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Rigid Indian plate: Constraints from GPS measurements
Gondwana Research 22 (2012) 1068–1072
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Rigid Indian plate: Constraints from GPS measurements P. Mahesh a,⁎, J.K. Catherine a, V.K. Gahalaut a, Bhaskar Kundu a, A. Ambikapathy a, Amit Bansal a, L. Premkishore a, M. Narsaiah a, Sapna Ghavri a, R.K. Chadha a, Pallabee Choudhary b, D.K. Singh c, S.K. Singh c, Subhash Kumar c, B. Nagarajan c, B.C. Bhatt d, R.P. Tiwari e, Arun Kumar f, Ashok Kumar g, Harsh Bhu h, S. Kalita i a
National Geophysical Research Institute (CSIR), Hyderabad 500007, India Institute of Seismological Research, Ahmedabad, India c Survey of India, Hyderabad, India d Indian Institute of Astrophysics, Kodaikanal, Tamilnadu, India e Department of Geology, Mizoram University, Aizwal, India f Department of Earth Sciences, Manipur University, Imphal, India g Department of Physics, Tezpur University, Tezpur, India h Department of Geology, Sukhadia University, Udaipur, India i Department of Environmental Sciences, Guwahati University, Guwahati, India b
a r t i c l e
i n f o
Article history: Received 27 September 2011 Received in revised form 11 January 2012 Accepted 13 January 2012 Available online 14 February 2012 Handling Editor: A. Aitken Keywords: Global positioning system India Plate motion Euler pole Lithospheric plate interiors Failed rift regions
a b s t r a c t We analyze GPS data from 26 sites located on the Indian plate and along its boundary. The large spatial coverage of the Indian plate by these sites and longer data duration helped us in refining the earlier estimates of the Euler pole for the Indian plate rotation. Our analysis suggests that the internal deformation of the Indian plate is very low (b1– 2 mm/year) and the entire plate interior region largely behaves as a rigid plate. Specifically, we did not infer any significant difference in motion on sites located north and south of the Narmada Son failed rift region, the most prominent tectonic feature within the Indian plate and a major source of earthquakes. Our analysis also constrains the motion across the Indo-Burmese wedge, Himalayan arc, and Shillong Plateau and Kopili fault in the NE India. © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction Precise estimation of plate motion improves our understanding of plate dynamics that governs plate boundary deformation, regional neotectonics, earthquake occurrence processes, and stability of plate interior regions. The geodetic methods, particularly the GPS measurement, have become very effective tool in characterizing plate motion and crustal deformation. However, it requires good number of evenly distributed observation sites on and across the plate to constrain its motion. In the past 10 years, several permanent GPS sites have been established in the Indian region. We use these data to estimate the plate motion and to assess the rigidity of the plate. In the past, a few such efforts have been made. Most of the previous studies used GPS data in the stable Indian region from two sites only, namely HYDE and IISC (Sella et al., 2002; Bettinelli et al., 2006). Hence these studies were confined to estimate the plate motion at global
scale and relative motion between two plates at plate boundaries. Jade et al. (2007) used GPS data from five sites within the plate interior from 2003 to 2006. The data from two sites, namely, IISC at Bangalore and DELH at Delhi, were for the period from 1997/98 to 2006. All the sites used by them are located along a north–south transect and there is not much coverage in the east–west direction. Banerjee et al. (2008) used the most comprehensive data set from 12 GPS sites located in the Indian plate interior. Along with the estimation of the Euler pole for the Indian plate motion, they opined that the almost east–west trending Narmada Son failed rift zone accommodates about 2±1 mm year− 1 of shortening in the north–south direction. Here in this article, we analyze the GPS data from more than 26 GPS sites in the Indian plate region (Fig. 1) to constrain the Indian plate motion, to assess the stability of the Indian plate and to estimate the deformation in the plate boundary regions. 2. GPS data and analysis
⁎ Corresponding author. E-mail address: [email protected] (P. Mahesh).
We processed the GPS data from continuous GPS sites of the Indian National GPS network along with the campaign data of good quality and
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P. Mahesh et al. / Gondwana Research 22 (2012) 1068–1072
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Fig. 1. GPS site velocities in Indian reference frame. Relatively larger error ellipse at a few sites (e.g. at Delhi, DELH) is due to less duration of the data. Note very low deformation within the plate interiors. NSFR, Narmada Son failed rift; PGFR, Pranhita Godavari failed rift; SP, Shillong Plateau; KF, Kopili fault; IBW, Indo-Burmese wedge. Earthquakes (M ≥ 4) are shown with pink circles from the USGS catalogue.
longer duration collected from the Indian plate interior regions. The campaign mode GPS sites are occupied annually for at least 3 days. The continuous and campaign mode GPS observations of 26 sites (Fig. 1) from 2003 to 2011 are processed together with 21 IGS sites (namely, BAHR, COCO, DARW, DGAR, GUAM, HYDE, HRAO, IISC, KARR, KIT3, KUNM, LHAS, LHAZ, MALD, MALI, PERT, POL2, SELE, SEY1, WUHN and URUM) surrounding the Indian plate and available from the Scripps Orbital and Positioning Centre (SOPAC). We used GAMIT, version 10.4 (Herring et al., 2010a,b), to estimate the time series of site coordinates and their velocities. The site position estimates and their rates were estimated in ITRF2008 (Altamimi et al., 2011) by stabilizing more stable continuous sites and core IGS reference sites using GLOBK, GLORG (Herring et al., 2010a,b). Coseismic corrections were applied to remove the coseismic offsets from the GPS sites that were affected by the 26 December 2004 Sumatra–Andaman earthquake (Mw 9.2). In the supplementary figure the correction applied to IISC time series is demonstrated. Station velocity uncertainties are estimated using site specific estimate of white noise and an assumed random walk noise of 0.75 mm year− 1/2 (Mao et al., 1999). The velocity uncertainties are not significantly affected by applying the random walk. Indian reference frame is realized by determining the plate rotation parameters that minimize the velocities of 13 sites which include 10 continuous and 3 campaign mode GPS sites, all representing the stable Indian plate located far away from the deforming boundary regions. 3. Results We estimated the velocity at sites located on the Indian plate and estimated the Euler pole of Indian plate rotation by stabilizing the site velocity of 13 sites on the stable India plate region. Our estimated pole of rotation for the Indian plate is located at 51.41 ± 0.07°N, 8.97 ± 0.8°E with an angular velocity of 0.539 ± 0.002°/Myr. Such a
pole gives a velocity of 54.05 ± 0.1 mm year − 1 towards N49.4° at HYDE IGS site. The pole is located about 8° east of the previous estimates of the Euler poles and hence closer to the Indian plate. At the same time there is about 13% increase in the angular velocity as compared to that by Banerjee et al. (2008) (Table 1). To assess the reliability of our estimated pole, we estimated the average root mean square residuals after subtracting the predicted motion of the Indian plate corresponding to the estimated Euler pole from the GPS derived velocity for the sites in the stable India plate region (Fig. 1). We did it for all the previous studies using their estimates of site velocity and Euler pole. We found that in our case the average residual with 0.76 mm year − 1 was the minimum. In case of Banerjee et al. (2008) and Jade et al. (2007), we found average residual as 1.71 and 3.47 mm year − 1, respectively. Such an analysis assured us that the Table 1 Comparison of Euler poles for Indian plate motion. Euler pole Reference
Latitude °N
Longitude °E
Angular velocity Degree/Myr
India/ITRF2008 Present study India/ITRF2000 Banerjee et al. (2008) India/ITRF2000 Jade et al. (2007) India/ITRF2000 Bettinelli et al. (2006) India/ITRF2000 Socquet et al. (2006) India/ITRF1997 Sella et al. (2002) India/ITRF2000 SOPAC Website
51.4 ± 0.07
8.9 ± 0.8
0.539 ± 0.002
52.9 ± 0.21
− 0.2 ± 3.7
0.499 ± 0.008
51.7 ± 0.5
− 15.1 ± 1.5
0.469 ± 0.01
51.4 ± 1.6
− 10.9 ± 5.6
0.483 ± 0.01
50.9 ± 5.1
− 12.1 ± 0.6
0.486 ± 0.001
53.7
− 13.9
0.483
53.1
2.2
0.519 ± 0.019
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Table 2 Estimated velocity at sites in ITRF08 and Indian reference frame. Site
ITRF 2008
Indian reference frame
Long.
Lat.
Ve
Vn
Sig.E
Sig.N
Ve
Vn
RHO
LUMA IMPH TZPR AZWL SHIL GWHT BHUB ALHB PLVM GSIL JBPR TONK HANL HYDE DEHR RSCL IISC KDKL BHOP DELH ARKW PUNE GOKL UDAI IITB ISRG
94.475 93.925 92.78 92.73 91.885 91.661 85.792 81.808 81.645 80.943 79.876 79.602 78.973 78.551 78.055 77.6 77.57 77.465 77.447 77.126 73.939 73.882 73.727 73.713 72.916 72.66
26.22 24.749 26.618 23.724 25.566 26.153 20.263 25.309 17.273 26.891 23.129 19.51 32.779 17.417 30.325 34.128 13.021 10.232 23.209 28.482 17.23 18.558 17.4 24.58 19.133 23.215
37.78 30.47 41.14 35.11 39.26 40.02 37.9 37.8 40.48 34.95 39.09 39.31 28.11 41.06 33.9 24.7 42.4 43.92 37.35 31.96 40.62 40.55 40.48 36.05 38.56 36.13
23.36 19.77 26.78 29.01 30.93 29.53 37.55 36.54 35.37 37.15 34.29 34.9 28.91 35.2 34.07 20.29 35.19 34.49 34.31 36.36 34.63 34.17 34.17 34.17 33.43 32.35
0.14 0.15 0.13 0.59 0.29 0.14 0.16 0.37 0.36 0.14 0.23 0.3 3.44 0.15 0.25 0.3 0.13 0.28 0.23 0.59 0.19 0.33 0.13 0.27 0.15 0.21
0.13 0.14 0.12 0.44 0.26 0.13 0.15 0.34 0.31 0.13 0.21 0.27 3.31 0.14 0.25 0.29 0.12 0.24 0.2 0.52 0.17 0.27 0.12 0.25 0.14 0.19
− 3.07 − 10.81 0.95 − 6.26 − 1.13 − 0.05 − 3.21 0.04 − 1.06 − 1.73 0.68 − 0.83 − 4.52 0.15 0.06 − 6.6 − 0.28 0.13 − 0.43 − 2.72 0.44 1.06 0.42 − 0.05 − 0.44 − 0.53
− 13.95 − 17.51 − 10.42 − 8.2 − 6.21 − 7.59 1.11 0.79 − 0.36 1.57 − 1.08 − 0.41 − 6.26 0.12 − 0.88 − 14.56 0.33 − 0.35 − 0.51 1.63 0.71 0.27 0.31 0.32 − 0.19 − 1.2
0.007 0.012 0.011 0.03 0.01 0.007 0.018 0.038 0.061 0.01 0 0.048 0.049 0.027 0.028 0.053 0.017 0.021 0.023 0.017 0.011 0.016 0.011 0.027 0.02 0.013
IGS sites DGAR KIT3 SEY1 BAHR MALI HRAO GUAM DARW KARR PERT WUHN KUNM COCO LHAS LHAZ URUM SELE POL2 MALD
72.37 66.885 55.479 50.608 40.194 27.687 144.868 131.133 117.097 115.885 114.357 102.797 96.834 91.104 91.104 87.601 77.017 74.694 73.526
− 7.27 39.135 − 4.674 26.209 − 2.996 − 25.89 13.589 − 12.844 − 20.981 − 31.802 30.532 25.03 − 12.188 29.657 29.657 43.808 43.179 42.68 4.189
47.37 27.76 25.03 30.54 25.45 18.14 − 9.9 35.51 38.9 39.41 31.6 30.56 45.29 46.58 46.58 32.66 28.08 27.2 50.41
33.55 4.91 11.46 29.99 16.52 18.92 5.71 59.87 58.41 57.08 − 12.55 − 19.23 52.89 15.42 15.42 4.2 3.52 4.07 36.6
0.13 0.11 0.13 0.15 0.2 0.19 0.13 0.12 0.11 0.13 0.12 0.13 0.17 0.11 0.11 0.21 0.1 0.1 0.5
0.13 0.11 0.12 0.14 0.18 0.2 0.12 0.11 0.11 0.12 0.11 0.12 0.15 0.1 0.1 0.22 0.11 0.11 0.37
− 1.3 3.79 − 23.84 0.69 − 23.09 − 39.52 − 61.81 − 5.86 − 0.8 5.19 − 13.88 − 13.06 − 0.89 8.26 8.26 3.82 3.33 3.02 4.77
0.07 − 26.77 − 15.7 5.13 − 2.89 6.91 − 20.35 28.17 22.84 21.29 − 48.62 − 56.58 15.48 − 21.66 − 21.66 − 32.45 − 31.16 − 30.02 2.79
0.031 − 0.03 0.094 − 0.012 0.15 0.214 − 0.169 − 0.065 − 0.059 − 0.045 − 0.002 0.006 − 0.072 0.037 0.037 0.078 − 0.005 − 0.016 0.066
Euler pole estimated by us is the most appropriate and represents plate motion all over the Indian plate. The velocities of sites in the stable Indian region in Indian reference frame are quite small (Fig. 1 and Table 2), suggesting low internal deformation of the plate interior regions which is below the resolution of present day uncertainties associated with the geodetic data (b1–2 mm/year). A few sites located near the plate boundary region show differential movement consistent with the convergence accommodated at that plate boundary (Figs. 1 and 2). 4. Discussions 4.1. Internal deformation of the plate interiors The plate interior regions of the Indian plate experience minimum deformation, which causes rare occurrence of earthquakes in these regions. Important of these earthquakes that occurred in recent past are the 1993 Latur earthquake (Mw 6.2), 1997 Jabalpur earthquake (Mw 5.8), and 2001 Bhuj earthquake (Mw 7.6). The last two of these earthquakes occurred in the failed rift regions within the stable Indian plate. Occurrence of these earthquakes may imply rapid
deformation of Indian plate. Paul et al. (1995) reoccupied the triangulation monuments of 1860s in south India using GPS and suggested no significant shear strain. Using repeat GPS measurements of a few sites in the stable India region, Paul et al. (2001) estimated north– south shortening strain rate as low as 2–6 × 10 − 9/year. Recently, Banerjee et al. (2008) tested the hypothesis that the Indian plate motion can be described by two plates wherein the east west trending Narmada Son failed rift zone across the central India segments the plate in two parts, although they found that such a model better fit the data but statistically it was not significant (Banerjee et al., 2008). We also tested the hypothesis of two plate model and found that it does not improve the data fitting in any significant way and hence we suggest that the Narmada Son failed rift zone or any other fault or the failed rift in the Indian plate do not segment the plate in two or more plates. Specifically, we did not find any significant change in velocity in north–south direction (Fig. 2), i.e. across the Narmada Son failed rift zone in the Indian plate. The Indian plate motion can be best described by a single Euler pole and the plate behaves as a rigid plate with low strain accumulation which is below the resolution of present day geodetic data (Figs. 1 and 3). We suggest
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Fig. 2. A north–south transect showing north–south component of the site velocity in the Indian reference frame. All the sites within the plate interior regions (south of Delhi) show velocity less than ±2 mm/year (shown with gray shading), suggesting that the plate behaves as a rigid plate and the strain accumulation rate, if any, is below the resolution of present day GPS observations. There is no apparent deformation across NSFR. RSCL is located to the north of the Higher Himalaya and its site motion represents surface convergence rate across the Himalaya. DEHR, though located in the Outer Himalaya, shows negligible velocity, suggesting strain accumulation on the subsurface detachment. Sites located in the deforming NE India region are enclosed in a rectangle. Note relative motion between TZPR and GWHT suggesting motion across the Kopili fault. Relative motion between IMPH and AZWL represents accommodation of part of the relative motion between India and Sunda plates. About 6 ± 1.5 mm/year of convergence is accommodated between Shillong plateau and Indian plate.
that the occasional occurrence of earthquakes in the plate interiors and in the failed rift regions is due to the localized deformation, probably caused by weak rheology, and such deformation does not affect the velocity at sites located far from the earthquake region (Mazzotti, 2007). Thus, although the failed rifts of the Indian plate
are the locales of earthquakes, they do not affect the Indian plate motion and do not segment the plate in two or more plates. 4.2. Motion across plate boundary regions 4.2.1. Motion across the Indo-Burmese wedge The predominantly northward motion of about 36 mm/year of the India plate with respect to the Sunda plate along its eastern boundary in the NE India (Socquet et al., 2006) is accommodated through dextral motion along the Sagaing fault in the east and in the IndoBurmese wedge in the west. A few sites located in the Indo-Burmese wedge indicate such motion. The plate boundary in the Indo-Burmese wedge appears to be between Aizwal and Imphal sites and it accommodates about 16 ± 2 mm year− 1 of dextral motion (Figs. 1 and 2) (Kundu et al., submitted for publication). Such a motion is consistent with the earthquake occurrence in the region (Kundu and Gahalaut, 2012).
Fig. 3. Residuals of site velocity in Indian reference frame for the sites located in the stable Indian plate which are stabilized to estimate Euler pole of rotation for the Indian plate. The circle represents velocity of ± 2 mm/year. Residual velocity at all the sites is less than 2 mm/year and is close to 1 mm/year, which indicates that the Indian plate is rigid and there is very little deformation in the plate interior regions and across major tectonic structures.
4.2.2. Deformation in the Shillong Plateau region The Shillong plateau is assumed to accommodate some of the India–Eurasia plate motion which is evident from previous GPS measurements (Jade et al., 2007; Banerjee et al., 2008), occurrence of the great 1897 Shillong Plateau earthquake (Mw 8; Bilham and England, 2001) and low temperature thermometry data (Clark and Bilham, 2008). Previous GPS measurements suggested 4–7 mm/year southward motion of the Shillong plateau in India fixed reference frame. We also obtained a similar velocity (about 6±1.5 mm year− 1) of sites located on and north of the Shillong plateau (Fig. 1). It is to be ascertained whether the motion across the Shillong Plateau affects convergence in the Himalayan region and whether the low seismicity in the Bhutan region (immediately north of the Shillong Plateau) is due to such process (Gahalaut et al., 2011). 4.2.3. Kopili fault A prominent fault, referred as the Kopili fault, is located between the Shillong Plateau and Mikir hills, the two main topographic
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features of the region. The approximately northwest–southeast oriented lineament is considered to be seismically active and extends up to the Himalaya in the north (Kayal et al., 2006). The two GPS sites, GWHT and TZPR, separated by a distance of about 120 km, are located on either sides of the Kopili fault. These sites show differential motion of about 3.0 ± 1.5 mm year − 1 and imply dextral motion on the northwest–southeast trending fault. The geodetically inferred motion across the Kopili fault is consistent with the earthquake focal mechanisms in the region. Two focal mechanism solutions are available in the Harvard CMT catalogue corresponding to earthquakes on October 5, 1999 (Mw 5.2) and February 23, 2006 (Mw 5.4) which occurred close to the Kopili fault (within 25–30 km). Additionally, Kayal et al. (2010) reported focal mechanism of an earthquake which occurred on August 19, 2009 (Mw 5.1) on Kopili fault. All these focal mechanisms are quite similar to each other and indicate dextral motion on the NWSE oriented nodal plane. Thus the GPS derived differential motion on the Kopili fault is consistent with the earthquake focal mechanisms. This is the first geodetic evidence in support of the active Kopili fault. 4.2.4. Himalayan arc One of the sites (RSCL at Leh) is located north of the Higher Himalaya and shows a motion of about 16.2 ± 0.2 mm year− 1 with respect to the Indian plate. This rate is consistent with the estimate of Holocene rate of convergence of about 18 mm year− 1 accommodated in the Himalaya (Molnar, 1990; Bilham et al., 1997; Avouac, 2003). The other site (DEHR) located in the Outer Himalaya shows negligible motion with respect to India, suggesting that the detachment under the Outer and Lesser Himalaya is locked and is accumulating strain (Banerjee and Bürgmann, 2002) for future large Himalayan earthquake. 5. Conclusions Our analysis of the GPS data from various sites located on the Indian plate results in estimation of a new and improved Euler pole of rotation for the Indian plate which is located at 51.44 ± 0.07°N, 8.9 ± 0.8°E with an angular velocity of 0.539 ± 0.002°/Myr. We find that the internal deformation across the plate and across major structures within the plate interior regions is very low (b1–2 mm/year) and the plate behaves as a single rigid plate. Motion at sites located in the Indo-Burmese wedge along the eastern boundary of the Indian plate suggests that part of the India–Sunda motion is accommodated in the Indo-Burmese wedge as dextral motion. The sites across the Shillong plateau show a relative convergence of about 6 ± 1.5 mm year− 1 with respect to the Indian plate. A well known fault in the region, the Kopili fault, also shows dextral motion of 3 ± 1.5 mm year− 1. Sense of motion across this fault is consistent with the earthquake focal mechanisms. Motion at sites located in the Himalaya arc is consistent with the Holocene convergence rate of about 18 mm/year and model of strain accumulation on the Himalayan detachment. Acknowledgements Comments from Roland Bürgmann and two anonymous reviewers helped us in improving the article. We acknowledge financial support from Seismology Division, MoES. We thank Survey of India, Dehradun for providing GPS data from several sites. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.gr.2012.01.011.
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Rigid Indian plate: Constraints from GPS measurements P. Mahesh a,⁎, J.K. Catherine a, V.K. Gahalaut a, Bhaskar Kundu a, A. Ambikapathy a, Amit Bansal a, L. Premkishore a, M. Narsaiah a, Sapna Ghavri a, R.K. Chadha a, Pallabee Choudhary b, D.K. Singh c, S.K. Singh c, Subhash Kumar c, B. Nagarajan c, B.C. Bhatt d, R.P. Tiwari e, Arun Kumar f, Ashok Kumar g, Harsh Bhu h, S. Kalita i a
National Geophysical Research Institute (CSIR), Hyderabad 500007, India Institute of Seismological Research, Ahmedabad, India c Survey of India, Hyderabad, India d Indian Institute of Astrophysics, Kodaikanal, Tamilnadu, India e Department of Geology, Mizoram University, Aizwal, India f Department of Earth Sciences, Manipur University, Imphal, India g Department of Physics, Tezpur University, Tezpur, India h Department of Geology, Sukhadia University, Udaipur, India i Department of Environmental Sciences, Guwahati University, Guwahati, India b
a r t i c l e
i n f o
Article history: Received 27 September 2011 Received in revised form 11 January 2012 Accepted 13 January 2012 Available online 14 February 2012 Handling Editor: A. Aitken Keywords: Global positioning system India Plate motion Euler pole Lithospheric plate interiors Failed rift regions
a b s t r a c t We analyze GPS data from 26 sites located on the Indian plate and along its boundary. The large spatial coverage of the Indian plate by these sites and longer data duration helped us in refining the earlier estimates of the Euler pole for the Indian plate rotation. Our analysis suggests that the internal deformation of the Indian plate is very low (b1– 2 mm/year) and the entire plate interior region largely behaves as a rigid plate. Specifically, we did not infer any significant difference in motion on sites located north and south of the Narmada Son failed rift region, the most prominent tectonic feature within the Indian plate and a major source of earthquakes. Our analysis also constrains the motion across the Indo-Burmese wedge, Himalayan arc, and Shillong Plateau and Kopili fault in the NE India. © 2012 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.
1. Introduction Precise estimation of plate motion improves our understanding of plate dynamics that governs plate boundary deformation, regional neotectonics, earthquake occurrence processes, and stability of plate interior regions. The geodetic methods, particularly the GPS measurement, have become very effective tool in characterizing plate motion and crustal deformation. However, it requires good number of evenly distributed observation sites on and across the plate to constrain its motion. In the past 10 years, several permanent GPS sites have been established in the Indian region. We use these data to estimate the plate motion and to assess the rigidity of the plate. In the past, a few such efforts have been made. Most of the previous studies used GPS data in the stable Indian region from two sites only, namely HYDE and IISC (Sella et al., 2002; Bettinelli et al., 2006). Hence these studies were confined to estimate the plate motion at global
scale and relative motion between two plates at plate boundaries. Jade et al. (2007) used GPS data from five sites within the plate interior from 2003 to 2006. The data from two sites, namely, IISC at Bangalore and DELH at Delhi, were for the period from 1997/98 to 2006. All the sites used by them are located along a north–south transect and there is not much coverage in the east–west direction. Banerjee et al. (2008) used the most comprehensive data set from 12 GPS sites located in the Indian plate interior. Along with the estimation of the Euler pole for the Indian plate motion, they opined that the almost east–west trending Narmada Son failed rift zone accommodates about 2±1 mm year− 1 of shortening in the north–south direction. Here in this article, we analyze the GPS data from more than 26 GPS sites in the Indian plate region (Fig. 1) to constrain the Indian plate motion, to assess the stability of the Indian plate and to estimate the deformation in the plate boundary regions. 2. GPS data and analysis
⁎ Corresponding author. E-mail address: [email protected] (P. Mahesh).
We processed the GPS data from continuous GPS sites of the Indian National GPS network along with the campaign data of good quality and
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P. Mahesh et al. / Gondwana Research 22 (2012) 1068–1072
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Fig. 1. GPS site velocities in Indian reference frame. Relatively larger error ellipse at a few sites (e.g. at Delhi, DELH) is due to less duration of the data. Note very low deformation within the plate interiors. NSFR, Narmada Son failed rift; PGFR, Pranhita Godavari failed rift; SP, Shillong Plateau; KF, Kopili fault; IBW, Indo-Burmese wedge. Earthquakes (M ≥ 4) are shown with pink circles from the USGS catalogue.
longer duration collected from the Indian plate interior regions. The campaign mode GPS sites are occupied annually for at least 3 days. The continuous and campaign mode GPS observations of 26 sites (Fig. 1) from 2003 to 2011 are processed together with 21 IGS sites (namely, BAHR, COCO, DARW, DGAR, GUAM, HYDE, HRAO, IISC, KARR, KIT3, KUNM, LHAS, LHAZ, MALD, MALI, PERT, POL2, SELE, SEY1, WUHN and URUM) surrounding the Indian plate and available from the Scripps Orbital and Positioning Centre (SOPAC). We used GAMIT, version 10.4 (Herring et al., 2010a,b), to estimate the time series of site coordinates and their velocities. The site position estimates and their rates were estimated in ITRF2008 (Altamimi et al., 2011) by stabilizing more stable continuous sites and core IGS reference sites using GLOBK, GLORG (Herring et al., 2010a,b). Coseismic corrections were applied to remove the coseismic offsets from the GPS sites that were affected by the 26 December 2004 Sumatra–Andaman earthquake (Mw 9.2). In the supplementary figure the correction applied to IISC time series is demonstrated. Station velocity uncertainties are estimated using site specific estimate of white noise and an assumed random walk noise of 0.75 mm year− 1/2 (Mao et al., 1999). The velocity uncertainties are not significantly affected by applying the random walk. Indian reference frame is realized by determining the plate rotation parameters that minimize the velocities of 13 sites which include 10 continuous and 3 campaign mode GPS sites, all representing the stable Indian plate located far away from the deforming boundary regions. 3. Results We estimated the velocity at sites located on the Indian plate and estimated the Euler pole of Indian plate rotation by stabilizing the site velocity of 13 sites on the stable India plate region. Our estimated pole of rotation for the Indian plate is located at 51.41 ± 0.07°N, 8.97 ± 0.8°E with an angular velocity of 0.539 ± 0.002°/Myr. Such a
pole gives a velocity of 54.05 ± 0.1 mm year − 1 towards N49.4° at HYDE IGS site. The pole is located about 8° east of the previous estimates of the Euler poles and hence closer to the Indian plate. At the same time there is about 13% increase in the angular velocity as compared to that by Banerjee et al. (2008) (Table 1). To assess the reliability of our estimated pole, we estimated the average root mean square residuals after subtracting the predicted motion of the Indian plate corresponding to the estimated Euler pole from the GPS derived velocity for the sites in the stable India plate region (Fig. 1). We did it for all the previous studies using their estimates of site velocity and Euler pole. We found that in our case the average residual with 0.76 mm year − 1 was the minimum. In case of Banerjee et al. (2008) and Jade et al. (2007), we found average residual as 1.71 and 3.47 mm year − 1, respectively. Such an analysis assured us that the Table 1 Comparison of Euler poles for Indian plate motion. Euler pole Reference
Latitude °N
Longitude °E
Angular velocity Degree/Myr
India/ITRF2008 Present study India/ITRF2000 Banerjee et al. (2008) India/ITRF2000 Jade et al. (2007) India/ITRF2000 Bettinelli et al. (2006) India/ITRF2000 Socquet et al. (2006) India/ITRF1997 Sella et al. (2002) India/ITRF2000 SOPAC Website
51.4 ± 0.07
8.9 ± 0.8
0.539 ± 0.002
52.9 ± 0.21
− 0.2 ± 3.7
0.499 ± 0.008
51.7 ± 0.5
− 15.1 ± 1.5
0.469 ± 0.01
51.4 ± 1.6
− 10.9 ± 5.6
0.483 ± 0.01
50.9 ± 5.1
− 12.1 ± 0.6
0.486 ± 0.001
53.7
− 13.9
0.483
53.1
2.2
0.519 ± 0.019
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Table 2 Estimated velocity at sites in ITRF08 and Indian reference frame. Site
ITRF 2008
Indian reference frame
Long.
Lat.
Ve
Vn
Sig.E
Sig.N
Ve
Vn
RHO
LUMA IMPH TZPR AZWL SHIL GWHT BHUB ALHB PLVM GSIL JBPR TONK HANL HYDE DEHR RSCL IISC KDKL BHOP DELH ARKW PUNE GOKL UDAI IITB ISRG
94.475 93.925 92.78 92.73 91.885 91.661 85.792 81.808 81.645 80.943 79.876 79.602 78.973 78.551 78.055 77.6 77.57 77.465 77.447 77.126 73.939 73.882 73.727 73.713 72.916 72.66
26.22 24.749 26.618 23.724 25.566 26.153 20.263 25.309 17.273 26.891 23.129 19.51 32.779 17.417 30.325 34.128 13.021 10.232 23.209 28.482 17.23 18.558 17.4 24.58 19.133 23.215
37.78 30.47 41.14 35.11 39.26 40.02 37.9 37.8 40.48 34.95 39.09 39.31 28.11 41.06 33.9 24.7 42.4 43.92 37.35 31.96 40.62 40.55 40.48 36.05 38.56 36.13
23.36 19.77 26.78 29.01 30.93 29.53 37.55 36.54 35.37 37.15 34.29 34.9 28.91 35.2 34.07 20.29 35.19 34.49 34.31 36.36 34.63 34.17 34.17 34.17 33.43 32.35
0.14 0.15 0.13 0.59 0.29 0.14 0.16 0.37 0.36 0.14 0.23 0.3 3.44 0.15 0.25 0.3 0.13 0.28 0.23 0.59 0.19 0.33 0.13 0.27 0.15 0.21
0.13 0.14 0.12 0.44 0.26 0.13 0.15 0.34 0.31 0.13 0.21 0.27 3.31 0.14 0.25 0.29 0.12 0.24 0.2 0.52 0.17 0.27 0.12 0.25 0.14 0.19
− 3.07 − 10.81 0.95 − 6.26 − 1.13 − 0.05 − 3.21 0.04 − 1.06 − 1.73 0.68 − 0.83 − 4.52 0.15 0.06 − 6.6 − 0.28 0.13 − 0.43 − 2.72 0.44 1.06 0.42 − 0.05 − 0.44 − 0.53
− 13.95 − 17.51 − 10.42 − 8.2 − 6.21 − 7.59 1.11 0.79 − 0.36 1.57 − 1.08 − 0.41 − 6.26 0.12 − 0.88 − 14.56 0.33 − 0.35 − 0.51 1.63 0.71 0.27 0.31 0.32 − 0.19 − 1.2
0.007 0.012 0.011 0.03 0.01 0.007 0.018 0.038 0.061 0.01 0 0.048 0.049 0.027 0.028 0.053 0.017 0.021 0.023 0.017 0.011 0.016 0.011 0.027 0.02 0.013
IGS sites DGAR KIT3 SEY1 BAHR MALI HRAO GUAM DARW KARR PERT WUHN KUNM COCO LHAS LHAZ URUM SELE POL2 MALD
72.37 66.885 55.479 50.608 40.194 27.687 144.868 131.133 117.097 115.885 114.357 102.797 96.834 91.104 91.104 87.601 77.017 74.694 73.526
− 7.27 39.135 − 4.674 26.209 − 2.996 − 25.89 13.589 − 12.844 − 20.981 − 31.802 30.532 25.03 − 12.188 29.657 29.657 43.808 43.179 42.68 4.189
47.37 27.76 25.03 30.54 25.45 18.14 − 9.9 35.51 38.9 39.41 31.6 30.56 45.29 46.58 46.58 32.66 28.08 27.2 50.41
33.55 4.91 11.46 29.99 16.52 18.92 5.71 59.87 58.41 57.08 − 12.55 − 19.23 52.89 15.42 15.42 4.2 3.52 4.07 36.6
0.13 0.11 0.13 0.15 0.2 0.19 0.13 0.12 0.11 0.13 0.12 0.13 0.17 0.11 0.11 0.21 0.1 0.1 0.5
0.13 0.11 0.12 0.14 0.18 0.2 0.12 0.11 0.11 0.12 0.11 0.12 0.15 0.1 0.1 0.22 0.11 0.11 0.37
− 1.3 3.79 − 23.84 0.69 − 23.09 − 39.52 − 61.81 − 5.86 − 0.8 5.19 − 13.88 − 13.06 − 0.89 8.26 8.26 3.82 3.33 3.02 4.77
0.07 − 26.77 − 15.7 5.13 − 2.89 6.91 − 20.35 28.17 22.84 21.29 − 48.62 − 56.58 15.48 − 21.66 − 21.66 − 32.45 − 31.16 − 30.02 2.79
0.031 − 0.03 0.094 − 0.012 0.15 0.214 − 0.169 − 0.065 − 0.059 − 0.045 − 0.002 0.006 − 0.072 0.037 0.037 0.078 − 0.005 − 0.016 0.066
Euler pole estimated by us is the most appropriate and represents plate motion all over the Indian plate. The velocities of sites in the stable Indian region in Indian reference frame are quite small (Fig. 1 and Table 2), suggesting low internal deformation of the plate interior regions which is below the resolution of present day uncertainties associated with the geodetic data (b1–2 mm/year). A few sites located near the plate boundary region show differential movement consistent with the convergence accommodated at that plate boundary (Figs. 1 and 2). 4. Discussions 4.1. Internal deformation of the plate interiors The plate interior regions of the Indian plate experience minimum deformation, which causes rare occurrence of earthquakes in these regions. Important of these earthquakes that occurred in recent past are the 1993 Latur earthquake (Mw 6.2), 1997 Jabalpur earthquake (Mw 5.8), and 2001 Bhuj earthquake (Mw 7.6). The last two of these earthquakes occurred in the failed rift regions within the stable Indian plate. Occurrence of these earthquakes may imply rapid
deformation of Indian plate. Paul et al. (1995) reoccupied the triangulation monuments of 1860s in south India using GPS and suggested no significant shear strain. Using repeat GPS measurements of a few sites in the stable India region, Paul et al. (2001) estimated north– south shortening strain rate as low as 2–6 × 10 − 9/year. Recently, Banerjee et al. (2008) tested the hypothesis that the Indian plate motion can be described by two plates wherein the east west trending Narmada Son failed rift zone across the central India segments the plate in two parts, although they found that such a model better fit the data but statistically it was not significant (Banerjee et al., 2008). We also tested the hypothesis of two plate model and found that it does not improve the data fitting in any significant way and hence we suggest that the Narmada Son failed rift zone or any other fault or the failed rift in the Indian plate do not segment the plate in two or more plates. Specifically, we did not find any significant change in velocity in north–south direction (Fig. 2), i.e. across the Narmada Son failed rift zone in the Indian plate. The Indian plate motion can be best described by a single Euler pole and the plate behaves as a rigid plate with low strain accumulation which is below the resolution of present day geodetic data (Figs. 1 and 3). We suggest
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Fig. 2. A north–south transect showing north–south component of the site velocity in the Indian reference frame. All the sites within the plate interior regions (south of Delhi) show velocity less than ±2 mm/year (shown with gray shading), suggesting that the plate behaves as a rigid plate and the strain accumulation rate, if any, is below the resolution of present day GPS observations. There is no apparent deformation across NSFR. RSCL is located to the north of the Higher Himalaya and its site motion represents surface convergence rate across the Himalaya. DEHR, though located in the Outer Himalaya, shows negligible velocity, suggesting strain accumulation on the subsurface detachment. Sites located in the deforming NE India region are enclosed in a rectangle. Note relative motion between TZPR and GWHT suggesting motion across the Kopili fault. Relative motion between IMPH and AZWL represents accommodation of part of the relative motion between India and Sunda plates. About 6 ± 1.5 mm/year of convergence is accommodated between Shillong plateau and Indian plate.
that the occasional occurrence of earthquakes in the plate interiors and in the failed rift regions is due to the localized deformation, probably caused by weak rheology, and such deformation does not affect the velocity at sites located far from the earthquake region (Mazzotti, 2007). Thus, although the failed rifts of the Indian plate
are the locales of earthquakes, they do not affect the Indian plate motion and do not segment the plate in two or more plates. 4.2. Motion across plate boundary regions 4.2.1. Motion across the Indo-Burmese wedge The predominantly northward motion of about 36 mm/year of the India plate with respect to the Sunda plate along its eastern boundary in the NE India (Socquet et al., 2006) is accommodated through dextral motion along the Sagaing fault in the east and in the IndoBurmese wedge in the west. A few sites located in the Indo-Burmese wedge indicate such motion. The plate boundary in the Indo-Burmese wedge appears to be between Aizwal and Imphal sites and it accommodates about 16 ± 2 mm year− 1 of dextral motion (Figs. 1 and 2) (Kundu et al., submitted for publication). Such a motion is consistent with the earthquake occurrence in the region (Kundu and Gahalaut, 2012).
Fig. 3. Residuals of site velocity in Indian reference frame for the sites located in the stable Indian plate which are stabilized to estimate Euler pole of rotation for the Indian plate. The circle represents velocity of ± 2 mm/year. Residual velocity at all the sites is less than 2 mm/year and is close to 1 mm/year, which indicates that the Indian plate is rigid and there is very little deformation in the plate interior regions and across major tectonic structures.
4.2.2. Deformation in the Shillong Plateau region The Shillong plateau is assumed to accommodate some of the India–Eurasia plate motion which is evident from previous GPS measurements (Jade et al., 2007; Banerjee et al., 2008), occurrence of the great 1897 Shillong Plateau earthquake (Mw 8; Bilham and England, 2001) and low temperature thermometry data (Clark and Bilham, 2008). Previous GPS measurements suggested 4–7 mm/year southward motion of the Shillong plateau in India fixed reference frame. We also obtained a similar velocity (about 6±1.5 mm year− 1) of sites located on and north of the Shillong plateau (Fig. 1). It is to be ascertained whether the motion across the Shillong Plateau affects convergence in the Himalayan region and whether the low seismicity in the Bhutan region (immediately north of the Shillong Plateau) is due to such process (Gahalaut et al., 2011). 4.2.3. Kopili fault A prominent fault, referred as the Kopili fault, is located between the Shillong Plateau and Mikir hills, the two main topographic
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features of the region. The approximately northwest–southeast oriented lineament is considered to be seismically active and extends up to the Himalaya in the north (Kayal et al., 2006). The two GPS sites, GWHT and TZPR, separated by a distance of about 120 km, are located on either sides of the Kopili fault. These sites show differential motion of about 3.0 ± 1.5 mm year − 1 and imply dextral motion on the northwest–southeast trending fault. The geodetically inferred motion across the Kopili fault is consistent with the earthquake focal mechanisms in the region. Two focal mechanism solutions are available in the Harvard CMT catalogue corresponding to earthquakes on October 5, 1999 (Mw 5.2) and February 23, 2006 (Mw 5.4) which occurred close to the Kopili fault (within 25–30 km). Additionally, Kayal et al. (2010) reported focal mechanism of an earthquake which occurred on August 19, 2009 (Mw 5.1) on Kopili fault. All these focal mechanisms are quite similar to each other and indicate dextral motion on the NWSE oriented nodal plane. Thus the GPS derived differential motion on the Kopili fault is consistent with the earthquake focal mechanisms. This is the first geodetic evidence in support of the active Kopili fault. 4.2.4. Himalayan arc One of the sites (RSCL at Leh) is located north of the Higher Himalaya and shows a motion of about 16.2 ± 0.2 mm year− 1 with respect to the Indian plate. This rate is consistent with the estimate of Holocene rate of convergence of about 18 mm year− 1 accommodated in the Himalaya (Molnar, 1990; Bilham et al., 1997; Avouac, 2003). The other site (DEHR) located in the Outer Himalaya shows negligible motion with respect to India, suggesting that the detachment under the Outer and Lesser Himalaya is locked and is accumulating strain (Banerjee and Bürgmann, 2002) for future large Himalayan earthquake. 5. Conclusions Our analysis of the GPS data from various sites located on the Indian plate results in estimation of a new and improved Euler pole of rotation for the Indian plate which is located at 51.44 ± 0.07°N, 8.9 ± 0.8°E with an angular velocity of 0.539 ± 0.002°/Myr. We find that the internal deformation across the plate and across major structures within the plate interior regions is very low (b1–2 mm/year) and the plate behaves as a single rigid plate. Motion at sites located in the Indo-Burmese wedge along the eastern boundary of the Indian plate suggests that part of the India–Sunda motion is accommodated in the Indo-Burmese wedge as dextral motion. The sites across the Shillong plateau show a relative convergence of about 6 ± 1.5 mm year− 1 with respect to the Indian plate. A well known fault in the region, the Kopili fault, also shows dextral motion of 3 ± 1.5 mm year− 1. Sense of motion across this fault is consistent with the earthquake focal mechanisms. Motion at sites located in the Himalaya arc is consistent with the Holocene convergence rate of about 18 mm/year and model of strain accumulation on the Himalayan detachment. Acknowledgements Comments from Roland Bürgmann and two anonymous reviewers helped us in improving the article. We acknowledge financial support from Seismology Division, MoES. We thank Survey of India, Dehradun for providing GPS data from several sites. Appendix A. Supplementary data Supplementary data to this article can be found online at doi:10. 1016/j.gr.2012.01.011.
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