Anisotropy of the Indian continental lithospheric mantle

Geophysical Journal International Geophys. J. Int. (2009) 179, 1341–1360

doi: 10.1111/j.1365-246X.2009.04395.x

Anisotropy of the Indian continental lithospheric mantle

1 University

of Cambridge, Department of Earth Sciences, Bullard Laboratories, Madingley Road, CB3 0EZ Cambridge, UK. E-mail: [email protected] 2 National Geophysical Research Institute, Uppal Road, Hyderabad 500007, India 3 Indian Institute of Astrophysics, Koramangala, Bangalore 560 034, India 4 CSIR Centre for Mathematical Modelling, Bangalore 560 037, India

Accepted 2009 September 21. Received 2009 September 1; in original form 2008 August 27

SUMMARY Due to the paucity of seismological data available in the public domain, the structure of the Indian lithosphere is still little known. We investigate the lithospheric structure and potential mechanical coupling between the crust and upper mantle along the Himalayan arc and underneath peninsular India using seismic anisotropy. Shear wave splitting measurements are performed on core-refracted phases. For each event recorded at a given seismological station we measured the orientation of the polarization plane of the fast S wave (phi), assumed to be a proxy for the orientation of the a axis of olivine, and the delay (dt) between the arrival time of the fast and slow S waves. We present a very comprehensive data set recorded at 86 seismological stations, deployed from the Himalayas to the southern tip of the Indian peninsula, in a joint effort by the National Geophysical Research Institute, Hyderabad, India, the University of Cambridge and the Indian Institute of Astrophysics. The unprecedented data set we present sheds light on the mechanisms involved in the India–Eurasia continental collision in a region along the Himalayan arc, south of the Indus-Tsangpo suture zone. At the scale of the Indian plate, the majority of the stations show a NNE–SSW orientation of phi over hundreds of kilometres, from Sri Lanka to the northern part of the Dharwar craton. This direction closely parallels the trend of the Indian plate motion, with respect to a fixed Eurasian plate, as defined through the NUVEL1A plate model. Along the Himalayan arc, from Ladakh in the northwest, to Bhutan and the Shillong plateau in the east, the orientation of phi rotates to become ∼EW, perpendicular to the plate motion as defined through NUVEL1A. Unlike previous studies, we do find strong evidence for seismic anisotropy south of the Indus Tsangpo suture zone. A large number of null results have been computed, with consistent orientation of the two fast polarization directions (phi) across the subcontinent. We demonstrate the potential value of the too often neglected null measurements in the interpretation of seismic anisotropy. From these results, we infer the dominance, beneath the Indian lithosphere, of the asthenospheric flow in aligning minerals in the sheared lithosphere–asthenosphere boundary layer, masking any compression induced anisotropy expected to be normal to this direction. Closer to the collision front in northern India, the anisotropy may in part, be due to the foliation planes of the Himalayan fold and thrust belt aligning the a axis of olivine perpendicular to the compression axis, but more likely to the turning of the relative asthenospheric flow along the strike caused by the downthrusting Indian lithosphere acting as a barrier. The continent-wide consistency of results strengthens the understanding that the Indian lithosphere has distinct anisotropic signatures, contrary to the hitherto assumed isotropy and allows one to interpret the results in a coherent framework of Indo-Eurasian convergence. Key words: Plate motions; Body waves; Seismic anisotropy; Continental tectonics: compressional; Asia.

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GJI Geodynamics and tectonics

Maggy Heintz,1 V. Pavan Kumar,2 Vinod K. Gaur,3,4 Keith Priestley,1 Shyam S. Rai2 and K. Surya Prakasam2

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I N T RO D U C T I O N Over the past two decades, seismic anisotropy has become a valuable tool in investigating the upper mantle structure and mechanical coupling with the crust. The preferential propagation direction of seismic waves (ie seismic anisotropy) is mainly attributed to the lattice preferred orientation (LPO) of olivine in the upper mantle down to 410 km depth (Silver 1996; Savage 1999) and to shape preferred orientation (SPO) of melt pockets or compositional layering in the crust. The shear wave splitting technique is the most commonly used method to detect azimuthal anisotropy in continental areas. A shear wave propagating through an anisotropic medium will split into a fast and a slow perpendicularly polarized quasi-S waves. At the seismological station, we will measure the orientation of the polarization plane of the fast S wave (phi), assumed to be a proxy for the orientation of the a axis of olivine, and the delay dt between the arrival time of the fast and slow S waves. dt is proportional to the thickness and intrinsic anisotropy of the anisotropic layer, and is 1s on average in continental areas (Silver 1996). Shear wave splitting measurements allow good lateral resolution of less than 50 km, although this trades off against poor vertical resolution unless adequate quality surface wave data over a range of periods is available to examine the depth distribution of anisotropy. Measurements are usually performed assuming a single anisotropic layer and the results express apparent anisotropy. Shear wave splitting is investigated using core refracted shear waves (SK(K)S and PK(K)S), primarily S and P waves travelling through the liquid outer core as a P wave and converting to a S wave at the core–mantle boundary (CMB) on the receiver side. They arrive at the seismological station with a nearly vertical incidence. Due to the P-to-S conversion at the CMB, these phases are initially polarized along the radial direction. Evidence for an anisotropic layer between the CMB and the surface is expressed by the presence of energy on the transverse component associated with the arrival of the core-refracted phase on the radial component. The development of olivine LPO is due to both past and present deformation of the upper mantle. In continental areas, Vinnik et al. (1992) attributed most of the observed anisotropy to the viscous shear in the asthenosphere that accommodates or causes the relative motions between the Earth’s tectonic plates and the underlying upper mantle, whereas Silver (1996) argued that the directions of anisotropy have been ‘frozen’ in the lithosphere as a result of post-tectonic thermal relaxation. This latter hypothesis allows the investigation of mechanical coupling between the crust and the mantle. Savage (1999) reviewed the two end members hypothesis and recognized that both asthenospheric and lithospheric sources of anisotropy play a combined role in many studies. From numerical modelling of global mantle flow and comparison with seismic anisotropy, Conrad et al. (2007) inferred that the fast polarization directions of split S waves observed through shear wave splitting measurements can constrain the mantle flow pattern in the asthenosphere, if the contribution from the anisotropic lithosphere is small. Asthenospheric anisotropy will contribute to shear wave splitting in oceanic domains as well as in continental areas. The signal is however often masked by a strong lithospheric component for the continents, overlain by a much thicker and deformed lithosphere compared to oceanic domains, and related to fossil and/or current active deformation. A recent review of continental anisotropy has been carried out by Fouch & Rondenay (2006), but this study does not reflect results for Australia obtained by Heintz & Kennett (2005) and Heintz & Kennett (2006). Bormann et al. (1996) investigated the

effect of the topography of the lithosphere–asthenosphere boundary (LAB) on shear wave splitting measurements in the case of an asthenospheric source of anisotropy. In areas of low LAB topography, the measured directions of anisotropy will closely follow the direction of the absolute plate motion (APM) caused by the basal drag of the moving lithospheric plate, while in areas of high LAB topography, phi will be subparallel to the contour line trends of the LAB. This is especially obvious in the presence of deep cratonic roots implying jumps in the thickness of the surrounding lithosphere (Bormann et al. 1996). In order to gain insights into the deep deformation processes involved in the India–Eurasia collision, several investigators have addressed the state of seismic anisotropy in the mantle beneath the southern Himalaya–Tibet collision zone. Sandvol et al. (1997) analysed data obtained through the INDEPTH-II experiment in the eastern Himalayas and Tibet, and found no evidence for anisotropy in this area, except 100 km north of the Indus-Tsangpo suture zone, the collision zone between the Indian plate and the Ladakh batholith (see Fig. 1 for location). Considering the Indian lithosphere to be characterized by consistent NNE–SSW directions of anisotropy based on results published by Ramesh & Prakasam (1995), Sandvol et al. (1997) explained the lack of anisotropy near the suture by a nearly vertical plunge of the olivine a axis related to the downwelling of the lithospheric mantle beneath the Himalayas and southern Tibet, resulting in apparent transverse isotropy. Chen & Ozalaybey (1998) regarded the Indian subcontinent as purely isotropic based on results obtained at stations HYB (Hyderabad) and SHIO (located on the Shillong plateau, see Fig. 1). They suggest that the onset of significant shear wave splitting near 30◦ N, ∼100 km north of the Indus-Tsangpo suture zone, represents a marker of the termination of the intact Indian lithosphere thrusting under the Himalayas. Both these studies highlight the need to carefully investigate seismic anisotropy underneath the Indian peninsula. A purely isotropic fast moving Indian plate, undergoing major continental collision and lithospheric deformation, is hardly conceivable. If the India–Eurasia collision zone and the resulting Himalaya and Tibetan plateau have been extensively studied by all means of geophysics, the structure of the upper mantle under India is still poorly known. A recent study published by Kumar & Singh (2008) presents results of seismic anisotropy obtained at 35 stations spread across India. 17 of those stations are located along the coast. The results obtained have mostly been explained by absolute plate motion related strain, considering the basal topography of the lithosphere in a no-net rotation frame. In the southern part of the subcontinent, the authors acknowledge evidence of fossilized anisotropy. The paucity of previous comprehensive investigations of seismic anisotropy at the scale of the subcontinent leaves room for both studies, especially because the data sets considered are extremely different. Of the 35 stations analysed by Kumar & Singh (2008), only eight are duplicated in this study, with some discrepancies in the results. Apart from the measurements of shear wave splitting obtained across the Indian subcontinent, we introduce here an extensive and unprecedented analysis of seismic anisotropy along the Himalayan arc. This study investigates the deformation of the Earth’s crust and upper mantle from the Indus-Tsangpo suture zone, in a region stretching along the Himalayan arc from Ladakh in the west to Bhutan in the east, down to the southern tip of the Indian subcontinent and in Sri Lanka. We therefore gain an unprecedented insight on the structure of the plate which collision with Eurasia gave rise to the most spectacular tectonic event on Earth. C

2009 The Authors, GJI, 179, 1341–1360 C 2009 RAS Journal compilation

Seismic anisotropy underneath India

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Figure 1. Location of the 86 seismological stations in India, Sri Lanka, Nepal, Bhutan and southern Tibet which provided data for this study. The stations are colour coded according to the network they belong to. Key geological units include: DWC – Dharwar Craton, BC – Bastar Craton, SC – Singhbhum Craton, AC – Aravalli Craton, SP – Shillong Plateau, GT – Granulite Terrains, VB – Vindhyan Basin, GB – Godavari Basin, KBB – Kaladgi Bhima Basin, CB – Cuddapah Basin, BB – Bengal Basin, DT – Deccan Traps. The dashed line shows the location of the Narmada-Tapti-Son-Damodar lineament, the dotted lines indicate some of the major mapped faults and the solid white line shows the location of the Indus-Tsangpo suture zone. The white arrows represent the motion of the Indian plate relative to a fixed Eurasian plate in the NUVEL1A plate model (DeMets et al. 1990), while the black arrows show the motion of the Indian plate in the hotspot HS3-NUVEL1A reference frame (Gripp & Gordon 1990).

R E G I O NA L S E T T I N G A N D T E C TO N I C S The Indian continental lithosphere is largely surfaced by a shield, itself an assemblage of Precambrian cratons and mobile belts, bearing a few sedimentary basins and a large cover of flood basalts in the central west (Fig. 1). The Indian subcontinent is subdivided into three main geographical regions, from north to south, respectively: the Himalayan front, the Indo-Gangetic plain, and the peninsular shield (Fig. 1). Palaeomagnetic data (Klootwijk et al. 1985) reveal that greater India’s indentation into southern Asia began in the west, in the Hindukush region, at equatorial-to-low northern palaeolatitudes (Klootwijk et al. 1994), The suturing ended in the Lhasa region with final subduction of oceanic material in the east. It was followed by the dramatic reduction in the velocity of the Indian C


plate but convergence continued at a pace of ∼5 m a century (Paul et al. 2001). The Indian shield covers two thirds of the subcontinent and constitutes one of the largest Precambrian shield areas in the world. It is a complex unit reflecting the long tectonic history of collage between cratonic blocks and intervening mobile belts assembled between the mid-Archean and the neo-Proterozoic time. The largest constituent is the Dharwar craton (Fig. 1). North of the Dharwar craton lies the Narmada-Tapti-Son-Damodar lineament, a palaeorift reactivated during Proterozoic. The Dharwar craton is surrounded from west to east by the Aravalli, Singhbhum and Bastar cratons. The Shillong plateau, located in the northeastern part of the country, is an outpost separated from the main shield area by the Bengal basin, and from the Himalaya by the Brahmaputra river. Some other large

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sedimentary basins lying on top of the Dharwar craton or along the collision front between two blocks include, from north to south, the Vindhyan, Godavari, Kaladgi Bhima and Cuddapah basins (Fig. 1). The Deccan traps formed at the end of the Cretaceous, 65 Ma, and are commonly attributed to the upwelling of a deep mantle plume. They represent one of the largest igneous province (LIP) on Earth and are understood to have taken place rapidly, in less than a million years. The eastern and western Ghats run NE–SW and NW–SE along the eastern and western coast of the subcontinent, respectively. While the western Ghats are believed to be a consequence of Cenozoic uplift, rifting, and down faulting (Gunnell & Fleitout 2000), the eastern Ghats are attributed to several stages of major EW compression (Navqi & Rogers 1987). Several studies have focused on the deep structure of the subcontinent by means of surface waves group/phase velocity dispersion (e.g. Mohan et al. 1992; Singh et al. 1999; Mitra et al. 2006b and Ritzwoller & Levshin 1998). From their tomographic model of Eurasia, Ritzwoller & Levshin (1998) highlighted high Rayleigh wave group velocity anomalies down to approximately 150 s, giving a reasonable estimate of the depth extent of the Indian shield. Focusing the area of interest on India and investigating the upper part of the upper mantle, Mitra et al. (2006b) inferred the southern part of India, up to the Aravalli craton, to be characterized by high Rayleigh waves group velocities at all periods (up to 45 s), while the northern portion of the subcontinent is underlain by low group velocity at all periods. The large scale of the structures prevents any rapid variations in velocity, but is in good agreement with Ritzwoller & Levshin (1998). From Rayleigh waves phase velocity measurements (Mitra et al. 2006a), the thickness of the lithosphere under the Indian shield has been estimated to be around 155 km, in agreement with Ritzwoller & Levshin (1998) and with the multimode Rayleigh wave tomographic model of Priestley et al. (2006). These results suggest a somewhat thinner cratonic root than that found for many other cratons in different parts of the world. Mitra et al. (2006a) suggest the rapid motion of the Indian plate across the Indian Ocean as a potential source of thinning of the cratonic root of the south Indian shield.

D ATA A N D M E T H O D We selected core refracted shear waves (SK(K)S and PK(K)S) recorded at 86 broad-band seismological stations spanning the Indian subcontinent and the Himalayan collision zone (Fig. 1). Two stations are permanent, operated by GEOSCOPE (HYB – Hyderabad) and IRIS (PALK – Pallekele, Sri Lanka), respectively, but most are broad-band seismometers installed for a short duration (6–24 months) by NGRI (NW Himalaya and Uttaranchal profiles), the University of Cambridge and the Indian Institute of Astrophysics or PASSCAL (the Nepal–Tibet HIMNT network, 28 stations operated by the University of Colorado for 18 months between 2001 and 2003 and the Bhutan XA network, five stations operated by the University of Texas at El Paso from 2002 January to 2003 March). Shear wave splitting measurements have already been made for data recorded at the HIMNT and XA networks, but for consistency we have re-analysed these data in the present study. Suitable events for shear wave splitting studies have an epicentral distance between 85◦ and 145◦ , with a body wave magnitude higher than 5.5 and a good signal-to-noise ratio (visually inspected). Contamination with other S phases (direct S, ScS or Sdiff) can be an issue at epicentral distances less than 90◦ . Only 37 events, includ-

Figure 2. Epicentral locations of the teleseismic events used in this study. All events have a body wave magnitude mb > 5.5 and an epicentral distance ranging from 85◦ to 145◦ . The black circles represent the events recorded by the temporary stations while the white circles represent the events recorded by the permanent stations. Grey circles symbolize events recorded by both permanent and temporary stations. The backazimuthal coverage is similar for both permanent and temporary stations. The region under study is highlighted in grey.

ing 20 from the HIMNT network (Table 1), have an epicentral distance below 90◦ , thus ruling out contamination with other S phases. Those measurements are in very good agreement with other values obtained at the same stations for events with an epicentral distance well over 90◦ . The data have been filtered using a Butterworth bandpass filter with corner frequencies at 0.03 and 0.3 Hz. A total of 153 events have been selected at the temporary stations and 86 at the permanent stations (Fig. 2). 14 and 6 years of data have been analysed at the permanent HYB and PALK stations, respectively. In order not to bias the reader’s appreciation of the backazimuthal coverage, we colour coded the events with respect to the type of stations they were recorded at (Fig. 2). The backazimuthal coverage is sensibly equivalent whatever the kind of stations considered, and except a lack of coverage northeast and northwest of India, the distribution of the epicentres is reasonably good. Shear wave splitting measurements were performed using the automated version (Teanby et al. 2004) of Silver & Chan (1988) algorithm. Phi and dt are determined through a grid search procedure by correcting the observed components for the anisotropy effect so as to minimize the energy on the transverse component associated with the arrival of the core-refracted phase on the radial component (see Fig. 3 for details). A single horizontal anisotropic layer is assumed. The advantage of using the automated version of the code is to check for consistency of the parameters over a large number of measurement windows and to remove subjectivity associated with manual window picking. The user chooses an initial measurement window, and an increment is set up to automatically vary its start and end time. We used 250 windows. A reliable solution will be characterized by a plateau (Fig. 3D) for phi and dt values. Stable regions are then identified through a cluster analysis. Two criteria based on C



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Figure 3. Example of a shear wave splitting measurement performed using the automated code of Teanby et al. (2004) for data from station BSP. (A) Radial and transverse components before (top traces) and after (bottom traces) rotation and time shifting of the traces. Note, after correction of the anisotropy, the removal of the energy on the transverse component associated with the arrival of the core refracted shear wave on the radial one. (B) Waveforms and particle motion of the two quasi-S waves before (left-hand side) and after (right-hand side) removal of the anisotropy. Before correction, the two waveforms are similar but shifted by dt, and the particle motion is elliptical. After correction, the two waveforms become perfectly superimposed, and the particle motion is linearized. (C) Diagram of energy representing phi versus dt. The thick contour indicates the 95 per cent confidence interval and the cross shows the best-fitting solution. (D) Representation of the phi and dt values obtained from the analysis of 250 measurement windows. Stable solutions correspond to a plateau associated with small error bars. The red cross indicates the solution that was chosen from the cluster analysis. (E) Plot of phi versus dt clusters computed for the 250 measurement windows. The plateau in D is condensed in a tight cluster of points in E. A cluster analysis is performed: the triangles show the clusters positions and the cross highlights the optimum cluster, associated with the lowest variance.

the variance of clusters and measurements are used to determine the optimum cluster, and from this cluster, the analysis window with the smallest errors on phi and dt is selected. Each measurement is then carefully manually checked and sorted into three different categories, namely good, fair or null. The criteria used to sort the data are based on (1) the quality of the signal-to-noise ratio, (2) the ellipticity of the particle motion in the horizontal plane when anisotropy is present, (3) its linearization by anisotropy removal, (4) the waveform coherence between the fast and slow split shear waves and (5) the stability of the measurement over the whole set of windows. Measurements satisfying all five criteria are rated as ‘good’ and those for which only four criteria are filled are rated as ‘fair’. The selection has been rather critical, giving good confidence in the data set obtained. A particular category of the data concerns the null measurements. A ‘null’ does not exhibit any energy on the transverse component (see Fig. 4), due either to an absence of anisotropy, an initial polarization of the incoming wave parallel or orthogonal to the fast anisotropic direction, or to low signal-to-noise ratio. In the absence of anisotropy, the estimated fast polarization direction reflects the initial polarization of the incoming wave, which for SKS corresponds to the backazimuth. Purely isotropic media will be characterized by nulls from all backazimuths, whereas strongly anisotropic media will have nulls coming from only 4 main directions, parallel and perpendicular to the backazimuth of the event. By its nature of C


undetermined solution, a null measurement is often discarded. In this case, 222 reliable null measurements, showing a good signalto-noise ratio (visually inspected), have been performed on the data set for peninsular India: 90 and 19 at the permanent stations HYB and PALK, respectively, and 113 measurements at all the temporary stations considered (peninsular India). We will see in the following that such measurements are not meaningless.

R E S U LT S We display the individual measurements for peninsular India in Fig. 5 whereas the results obtained at stations deployed along the Himalayan arc are presented separately in Figs 8 and 9. Table 1 summarizes all the measurements obtained by geographical area. We briefly discuss below our measurements from Sri Lanka and the southern tip of the Indian peninsula up to the Himalayan arc.

Sri Lanka The agreement between null and non-null results for events coming from various backazimuths indicates that the medium beneath PALK is well characterized by a polarization direction trending N-5◦ /34◦ , that is, NNE–SSW (Table 1 and Fig. 5).

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Figure 4. Example of a null measurement performed at HYB. (A) A null measurement is characterized by an absence of energy on the transverse component associated with the arrival of the core-refracted phase on the radial component. (B) The particle motion is linear before as well as after correction of the potential anisotropy. (C) A confidence interval elongated along the time axis, and values of dt, reaching the limit of calculation (6 s in the present case), are characteristic of null measurements. The solution is unconstrained in dt and two possible perpendicular fast polarization directions are observed. These correspond to the elongated areas outlined by the 95 per cent contour interval (bold black line) (D) Large errors characteristic of an undetermined solution are associated with the parameters measured. (E) The solution is unconstrained in dt and tends towards the maximum, while one possible trend in phi is highlighted by the position of the cluster.

Peninsular India Southern granulite terrains KOD is our southernmost station in peninsular India. All non-null results show phi trending N-3◦ /5◦ and the 21 null values show phi between N13◦ and N50◦ (and perpendicular). In an earlier study, Ramesh & Prakasam (1995) observed an orientation of phi in agreement with our measurements, based on a shear wave splitting analysis performed on digitized WWSSN data. Kumar & Singh (2008) however report two different orientations of phi (N142◦ and N60◦ ), based on the analysis of only four events.

Southern Dharwar craton At GBA, 12 null measurements consistently show values of phi between N-5◦ and N13◦ , with one exception at N57◦ ± 22.5◦ . Two non-null measurements of fair quality give phi of 4◦ and 17◦ for events with backazimuths of 241◦ and 113◦ , respectively. These 15 measurements all roughly agree for an orientation of phi trending NNE–SSW. Station BGL, which lies ∼100 km south of GBA, is characterized by a good backazimuthal coverage and measurements of shear wave splitting accounting for an average anisotropic direction oriented NE–SW (∼N30◦ ).

In and around the Cuddapah basin The large scatter in the results observed at station SLM, located SE of HYB, together with the lack of consistency between the values of phi and the backazimuths of the events is characteristic of an apparently isotropic medium. At station LTV, southwest of SLM, phi varies with respect to the backazimuth, showing evidence for a potentially complex twolayers anisotropic system. The directions of anisotropy measured at LTV (N-34◦ /-25◦ or N8◦ /20◦ ) are either parallel to the motion of the Indian plate as defined through the NUVEL1A model considering a fixed Eurasia (DeMets et al. 1990) (NNE–SSW) or through the HS3-NUVEL1A model (Gripp & Gordon 2002) (NNW–SSE). This could reflect a contribution from both the asthenosphere (prior to collision) and the lithosphere (subsequent to collision) to the measured directions.

Central Dharwar craton 90 null measurements are obtained at station HYB for events showing a good backazimuthal coverage. The medium beneath HYB is very likely to be apparently isotropic, in good agreement with previous studies (Chen & Ozalaybey 1998; Barruol & Hoffmann 1999). The nearby NND station, in the Deccan traps northwest of HYB, is characterized by the same pattern of anisotropic directions (Fig. 5B). C



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Figure 5. Maps showing the individual results (A – good and B – nulls) obtained at the 29 seismological stations south of the Himalayan collision zone. Black and white arrows represent the direction of the Indian plate motion as defined through the HS3-NUVEL1A (Gripp & Gordon 1990) and NUVEL1A (DeMets et al. 1990) plate models, respectively. (A) The orientations of the red lines represent the fast polarization directions. The length of each line is proportional to the delay time dt between the fast and slow S waves. (B) Individual null results computed at each seismological station. Due to the 90◦ uncertainty inherent to the definition of a null (see text for details), the two potential perpendicular directions of anisotropy are materialized by a cross.

We show on Figs 4 and 6 an example of null/non-null measurements, respectively, obtained for two events reaching station HYB with similar backazimuth. It does seem incompatible to obtain nonnull measurements at a station otherwise characterized by apparent isotropy. This observation is often neglected, mainly because null results are rarely considered in shear wave splitting studies, but also because to our knowledge, there is not so far an explanation. This could be due to an oddity on the ray path or to horizontal heterogeneities. It is not unreasonable to expect the nearby KIL and MBN stations to be characterized by apparent isotropy, unless the lithosphere is heterogeneous and its properties vary over short distances. The agreement between the null (11) and non-null (2) measurements obtained at KIL for events with different backazimuths leads us to infer a fast polarization direction trending N30◦ /40◦ . Without considering the nulls, no conclusion would have been drawn on the basis of a single measurement. At station MBN, null measurements give an orientation of phi parallel or perpendicular to the backazimuth of the events. Two events with almost opposite backazimuths (33◦ and 245◦ ) give directions of phi consistent with the two potential directions derived from the null results (respectively N-80◦ and N-2◦ ). A two-layers medium could be considered to explain the results, but there is not enough data to compute apparent phi and dt parameters obtained through a two-layers anisotropic system. Stations SNI and KNP are located in close vicinity of the Narmada-Tapti-Son-Damodar lineament, in a region where its superficial expression is trending NE. While KNP is only characterized by one measurement showing N47◦ , station SNI is very well characterized, with phi trending N51◦ /55◦ , subparallel to the crustal expression of the lineament. C


Ganga basin Measurements performed at stations RWA and ALB differ by almost 90◦ from those obtained by Kumar & Singh (2008). Whereas the authors characterized the medium underneath RWA by phi trending N37◦ on the basis of five events, three good measurements lead us to infer a direction ranging N89◦ –113◦ . If station ALB is characterized by two measurements highlighting an orientation of N36◦ in Kumar & Singh (2008), we are much more confident in our six good measurements, showing a well-constrained orientation of phi trending N94◦ –101◦ (see Fig. 7 for an example of the data quality at station ALB). Around New Delhi, stations MRT, NPL and GRG are characterized by 15 very well-constrained orientations of phi trending between N36◦ and N54◦ , with one value up to N68◦ at GRG. This is in good agreement with the average value of N46◦ obtained at NDI by Kumar & Singh (2008).

On and around the Shillong plateau At CHP, 3 measurements consistently lead to null results with a potential direction of anisotropy N21◦ /36◦ (or N111◦ /126◦ ) for events with a backazimuth ∼N110◦ . The medium underneath CHP cannot be characterized. When compared to previous results obtained at two stations deployed NW of CHP and highlighting phi N49◦ /55◦ (Singh et al. 2006), the results we obtain, if accounting for anisotropy, could fit well with the surroundings. BAI, north of CHP, is characterized by four good measurements with phi trending ENE-WSW (N57◦ /86◦ ) and values of dt much smaller than the continental average of 1 s. The values we compiled at BAI represent a smooth transition between the NE–SW and EW

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Figure 6. Example of a very good measurement performed at station HYB otherwise characterized by apparent isotropy. (A) The traces are represented in the radial-transverse frame, before (top two traces) and after (bottom two traces) correction of anisotropy. (B) Waveforms and particle motions before (left-hand side) and after (right-hand side) rotation and time shifting of the traces. (C) Diagram of energy showing the 95 per cent confidence interval (thick black line). (D) Phi and dt values obtained from the analysis of 250 measurement windows. (E) Cluster analysis.

orientations obtained by Singh et al. (2006) at stations located NW and NE of the Shillong plateau, respectively. Results obtained by Singh et al. (2006) at station RUPA, located several hundreds of metres away from station BMD, show phi trending N100◦ with a dt of 0.7 s, using the multi-events method (Wolfe & Silver 1998) on 11 phases. At station BMD, two null measurements show an orientation of phi trending either N26◦ /30◦ or N116◦ /120◦ . If the latter direction could be confirmed by non-null measurements, it would be in very good agreement with results obtained by Singh et al. (2006) at the nearby RUPA station. Himalayan arc: from Bhutan to northwest Himalaya Bhutan For consistency sake with Singh et al. (2007), we reprocessed the data recorded in Bhutan through the XA network (see Fig. 8 for comparison between both studies). Unlike Singh et al. (2007), we did not obtain any good quality at station BUMT. At CHUK however, two good measurements give an orientation of phi trending N53◦ /57◦ , in reasonable agreement with the single measurement obtained by Singh et al. (2007) showing a value of N71◦ . One of the two events recorded at CHUK has been recorded at stations PARO and DOCH and led to measurements of good quality, accounting for phi trending N31◦ and N19◦ , respectively. The error on the latter measurement is however 20.5◦ .

Eastern Nepal and southern Tibet We also reprocessed the data acquired by the HIMNT network in eastern Nepal and southern Tibet. When comparing our results with Singh et al. (2007, see Fig. 8), we do overall find a good agreement and only discrepancies are reported below. In the northern part of the network, whereas Singh et al. (2007) did not make any measurement at station DINX, we measured a well constrained orientation of phi trending N5◦ /32◦ . We measure an orientation of phi between N68◦ and 86◦ at NAIL compared to N98◦ (Singh et al. 2007), and ∼N60◦ on a single measurement at XIXI compared to N95◦ (Singh et al. 2007). At station SSAN, towards the north, a single value of phi gives N-60◦ whereas phi seems to be trending N88◦ on the basis of two data (Singh et al. 2007). At TUML, phi varies between N58◦ and N100◦ , based on the analysis of 12 good measurements. Singh et al. (2007) report an average value of N52◦ at TUML associated with an error of 21.5◦ . Similarly, if PHAP seems to be characterized by an orientation of phi around N96◦ in Singh et al. (2007), we do find a range of values varying between N42◦ and N71◦ . In the central part of the array, the single measurement at JIRI is in very good agreement with the average value of N46◦ obtained by Singh et al. (2007), although they report an associated error of 35.5◦ , on only two measurements. We did not retain any value at BUNG and NAMC.

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Figure 7. Example of a measurement performed at station ALB, located in the Indo-Gangetic plain, showing a very well-constrained fast polarization direction trending N-86◦ ± 2.75◦ , in complete disagreement with Kumar & Singh (2008). Kumar & Singh (2008) characterize station ALB by a fast polarization direction trending N36◦ ± 12◦ , values obtained using the multiple events method of Wolfe & Silver (1998) on two events. The six measurements we performed at this station consistently show a fast polarization direction trending N94◦ /101◦ .

Figure 8. Comparison between the average fast polarization directions obtained by Singh et al. (2007) (white) and the individual measurements performed in this study (red) at stations of the HIMNT network in eastern Nepal and southern Tibet (black circles) and of the XA network in Bhutan (blue circles). The length of each line is proportional to the delay time between the fast and slow S waves. National boundaries are shown in black. Black and white arrows represent the direction of the Indian plate motion as defined through the HS3-NUVEL1A (Gripp & Gordon 1990) and NUVEL1A (DeMets et al. 1990) plate models, respectively.

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M. Heintz et al. further towards the northeast is characterized by several nulls and two quite different good measurements (N-74◦ and N67◦ ). At the northeastern end of the profile, station JLM appears to be very well characterized, with phi trending N-19◦ /6◦ . Regarding the outlying stations, PTG in the southeast is extremely well characterized, with phi varying between N30◦ and N38◦ , whereas single measurements are performed at stations DCL (north of PTG) and DRS (west of the profile) showing phi trending N32◦ and N46◦ , respectively. We report two very different values of phi (N-20◦ and N-73◦ ) at station MNY, for events with backazimuths of 154◦ and 101◦ , respectively. Station HSL, towards the north, is characterized by phi ∼N-79◦ based on two measurements. Northwest Himalaya The northwestern Himalaya network is located in the Himanchal province and Ladakh (Jammu and Kashmir province), India, and is composed of 14 stations operated by NGRI between 2002 and 2003, and station HNL and LEH operated by the Indian Institute of Astrophysics and the University of Cambridge since 2000. Most stations along the profile (KUK, BDI, BSP, KTH, RTS, LEH, KDG and TKS) show an orientation of phi trending ENE–WSW to EW, with values ranging from N69◦ to N96◦ . At stations GHR, KUL, DCH and HMS measurements highlight a more NE–SW trend with phi ranging between N30◦ and N61◦ . The large variation of values obtained at HNL together with several null results leads towards an apparently isotropic medium. P L AT E M O D E L S F O R I N D I A

Figure 9. Map showing the individual good measurements obtained at the stations belonging to the NW Himalayan network (white circles), to the Uttaranchal network (orange circles), and three stations located around New Delhi (black circles). The length of each line is proportional to the delay time between the fast and slow S waves. Black and white arrows represent the direction of the Indian plate motion as defined through the HS3-NUVEL1A (Gripp & Gordon 1990) and NUVEL1A (DeMets et al. 1990) plate models, respectively.

Uttaranchal The Uttaranchal network, west of Nepal, consists of 19 stations oriented along a NE–SW profile, and several outlying stations (Fig. 9). Due to the very close spacing of the stations along the profile compared to the lateral resolution obtained through shear wave splitting measurements, we only processed data at a total of 18 seismological stations, including six outlying stations. Overall, except for stations located at the northeastern end of the profile, there is a good consistency in terms of the orientation of phi measured across the profile at stations KTD, DKL, PPL, SYT, NND, NTI and HLG (Fig. 9). Phi appears to be roughly ENE–WSW oriented (N58◦ –94◦ ), with some local variations between nearby stations. Station MRG for instance, located between DKL and PPL, differs by an orientation of phi ranging N-70◦ /-61◦ , whereas station GTH,

In studies dealing with seismic anisotropy, various absolute plate motion (APM) models are used for comparison with the fast polarization directions, usually with no justification for the model selection. We investigate here two potential plate models for India: HS3-NUVEL1A (Gripp & Gordon 2002) and NUVEL1A (DeMets et al. 1990). HS3-NUVEL1A (Gripp & Gordon 2002) is a widely used absolute reference frame, strongly constrained by the hotspot migration rate in the Pacific Ocean. When seismic anisotropy is interpreted in terms of the relative velocity of the lithosphere with respect to the deep mantle, it is usually assumed that this velocity is given by some model of absolute plate motion; this suggests that the anisotropy is generated by shear at the base of the lithosphere as it moves relative to the deep mantle. The hotspot reference model gives directions and velocities averaged over the past 5.8 Ma, and an average N340–355◦ E direction for the absolute motion of the Indian plate. NUVEL1A (DeMets et al. 1990) is a global model of current relative plate velocities, assuming constant velocities over the past 3 Myr. The N10◦ –20◦ E direction of the Indian plate with respect to fixed Eurasia obtained through NUVEL1A, is in good agreement with several studies dealing with geodetic measurements (Freymueller et al. 1996; Reddy et al. 2000) accounting for current tectonic motions between India and Eurasia. Due to the very unique tectonic setting of the India–Eurasia collision, it is reasonable to expect motions of the Indian lithosphere due to its Tertiary deformation resulting from the collision to be larger than those due to the absolute motion of India over the same period of time (past ∼50 Myr), that is, ∼NNE–SSW. If anisotropy is predominately within the lithosphere, and caused by in-plane compressive stress, then the directions should be perpendicular to NUVEL1A (DeMets C


Seismic anisotropy underneath India et al. 1990). If the anisotropic directions are parallel to NUVEL1A (DeMets et al. 1990), and caused by shearing at the base of the lithosphere, then the relative asthenospheric flow must also be in the same direction as the relative plate motion. DISCUSSION The first significant result of our analysis is that much of the south Indian shield is characterized by a NNE–SSW trending fast axis of anisotropy. Our second significant result is the turning of the fast anisotropy direction by 90◦ as one approaches the Himalayan foredeep. Null results Our interpretation draws considerable strength from the consistent polarization planes of an unprecedented number of null measurements (222 in total) obtained at sites in peninsular India and Sri Lanka: 90 and 19 at the permanent stations HYB and PALK (Sri Lanka), respectively, and 113 measurements for all the temporary stations deployed across the peninsula. Whilst null results do not allow a definitive interpretation, when they are interpreted properly, they can provide valuable information about the anisotropic properties of a region. The majority of the events used in this study have a backazimuth between N100◦ and N120◦ , and they result in null as well as non-null results. According to Wustefeld & Bokelmann (2007), favourable backazimuths for splitting measurements should be between N35◦ and N45◦ of these backazimuths, that is, ∼N55◦ /85◦ . We unfortunately do not record any events from these backazimuths with a suitable epicentral distance in India (Fig. 2). The quality of our non-null measurements is undeniable and they cannot be considered as near-null measurements. The agreement between the fast polarization directions obtained through the null and non-null results, allows us to confidently infer directions of anisotropy at stations otherwise characterized by a too small amount of non-null results to deduce any preferential orientation. This overwhelming preponderance of null measurements could be explained by the fact that the source of anisotropy lies in the sheared boundary layer between the lithosphere and the asthenosphere, sustained by the relative asthenospheric flow with respect to the NNE–SSW motion of the Indian plate. This relative flow would create foliations in the horizontal plane with a fast direction orientated along the flow. The relative paucity of non-null measurements would, therefore, appear more to be the result of the orthogonality of the incident shear wave phases (arriving at the stations with a backazimuth between N100◦ and N120◦ ) and the foliation planes, than to an absence of anisotropy, which is more difficult to conceive given the fast motion of the Indian plate. The apparently isotropic pattern obtained at station HYB confirms previous studies (Chen & Ozalaybey 1998; Barruol & Hoffmann 1999). Stations NND and SLM, both located in the vicinity of HYB, also exhibit complex patterns of anisotropy, mostly accounting for apparent isotropy and/or a very complex medium. SLM, NND and HYB are located along a NW–SE line, almost parallel to the Godavari graben (see Fig. 1 for location). We should not read too far in this observation, but it is worth mentioning that this ‘line’ also corresponds to the contrast between a high velocity anomaly in P-wave traveltime under the Dharwar craton and Cuddapah basin (stations SLM, NND and HYB) and a low velocity anomaly beneath the Eastern Ghat metamorphic belt and the Godavari basin (station KDM) (Prakasam & Rai 1998). The authors C


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interpreted the difference in terms of velocity anomaly as a thinning of the lithosphere between the Dharwar craton (>200 km) and the eastern margin ( 70◦ (16.3 per cent). On Fig. 11, we plot the values of ßNUVEL with respect to the latitude of the stations. We observe nearly all the values of ßNUVEL between 70◦ and 90◦ , at latitudes higher than N25◦ . In other words, north of N25◦ latitude, a large proportion of the fast polarization directions rotate to become almost perpendicular to the motion of the Indian plate colliding with Eurasia (see Fig. 5). Our results show a continent-wide consistency of the fast anisotropic directions (NNE–SSW) at most sites on the Indian shield, aligned with the Indian plate velocity referenced to fixed Eurasia. That this direction is parallel rather than orthogonal to the compression axis, as would be expected for in-plane compressive stresses within the lithosphere, indicates that the anisotropy is most likely due to the relative asthenospheric flow, parallel to the relative plate velocity. Furthermore, if the shearing of the lithosphere–asthenosphere boundary layer is the source of the anisotropy, a depth differentiated correlation should reveal a weak correlation between the orientation of the plate motion as defined through NUVEL1A (DeMets et al. 1990) and the anisotropy at shallower depths in the upper mantle, and a higher correlation should be seen at greater depth. The global anisotropic surface wave tomographic model of Debayle et al. (2005) corroborates these as-

sumptions: the authors observe a weak correlation at 50 km depth under India between the fast polarization directions of the azimuthal anisotropy and the plate motion derived from NUVEL1A (DeMets et al. 1990), and a much better one at 150 km. Towards the Himalaya–Tibet collision zone, the orientation of the fast polarization directions turns to become perpendicular to the relative plate velocity as defined through NUVEL1A. Previous studies also account for ∼EW orientation of phi north of the IndusTsangpo suture zone (McNamara et al. 1994; Hirn et al. 1995; Guilbert et al. 1996; Sandvol et al. 1997; Huang et al. 2000), thus validating our observation. Compression along a NNE–SSW axis is expected to align the a axis of olivine at right angles to it (ESE–WNW) so as to minimize the strain energy. The expected orientation of the anisotropy axis induced by compression at the collision front should be pervasive throughout the lithosphere. However, there is apparently little detectable evidence of this in the Indian shield to the south. What is observed, therefore, is more likely an expression of ongoing processes, rather than frozen orientations in the lithosphere during post-tectonic thermal relaxation. Beneath the shield, it parallels the converging plate velocity, but beneath the collision zone it is quite likely forced to turn parallel to the underthrusting lithosphere acting as a barrier. Such along strike fast polarizations have indeed been observed in some subduction zones (Savage 1999). We present on Fig. 12 a 3-D cartoon showing an interpretation of the formation of seismic anisotropy resulting from the India–Eurasia collision and summarising the above discussion. Further north at the trans-Himalayan Ladakh stations, the fast anisotropy directions have highly variable directions intermediate between the NUVEL 1A (DeMets et al. 1990) plate model and normal to the convergence front. A coherent explanation for this wide variation is at present not warranted by the data, particularly in the absence of well-constrained crustal anisotropy in the region. We discuss below the limited results available in terms of crustal anisotropy at the scale of the Indian continent. In a study based on shear wave splitting measurements of Moho converted Ps phases, Rai et al. (2008) determined the directions of crustal seismic anisotropy at 18 sites south of the transcontinental Narmada-Tapti-Son-Damodar lineament down to Sri Lanka. Their results indicate a delay time of ∼0.3 s at most of these stations, about a third of the total measured in the present study, which warrants our interpretation that our results largely reflect the anisotropic effects of sources in the mantle. Furthermore, their crustal anisotropy fast directions have a bimodal distribution of azimuths: approximately N–S in the Dharwar craton closely paralleling the structural

Figure 12. 3-D cartoon showing an interpretation of the formation of seismic anisotropy resulting from the India–Eurasia collision. C



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Table 1. Events information (backazimuth (Baz)), and station to event great circle arc length in degrees (Distance) and results of shear wave splitting measurements performed at each seismological station, sorted by geographical locations and/or networks. Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

105.08 105.8

103.83 104.19

−76 −73

−0.75 4.75

1.593 1.2

0.054 0.162

vg?? vg ??

107.15 109.46 104.45 107.69 102.32 259.77 108.75 107.8 106.36

88.6 90.51 107.21 89.13 105.69 156.45 102.13 102.98 103.49

41 −86 −82 43 53 36 81 74 67

12.75 13 8.25 11.25 12.5 9 11 9.5 15.75

0.85 1.05 1.093 0.75 0.675 0.5 0.775 0.775 0.5

0.268 0.275 0.46 0.179 0.15 0.056 0.181 0.087 0.125

G G F G G G G G G

105.287 105.89 106.61

104.52 102.34 102.7

0 8 12

14.5 7.25 5.5

0.775 0.718 0.875

0.281 0.226 0.206

G G G

105.14 108.52 106.59 107.57 104.87 106.71 105.39 106.13

87.84 102.64 106.05 103.49 105.88 103.62 103.68 104.01

−77 −78 −89 76 −81 74 −85 −88

4.25 5.75 8.25 11.25 3.75 11.75 10.75 5.25

1.218 1.25 1.15 0.325 0.9 0.325 0.725 0.925

0.304 0.3 0.156 0.068 0.175 0.068 0.218 0.225

G G F G G G G G

104.58 110.14

90.05 91.76

−60 −67

6.25

0.725

0.187

G N

108.3 110.15 107.34

103.06 93.32 103.91

−79 −72 −79

2 5.5

1.343 2.12

0.156 0.34

G F N

107.45 107.99 104.79 105.75 102.58 259.86 109 108.05 105.35 107.2 105.88

88.06 88.59 89.12 86.77 105.13 157.02 101.59 102.43 104.81 102.56 102.61

106.61

102.94

−80 −83 79 58 40 55 −86 72 73 64 72 77 72

8.25 8.5 10.75 11.5 16.75 6.25 13.5 5.75 9 6 10.25 6 11.25

1.062 0.8 0.55 0.825 0.968 0.9 0.75 0.75 0.75 0.843 0.75 1.125 0.7

0.148 0.106 0.081 0.193 0.414 0.15 0.225 0.043 0.075 0.078 0.093 0.068 0.1

G G G G F G G VG G G G G G

104.94 106.26 105.67

104.27 104.24 104.61

16 30 60

10.25 16.75 15.25

0.725 0.625 0.468

0.262 0.234 0.117

F F G

Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

DOCH 2002.091.19.59.32

258.11

163.18

19

20.5

0.39

0.337

G

NE NEPAL BIRA 2001.294.00.29.21 2002.021.15.42.35 2002.181.21.29.36 2002.231.11.01.01 DINX 2001.336.02.47.56 2001.346.12.53.18 2001.361.10.54.51 2002.002.17.22.48 2002.031.16.27.19 2002.128.05.26.00 2002.167.06.55.13 GAIG 2001.336.02.47.56 2001.346.12.53.18 2001.361.10.54.51 2002.002.17.22.48 ILAM 2002.277.19.05.48 2001.294.00.29.21 JIRI 2002.145.05.36.32 MAZA 2002.250.08.14.19 2002.290.04.23.55 2002.295.11.39.04 MNBU 2002.224.02.59.24 2002.231.11.01.01 NAIL 2002.290.04.23.55 2002.295.11.39.04 PHAP 2002.128.05.26.00 2002.181.21.29.36 2002.231.11.01.01 2002.290.04.23.55 2002.295.11.39.04 PHID 2001.361.10.54.51 2002.021.15.42.35 2002.031.16.27.19 2002.113.15.05.31 2002.128.05.26.00 2002.181.21.29.36 2002.231.11.01.01 2002.250.08.14.19 2002.277.19.05.48 2002.290.04.23.55 RBSH 2001.346.12.53.18 2002.021.15.42.35 2002.031.16.27.19

123.48 108 109.19 108.25

106.742 88.277 101.26 102.117

71 40 −72 16

8 5.25

106.1 109.43 107.45 109.71 104.76 102.19 103.88

87.07 90.173 88.529 90.49 89.53 105.49 101.92

26 23 27 20 32 22 5

6.5 1.25 2.75

0.968 1.175 1.075

0.289 0.106 0.187

3.75 2.5 5.25

0.825 1.325 1.625

0.125 0.181 0.168

G G G N G G G

105.78 109.2 107.18 109.5

86.98 89.97 88.39 90.28

49 50 38 52

15 14.25 11.25 16.75

0.45 0.65 0.825 0.6

0.156 0.206 0.225 0.25

G G G G

107.57 123.56

101.81 106.48

26 58

3.75 12.75

1 0.975

0.21 0.306

F G

49

8.25

1.3

0.112

G

−4 5 −11

12.25 11 12.5

0.45 0.7 0.375

0.143 0.2 0.093

G G G

33.73

80.277

0.775 0.975

0.062 0.156

RC14 2002.290.04.23.55 2002.295.11.39.04 RUMJ 2001.361.10.54.51 2002.002.17.22.48 2002.013.11.04.19 2002.021.15.42.35 2002.128.05.26.00 2002.148.04.04.22 2002.181.21.29.36 2002.231.11.01.01 2002.295.11.39.04 SAJA 2002.250.08.14.19 2002.290.04.23.55 2002.295.11.39.04 SIND 2002.113.15.05.31 2002.181.21.29.36 2002.219.04.50.06 2002.231.22.01.01 2002.250.08.14.19 2002.277.19.05.48 2002.290.04.23.55 2002.295.11.39.05 SSAN 2002.031.16.27.19 2001.272.02.40.07 THAK 2002.181.21.29.36 2002.224.02.59.24 2002.231.11.01.01 TUML 2001.361.10.54.51 2002.021.15.42.35 2002.031.16.27.19 2002.113.15.05.31 2002.128.05.26.00 2002.148.04.04.22 2002.181.21.29.36 2002.231.11.01.01 2002.250.08.14.19 2002.277.19.05.48 2002.290.04.23.55 2002.290.04.23.55 2002.295.11.39.04 XIXI 2002.290.04.23.55 2002.277.19.05.48 2002.295.11.39.04

G G N N

105.27 105.87 106.59

104.6 102.41 102.77

110.36 107.31

93.222 103.74

70 72

16 18.5

0.75 0.625

0.218 0.296

F F

105.26 105.99

103.63 103.98

68 86

14 9.25

0.281 0.593

0.078 0.156

G G

102.28 108.72 107.77 105.59 106.33

105.71 102.17 103.01 103.18 103.52

42 71 42 62 52

20 14.25 15.5 17 14.5

0.6 0.375 0.375 0.3 0.4

0.9 0.1 0.143 0.112 0.106

F G G G G

107.71 108.25 105.04 106 102.85 109.26 108.31 105.61 107.46 106.15

87.526 88.061 88.59 86.23 104.6 101.06 101.9 104.27 102.02 102.07

81 −89 72 62 −84 73 88 71 48 68

13.25 8.75 15 15 6.25 13.25 11.5 10.75 11.5 12

0.5 0.775 0.425 0.4 1.218 0.575 0.8 0.593 0.531 0.5

0.132 0.162 0.143 0.106 0.242 0.131 0.206 0.109 0.093 0.075

G G G G G G G G G G

109.29 107.83 104.62

90.258 89.17 89.66

−72 −70 −65

5

0.15

N N G

Baz

Distance

Phi

Err

Dt

Err

Qual.

256.81 259.42

163.01 159.04

53 57

16.25 13.5

0.99 0.96

0.3 0.202

G G

258.39

162.92

31

16.5

0.39

0.127

G

0.6

BHUTAN Event CHUK 2002.091.19.59.32 2002.148.04.04.22 PARO 2002.091.19.59.32

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M. Heintz et al.

Table 1. (Continued.) Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

104.09 54.27 104.22 108.4 103.85 104.49

113.85 106.48 95.35 99.43 96.62 95.96

−60 66 10 17 8 10

5.25 14

1 0.97

0.13 0.27

G G N N N N

108.31 105.58 103.41 110.7 103.63 104.06 310.54 106.22 103.67

102.3 97.87 96.22 115.74 96.42 97.05 124.34 99.49 96.23

75 −88 −87 86 −86 89 82 17 82

9.5 8 8.25 3.75 9.5 6 12.75

0.675 1.42 1.22 1.3 1.3 0.975 0.97

0.16 0.35 0.31 0.13 0.38 0.21 0.15

8.5

0.5

0.13

G G G VG G VG G N G

102.87 102.22 154.46 101.76 108.89

93.74 108.2 106.05 92.22 99.54

−73 −69 −20 −73 10

5 3 3.75 1.75

1.1 1.125 0.9 0.85

0.25 0.24 0.168 0.05

F/G F G G N

219.73 110.71 110.71 102.02

123 115.62 115.62 112.83

−70 −76 −61 −63

14 19 2

0.675 0.975 1.45

0.13 0.43 0.16

G F VG N

270.71 101.82 108.41 108.33 108.72

94.09 109.17 100.42 100.48 100.51

67 90 52 −90 −66

7 14.5 19.25 16.5

1.1 0.75 0.32 0.5

0.19 0.24 0.26 0.17

G F F F N

104.56 54.23 104.19 108.37

96.2 106.47 95.39 99.48

9 78 −82 −79

12.75

0.77

0.175

105.66 147.34 105.94 103.76 104.18 102.03 103.79

97.54 101.07 110.61 96.2 96.83 112.75 96.01

10 79 88 −78 −86 84 15

6.5 4

0.875 1.1

0.13 0.13

3 13

0.95 0.72

0.12 0.26

102.95 104.06 104.2 104.83 109.02 108.94 109.33

93.53 93.95 95.95 95.28 99.29 99.34 99.37

35 37 35 30 35 33 38

5.25 6 4.25 2.25 8.25 3.25 13

0.82 0.9 0.82 1.07 0.95 1.17 0.85

0.15 0.25 0.16 0.137 0.35 0.28 0.31

219.84 105.77 105.96 311.18 53.86

123.33 97.55 110.5 124.43 107.38

78 −76 58 70 87

14

0.55

0.1

10 10 12.5

0.4 0.6 0.67

0.08 0.1 0.15

UTTARANCHAL ALI 2006.023.06.02.59 2006.066.06.28.55 2006.090.13.21.00 2006.107.23.49.59 2006.219.22.18.54 2006.276.18.03.14 BHT 2007.271.11.16.39 DCL 2007.094.11.00.27 2007.124.12.06.52 2007.196.09.27.34 2007.213.17.08.51 2007.270.19.57.49 2007.271.01.01.48 DRS 2007.124.12.06.52 2007.213.17.08.51 2007.215.00.41.14 2007.270.19.57.49 DKL 2005.138.09.10.53 2005.163.19.26.24 2005.268.12.55.46 2006.057.03.08.27 2006.090.13.21.00 2006.107.23.49.59 2006.219.22.18.54 2006.272.13.08.25 GTH 2005.168.06.21.42 2005.341.23.32.51 2006.057.03.08.27 2006.057.03.08.27 2006.090.13.21.00 2006.227.23.53.47 2006.291.10.45.32 2006.340.20.39.49 2007.059.23.13.15 HLG 2005.101.17.08.53 2006.057.03.08.27 2006.219.22.18.54 2006.227.23.53.47 HSL 2007.213.17.08.51 2007.228.08.39.27 2007.270.19.57.49 2007.271.01.35.51 JLM 2006.057.03.08.27 2006.219.22.18.54 2006.281.13.50.24 2007.023.17.16.20 2007.271.11.16.39

105.93 103.83 110.74 102.27 104.46 106.59

97.09 95.52 115.16 93.62 96.36 98.84

−74 10 −67 −74 15 −76

108.32

101.22

13

2.75

1.85

0.3

F

108.83 271.45 104.24 104.87 109.05 109.06

98.7 95.38 95.89 95.22 99.25 99.34

−72 32 −77 −78 −79 −74

1.25 11.25

1.85 0.95

0.3 0.15

F G N N N N

270.41 103.75 153.87 107.93

93.47 97.24 107.21 101.28

44 12 46 23

219.54 219.68 105.62 105.91 110.71 101.85 104.09 310.65

122.73 122.86 97.8 110.75 115.68 94.27 96.98 124.37

73 80 10 0 81 −83 8 86

19.99 111.41 105.97 105.97 110.71 102.05 103.9 142.22 221.28

105.46 114.91 110.45 110.45 115.42 112.58 95.87 108.33 124.48

30 30 71 25 25 45 −79 67 −74

N N N N (G) N N

6.75

0.9

0.08

4 6.75

0.67 1.05

0.075 0.3

9.25 13

0.575 0.75

0.068 0.13

10.75

0.9

0.175

0.7

0.068

8

18.75

0.52

0.18

18.75

0.42

0.16

6 16.2

1.42 0.65

0.23 0.29

108.59 106.03 104.43 102.11

101.73 110.21 96.41 112.31

15 77 84 72

103.92 103.16 108.06 107.98

96.96 87.14 101.04 101.09

13 8 −79 −79

1.75 1.75

106.12 104.57 104.1 102.62 108.92

109.98 96.18 113.87 94.21 100.15

3 6 −12 −19 5

0.75 15.25 6.75 13 6.75

6 10.75 4

F N G F VG G N N G N (VG) N (VG) VG N N F N N F N G G

0.0625 0.13 0.05

N G VG VG

2.725 2.9

0.162 0.29

N N G G

1.25 0.8 0.7 0.35 0.8

0.043 0.343 0.162 0.19 0.25

VG G G G G

0.625 0.52 0.77

KSP 2006.281.13.50.24 2006.288.17.07.49 2006.291.10.45.32 2007.094.11.00.27 2007.196.09.27.34 2007.213.17.08.51 KTD 2005.101.17.08.53 2005.268.12.55.46 2006.066.06.28.55 2006.090.13.21.00 2006.120.08.17.34 2006.219.22.18.54 2006.272.13.08.25 2006.276.18.03.14 2006.291.10.45.32 MNY 2007.023.17.16.20 2007.126.21.11.52 2007.215.00.41.14 2007.224.12.05.19 2007.270.19.57.49 MRG 2005.163.19.26.24 2006.090.13.21.00 2006.090.13.21.00 2006.227.23.53.47 NND 2007.124.12.06.52 2007.126.21.11.52 2007.270.19.57.49 2007.271.01.35.51 2007.271.11.16.39 NTI 2006.219.22.18.54 2006.288.17.07.49 2006.291.10.45.32 2007.094.11.00.27 PPL 2006.023.06.02.59 2006.039.05.04.53 2006.057.03.08.27 2006.120.08.17.34 2006.219.22.18.54 2006.227.23.53.47 2006.291.10.45.32 PTG 2007.023.17.16.20 2007.115.13.34.14 2007.196.09.27.34 2007.213.17.08.51 2007.270.19.57.49 2007.271.01.35.51 2007.271.11.16.39 SYT 2005.163.19.26.24 2005.268.12.55.46 2006.057.03.08.27 2006.272.13.08.25 2006.288.17.07.49

C

N G N N N G G N G G N (G) VG G G VG G G G G N G G G



1355

Table 1. (Continued.) Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

100.36 102.28 106.5

112.43 81.4 102.9

−87 51 −82

5.75

1.32

0.212

N G N

323.6 131.03 105.83

121.46 110.37 101.07

59 32 −88

99.42 99.8 106.4

111.93 113.13 103.05

100.6 103.39

NW HIMALAYA BDI 2003.095.22.03.32 2003.117.16.03.40 BSP 2003.117.16.03.40 2003.117.16.03.40 2003.310.10.38.04 CHD 2003.095.22.03.32 2003.117.16.03.40 2003.310.10.38.04 DCH 2003.187.21.34.15 2003.203.04.21.40 2003.208.02.04.11 2003.245.18.28.00 RTS 2002.251.13.15.55 2003.117.16.03.40 2003.245.18.28.00 MTH 2003.117.16.03.43 2003.004.05.15.03 2003.073.12.54.12 KTH 2003.187.21.34.15 2003.208.02.04.11 2003.273.14.08.37 2003.280.04.55.28 2003.310.10.38.04 KUL 2003.095.22.03.32 2003.117.16.03.40 2003.208.02.04.11 2003.208.02.04.11 2003.265.04.45.36 2003.273.14.08.37 2003.310.10.38.04 KDG 2003.010.13.11.56 2003.095.22.03.32 2003.117.16.03.43 2003.187.21.34.15 2003.203.04.21.40 2003.208..02.04.11

103.37 106.71

99.07 102.92

88 69

7 11.25

0.875 0.8

0.156 0.13

G G

106.64 106.64 105.56

103.02 103.02 101.46

−86 88 88

2.75 6.5 2.5

1.62 1.175 1.2

0.2 0.118 0.11

G G VG

103.41 106.76 105.68

99.01 102.85 101.3

55 43 87

0.575 0.5 1.45

0.081 0.081 0.137

G G VG

102.98 103.56 100.21 92.95

98.5 97.28 114.32 114.4

54 61 49 40

9.75 9.75 9.25 8.25

0.775 0.825 0.975 0.675

0.1 0.093 0.118 0.087

G G VG G

103.86 106.45 92.8

111.22 103 114

82 70 −89

7 15.75

0.975 0.475

0.1 0.16

G G N

107.14 100.72 96.53

101.91 112.06 112.6

23 19 −81

3.75

1.22

0.28

G N N

103.02 100.36 110.1 92.79 105.56

98.44 114.27 117.54 117.56 101.4

−86 76 −84 −73 81

5.75 11.75 7.75 4.5 6.25

1.8 1.05 1.32 2.85 1.275

0.106 0.24 0.21 0.07 0.156

G G G F G

103.38 106.64 100.48 100.48 324.59 110.24 105.59

99.02 102.93 114.26 114.26 119.99 117.48 101.36

57 66 48 50 60 64 51

8 8.25 6.75 14.75

0.65 0.75 0.85 0.675

0.1 0.068 0.087 0.206

4.75 13.5

0.675 0.7

0.05 0.24

G VG VG G N G G

102.32 103.28 106.36 102.96 103.57 99.66

81.55 99.14 103.08 98.53 97.32 114.25

76 −90 85 83 −84 86

9.75 10.75 4.25 4.25 6 5.75

1.175 1.5 1.375 1.22 1.625 1.225

0.181 0.3 0.13 0.18 0.193 0.225

G G G G G G

9.5 11.5 3.5

HMS 2002.295.11.39.04 2003.010.13.11.56 2003.117.16.03.43 KUK 2003.265.04.45.36 2003.306.05.32.15 2003.310.10.38.04 LEH 2002.290.04.23.52 2003.004.05.15.03 2003.117.16.03.43 GHR 2003.004.05.15.04 2003.095.22.03.32 HNL 2002.250.08.14.19 2002.290.04.23.52 2002.352.14.12.21 2003.095.22.03.32 2003.117.16.03.43 2003.163.08.59.20 2003.165.18.28.49 2003.187.21.34.15 2003.203.04.21.40 2003.208.02.04.11 2003.245.18.28.00 TKS 2002.277.19.05.49 2002.290.04.23.55 2003.052.22.13.25 2003.095.22.03.32 2003.095.22.03.32 2003.117.16.03.43 2003.156.08.23.17 2003.124.20.08.45 2003.165.18.28.49 2003.187.21.34.15 2003.203.04.21.40 2003.208.02.04.11 2003.245.18.28.00

5

0.875

0.106

N N VG

74 69 73

5.75 2 3.5

1.2 1.1 1.075

0.29 0.09 0.175

G VG VG

113.05 99.01

30 45

12.25 8.75

0.72 0.52

0.16 0.11

G G

99.96 100.61 219.84 104.19 107.36 102.79 102.97 103.87 104.45 100.99 93.81

112.96 110.57 124.21 97.69 101.56 81.38 83.92 97.08 95.86 112.88 112.92

9 8 35 −77 −85 −22 54 −76 26 57 −80

6.25 8.25

0.625 0.675

0.18 0.2

10 16

1.175 0.275

0.212 0.075

6 16.25

0.975 0.42

0.27 0.11

G G N N (G) N (G) G G N (G) G G N

100.38 99.16 104.91 103.13 103.13 106.19 109.75 109.59 102.32 102.82 103.44 99.39 92.13

112.4 112.35 115.4 99.36 99.36 103.32 117.13 117.6 85.56 98.74 97.54 114.44 114.25

88 88 −89 −90 −89 74 89 89 89 −88 6 0 88

1.75 19.75

1.675 0.9

0.13 0.33

N (G) N N N (VG) N (G) G F N N N (VG) N N (G) N (VG)

INDIAN SUBCONTINENT AND SRI LANKA Event ABU 2002.168.21.26.22 2002.231.11.01.01 2002.231.11.08.24 2002.290.04.23.55 2006.219.22.18.54 2006.237.00.44.46 ALM 2002.180.02.39.00 2002.231.11.01.01 2002.231.11.08.24 2002.231.11.08.24 2003.208.02.04.11 2004.107.16.58.36

C

Baz

Distance

Phi

99.99 103.77 106.64 101.48 102.4 261.77

98.37 114.19 113.11 114.59 100.88 143.52

20 15 21 10 12 71

102.24 104.35 107.26 107.26 102.54 105.65

93.38 109.47 108.72 108.72 111.69 113.67

15 11 13 10 3 10


Err

4.75

Dt

1.9

Err

Qual.

0.28

N N N N N F N N N N N N

Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

SLM 1999.016.10.44.39 1999.028.08.10.05 1999.037.21.47.59 1999.053.01.00.32 1999.092.17.05.47 1999.099.12.16.01 1999.103.10.38.48 1999.103.10.38.48 1999.127.14.13.52 1999.127.14.13.52 2000.114.09.27.23 2000.125.20.36.32 2000.127.13.44.13

24.07 34.19 102.87 110.38 109.24 113.46 107.49 107.49 26.05 26.05 244.27 104.29 101.73

98.16 89.93 91.43 96.46 94.71 105.15 109.05 109.05 95.79 95.79 142.83 106.47 89.81

28 −82 18 −69 −69 49 −66 54 −71 −67 28 88 −75

2.5 2

1.31 0.86

0.16 0.07

1.5

1.8

0.18

21.5

0.86

0.4

7.25 14 4 4.5 8.25

0.6 1.2 1.95 0.787 0.63

0.11 0.46 0.38 0.056 0.11

G VG N G N F N VG G G G G N

1356

M. Heintz et al.

Table 1. (Continued.) Event 2004.250.12.42.59 2004.250.12.42.59 2004.255.21.52.38 2004.272.15.29.53 2006.023.06.02.59 2006.057.03.08.27 BAI 2002.285.20.09.11 2002.285.20.09.11 2002.277.19.05.48 2002.280.19.00.32 2003.020.19.04.50 BGL 1998.361.00.38.26 1999.028.08.10.05 1999.037.21.47.59 1999.045.21.12.24 1999.127.14.13.52 1999.127.14.13.52 1999.340.23.12.33 2000.008.16.47.20 2000.009.21.54.40 2000.026.13.26.50 2000.026.13.26.50 2000.056.01.43.58 2000.064.02.24.22 2000.065.23.57.03 2000.114.09.27.23 2000.133.18.43.18 BMD 2002.231.11.01.01 2002.231.11.08.24 CHP 2002.231.11.01.01 2002.231.11.08.24 2005.101.17.08.53 CUD 1999.340.23.12.30 2001.009.16.49.28 GBA 1998.361.00.38.26 1999.036.11.39.45 1999.037.21.47.59 1999.053.01.00.32 1999.053.01.00.32 1999.099.12.16.01 1999.099.12.16.02 1999.103.10.38.48 1999.103.10.38.48 1999.340.23.12.33 2000.008.16.47.20 2000.056.01.43.58 2000.114.09.27.23 2000.125.20.36.32 2000.133.18.43.18 GRR 2001.009.16.49.28 HNL 2000.296.20.26.39 2006.066.06.28.55 2006.078.04.36.52 2006.291.10.45.32 2007.115.13.34.14 LTV 1998.361.00.38.26 1999.028.08.10.05 1999.037.21.47.59

Baz

Distance

Phi

220.87 220.87 217.57 208.67 106.06 106.38

124.22 124.22 122.41 93.37 96.81 109.87

40 42 −81 24 17 10

Err

315.83 315.83 109.18 112.12 102.04

156.17 156.17 98.39 87.54 100.89

86 80 57 73 34

108.52 33.87 102.64 104.94 25.74 25.74 25.52 103.31 106.99 103.64 103.64 107.79 151.52 154.84 241.25 247.72

109.43 93.18 92 93.81 99.1 99.1 97.81 110.84 100.43 111.12 111.12 100.04 92.61 91.85 140.28 144.36

−72 39 15 36 30 30 31 15 16 20 48 46 −30 35 13 5

110.17 112.96

98.02 97.34

26 30

N N

110.15 112.92 114.38

97.92 97.14 89.77

21 20 36

N N N

25.85 104.76

96.01 92.14

46 16

108.31 102.28 102.58 110.32 110.32 113.87 113.87 108.11 108.11 25.41 103.07 107.66 241.89 104.75 248.47

109.8 92.52 92.32 96.97 96.97 105.5 105.5 109.7 109.7 97.41 111.12 100.4 140.36 107.28 144.37

8 9 4 9 11 13 17 9 13 57 8 10 4 5 −5

104.25

94.28

22

103.48 103.3 99.53 103.55 103.11

96.55 99.97 96.59 95.85

10 78 21 14 −76

107.83 33.75 102.5

110.29 91.76 92.69

−34 20 −25

8.25

Dt

0.35

Err

0.05

4.5 13 13.75 4.25 3

0.91 0.65 0.455 0.55 0.585

0.081 0.211 0.12 0.02 0.056

1.75

1.61

0.193

19.75

0.71

0.43

1.5

1.46

0.122

11.75 16

1.2 0.79

0.24 0.28

8.75 6.75 9.5

1.35 0.825 1.05

0.24 0.064 0.168

9.75

3.25

1.25

0.48

1.31

1.35

0.14

0.225

0.075

Qual.

Event

Baz

Distance

Phi

Err

Dt

Err

Qual.

N N VG N N N

2000.133.18.43.18 2000.166.02.15.25 2000.168.07.55.35 TPT 2001.009.16.49.28 NGP 2007.126.21.11.52 2007.213.17.08.51 2007.270.19.57.44 2007.278.07.17.52 2007.278.07.17.52 2007.286.17.45.53 2007.289.21.05.43 2007.322.05.40.12 2007.333.19.00.19 HYB 1990.224.21.25.22 1990.271.19.44.48 1990.271.19.44.48 1990.290.14.30.14 1991.144.20.50.55 1991.150.13.17.44 1991.160.07.45.06 1991.160.07.45.06 1992.148.05.13.41 1992.193.10.44.20 1992.193.10.44.20 1992.220.18.19.19 1992.241.18.18.45 1992.259.21.04.00 1992.259.21.04.00 1992.285.19.24.29 1992.292.15.11.59 1993.010.14.39.03 1993.010.14.39.03 1993.010.14.39.03 1993.080.05.04.59 1993.080.05.04.59 1993.106.14.08.38 1993.106.14.08.38 1993.136.21.44.50 1993.136.21.44.50 1993.144.23.51.22 1993.145.23.16.43 1993.219.17.53.27 1993.219.17.53.27 1993.219.17.53.27 1994.043.17.58.25 1994.068.23.28.07 1994.090.22.40.53 1994.090.22.40.53 1994.117.09.23.27 1994.117.09.23.27 1994.130.06.36.28 1994.300.22.20.31 1994.300.22.20.31 1994.309.02.16.01 1995.017.16.54.12 1995.017.16.54.12 1995.036.22.51.10 1995.036.22.51.10 1995.036.22.51.10 1995.180.12.24.03 1995.180.12.24.03 1995.276.01.51.24 1995.305.00.35.32 1996.077.14.48.56

251.28 112.65 231.84

146.63 104.85 146.96

40 −68 −28

3.5 2.25 4

0.937 1.61 1.5

0.056 0.18 0.196

G F F

104.32

93.94

−78

104.55 104.97 109.61 110.68 110.68 109.76 111.25 260.84 309.36

107.19 94.27 97.58 107.74 107.74 97.55 107.95 147.87 127.02

45 45 71 51 34 66 63 29 −51

108.35 103.21 103.21 278 267.22 30.38 105.67 105.67 101.46 108.28 108.28 21.14 269.67 103.78 103.78 108.25 313.11 212.36 212.36 212.36 103.89 103.89 103.69 103.69 100 100 254.1 29.61 110.23 110.23 110.23 109.39 103.68 108.16 108.16 106.52 106.52 245.59 112.28 112.28 147.31 106.99 106.99 124.66 124.66 124.66 108.34 108.34 299.56 241.85 104.32

96.09 92.46 92.46 149.62 150.6 92.02 109.64 109.64 90.08 108.06 108.06 97.68 92.31 92.93 92.93 95.92 145.6 112.38 112.38 112.38 107.11 107.11 106.73 106.73 111.36 111.36 146.9 92.26 106.88 106.88 106.88 96.69 107.1 107.02 107.02 112.28 112.28 143.11 106.85 106.85 98.73 107.11 107.11 108.39 108.39 108.39 96.27 96.27 152.66 150.25 93.14

20 15 20 −21 0 30 16 18 100 25 27 19 18 15 −66 22 −46 −53 30 34 42 −73 15 20 20 10 −20 −86 23 14 32 22 7 42 26 20 20 65 26 35 60 15 25 45 42 49 65 −53 30 63 15

G F F VG F N F N F N N F N N N G F N G G F

F N N N N N N N F N N N N N F N N N

16.5 7

0.39 0.95

0.14 0.19

6

1.07

0.26

N G F N G

0.82 1.2 0.6

0.33 0.168 0.168

F G F

13.25 1.25 14.75

C

N 5.5 6 11.25 7 4 7.25 5.75 14.25

11

0.9 0.95 0.75 0.62 1.07 0.675 0.45 0.455

0.1 0.14 0.09 0.106 0.14 0.05 0.025 0.121

0.41

0.10

7.5

3.075

0.38

2

2.36

0.23

9.75

0.65

0.162

6

0.412

0.075

9.5

1.01

0.26

0.2625 1.31

0.0375 0.14

11 6.25

G G G G G G G G N N N N F N N N N N N N N F N N N N F N N G N N N N N N F N N N N N F N N N N N N N N N N N N F G N N N



1357

Table 1. (Continued.) Event 1999.334.04.01.53 1999.340.23.12.33 1999.340.23.12.33 1999.341.00.19.49 2000.133.18.43.18 KOD 1998.084.12.17.22 1998.088.19.48.16 1998.136.02.22.03 1998.361.00.38.26 1999.028.08.10.05 1999.034.01.13.57 1999.037.21.47.59 1999.092.17.05.47 1999.103.10.38.48 1999.103.10.38.48 2000.114.09.27.23 2000.133.18.43.18 2000.166.02.15.25 2001.103.15.33.53 2001.118.04.49.53 2001.118.04.49.53 2002.231.11.01.01 2002.231.11.01.01 2002.231.11.08.24 2003.050.03.32.36 2003.073.12.54.12 2003.095.22.03.32 2005.078.17.34.46 2005.078.17.34.46 2005.101.17.08.53 KDM 2001.009.16.49.28 KIL 1999.028.08.10.05 1999.037.21.47.59 1999.045.21.12.24 1999.053.01.00.32 1999.053.01.00.32 1999.053.01.00.32 1999.103.10.38.48 1999.103.10.38.48 1999.127.14.13.52 1999.127.14.13.52 1999.334.04.01.53 1999.340.23.12.33 2000.008.16.47.20 MBN 1999.028.08.10.05 1999.053.01.00.32 1999.103.10.38.48 1999.103.10.38.49 1999.127.14.13.52 1999.334.04.01.53 1999.334.16.23.34 1999.340.23.12.33 2000.008.16.47.20 2000.009.21.54.40 2000.114.09.27.23 2000.125.20.36.32 2000.125.20.36.32 NND 1999.028.08.10.05 1999.092.17.05.47 1999.099.12.16.01 1999.099.12.16.01 1999.103.10.38.48

C

Baz

Distance

Err

Qual.

Event

Baz

Distance

Phi

258.2 25.32 25.32 25.36 250.28

147.69 96.22 96.22 96.25 144.78

Phi 15 15 8 10 16

6.75

1.4

0.162

2 4.25

2.125 0.937

0.2 0.075

G N N F F

155.09 105.42 110.26 109.42 33.96 107.89 102.71 109.37 109.21 109.21 238.48 244.29 113.83 210.79 105.74 105.74 109.72 109.72 112.13 32.059 104.91 105.77 109.92 109.92 111.08

90.62 105.73 105.77 108.63 95.54 110.42 91.49 94.09 108.54 108.54 138.8 143.14 103.82 105.35 107.82 107.82 105.73 105.73 104.08 97.6 109.43 93.2 105.71 105.71 96.64

46 22 22 22 35 30 −71 30 20 20 −2 5 22 −3 13 20 40 47 40 50 40 39 21 30 20

5.25

0.5

0.06

7.25 7.25

0.675 0.86

0.0937 0.12

8.5

0.715

0.089

105.18

91.18

43

33.53 102.14 104.29 109.55 109.55 109.55 106.36 106.36 25.11 25.11 263.04 24.96 101.16

89.53 93.99 95.98 99.19 99.19 99.19 111.72 111.72 94.97 94.97 147.55 93.67 112.82

37 15 26 23 20 23 17 19 30 31 41 30 5

33.84 109.99 107 107 25.56 261.25 109.97 25.37 101.87 106.3 245.48 103.8 103.8

89.96 97.84 110.41 110.41 95.62 148.39 97.27 94.32 111.6 101.45 142.08 107.81 107.81

−80 −66 −61 23 21 85 −73 30 13 24 −2 −78 −81

109.83 103.21 103.21 106.61 117.69 117.69 256.36 307.43 113.26 113.26 107.75 104.73 271.97 110.66 110.66 103.59 108.43 108.43 116.84 101.18 107.19 107.19 34.09 102.78 106.99 106.99 262.2 25.67 25.72 25.72 245.92 253.52 112.26 105.07 100.65 100.65 107.28 104.58 103.63 103.63 115.94 115.94 107.36 107.89 110.51 283.62 106.35 106.35 106.35 106.66 133.81 101.75 102.9

109.74 92.62 92.62 107.91 108.39 108.39 146.1 146.98 106.12 106.12 109.61 93.34 148.35 98.54 98.54 106.5 107.14 107.14 109.14 91 109.87 109.87 89.03 92.04 109.75 109.75 149.31 93.46 93.49 93.49 143.08 146.71 105.65 94.06 111.16 111.16 107.28 93.08 108.61 108.61 109.32 109.32 109.59 107.03 107.03 149.69 108.48 108.48 108.48 109.57 101.14 110.89 92.6

21 20 18 19 30 45 75 41 26 27 22 21 85 22 27 19 45 23 46 21 21 18 −73 16 20 22 77 22 24 20 −14 −15 26 17 15 5 30 20 15 18 47 31 19 22 24 1 44 19 25 26 48 10 19

33.75 108.51 112.34 112.34 106.12

88.3 97.13 107.73 107.73 111.39

84 17 25 25 17

1996.107.00.30.54 1996.162.01.04.46 1996.162.01.04.46 1996.218.22.38.22 1996.310.09.41.34 1996.310.09.41.34 1997.023.02.15.22 1997.245.12.13.22 1997.247.04.23.37 1997.247.04.23.37 1997.287.09.53.18 1997.319.18.59.24 1997.332.22.53.41 1998.004.06.11.59 1998.004.06.11.59 1998.088.19.48.16 1998.136.02.22.03 1998.136.02.22.03 1998.190.14.45.40 1998.197.11.56.36 1998.361.00.38.26 1998.361.00.38.26 1999.028.08.10.05 1999.037.21.47.59 1999.103.10.38.48 1999.103.10.38.48 1999.334.04.01.53 1999.340.23.12.33 1999.341.00.19.49 1999.341.00.19.49 2000.114.09.27.23 2000.133.18.43.18 2000.166.02.15.25 2000.222.00.08.41 2000.255.17.17.53 2000.255.17.17.53 2000.353.01.19.21 2001.009.16.49.28 2001.118.04.49.53 2001.118.04.49.53 2001.154.02.41.57 2001.154.02.41.57 2001.185.07.06.31 2002.231.11.01.01 2002.231.11.08.24 2002.285.20.09.11 2003.004.05.15.03 2003.004.05.15.03 2003.004.05.15.03 2003.208.02.04.11 2003.233.12.12.49 2004.025.11.43.11 2004.100.15.23.35 PAL 2000.353.01.19.21 2001.009.16.49.28 2001.059.12.30.14 2001.059.18.54.32 2001.118.04.49.53 2001.154.02.41.57 2001.185.07.06.31 2002.068.12.27.11 2002.101.21.56.56 2002.231.11.01.01 2002.285.20.09.11 2003.004.05.15.03 2003.117.16.03.40

110.07 105.16 111.75 18.66 106.77 118.7 110.52 213.75 104.56 110.62 266.01 109.37 110.76

102.05 88.45 92.22 121.96 103.93 102.81 104.31 106.05 88.88 101.71 152.67 103.38 91.69

25 26 23 24 18 34 19 −5 23 23 10 20 30


Err

4.25

2.25

4.75

Dt

0.75

1.35

0.45

0.065

0.1

0.018

4.75

0.525

0.075

3

2.92

0.27

2.75 2.75

12.5

4.25

2.17 0.637

0.33 0.065

0.937

0.178

1.95

0.33

F N N N N N N N N N VG G N VG N N N N N N N N N N N VG N N F N N N N N N N VG N N VG N N N F N N N N F G N N F N N F N

Err

Dt

Err

15.25

0.675

0.27

4.5

1.05

0.1593

2.75

0.725

0.0875

4.25

0.63

0.13

6

0.86

0.21

7

0.75

0.175

15.75 3.5

0.5 1.71

0.14 0.16

5.25

1.2

0.33

Qual. N N N N N N N N N N N N N N N N F N G N N N VG N N N N N N N F N N N N N N N N N F N N N N N G N N N N N N N N N N N N N F G N N N F

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Table 1. (Continued.) Event 1999.103.10.38.48 1999.110.19.04.08 1999.127.14.13.52 1999.334.04.01.53 1999.334.16.23.34 1999.340.23.12.33 1999.341.00.19.49 2000.008.16.47.20 2000.064.02.24.22 2000.072.22.21.30

Baz

Distance

Phi

Err

Dt

Err

Qual.

106.12 117.57 25.34 264.79 109.58 25.18 25.23 100.9 151.19 34.26

111.39 111.17 93.76 148.31 98.36 92.46 92.48 112.38 98.06 144.65

−77 40 23 84 22 23 32 8 59 64

3.25 14.25

1.91 0.97

0.31 0.31

F F N N N N N N N N

Event 2003.124.20.08.46 2003.310.10.38.04 2003.273.14.08.37 2003.288.02.19.43 2003.359.20.42.33 2003.361.04.55.25 2004.025.11.43.11 2004.025.11.43.11 2004.078.20.04.24 2004.100.15.23.35 2004.250.12.42.59

Baz

Distance

119.64 109.21 119.48 106.63 112.09 111.97 105.36 105.36 112.59 103.42 214.22

103.12 90.66 103.88 102.24 91.59 91.45 106.47 106.47 105.03 88.24 107.09

Phi

Baz

Distance

105.79 264.69 146.53 101.54 107.56 102.67 269.76 101.41 107.72

111.65 144.88 97.01 96.06 101.03 96.44 92.49 110.53 101.58

50 41 50 53 42 37 75 44 36

105.69 108.02 56.7 105.24 148.04 109.77

93.41 95.72 108.8 92.64 96.66 96.31

103.9 111.5 106.91 110.01 111.03 107.07

24 20 30 28 27 19 3 15 28 20 9

Err

Dt

Err

Qual.

1.1

0.31

11.25

0.84

0.16

N N N F N N N N N N G

Err

Dt

Err

Qual.

2.75 6.25

1.02 1.05

0.05 0.08

4.25 10.75 3.75 5 3 10

0.95 1.3 1.4 2.1 0.95 1.32

0.08 0.35 0.15 0.13 0.05 0.33

VG G N G G G F VG G

89 −85 −67 −71 −62 −85

6.5 17.75 7.5 2 21 7

0.43 0.65 1.1 2.75 0.25 0.65

0.06 0.3 0.05 0.58 0.4 0.13

G N G N N VG

93.28 98.2 107.64 99.02 98.3

55 56 51 52 67

1.75 6.25 5.5 4.25 8.25

1.2 1.18 1.1 1.25 1.28

0.02 0.16 0.11 0.11 0.18

G F VG VG F

107.96

−78

4.5

1.3

0.25

G

8

GANGA BASIN Event ALB 2006.002.22.13.40 2006.023.06.02.59 2006.055.14.15.45 2006.057.03.08.27 2006.066.06.28.55 2006.219.22.18.54 2006.276.18.03.04 2006.291.10.45.32 2007.093.20.26.09 2007.094.11.00.27 GRG 2005.324.12.53.02 2006.107.23.49.59 JHN 2005.078.17.34.46 2005.099.15.16.27 2006.023.06.02.59 2006.066.06.28.55 2006.116.01.46.03 2006.219.22.18.54 2006.291.10.45.32 KNP 2005.039.14.48.21 MRT 2006.057.03.08.27 2006.126.21.11.52 2007.196.09.27.34 2007.213.17.08.51 2007.270.19.57.44 2007.271.01.35.51

Baz

Distance

Phi

Err

Dt

Err

Qual.

104.26 107.32 102.92 108.46 105.12 105.81 108.11 105.37 109.71 109.85

107.14 93.87 105.19 106.89 92.46 93.25 95.59 92.48 96.32 96.22

−80 −89 −79 −79 −86 −85 17 −81 −81 −81

4.75 19 2 3 3.25 2.75 0.75 4.25 3.5 3

1.37 0.5 1.02 1.02 0.6 0.65 2.95 0.75 1.43 1.07

0.4 0.2 0.15 0.2 0.1 0.05 0.19 0.2 0.1 0.17

F F G VG G VG N N VG VG

31.51 101.19

82.7 95.42

68 39

7 7.75

0.625 1

0.03 0.19

G G

105.67 26.5 106.1 103.86 149.15 104.57 104.12

109.28 87.37 96.49 95.12 100.39 95.9 95.13

−75 −66 −76 −77 −40 −75 10

3.5 0.75 0.25 0.25 3.75 1.75 7

1.32 2.05 3.3 4 1.42 1.3 2.08

0.4 0.46 0.19 0.03 0.26 0.14 0.84

N N N N F N N

103.86

93.55

47

7.25

0.72

0.11

G

105.82 101.4 103 103.61 107.83 107.82

11.39 110.24 98.16 97.47 101.34 101.45

39 51 48 42 54 41

6.25 8.25 16 6 5.5 9.25

0.9 0.85 0.775 0.93 0.9 1.05

0.1 0.07 0.14 0.11 0.05 0.18

G G G G G G

Event NPL 2006.057.03.08.27 2006.317.01.26.35 2006.320.20.29.54 2007.023.17.16.20 2007.094.11.00.27 2007.115.13.34.14 2007.124.12.06.52 2007.126.21.11.52 2007.270.19.57.44 RWA 2006.219.22.18.54 2006.276.18.03.14 2006.288.17.07.49 2006.291.10.45.32 2007.030.04.54.50 2007.094.11.00.27 SNI 2005.039.14.48.21 2005.041.16.53.19 2005.078.17.34.46 2005.101.17.08.53 2005.223.09.08.46 WRD 2005.078.17.34.46

Phi

Note: The quality of the measurements is rated VG (very good), G (good), F (fair) and N (null). N (VG) and N(G) are very good nulls and good nulls respectively. See text for details.

fabric of the craton and WNW–ESE in the Pan African granulites of southern India and Sri Lanka, also paralleling their structural fabrics. Thus, while the sources of crustal anisotropy in the Indian lithosphere appear to lie in its frozen structural fabric induced by past tectonic events, those of mantle anisotropy are dominated by the ongoing motion of the Indian plate with respect to the underlying asthenosphere. This does not, however, warrant the inference that the Indian lithospheric crust is decoupled from the underlying mantle. C O N C LU S I O N In a constant effort to better understand the Indo-Eurasian collision process at depth, we present the results of shear wave splitting measurements performed at 86 stations deployed along the Himalayan arc, from Ladakh in the west to Bhutan in the east, and across the

Indian peninsula down to Sri Lanka. An unfavourable location of the Indian continent with respect to earthquake sources yields a much larger number of null results compared with non-null results; in this study, we exploited the too often neglected information derived from the former to add confidence to the latter. This approach has greatly enhanced the robustness of the results presented. Much of the south Indian shield is characterized by a NNE–SSW trending fast axis of anisotropy parallel to the direction of the Indian plate with respect to a fixed Eurasia in agreement with NUVEL1A (DeMets et al. 1990). The fact that this direction is parallel rather than orthogonal to the compression axis, as would be expected for in-plane compressive stresses within the lithosphere, indicates that the anisotropy is most likely due to the relative asthenospheric flow, parallel to the relative plate velocity. Furthermore, a weak correlation between the orientation of the plate motion as defined through NUVEL1A (DeMets et al. 1990) and the anisotropy at shallower

C


Seismic anisotropy underneath India depths in the upper mantle, and a higher correlation at greater depth, indicate that the shearing of the lithosphere–asthenosphere boundary layer is the potential source of the observed anisotropy. This relative flow would create foliations in the horizontal plane with a fast direction orientated along the flow (NNE–SSW) and the overwhelming preponderance of null measurements could be explained by the result of the orthogonality of the incident shear wave phases (arriving at the stations with a backazimuth between N100◦ and N120◦ ) and the foliation planes, rather than to an absence of anisotropy. As one approaches the Himalayan foredeep the fast anisotropy directions rotate by 90◦ to align with the strike of the orogen. This expected orientation of the anisotropy axis induced by compression at the collision front should be pervasive throughout the lithosphere. However, there is apparently little detectable evidence of this in the Indian shield to the south. What is observed towards the collision front is more likely the downthrusting of the Indian lithosphere acting as a barrier and orientating the asthenospheric flow along the strike, with an additional component resulting from the expected along strike foliation planes induced by the Himalayan fold and thrust belts.

AC K N OW L E D G M E N T S MH would like to thank S. Fishwick, and VKG would like to thank D. Mckenzie for stimulating discussions. The manuscript greatly benefited from comments of two anonymous reviewers. GEOSCOPE, PASSCAL and the IRIS Data Management Centre are acknowledged for providing us with data from stations HYB and PALK, respectively.

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