)
6.5 3.2 4.3 3.7 3.0 2.9 3.5 3.4 3.8
2.5 1.7 2.1 2.0 1.7 1.6 1.9 1.9 2.2
E component (mm) 136 25 9 7 14 8 1.5 3 8
N component (mm) 9 0 3 7 1 1 1 0 4
apparent increase in the sea level during our GPS field work after the earthquake in January 2005 [19,20]. Based on the reports of newly formed beaches, uplift has been inferred from the North Sentinel Island, which lies at about 60–70 km west of the eastern coast of the Southern Andaman island [19,20]. These observations imply that the island belt, particularly its eastern coast, lies close to the downdip edge of the rupture, where coseismic subsidence is expected due to reverse slip on the rupture, while North Sentinel island lies close to the updip edge of the rupture where coseismic uplift is expected. These observations are consistent with the assumed width of 100 km of the rupture in the Andaman (Fig. 3). The width of the rupture is assumed to increase up to 175 km in the south direction [3,8,10]. We assumed that the rupture had a dip of about 88, with its updip edge lying near the trench at 4 km depth. We started with a model in which the rupture was confined in the northern Sumatra region. The analysis of coseismic data at permanent GPS sites required that rupture extended up to a length of 1200 km with its northern and southern edges lying close to the 148 and 38 latitudes, respectively. Considering the geometry of the rupture and trench [7], inferred from aftershocks zone and bathymetry, respectively, we divided the entire rupture into three segments. The northernmost rupture segment of 400 100 km2 occupies regions of Andaman Islands. We estimated an oblique slip of 10 m (6 and 8 m as right lateral and reverse slip, respectively) on this segment. The middle segment of 450 140 km2 occupies the region of Nicobar Islands. The slip on this segment is estimated to be
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Fig. 2. Change in East and North components of coordinates of GPS sites. Note the change in scale in displacement for SAMP. Arrow indicates the earthquake of December 26, 2004. Step in the bold line shows the computed coseismic displacement at the site.
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Fig. 3. East–west profile showing the bathymetry (scale on left) and the computed coseismic elevation changes (scale on right) in the Andaman region, corresponding to rupture widths of 90, 100, 110, 120 and 130 km and an assumed reverse slip of 8 m on the rupture. The two shaded rectangles show the reported uplift and subsidence in the N. Sentinel and Andaman islands, respectively [19,20].
about 10.8 m (3 and 9 m as right lateral and reverse slip, respectively). The 11.2 m of slip on the southernmost segment of 350 175 km2 is predominantly reverse. The oblique slip on the northernmost segment makes the orientation of slip vector uniform along the entire earthquake rupture. The dextral slip component in slip on the northern rupture is essential, as this segment of the rupture is more oblique to the plate motion than the southern segment. The seismic moment, M o of our model is 5.3 1022 N m which yields M w = 9.1 for this earthquake. We calculated coseismic horizontal displacement assuming flat earth using the above rupture parameters. Estimated coseismic horizontal displacement at each site has been shown in Fig. 2 with a step which shows the magnitude of horizontal displacement in north and east directions. The computed values of coseismic horizontal displacement show a good agreement with the observed displacement derived from the GPS data (Table 1). It can be seen that the sites that lie to the west of the rupture and in the footwall (e.g., IISC, HYDE and DGAR) show eastward movement, while those that lie to the east and in the hanging wall (e.g., SAMP, NTUS, PIMO and KUNM), show westward movement, which is consistent with the predominantly thrust motion during this earthquake. The sites located on the northern and southern side of the rupture (e.g., COCO, BAKO) show almost null movement. The displacement observed at these far-field GPS sites is the plate motion deficit which accumulated through several years during the interseismic period of strain accumulation for this earthquake at the subduction boundary.
4. Discussion 4.1. Homogenous half space model vs. spherical layered earth model In our analysis, we have used homogenous half space to analyse the far-field GPS data. We agree that at great distances (N 400 km), it is implied that the earth should be considered spherical, which may change the estimates of slip on the rupture. However, sites located close to the rupture, such as SAMP, NTUS and COCO, will not be affected significantly. The predicted horizontal displacement at sites located at the far distance will be less using a half space model than that using a spherical earth model, for the same slip on the rupture in two models [10]. Thus, using a half space model, one will overestimate the slip on rupture. Introduction of layers and consideration of gentler rupture plane increase the slip estimate on the rupture [10]. 4.2. Near-field GPS data After the Sumatra earthquake, several organisations in India, e.g., Survey of India, Dehradun; CMMACS Bangalore [21] and CESS Trivandrum [22] have remeasured some of their previously established GPS sites in the Andaman and Nicobar region. These measurements suggest that sites in Nicobar islands shifted by 5–6 m and those in Andaman islands by 3–5 m towards SW due to the earthquake. We used these observations in our analysis. Our rupture model predicts coseismic displacements at
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these sites which is consistent with these observations (Fig. 1). One of the limitations in our analysis is that we had to consider the gentler dip for the rupture plane. Though the adopted value of 88 is consistent with the dip inferred from the fault plane solution of the 2004 Sumatra earthquake, the general dip of the subduction zone under Sumatra is reported to be about 208 [23]. In our model, steeper rupture increases farfield displacements [10]. Thus, consideration of a steeper rupture leads to lower estimate of rupture slip, which is inconsistent with the near-field GPS data. We agree that this problem can be overcome by considering the spherical earth. Low estimate of M w as 9.1 in our case is mainly due to this problem.
5. Conclusion The 2004 Sumatra earthquake caused permanent horizontal displacement as far as 4000 km from its epicentre, which are discernible in the GPS data at permanent sites. It caused horizontal displacement of 3–6 m in the Andaman–Nicobar region. Using an elastic half space earth model, we suggest that average slip of more than 10 m on a 1200 150-km2 rupture occurred to cause such widespread and large displacements. However, for accurate modelling of the far-field GPS data, the spherical earth model should be used.
Acknowledgement We thank Mark Murray, Maurizio Battaglia, Sridevi Jade and Roland Bu¨rgmann for introducing us to GPS data processing using GMIT/GLOBK. We greatly appreciate comments from Roland Bu¨rgmann and Roger Bilham. This and our GPS field work in Andaman–Nicobar region was financially supported by DST. We acknowledge support from Director, NGRI and R.K. Chadha.
References [1] S. Stein, E.A. Okal, Speed and size of the Sumatra earthquake, Nature 434 (2005) 581 – 582.
[2] K. Sieh, What happened and what’s next? Nature 434 (2005) 573 – 574. [3] T. Lay, et al., The great Sumatra–Andaman earthquake of 26 December 2004, Science 308 (2005) 1127 – 1133. [4] J. Park, et al., Earth’s free oscillations excited by the 26 December 2004 Sumatra–Andaman earthquake, Science 308 (2005) 1139 – 1144. [5] M. Simoes, J.P. Avouac, R. Cattin, P. Henry, The Sumatra subduction zone: a case for a locked fault zone extending into the mantle, J. Geophys. Res. 109 (2004) B10402, doi:10.1029/ 2003JB002958. [6] T.J. Fitch, Plate convergence, transcurrent faults and internal deformation adjacent to southeast Asia and the western Pacific, J. Geophys. Res. 77 (1972) 4432 – 4460. [7] J.R. Curray, Tectonics and history of the Andaman sea region, J. Asian Earth Sci. 25 (2005) 187 – 228. [8] C.J. Ammon, et al., Rupture process of the 2004 Sumatra– Andaman earthquake, Science 308 (2005) 1133 – 1139. [9] S. Ni, H. Kanamori, D. Helmberger, Energy radiation from the Sumatra earthquake, Nature 434 (2005) 582. [10] P. Banerjee, F.F. Pollitz, R. Bu¨rgmann, The size and duration of the Sumatra–Andaman earthquake from far-field static offsets, Science 308 (2005) 1769 – 1772. [11] F. Kru¨ger, M. Ohrnberger, Tracking the rupture of the Mw = 9.3 Sumatra earthquake over 1,150 km at teleseismic distance, Nature 435 (2005) 937 – 939. [12] M. Ishii, P.M. Shearer, H. Houston, J.E. Vidale, Extent, duration and speed of the 2004 Sumatra–Andaman earthquake imaged by the Hi-Net array, Nature 435 (2005) 933 – 936. [13] R. King, Y. Bock, Documentation for the GAMIT GPS analysis software, Release 9.94, Mass. Inst. of Technol., Cambridge, MA, and Scripps Inst. of Oceanogr. La Jolla, CA (2000). [14] T. Herring, Global Kalman Filter VLBI and GPS Analysis Program (GLOBK), version 5.0, Mass. Inst. of Technol, Cambridge, MA, 2000. [15] J. Freymueller, Preliminary analyses of the 12/26/2004 Mw 9.0 Earthquake in Indonesia, http://www.unavco.org/research_ science/science_highlights/sumatra_2004/sumatra_2004.html (2005). [16] M. Hashimoto, Crustal deformations associated with the Sumatra earthquake on December 26, 2004, http://www. rcep.dpri.kyoto-u.ac.jp/~hasimoto/Manabu/sumatraEQ_e.htm (2005). [17] S.A. Khan, O. Gudmundsson, GPS analyses of the Sumatra– Andaman earthquake. EOS Transactions, AGU. 86 (9) (2005) 89,94. [18] Y. Okada, Internal deformation due to shear and tensile faults in a half space, Bull. Seismol. Soc. Am. 82 (1992) 1018 – 1040. [19] V.K. Gahalaut, R.K. Chadha, Sumatra tsunamogenic earthquake of 26 December 2004, Deep Cont. Stud.—Dept. Sci. Technol. Newsl. 15 (1) (2005 (January)) 18 – 20. [20] R. Bilham, E.R. Engdahl, N. Feldl, S.P. Satyabala, Partial and complete rupture of the Indo-Andaman plate boundary 1847–2004, Seismol. Res. Lett. 76 (2005) 299 – 311.
J.K. Catherine et al. / Earth and Planetary Science Letters 237 (2005) 673–679 [21] J. Sridevi, M.B. Ananda, P. Dileep Kumar, S. Banerjee, Coseismic and postseismic displacements in Andaman and Nicobar Islands from GPS measurements, Curr. Sci. 88 (2005) 1980 – 1984. [22] C.P. Rajendran, A. Earnest, K. Rajendran, R. Bilham, J. Freymueller, A slow Mw z 8.7 earthquake embedded in the
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Andaman segment of the Great 2004 Sumatra rupture. Nature (submitted for publication). [23] H.J. Scho¨ffel, S. Das, Fine details of the Wadati–Benioff zone under Indonesia and its geodynamic implications, J. Geophys. Res. 104 (1999) 13101 – 13114.
Constraints on rupture of the December 26, 2004, Sumatra earthquake from far-field GPS observations Joshi K. Catherine, Vineet K. Gahalaut *, Vipul K. Sahu National Geophysical Research Institute, Uppal Road, Hyderabad 500 007, India Received 20 April 2005; received in revised form 23 June 2005; accepted 5 July 2005 Available online 10 August 2005 Editor: R.D. van der Hilst
Abstract We use Global Positioning System (GPS) data from nine permanent GPS sites surrounding the epicentre of the December 26, 2004, Sumatra earthquake to infer coseismic displacements at these sites. The results suggest that GPS site at SAMP, Medan in Sumatra Island, the nearest site from the epicentre, experienced a westward coseismic horizontal displacement of about 14 cm, while sites in southern India, namely, HYDE and IISC experienced predominantly eastward coseismic horizontal displacement of about 6–11 mm. We estimated the slip on the mainshock rupture and rupture extent in a uniform half space by analysing these coseismic displacements. We estimate an average reverse slip of about 11 m on the southern part of the rupture and an oblique slip of about 10 m on the northern part of the 1200 100–175-km2 rupture. Our results of estimated coseismic horizontal displacement in the Andaman and Nicobar region using the above rupture parameters are also consistent with the near-field GPS measured coseismic displacements in the region. D 2005 Elsevier B.V. All rights reserved. Keywords: GPS; coseismic displacement; Sumatra earthquake; coseismic slip
1. Introduction The December 26, 2004, Indian Ocean earthquake (Mw ~ 9.3), the greatest earthquake in past 40 years [1–4], nucleated just north of Simeulue island, off the western coast of northern Sumatra, Indonesia. The region lies at the western end of the earthquake belt along the subduction zone that accounts for 81% of
* Corresponding author. E-mail address: [email protected] (V.K. Gahalaut). 0012-821X/$ - see front matter D 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.epsl.2005.07.012
the world’s largest earthquakes. In the region of the earthquake, the Indian plate moves in the NNE direction at a rate of about 5 cm/year [3,5]. This results in oblique convergence at the Sunda and Andaman trench. The oblique motion is partitioned into thrustfaulting, which occurs on the frontal part of the subduction zone, and strike–slip faulting, which occurs a few hundred kilometres to the east of the trench in the back-arc region on Sumatra Fault System (SFS) [6,7]. The focal mechanism of the mainshock suggests thrust motion on a gently NE-dipping plane. Available models of the earthquake rupture suggest that during
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this earthquake, about 1200 km of subduction plate boundary in the Sumatra and Andaman–Nicobar region slipped up to 20 m [1,8,9]. High slip occurred in the northern Sumatra region, which decreased in the NNE and N direction. The slip did not occur instantaneously but took place in two phases over a period of several minutes [3,8,10–12]. The earthquake has prompted several investigators to analyse and estimate the size of the earthquake, slip on rupture, its duration, speed, spatial rupture extent, possible cause of its nucleation and Tsunami generation [1–12]. In this article, we analyse GPS data from far-field sites to see the effect of the earthquake and to provide constraints on the rupture and slip on it.
2. GPS data and analysis We used the GPS data from the permanent sites, which are located around the epicentre. The nearest site, SAMP, at Medan on the eastern coast of Sumatra, lies at about 330 km east of the epicentre. We used data from permanent GPS sites at SAMP, NTUS, BAKO, COCO, KUNM, PIMO, IISC, HYDE, LHAS, DGAR, MALD, SEY1 and KIT3 (Fig. 1). We processed daily observation files from these sites using GAMIT, GLOBK/GLORG [13,14] to estimate the coordinates of these sites in ITRF2000. We assumed that site KIT3, at Kitab in Uzbekistan, which lies very far away from the epi-
Fig. 1. Permanent GPS sites, identified with four letter words, around the December 26, 2004, Sumatra earthquake, shown with a star. Black arrows at the sites, tipped with 95% confidence ellipse, indicate the coseismic displacement derived from the GPS data. White arrows denote the computed coseismic horizontal displacement at that site. The scale for these arrows is given at the right bottom of the figure. Contours of computed horizontal displacement (in meters) due to slip on rupture are also shown. Note that the contour interval is not uniform. The black and white arrows in the Andaman–Nicobar region show the observed [21,22] and computed displacements at sites. The scale for these arrows is shown at the bottom left of the figure. Red dots denote the aftershocks of the mainshock.
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Table 1 Comparison of the observed and calculated coseismic displacement Site Name SAMP NTUS PIMO KUNM IISC HYDE COCO BAKO DGAR
Observed displacement Latitude 3.62 1.34 14.63 25.03 13.02 17.41 12.18 6.49 7.27
Longitude 98.71 103.68 121.08 102.79 77.57 78.55 96.83 106.85 72.37
Eoffset (mm)
Noffset (mm)
140 20 10 6 10 6 – – –
centre, is unaffected by the earthquake, and hence, this site was tightly constrained in our analysis. We estimated coseismic horizontal displacements at all these sites by estimating the change in the north and east coordinates averaged over 4 days before and 4 days after the earthquake (Table 1). Fig. 2 shows the change in east and north components of coordinates of GPS sites. For sites SEY1, LHAS and MALD, sufficient data before and after the earthquake were not available; hence, their time series are not shown here. Freymueller [15], Hashimoto [16] and Khan and Gudmundsson [17] reported similar changes at SAMP, NTUS and BAKO. Banerjee et al. [10] reported changes at several sites and our results are consistent with them.
3. Modelling of coseismic horizontal displacement derived from GPS data We used elastic dislocation formulation [18] to compute the displacement due to uniform slip on a rectangular rupture. The analytical expressions have been derived assuming the earth to be a flat homogenous elastic half space. From the observed coseismic horizontal displacement, we estimated the best fit model for slip on the rupture and rupture extent in the north and south direction by testing the range of models. The width of rupture in Andaman region is assumed to be 100 km which is justified from the observed subsidence and uplift in the Andaman region. After the earthquake, subsidence of about 1 m has been observed on the eastern coast of Andaman and Nicobar region which we noticed in the
13 0 4 6 0 3 – – –
Computed displacement Ej (mm)
Nj (mm)
6.5 3.2 4.3 3.7 3.0 2.9 3.5 3.4 3.8
2.5 1.7 2.1 2.0 1.7 1.6 1.9 1.9 2.2
E component (mm) 136 25 9 7 14 8 1.5 3 8
N component (mm) 9 0 3 7 1 1 1 0 4
apparent increase in the sea level during our GPS field work after the earthquake in January 2005 [19,20]. Based on the reports of newly formed beaches, uplift has been inferred from the North Sentinel Island, which lies at about 60–70 km west of the eastern coast of the Southern Andaman island [19,20]. These observations imply that the island belt, particularly its eastern coast, lies close to the downdip edge of the rupture, where coseismic subsidence is expected due to reverse slip on the rupture, while North Sentinel island lies close to the updip edge of the rupture where coseismic uplift is expected. These observations are consistent with the assumed width of 100 km of the rupture in the Andaman (Fig. 3). The width of the rupture is assumed to increase up to 175 km in the south direction [3,8,10]. We assumed that the rupture had a dip of about 88, with its updip edge lying near the trench at 4 km depth. We started with a model in which the rupture was confined in the northern Sumatra region. The analysis of coseismic data at permanent GPS sites required that rupture extended up to a length of 1200 km with its northern and southern edges lying close to the 148 and 38 latitudes, respectively. Considering the geometry of the rupture and trench [7], inferred from aftershocks zone and bathymetry, respectively, we divided the entire rupture into three segments. The northernmost rupture segment of 400 100 km2 occupies regions of Andaman Islands. We estimated an oblique slip of 10 m (6 and 8 m as right lateral and reverse slip, respectively) on this segment. The middle segment of 450 140 km2 occupies the region of Nicobar Islands. The slip on this segment is estimated to be
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Fig. 2. Change in East and North components of coordinates of GPS sites. Note the change in scale in displacement for SAMP. Arrow indicates the earthquake of December 26, 2004. Step in the bold line shows the computed coseismic displacement at the site.
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Fig. 3. East–west profile showing the bathymetry (scale on left) and the computed coseismic elevation changes (scale on right) in the Andaman region, corresponding to rupture widths of 90, 100, 110, 120 and 130 km and an assumed reverse slip of 8 m on the rupture. The two shaded rectangles show the reported uplift and subsidence in the N. Sentinel and Andaman islands, respectively [19,20].
about 10.8 m (3 and 9 m as right lateral and reverse slip, respectively). The 11.2 m of slip on the southernmost segment of 350 175 km2 is predominantly reverse. The oblique slip on the northernmost segment makes the orientation of slip vector uniform along the entire earthquake rupture. The dextral slip component in slip on the northern rupture is essential, as this segment of the rupture is more oblique to the plate motion than the southern segment. The seismic moment, M o of our model is 5.3 1022 N m which yields M w = 9.1 for this earthquake. We calculated coseismic horizontal displacement assuming flat earth using the above rupture parameters. Estimated coseismic horizontal displacement at each site has been shown in Fig. 2 with a step which shows the magnitude of horizontal displacement in north and east directions. The computed values of coseismic horizontal displacement show a good agreement with the observed displacement derived from the GPS data (Table 1). It can be seen that the sites that lie to the west of the rupture and in the footwall (e.g., IISC, HYDE and DGAR) show eastward movement, while those that lie to the east and in the hanging wall (e.g., SAMP, NTUS, PIMO and KUNM), show westward movement, which is consistent with the predominantly thrust motion during this earthquake. The sites located on the northern and southern side of the rupture (e.g., COCO, BAKO) show almost null movement. The displacement observed at these far-field GPS sites is the plate motion deficit which accumulated through several years during the interseismic period of strain accumulation for this earthquake at the subduction boundary.
4. Discussion 4.1. Homogenous half space model vs. spherical layered earth model In our analysis, we have used homogenous half space to analyse the far-field GPS data. We agree that at great distances (N 400 km), it is implied that the earth should be considered spherical, which may change the estimates of slip on the rupture. However, sites located close to the rupture, such as SAMP, NTUS and COCO, will not be affected significantly. The predicted horizontal displacement at sites located at the far distance will be less using a half space model than that using a spherical earth model, for the same slip on the rupture in two models [10]. Thus, using a half space model, one will overestimate the slip on rupture. Introduction of layers and consideration of gentler rupture plane increase the slip estimate on the rupture [10]. 4.2. Near-field GPS data After the Sumatra earthquake, several organisations in India, e.g., Survey of India, Dehradun; CMMACS Bangalore [21] and CESS Trivandrum [22] have remeasured some of their previously established GPS sites in the Andaman and Nicobar region. These measurements suggest that sites in Nicobar islands shifted by 5–6 m and those in Andaman islands by 3–5 m towards SW due to the earthquake. We used these observations in our analysis. Our rupture model predicts coseismic displacements at
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J.K. Catherine et al. / Earth and Planetary Science Letters 237 (2005) 673–679
these sites which is consistent with these observations (Fig. 1). One of the limitations in our analysis is that we had to consider the gentler dip for the rupture plane. Though the adopted value of 88 is consistent with the dip inferred from the fault plane solution of the 2004 Sumatra earthquake, the general dip of the subduction zone under Sumatra is reported to be about 208 [23]. In our model, steeper rupture increases farfield displacements [10]. Thus, consideration of a steeper rupture leads to lower estimate of rupture slip, which is inconsistent with the near-field GPS data. We agree that this problem can be overcome by considering the spherical earth. Low estimate of M w as 9.1 in our case is mainly due to this problem.
5. Conclusion The 2004 Sumatra earthquake caused permanent horizontal displacement as far as 4000 km from its epicentre, which are discernible in the GPS data at permanent sites. It caused horizontal displacement of 3–6 m in the Andaman–Nicobar region. Using an elastic half space earth model, we suggest that average slip of more than 10 m on a 1200 150-km2 rupture occurred to cause such widespread and large displacements. However, for accurate modelling of the far-field GPS data, the spherical earth model should be used.
Acknowledgement We thank Mark Murray, Maurizio Battaglia, Sridevi Jade and Roland Bu¨rgmann for introducing us to GPS data processing using GMIT/GLOBK. We greatly appreciate comments from Roland Bu¨rgmann and Roger Bilham. This and our GPS field work in Andaman–Nicobar region was financially supported by DST. We acknowledge support from Director, NGRI and R.K. Chadha.
References [1] S. Stein, E.A. Okal, Speed and size of the Sumatra earthquake, Nature 434 (2005) 581 – 582.
[2] K. Sieh, What happened and what’s next? Nature 434 (2005) 573 – 574. [3] T. Lay, et al., The great Sumatra–Andaman earthquake of 26 December 2004, Science 308 (2005) 1127 – 1133. [4] J. Park, et al., Earth’s free oscillations excited by the 26 December 2004 Sumatra–Andaman earthquake, Science 308 (2005) 1139 – 1144. [5] M. Simoes, J.P. Avouac, R. Cattin, P. Henry, The Sumatra subduction zone: a case for a locked fault zone extending into the mantle, J. Geophys. Res. 109 (2004) B10402, doi:10.1029/ 2003JB002958. [6] T.J. Fitch, Plate convergence, transcurrent faults and internal deformation adjacent to southeast Asia and the western Pacific, J. Geophys. Res. 77 (1972) 4432 – 4460. [7] J.R. Curray, Tectonics and history of the Andaman sea region, J. Asian Earth Sci. 25 (2005) 187 – 228. [8] C.J. Ammon, et al., Rupture process of the 2004 Sumatra– Andaman earthquake, Science 308 (2005) 1133 – 1139. [9] S. Ni, H. Kanamori, D. Helmberger, Energy radiation from the Sumatra earthquake, Nature 434 (2005) 582. [10] P. Banerjee, F.F. Pollitz, R. Bu¨rgmann, The size and duration of the Sumatra–Andaman earthquake from far-field static offsets, Science 308 (2005) 1769 – 1772. [11] F. Kru¨ger, M. Ohrnberger, Tracking the rupture of the Mw = 9.3 Sumatra earthquake over 1,150 km at teleseismic distance, Nature 435 (2005) 937 – 939. [12] M. Ishii, P.M. Shearer, H. Houston, J.E. Vidale, Extent, duration and speed of the 2004 Sumatra–Andaman earthquake imaged by the Hi-Net array, Nature 435 (2005) 933 – 936. [13] R. King, Y. Bock, Documentation for the GAMIT GPS analysis software, Release 9.94, Mass. Inst. of Technol., Cambridge, MA, and Scripps Inst. of Oceanogr. La Jolla, CA (2000). [14] T. Herring, Global Kalman Filter VLBI and GPS Analysis Program (GLOBK), version 5.0, Mass. Inst. of Technol, Cambridge, MA, 2000. [15] J. Freymueller, Preliminary analyses of the 12/26/2004 Mw 9.0 Earthquake in Indonesia, http://www.unavco.org/research_ science/science_highlights/sumatra_2004/sumatra_2004.html (2005). [16] M. Hashimoto, Crustal deformations associated with the Sumatra earthquake on December 26, 2004, http://www. rcep.dpri.kyoto-u.ac.jp/~hasimoto/Manabu/sumatraEQ_e.htm (2005). [17] S.A. Khan, O. Gudmundsson, GPS analyses of the Sumatra– Andaman earthquake. EOS Transactions, AGU. 86 (9) (2005) 89,94. [18] Y. Okada, Internal deformation due to shear and tensile faults in a half space, Bull. Seismol. Soc. Am. 82 (1992) 1018 – 1040. [19] V.K. Gahalaut, R.K. Chadha, Sumatra tsunamogenic earthquake of 26 December 2004, Deep Cont. Stud.—Dept. Sci. Technol. Newsl. 15 (1) (2005 (January)) 18 – 20. [20] R. Bilham, E.R. Engdahl, N. Feldl, S.P. Satyabala, Partial and complete rupture of the Indo-Andaman plate boundary 1847–2004, Seismol. Res. Lett. 76 (2005) 299 – 311.
J.K. Catherine et al. / Earth and Planetary Science Letters 237 (2005) 673–679 [21] J. Sridevi, M.B. Ananda, P. Dileep Kumar, S. Banerjee, Coseismic and postseismic displacements in Andaman and Nicobar Islands from GPS measurements, Curr. Sci. 88 (2005) 1980 – 1984. [22] C.P. Rajendran, A. Earnest, K. Rajendran, R. Bilham, J. Freymueller, A slow Mw z 8.7 earthquake embedded in the
679
Andaman segment of the Great 2004 Sumatra rupture. Nature (submitted for publication). [23] H.J. Scho¨ffel, S. Das, Fine details of the Wadati–Benioff zone under Indonesia and its geodynamic implications, J. Geophys. Res. 104 (1999) 13101 – 13114.