JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112, B10402, doi: /2007jb004928, 2007

JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 112,, doi:10.1029/2007jb004928, 2007 Burma plate motion Vineet K. Gahalaut 1 and Kalpna Gahalaut 1 Received 5 January 2007; revised 11 June 2007; accepted 13 July 2007; published 4 October 2007. [1] Plate motion in the Indo-Burmese Arc-Andaman-Sumatra region of Burma plate is poorly resolved. This is mainly due to lack of relevant data and complex tectonics of the region. We analyze (1) azimuths of coseismic displacements due to the 2004 Sumatra- Andaman and 2005 Nias earthquakes; (2) estimates of interseismic deformation in the Indo-Burmese Arc, Andaman, Sumatra, and Sagaing Fault regions (all based on GPS measurements); (3) long-term plate motion rates across Sumatra Fault System, Sagaing Fault, and Andaman Sea from geomorphological and other geophysical studies, and (4) the earthquake focal mechanisms in the region. We suggest that the SSW motion of Sunda plate with respect to Indian plate may be partitioned into the dextral strike-slip motion across the Sagaing Fault in the north and Sumatra Fault System in south in the back-arc region, and the arc-normal motion across the Sumatra subduction zone, which becomes oblique in Andaman and southern Indo-Burmese Arc region and dextral in the northern Indo-Burmese Arc region of the fore arc. Under the rigid plate approximation, we estimate a pole for India-Burma plate pair at 27 ± 1 N, 82 ± 1.1 E with an angular velocity of 0.845 ± 0.12 /Ma and for Burma-Sunda at 22.3 ± 1.1 N, 109.3 ± 2.5 E with an angular velocity of 0.67 ± 0.12 /Ma. Thus the plate motion in the northern and southern regions of Burma plate, namely, the Indo-Burmese Arc and Andaman-Sumatra Arc, may be explained by a single pole and does not require a boundary between the two. Citation: Gahalaut, V. K., and K. Gahalaut (2007), Burma plate motion, J. Geophys. Res., 112,, doi:10.1029/2007jb004928. 1. Introduction [2] Plate motions in many of the active deformation zones are poorly resolved and can be widely distributed [Bird, 2003]. This has also led to increase in defining the number of plates. In this article we focus on the region east of the eastern margin of the Indian plate. Extensive GPS measurements in Myanmar, Thailand, Indonesia, Malaysia etc. have indicated that the plate motion in this part differs from that of Eurasian plate and it requires a different pole, which moves with a higher angular velocity [Michel et al., 2000, 2001; Becker et al., 2000; Bock et al., 2003; Vigny et al., 2003; Socquet et al., 2006]. This block has been referred as Sunda plate. The western margin of the Sunda plate converges obliquely with India-Australia plate. The relative motion between the Sunda and India-Australia plates is partitioned between the predominantly arc-normal subduction in the fore arc and dextral motion along the Sumatra Fault System in Sumatra region and Sagaing Fault in the back-arc region [Fitch, 1972; McCaffrey, 1992]. The region bounded by the fore and back arc has been referred by various names, e.g., Burma platelet [Guzman-Speziale and Ni, 1996]; Burma microplate [Stein and Okal, 2005]; Andaman microplate [DeShon et al., 2005]; small Burma plate [Bird, 2003]; Burma or Myanmar sliver plate [Curray, 2005, 1 National Geophysical Research Institute, Hyderabad, India. Copyright 2007 by the American Geophysical Union. 0148-0227/07/2007JB004928$09.00 Socquet et al., 2006], Burma subplate [Lay et al., 2005], and Burma plate [Briggs et al., 2006], etc. Hereinafter, we refer it as Burma plate. Many investigators have suggested that the Burma plate terminates in the north near 16 N latitude, north of Andaman, [Bird, 2003; Stein and Okal, 2005; DeShon et al., 2005] and have estimated the pole of the India-Burma plate pair near the northern termination of the plate [Bird, 2003] or slightly east of it [Stein and Okal, 2005], whereas others have considered that the region lying further north, between the Indo-Burmese Arc and Sagaing Fault is also a part of it [Curray et al., 1979; Nielsen et al., 2004; Curray, 2005; Vigny et al., 2005; Lay et al., 2005; Briggs et al., 2006; Socquet et al., 2006]. It appears from the above that the motion and extent of Burma plate are not well constrained which has led to poor understanding of the plate kinematics and earthquake occurrence processes in the region. This has also led to poor assessment of the seismic hazard in the region. [3] In this article, we examine the GPS measurements of coseismic and interseismic deformation rates, long-term plate convergence rates, and earthquake focal mechanisms to understand the plate kinematics, partitioning of India- Sunda plate convergence and to define poles for India- Burma and Burma-Sunda plate pairs. 2. Prelude [4] For the definition of Burma plate, we follow Curray [2005] and define Burma plate sandwiched between India and Sunda plate. It is bounded in the east by Sumatra Fault 1of9

System in its southern part and Sagaing Fault in its northern part. In the west it is bounded by Sumatra Andaman fore-arc subduction zone in its southern part and Indo-Burmese Arc in its northern part [Nielsen et al., 2004]. Thus, according to this definition, the geographical region of Burma (now Myanmar) is also included in Burma plate. Further, we analyze all GPS measurements in the fore-arc region with reference to Indian plate. To estimate plate motion of Sunda plate with reference to Indian plate, we use the pole at 20.2 N and 26.1 E with an angular velocity of 0.37 /Ma [Socquet et al., 2006] that has been derived from extensive GPS measurements spanning 11 a from 190 sites in Asia, Nepal, and eastern Indonesia including Myanmar. 3. GPS Measurements of Coseismic Displacements [5] The 2004 Sumatra-Andaman and 2005 Nias earthquakes are among the best GPS monitored great subduction zone earthquakes [Jade et al., 2005; Earnest et al., 2005; Subarya et al., 2006; Gahalaut et al., 2006; Gahalaut and Catherine, 2006; Briggs et al., 2006; Kreemer et al., 2006; Vigny et al., 2005]. GPS measurements before and after the two great earthquakes provided the most robust estimates of coseismic displacements and slip on the earthquake ruptures. The two earthquakes caused horizontal displacements reaching to 6 m in approximately WSW direction, Figure 1 and Table 1 [Subarya et al., 2006; Gahalaut et al., 2006; Gahalaut and Catherine, 2006; Briggs et al., 2006; Kreemer et al., 2006]. We considered the azimuths of these large coseismic displacements at sites located above the rupture only and avoided sites, which lie on the edges of the rupture or far from it. This procedure ensures that the azimuth of coseismic displacement vectors represent the azimuth of coseismic slip vectors. In fact the coseismic displacement azimuths at sites above the rupture have been found to be consistent with that of the rake of the modeled coseismic slip on the rupture [e.g., Subarya et al., 2006; Gahalaut et al., 2006; Gahalaut and Catherine, 2006; Briggs et al., 2006; Kreemer et al., 2006; Chlieh et al., 2007]. In the Nias region, the direction of coseismic displacement is almost perpendicular to the NW SE trending trench, while in Andaman it becomes oblique as the trench becomes N S (Figure 1). At some sites the direction of coseismic displacement differs from the nearby sites, e.g., at a site, HB (Table 1) in Little Andaman shows displacement in SW direction, whereas sites in the regions lying north or south of it, show motion in WSW direction. This could be due to slip variation on the rupture. We ignore this site in our analysis. GPS measurements of postseismic deformation at sites located in the source zone of the two earthquakes in the Andaman, Nicobar and Sumatra regions indicate that deformation continues to occur approximately in the same direction [Gahalaut et al., 2006; Banerjee et al., 2007; Hsu et al., 2006; V. K. Gahalaut et al., Afterslip in the Andaman-Nicobar region following the 26 December 2004 earthquake, manuscript in preparation, 2007]. [6] The azimuths of interseismic deformation in the Andaman-Sumatra region (discussed in section 4) agree, within ±5, with that of coseismic deformation. The consistency in the azimuths of coseismic, postseismic, and interseismic deformation suggests that the average azimuth of the coseismic displacements in the region may be considered as the direction of frontal arc motion with respect to Indian plate. We agree that this assumption may not be entirely correct as these measurements span a short time period. However, we emphasize that since the measurements are made in three essential and integral phases of deformation cycle, the average azimuth of the deformation should represent azimuth of fore-arc motion. In fact our subsequent analysis prove that the above assumption is true, at least in this case. 4. GPS Measurements of Interseismic Deformation and Other Long-Term Rates [7] Interseismic deformation rates have been reported from Myanmar, Andaman and Sumatra regions. Vigny et al. [2003] and Socquet et al. [2006] reported GPS measurements of interseismic deformation across the Sagaing Fault and at a few sites west of it. These observations have been used to estimate interseismic slip rate across the Sagaing Fault and Indo-Burmese Arc [Socquet et al., 2006; Sahu et al., 2006]. It has been reported that out of relative plate motion of about 36 mm/a between the Sunda and Indian plates at about 20 N latitude, dextral motion of about 18 mm/a occurs across the Sagaing Fault. This estimate is consistent with the long-term estimate of 18 mm/a derived from neotectonic studies [Bertrand et al., 1998]. The remaining motion of about 20 23 mm/a is accommodated obliquely across the Indo-Burmese Arc, through dextral motion of about 18 mm/a and thrust motion of about 13 mm/a [Socquet et al., 2006]. [8] Extensive GPS measurements have been undertaken off the western coast of Sumatra. The measurements taken during 1989 2001 are consistent with strain accumulation along the fore arc [Prawirodirdjo et al., 1997; Bock et al., 2003]. With respect to Indian plate the direction of motion at sites located on the island belt is generally toward WSW and perpendicular to the local strike of the trench axis (Figure 1). McCaffrey [2002] analyzed these GPS observations and used a variable slip model in which maximum interseismic slip was about 50 mm/a. Simoes et al. [2004] analyzed these and uplift rates derived from coral dating. They estimated that a long-term slip rate of 44 68 mm/a on the Main Thrust Zone (MTZ) of the subduction zone is required to explain these data. [9] Paul et al. [2001] and Jade [2004] reported campaign mode GPS measurements during 1996 1999, at a single site, CARI, near Port Blair, Andaman, which suggest an interseismic deformation rate of 13.3 ± 3 mm/a toward N250 with respect to India. If we consider the locking of MTZ, having a dip of 12 and width of 120 km, to be full, it corresponds to a convergence slip rate of about 30 ± 6 mm/a toward N250. Gahalaut et al. [2006] reported another set of measurements at Port Blair by Survey of India, made in 1994 1995 and April 2004. The 1995 2004 observations at Port Blair provide a relative velocity of about 63 ± 6 mm/a toward N277 with reference to Indian plate. Large errors in the estimate are due to the shorter duration (less than 7 hours) of GPS measurements during the 1995 campaign and also due to single measurement. Though the two estimates vary significantly in their magnitudes, probably reflecting intense variation in strain accumulation both in 2of9

Figure 1. Simplified tectonic map of the eastern margin of the Indian plate. Solid black arrows denote the coseismic displacements due to the 26 December 2004 Sumatra-Andaman earthquake [Gahalaut et al., 2006; Subarya et al., 2006]. Sites from northern Sumatra having less displacement (<1 m) have been ignored. Grey arrows show the coseismic displacements due to 28 March 2005 Nias earthquake [Gahalaut and Catherine, 2006; Briggs et al., 2006; Kreemer et al., 2006]. The red arrow in the Andaman shows interseismic deformation during 1996 1999 at a site near Port Blair [Paul et al., 2001; Jade, 2004]. Red arrows in Nias region show interseismic deformation during 1989 1993. Pairs of pink arrows denote direction of horizontal principal stress, estimated from the inversion of focal mechanisms of earthquakes that occurred before the 2004 Sumatra-Andaman earthquake, whereas pairs of purple arrows of smaller size denote direction of horizontal principal stress, estimated from the inversion of focal mechanisms of aftershocks of the two great earthquakes. Blue rectangle denotes the location of Sagaing Fault GPS network [Vigny et al., 2003], which yielded a dextral motion of 18 mm/a of the Sagaing Fault. Blue arrows, marked with rates of 38 and 23 mm/a, denote the motion of Sunda plate and Indo-Burmese Arc with reference to Indian plate [Socquet et al., 2006]. Numbers in the fore-arc region denote rates of plate convergence with reference to Indian plate, derived from GPS measurements during interseismic period. Numbers in the back-arc region denote opening rate in the Andaman Sea (16 38 mm/a [Kamesh Raju et al., 2004]) and rate of dextral motion across Sumatra Fault System (25 mm/a). The inset on top left shows the location of Sunda and other nearby plates. The arrows show plate motion in no net rotation [Kreemer et al., 2003]. 3of9

Table 1. GPS Site Description and Horizontal Coseismic Displacements at Sites Where It Exceeds 1 m a Observed Displacement Site Region Site Code Longitude Latitude Displacement, m b deg Horizontal Coseismic Displacements Due to 26 December 2004 Earthquake 1 North Andaman Aerial Bay AB 93.027 13.278 4.75 235 2 North Andaman East Island EI 93.047 13.631 4.41 235 3 Middle Andaman Long Island LI 92.932 12.376 2.25 241 4 Middle Andaman Udaygarh UG 92.773 12.216 2.91 235 5 Middle Andaman Govindgarh GG 92.983 12.036 1.66 235 6 South Andaman Port Blair PB 92.721 11.649 3.24 251 7 South Andaman Passage Island PI 92.676 11.178 3.14 248 8 c Little Andaman Hut Bay HB 92.569 10.696 4.21 231 9 Car Nicobar Car Nicobar CN 92.804 9.225 6.47 243 10 Nicobar Islands Teresa Island TI 93.124 8.302 6.61 242 11 Nicobar Islands Kardip KD 93.549 8.036 4.33 246 12 Nicobar Islands Miroe Island MI 93.541 7.514 5.67 240 13 Great Nicobar Campbell Bay CB 93.934 7.004 4.73 240 14 Sumatra region R178 95.333 5.858 2.05 231 15 Sumatra region R176 95.057 5.713 2.76 232 16 Sumatra region K515 95.487 5.568 2.13 231 17 Sumatra region K505 95.271 5.480 2.71 230 18 Sumatra region K504 95.243 5.433 2.75 230 19 Sumatra region PIDI 95.933 5.331 1.69 235 20 Sumatra region R175 95.203 5.241 3.19 230 21 Sumatra region R174 95.365 4.842 3.67 229 22 Sumatra region R173 95.518 4.607 3.71 230 23 Nias region R171 95.387 2.959 5.77 222 Horizontal Coseismic Displacements Due to 28 March 2005 Earthquake 24 Nias region BSIM 96.326 2.409 2.34 230 25 Nias region LHWA 97.134 1.383 4.53 223 a Coseismic displacements at sites 1 13 are reported by Gahalaut et al. [2006], 14 23 by Subarya et al. [2006], and 24 25 by Gahalaut and Catherine [2006]. b Error in these estimates does not exceed 5 [Gahalaut et al., 2006; Gahalaut and Catherine, 2006; Subarya et al., 2006; Chlieh et al., 2007]. c We did not consider the azimuth of coseismic displacement at this site in our analysis as it shows motion toward SW, whereas other nearby sites show motion toward WSW. time and space, they are almost consistent in direction. However, possibility of error in this estimate appears strong as it is based on only two measurements with one of them spanning only seven hours of measurements. Thus we prefer the estimate by Paul et al. [2001]. [10] Geologic rates of crustal deformation in Andaman Sea and across Sumatra Fault System have also been reported. Kamesh Raju et al. [2004] analyzed magnetic anomalies and reported an opening rate of 16 38 mm/a in the Andaman Sea region. Sieh and Natawidjaja [2000] estimated dextral motion of about 25 mm/a across the Sumatra Fault System relying on offsets of geomorphic features, with a minimum of about 10 mm/a in the southern Sumatra and 37 mm/a north of Sumatra. 5. Analysis: India-Burma and Burma-Sunda Plate Pair Poles [11] Dextral motion in the back arc and thrust motion in the fore arc, which becomes oblique in Andaman and Indo- Burmese Arc, appear to be consistent with the view that the fore arc of Burma plate moves clockwise with respect to India with a pole located in central India. We performed a grid search in which we estimated the location of pole. The angular motion along this pole predicts such a linear motion in the fore arc, which provides minimum discrepancy between the average azimuth of reported coseismic and interseismic displacements and the direction of predicted displacement. Considering the scatter in the azimuths of coseismic displacement vectors, we considered their average directions, which are N230, N242 and N240 in the fore-arc regions of Sumatra (at 3 N, 96 E), Nicobar (at 8 N, 93 E) and Andaman (at 13 N, 93 E). In the Indo-Burmese Arc we considered an azimuth of N215 at 20 N, 94 E [Socquet et al., 2006]. As discussed earlier, at least one site, HB, in Little Andaman shows significant deviation in the azimuth of coseismic displacement from that in the northern and southern part, we ascribe this deviation to heterogeneity in the coseismic slip on the earthquake rupture and do not use this observation. For the assumed pole location within the search region, we computed the difference in the predicted and observed azimuth of displacement vector for each region, e.g., for the Indo-Burmese Arc region, we assumed the pole location within the search region and for each pole location we computed the azimuth of the predicted linear displacement at 20 N and 94 E and then plotted the difference between the predicted azimuth and the observed azimuth of N215 (Figure 2a). We performed such grid search for all the regions and then averaged the difference to obtain a common minimum (Figure 2e). The magnitude of rotation is constrained by the available estimate of 23 mm/a across the Indo-Burmese Arc [Socquet et al., 2006], 30 mm/a across Andaman and about 40 mm/a across Sumatra frontal arc region. The grid search approach suggests that the fore-arc motion can be explained by a rotation of 0.845 ± 0.13 /Ma through a pole located at 27.05 ± 1 N and 82.05 ± 1.1 E. The error bars in location correspond to an average 5 discrepancy in the azimuth of 4of9

Figure 2. Results of grid search for estimating the India-Burma pole. We computed azimuth of motion in the fore-arc regions of (a) North Sumatra (at 3 N, 96 E), (b) Nicobar (at 8 N, 93 E), (c) Andaman (at 13 N, 93 E), and (d) Indo-Burmese arc (at 20 N, 94 E) by assuming poles located at every 1 interval within the search region. The difference in degree between the azimuths of predicted displacement and observed average direction of coseismic and/or interseismic displacement (N215, N240, N242, and N230 in Indo-Burmese Arc, Andaman, Nicobar, and north Sumatra region) is contoured. (e) Average discrepancy. The minimum occurs at 27.05 N and 82.05 N, which we adopt as the preferred location of pole for India-Burma plate pair. GPS measurements [e.g., Chlieh et al., 2007; Briggs et al., 2006] and that in the magnitude of rotation to an uncertainty of ±5 mm/a in the slip rate estimate in various regions. A higher rate will provide correspondingly higher rate in the Sumatra fore-arc region, which may be consistent with the reported rate but it may also cause higher rate across Indo- Burmese Arc making it inconsistent with what is reported [Socquet et al., 2006]. [12] We added the India-Sunda [Socquet et al., 2006] and the above estimated India-Burma Euler pole and calculated the Burma-Sunda pole at 22.3 ± 1.1 N and 109.3 ± 2.5 E with an angular velocity of 0.67 ± 0.12 /Ma. The Burma- Sunda pole predicts velocity in the back arc (Figure 3), which are consistent with the reported results. The pole predicts a dextral motion of 28 mm/a on the Sumatra Fault System, a dextral motion of 16 17 mm/a on the Sagaing Fault and an opening rate of 21 mm/a in the Andaman Sea. These rates are consistent with the reported estimates of long-term motion in these regions [Sieh and Natawidjaja, 5of9

Figure 3. (left) Prediction of the fore- and back-arc motion corresponding to the India-Burma pole at 27.05 N, 82.05 E with an angular velocity of 0.845 /Ma and Burma-Sunda pole at 22.3 N, 109.3 E with an angular velocity of 0.67 ± 0.12 /Ma. India-Burma pole is shown with 1s error, which approximately corresponds to 5 scatter in the azimuth of coseismic displacement vectors. The motion of Sunda with respect to India is calculated using the pole at 20.2 N and 26.1 E with an angular velocity of 0.37 /Ma [Socquet et al., 2006]. (right) Partitioning of India-Sunda motion in Indo-Burmese Arc, Andaman- Nicobar, and Nias-Sumatra regions. 2000; Vigny et al., 2003; Socquet et al., 2006; Bertrand et al., 1998; Kamesh Raju et al., 2004]. [13] We emphasize here that we did not use the deformation rate data from back-arc region to constrain the location of any of the poles and motion across them. Thus the agreement between the predicted and reported values of motion in the back-arc region further establishes the soundness and appropriateness of our model of poles in the region. It also validates the assumption of considering the azimuths of the interseismic, coseismic, and postseismic deformation to be the azimuth of long-term plate convergence in the fore-arc region. The partitioning of the rate of Sunda plate with respect to Indian plate into the rate of Burma plate with respect to Indian plate (i.e., fore-arc motion) and the rate of Burma plate with respect to Sunda plate (i.e., back-arc motion) is also shown in Figure 3. Partitioning in the Indo-Burmese Arc region is consistent with that by Vigny et al. [2003], Nielsen et al. [2004], and Socquet et al. [2006]. 6. Stress Directions From Plate Motion and Earthquake Focal Mechanisms [14] Here we assess whether the derived poles will cause the right kind of orientation of principal stresses that would produce slip in the observed direction on the faults. The plate motion in the fore-arc region of Burma plate, derived from the India-Burma pole should produce a stress state in 6of9

Table 2. Results of Inversion of Focal Mechanisms for Estimating the Principal Stress Directions a Fore Arc: Subduction Zone and Indo-Burmese Arc s 1 s 2 s 3 Back Arc: Sumatra Fault System, Andaman Sea, and Sagaing Fault s 1 s 2 s 3 Indo-Burmese region (14 26 N) 206, 20 311, 36 93, 47 41, 4 157, 82 310, 7 Andaman region (10 14 N) 260, 13 356, 24 143, 63 248, 39 41, 48 147, 14 Aftershocks 243, 40 101, 43 351, 20 - - - Nicobar-Sumatra (3 10 N) 204, 29 302, 15 57, 57 12, 1 277, 80 102, 10 Aftershocks 166, 7 263, 43 69, 46 210, 67 16, 23 108, 5 Sumatra (3 N 3 S) 219, 42 313, 5 48, 47 186, 22 291, 33 68, 48 Aftershocks 224, 28 314, 1 45, 62 - - - a For inversion, earthquake focal mechanisms for the period from 1973 to 25 December 2004 (1 day prior to the 2004 Sumatra-Andaman earthquake) have been used. We also inverted aftershock focal mechanism solutions. which maximum principal stress (s 1 ) should be almost NNE SSW oriented to cause a predominantly dextral motion in the northern Indo-Burmese Arc region, NE SW oriented to cause oblique (thrust plus dextral) and thrust motion in Andaman and Sumatra region, respectively. Similarly, in the back-arc region the Burma-Sunda pole should produce a stress state in which s 1 is NNE SSW oriented to cause dextral motion on Sagaing Fault and a subvertical s 1 to cause a tensile regime in the Andaman Sea where opening is reported, and almost N S oriented s 1 in the Sumatra Fault System region to cause dextral motion along it. We verify whether the available earthquake focal mechanism solutions are consistent with this view. [15] We used earthquake focal mechanism solutions from centroid moment tensor (CMT) catalogue (auxiliary material 1 Figure S1) to estimate the directions of principal stress in various parts of Burma plate. In the back-arc region of Sagaing Fault and Sumatra Fault System, predominantly dextral motion occurs on the approximately N S to NW SE oriented nodal planes. In the Andaman sea, earthquakes occur through normal and strike-slip motion on predominantly NE SW oriented planes. In the fore arc, predominantly thrust motion occurs in the Andaman- Nicobar and Sumatra region. However, a few earthquakes with normal motion have been reported in Sumatra region. These regions approximately coincide with the zones of transtension, which formed during the geological evolution of the Sumatra Fault System [Sieh and Natawidjaja, 2000]. The West Andaman Fault (WAF) in the Sumatra offshore region is characterized by thrust and strike-slip faulting and has been considered as a lithospheric-scale boundary which probably controlled the eastern limit of the 2004 Sumatra- Andaman earthquake rupture [Singh et al., 2005; Kamesh Raju et al., 2007]. In north, in the Indo-Burmese Arc region, strike-slip and thrust motion-type earthquakes are reported [Guzman-Speziale and Ni, 1996; Rao and Kumar, 1999; Satyabala, 2003; Rao and Kalpna, 2005]. We carefully segregated the earthquakes in fore and back-arc regions having distinct focal mechanisms and used the linear least squares inversion approach [Michael, 1984, 1987] to estimate the best fitting maximum (s 1 ), intermediate (s 2 )andminimum (s 3 ) stress directions (Figure 1 and Table 2). In the back-arc region, approximately NNE SSW direction of s 1 is consistent with dextral motion on the Sagaing Fault and 1 Auxiliary materials are available in the HTML. doi:10.1029/ 2007JB004928. Sumatra Fault System. In Andaman sea, we show subhorizontal s 3,whichalongwithsteeps 1 is consistent with the predominant normal motion and opening up of Andaman Sea. In the Sumatra fore-arc region, s 1 is almost perpendicular to the trench. However, in the Nicobar region it is very oblique to the trench axis. This could be because of the fact that in this region the distance between the back arc and fore arc is the least (Figure S1), and hence there could be problem in segregating the fault plane solutions in the two regions if the epicentral locations of the earthquakes contain large errors. Hence there could be some influence of earthquakes of the back-arc region on the fore-arc region due to the mixing up of the two. It could be more so as the earthquakes are more frequent in the back-arc region. In the Andaman, the direction of s 1 suggests predominance of thrust faulting, rather than oblique faulting. However, the direction is similar to the coseismic displacement due to the 2004 earthquake and with that of the interseismic deformation at CARI [Paul et al., 2001; Jade, 2004]. Further, north in the Indo-Burmese Arc, direction of s 1 is NNE SSW, which is consistent with the predominant dextral motion along N S planes and thrust motion across E W planes. We inverted the focal mechanisms of the aftershocks of the two earthquakes also (Table 2). Directions of s 1 and s 3 do not change appreciably in this case, except in the fore-arc region of Nicobar. In Andaman region the azimuth of s 1 is more consistent with azimuth of estimated coseismic displacements. In Sumatra and back-arc region of Nicobar, it does not change in any significant way, whereas in Nicobar, direction of s 1 is almost parallel to the trench axis. [16] In summary, the principal stress directions in the fore and back-arc regions, derived from the earthquake focal mechanisms, are generally (except in Nicobar region) consistent with that expected to arise from the motion predicted by the two poles. Thus the principal stress directions are generally consistent with the coseismic displacements of the two earthquakes and the direction of convergence derived from GPS measurements in the interseismic period. We suggest that the derived stress directions from the focal mechanisms may contain relatively large errors. An error in location of earthquake may lead to poor segregation of focal mechanisms in the fore and back arc. Uncertainties of ±15 in trend and ±5 in plunge of the slip vector from an individual earthquake are typical in the CMT focal mechanisms [McCaffrey, 1992]. We may expect similar error in our analysis of principal stress determination. 7of9

[17] McKenzie [1969] pointed out that it is not necessary that the stress state derived from the inversion of focal mechanisms should be consistent with the present-day motion as the present-day motion on these faults is the motion along the preexisting faults. Thus the stress state inferred from the focal mechanisms may not be consistent with the fault kinematics. It is more appropriate to estimate the orientation of principal stress, which is consistent with the estimated or measured direction of slip on the fault. In the present case, the direction of slip on faults appears to be consistent with the principal stress directions estimated from focal mechanisms. 7. Concluding Discussion [18] We suggest that a single pole located in north central India can explain the relative motion between the Burma fore arc and the Indian plate. The model does not require that the Andaman and Sumatra fore arc be terminated in the north at about 16 N or that the fore arc is fragmented into independent blocks along these two regions. The analysis allowed us to partition the India-Sunda motion into fore and back-arc motion of Burma plate that are consistent with that estimated in these regions. The model implies that part of the oblique convergence between India and Sunda plates in Andaman and southern Indo-Burmese Arc region is actually accommodated through oblique subduction in the fore-arc region of Burma plate. Thus the slip partitioning in this region is partial. Convergence in the northern Indo-Burmese Arc region is accommodated through predominantly dextral strike-slip motion. Increasing obliqueness of subduction in the north probably is one of the reasons why the 2004 earthquake rupture terminated north of Andaman. [19] Relocated aftershocks of the 2004 Sumatra-Andaman and 2005 Nias earthquakes show a sharp boundary at about 1 2 N latitude [DeShon et al., 2005]. DeShon et al. [2005] proposed this as the southern extent of the Burma plate, which is about 50 100 km further northward of that proposed earlier [Bird, 2003]. From our analysis we could not address this issue, but we are of the opinion that in the southern direction, the Burma plate extends at least to this region. Since the pole estimated here for the Burma-India plate pair can also explain the convergence rate in the island belt, west of Sumatra, there may be a possibility that the plate extends further south, till the southern tip of the Sumatra and Sumatra Fault System near 7 S. [20] The equatorial region of Indian Ocean that lie west of Sunda trench is now referred as the diffused plate boundary zone between the India and Australia plates [Gordon et al., 1990; Deplus, 2001; DeMets et al., 2005]. The zone is as wide as 1500 km [Chamot-Rooke et al., 1993; Krishna et al., 1998]. Relative motion of Australia with reference to Indian plate in the Simeulue and Nias islands region is no more than 1 cm/a toward N10 E, which increases to about 3 cm/a on the Australian mainland [Delescluse and Chamot- Rooke, 2007]. Off the west coast of Sumatra, though the relative motion is small, it implies that motion here is neither purely Australian nor purely Indian [Vigny et al., 2005; Delescluse and Chamot-Rooke, 2007]. Thus it may imply that our model, based on India-Sunda plate motion, represents plate motion in the northern Sumatra and further north of it more appropriately as in the region south of it, plate motion should be governed by the Sunda-Australia plate motion. However, we suggest that the motion of northern Sumatra fore arc is closer to Sunda-India motion rather than Sunda-Australia motion. The azimuth of interseismic deformation rates in the Sumatra-Nias region, when estimated with reference to Indian plate agrees with the azimuth of coseismic movements. However, with reference to Australia plate the azimuths of interseismic motion suggest southward motion of the fore arc. This kind of motion becomes oblique to trench axis and also is inconsistent with the azimuth of coseismic displacement and earthquake focal mechanisms. Thus we suggest that the fore-arc motion in the region off the west coast of Sumatra may be closer to Sunda-India motion rather than Sunda- Australia motion. [21] Besides the diffused plate boundary zone between the India-Australia plates, another important feature of the region is the 90 E Ridge (NER). Earlier work suggested that NER accommodated as much as 2 cm/a of sinistral shear [Stein and Okal, 1978]. However, after the introduction of concept of diffused plate boundary [Wiens et al., 1985], the role of NER in controlling the deformation is minimized and motion along the NER is about 3 mm/a only [DeMets et al., 1988; Petroy and Wiens, 1989]. Krishna et al. [1998], Deplus et al. [1998], and Tinnon et al. [1995] reported evidence of deformation close to NER which is suggestive of weak rheology for the NER [Delescluse and Chamot- Rooke, 2007]. It implies that plate motion of India-Australia in the NER and Wharton Basin, which lies to the east of NER, is affected by deformation in the region. However, in this article we did not consider its effect, assumed a rigid plate and adopted the India-Sunda pole of Socquet et al. [2006] to estimate the poles for India-Burma fore arc and Sunda-Burma back arc and fore- and back-arc motion of Burma plate. [22] Acknowledgments. 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