Upper Mantle Shear Velocity Structure below ... - CiteSeerX

Bulletin of the Seismological Society of America, Vol. 105, No. 5, pp. 2713–2723, October 2015, doi: 10.1785/0120140347

Upper Mantle Shear Velocity Structure below Northwestern India Based on Group Velocity Dispersion by Prantik Mandal

Abstract

We derive a well-resolved 1D model of shear-wave velocity (V S ) through both linear, and nonlinear joint inversions of Rayleigh- and Love-wave group velocity dispersion data. Our best model from the deterministic approach suggests a two-layered crust over a half-space: the upper crust is 13.8 km thick with a shear velocity of 3:22 km=s; for the lower crust they are 24.9 km and 3:62 km=s, respectively. Finally, we use a global optimization method (very fast simulated annealing) to estimate shearwave velocity. Multiple models give acceptable fits to the observed data. A common feature of these models is a low-velocity zone (LVZ) with its upper boundary at 60– 70 km depth. At this depth, the temperature–depth profile of Kachchh cuts the wet peridotite geotherm, resulting in increased water content due to partial melting, and thus the decreased shear velocity. This LVZ extends down to a depth of 120–130 km, where the temperature–depth profile of Kachchh cuts the dry peridotite geotherm that leads to a sharp decrease in water content, thereby, a sharp increase in shear velocity. The maximum V S perturbation of −3% in the upper mantle can be explained by the presence of 100–150 K excess temperature beneath northwestern India at 70–120 km depth relative to the adjacent mantle along with the presence of 1% penny-shaped melt inclusions (probably CO2 -rich carbonatite melts) in the upper mantle. This is likely related to the imprints of the initial Deccan/Reunion plume head that might have moved with India since the Deccan volcanism at 65 Ma.

Introduction The Indian lithosphere has undergone several major tectonic deformation phases, which include rifting from Africa at 184 Ma (separation of proto-Indian continent from Africa), the breakup from Madagascar at 88 Ma, and the Deccan flood volcanism at 65 Ma (interaction with Deccan/Reunion mantle plume) as India passed over a large plume head, which was located where the Reunion hot spot is presently situated in the Indian Ocean (e.g., Mahoney et al., 2002). The latter event resulted in outpouring of a large volume of basaltic lava (covering an area of 0:5 million km2 with 2000 m thick basaltic lava) in the western and central parts of the Indian subcontinent within a short duration of time (Courtillot et al., 1986). Consistent with the plume theory, the P-wave seismic tomography of northwestern India reveals a low-seismic velocity anomaly in the upper mantle extending to a depth of 600 km beneath north of the gulf of Cambay and the present exposure of the Deccan flood basalts (Kennett and Widiyantoro, 1999). This low-velocity anomaly is centered around Mer-Mundwara and Sarnu-Dardali in Rajasthan, characterized by alkaline basalts with a high 3 He=4 He ratio (Basu et al., 1993), representing the fossil plume head (Kennett and Widiyantoro, 1999). This plume model is also supported by high (53–90 mW=m2 ) regional heat-flow val-

ues and the presence of low Lg Q as well as low V S in the upper mantle below northwestern India (Roy, 2004; Mitchell et al., 2008). However, resolution in the study of Kennett and Widiyantoro (1999) was limited due to poor station coverage in the central region of their low-velocity anomaly, and this necessitated a fresh attempt to map deep crustal and uppermost mantle structure below northwestern India. Prior crustal velocity investigations in northwestern India suggest a significant variation in the estimated thickness of upper granitic and lower basaltic crustal layers (Kaila et al., 1990; Bodin and Horton, 2004; Mandal et al., 2004; Sarkar et al., 2007). The deep seismic sounding studies imaged crustal magmatic underplating with a high-velocity layer below the Saurashtra region (Kaila et al., 1990) while crustal intrusive bodies/underplating have also been delineated below the Kachchh region by local earthquake tomography, by modeling of gravity and magnetic data, and by refraction data (Kayal et al., 2002; Mandal et al., 2004; Mishra et al., 2005; Sarkar et al., 2007; Mandal and Chadha, 2008). Consistent with the intrusive theory, a P-receiver function study delineates an updoming of the Moho (4–7 km) as well as the asthenosphere (6–12 km) beneath the central Kachchh rift zone (KRZ), relative to the surrounding unrifted regions

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(Mandal, 2011). Mandal and Pandey (2010) proposed a compositionally altered lower crust (at 24–34 km depth) and a subcrustal eclogite layer (at 34–42 km depth) overlying a thin lithosphere (∼70 km) beneath the KRZ, which might have resulted from the lithosphere and Deccan/Reunion mantle plume interaction at 65 Ma. Through inversion modeling of group velocity dispersion data, Mandal et al. (2007) delineated a two-layered crust beneath the Kachchh region, characterized by a 13.8 km thick upper crust with shear-wave velocity (V S ) of 3:2 km=s and a 24.9 km thick lower crust with V S of 3:7 km=s. Their study also reveals an upper mantle with V S of 4:65 km=s. However, their study samples crust and upper mantle beneath a narrow zone (with azimuths ranging from 0° to 30°) between Kachchh, Gujarat, and Kashmir Himalaya, which covers a small part of northwestern India. To understand the deeper geodynamic processes, which have shaped the diverse geology of northwestern India and resulted from the rifting and plume tectonics, it is necessary to reinterpret the detailed crustal and upper mantle structure beneath the region in the light of recently collected regional broadband data from large earthquakes. To date, no attempt has been made to study the deeper geodynamical process underlying northwestern India except P-wave tomography of Kennett and Widiyantoro (1999), which reveals a low-seismic velocity anomaly in the upper mantle extending down to a depth of 600 km, probably representing the imprints of fossil mantle plume. In this article, models of the crust and upper mantle are constructed from the joint inversion modeling of Rayleigh- and Love-wave group velocity dispersion data. This has been achieved by analyzing data recorded at a specially designed seismic network of 15 portable three-component broadband seismographs in the Kachchh seismic zone (KSZ) during the years from 2001 to 2012 (Fig. 1). The 11-year long experiment recorded several regional earthquakes, which resulted in an excellent sampling of the region below northwestern India in general and the mantle low V P zone as mapped through seismic tomography in particular (Figs. 2a,b; Kennett and Widiyantoro, 1999). Here, we present the group velocity dispersion characteristics and shear velocity structure estimated using broadband data of 10 regional earthquakes (M w 5.0–7.7) recorded by 15 portable stations from Kachchh, Gujarat, India, and six Indian Meteorological Department (IMD), Delhi, India, stations from Peninsular India. In addition, we attempt to provide a feasible interpretation of our results in terms of geodynamic processes associated with northwestern India covering the Deccan volcanic province (DVP).

Geology and Thermal State of the Area Deccan basalts of 65 Ma occupy an area of 0.5 million square km of northwestern India, covering most of the rifts except Godavari and Mahanadi. In the DVP, normal low heatflow values ranging from 40 to 70 mW=m2 characterize the stable southern Deccan (Gupta and Gaur, 1984), whereas higher heat flows of 75–116 mW=m2 are found in the north-

Figure 1.

Location of the 15 mobile broadband stations (marked by solid black triangles) along with the 2001 Bhuj mainshock epicenter (gray star symbol). Stations: KNP, Khingarpar; VJP, Vajepar; TAP, Tapar; GDM, Gandhidham; MTP, Motapaya; GDD, Gadhada; BHA, Bhachau; BEL, Bela; RAM, Ramvav; VND, Vondh; NGR, Nagor; BHU, Bhuj; NPR, Naranpar; VGH, Vaghura; TPM, Tappar; Mundra; and KMU, Kachchhmainland uplift. Major faults (solid lines): ABF, Allah Bund fault; IBF, Island belt fault; KMF, Kachchh mainland fault; KHF, Katrol hill fault; NPF, Nagar Parkar fault; NKF, North Kathiawar fault; BF, Banni fault; and NWF, North Wagad fault, the causative fault for 2001 Bhuj earthquake and Gedi fault, are shown by dotted lines. The location of Rajkot city is also shown by a black square symbol. The PU, KU, BU, and WU show Patcham, Khadir, Bela, and Wagad uplifts, respectively. An elliptical area shown by the dotted line marks the location of the central Kachchh rift zone covering KNP, TAP, BHA, NDD, and VJP sites. The inset shows the key map for the area, where, the study area is shown by a square symbol and an arrow shows the location of Cambay. The epicentral location of the 1993 Killari earthquake is also shown by a small filled gray squared symbol. The black filled portion marks the areal extent of Deccan volcanic province (DVP) in India. The color version of this figure is available only in the electronic edition.

ern part of Cambay graben, Surat, and Saurashtra (Pandey and Negi, 1995; Sheth, 2005). The Aravalli craton in Rajasthan and Kachchh in Gujarat are very important tectonic units where evidence of a high-helium isotopic ratio of 10–14 and mantle xenoliths is reported that suggest a deep mantle origin (Basu et al., 1993; Sen et al., 2009). In the Aravalli craton, heat-flow values range from 46 to 96 mW=m2 , whereas a minimum surface heat flow of 60–70 mW=m2 has been projected for the Kachchh region based on mantle-derived spinellherzolite geotherms (Mukherjee and Biswas, 1988). The estimated temperatures at Moho (∼40 km) and lithosphere– asthenosphere boundary (LAB) (∼70 km) below the KSZ are

Upper Mantle Shear Velocity Structure below Northwestern India Based on Group Velocity Dispersion

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the partial melting of CO2 -rich lherzolite at about 2–2.5 GPa (at 70–90 km depth) from a mixed source of asthenosphere and Reunion-like plume material (Sen et al., 2009). Such carbonatite partial melts perhaps could have ascended along deep lithospheric rift faults resulting in the metasomatized lithosphere beneath northwestern India in general and Kachchh in particular (Pandey and Agrawal, 1999; Sen et al., 2009). Seismic Network and Data

Figure 2. (a) Ray coverage of the surface-wave dispersion study. Open triangles mark the stations, whereas solid circles represent the epicenters of the considered 10 events for the surface-wave dispersion study. Z and Q mark the Zhob and Quetta in Pakistan, respectively, where alkaline Reunion basalts of 72–73 Ma are found. M and S represent the Mer-Mundwara and Sarnu-Dendali alkaline magmatism in Rajasthan whereas D marks the Dhandhuka in Saurashtra, where primitive picritic basalts are found. The dotted circular region shows the main low P-wave velocity anomaly of Kennett and Widiyantoro (1999). (b) Map view of the low P-wave velocity anomaly as mapped through tomography at 80 km depth (modified after Kennett and Widiyantoro, 1999). The color version of this figure is available only in the electronic edition. found to be 700° C and 1250° C, respectively (Fig. 3a). Such temperatures suggest the probable presence of carbonatite melts in the upper mantle underlying northwestern India. The bulk of the Deccan lavas are tholeiitic with less than 7% MgO including phenocrysts of olivine and plagioclase (Sen, 2001). Carbonatites and other mafic alkaline lavas are also reported to be present in Deccan traps in minor amounts (Sen, 2001). Quantitative modeling of these Deccan melts suggested that the melting range might have been 100–60 km below the surface (Sen, 2001; Sen et al., 2009). Recent study of isotopic ratios of xenoliths proposed that alkalic basalts found in Kachchh probably are the alkalic melts generated by

The Mesozoic KRZ is part of a network of rifts that developed along the western margin of India following the breakup of Gondwanaland (Biswas, 1987). The Quaternary/ Tertiary sediments, Deccan volcanic rocks and Jurassic sandstones resting on the Archaean basement characterize the geologic sequence of the Kachchh region (Gupta et al., 2001). The rift zone is bounded by a north-dipping Nagar Parkar fault in the north and a south-dipping Kathiawar fault in the south. Other major faults in Kachchh are the east–west-trending Allah Bund fault, Island belt fault, Kachchh mainland fault, and Katrol Hill fault (Fig. 1). The data used in this article correspond to digital waveforms of nine regional earthquakes recorded during February 2001–December 2012 by the Kachchh seismic network of the National Geophysical Research Institute (NGRI), Hyderabad, India, that consists of 15 mobile broadband seismographs (Fig. 1). Each seismograph station is equipped with a 24-bit REFTEK digital recorder and a three-component broadband (natural period 30–120 s) Güralp sensor for recording local, regional, and teleseismic events. The data are recorded in continuous mode with a sampling rate of 100 samples=s. Most of these stations are located on hard sediments or rocks (Jurrassic sediments or basalt). However, a few of these stations are on a thin soil layer. We also used regional seismograms of the 2001 Bhuj mainshock of M w 7.7 from six broadband IMD stations in Peninsular India. All IMD stations are equipped with a Streckeisen, three-component, STS-2 broadband sensor (response flat to velocity about from 0.008 to 50 Hz) and a Kinemetrics FBA-23 accelerometer (response flat to acceleration from ∼0 to 45 Hz) connected to a six-channel, 24-bit Quanterra digitizer. For IMD stations, data are recorded in both continuous (20 samples=s) and triggered (80 samples=s) mode. For both NGRI and IMD stations, the time tagging has been done using a GPS-derived time. Table 1 shows the hypocentral parameters of regional events used in the surfacewave analysis. A subset of 15 broadband stations, which are located in Kachchh, provides high-resolution data for a better V S model for northwestern India based on the ray coverage shown in Figure 2.

Method Surface-Wave Group Velocity Dispersion The conventional frequency–time analysis (FTAN) of broadband seismograms of 10 regional earthquakes from

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Figure 3. (a) Computed temperature–depth profiles for the Kachchh seismic zone (KSZ) and New Madrid seismic zone (New Madrid, United States) (Liu and Zoback, 1997). The BDS line represents the dry basalt solidus, (b) average stacked Rayleigh-wave group velocity dispersion curves versus time period, and (c) average stacked Love-wave group velocity dispersion curves versus time period. Error bars show standard deviations. 15 stations in Kachchh and 6 IMD broadband stations in Peninsular India is performed to estimate the fundamental mode group velocity dispersion of Rayleigh and Love waves (Table 1). A detailed description of the FTAN is given by Levshin et al. (1992), and papers referred to therein. Table 1 shows the earthquake parameters of the events used in the surface-wave analysis. The ray coverage for the surfacewave dispersion study is shown in Figure 2a, which has good sampling of the lithospheric volume beneath northwestern

India in general and the mantle low V P zone as imaged from seismic tomography of Kennett and Widiyantoro (1999) in particular (Figs. 2a,b). A stacking technique is applied in the frequency–time domain to measure group velocities of the fundamental mode Rayleigh as well as Love waves (Dziewonski et al., 1969; Campillo et al., 1996; Shapiro et al., 1997). The rapid fall off of the amplitude spectra at long periods leads to a systematic error in the FTAN. We have corrected such error using a procedure given by Shapiro and Singh (1999). As we are interested in estimating an average dispersion curve, a logarithmic stacking in the period-group velocity domain is used, which permits an estimation of the average dispersion curves. The average group velocity dispersion curves along with standard deviations are shown in Figure 3b,c, which shows stable group velocity dispersion for both Rayleigh (5–75 s) and Love waves (9–75 s). The maximum errors of 0:3–0:5 km=s are obtained for the longer period (50–75 s) for both Rayleigh- and Love-wave group velocity dispersion curves. At 5–75 s period, the Rayleighwave group velocity varies from 2.8 to 3:6 km=s (Fig. 3b) and between 9–75 s, Love wave group velocity ranges from 3.1 to 3:9 km=s (Fig. 3c). These stable group velocity dispersion curves of fundamental mode surface waves at periods between 5–70 s for Rayleigh and 9–75 s for Love waves are used as inputs for joint inversion to estimate shear velocity structure beneath northwestern India. Rayleigh- and Lovewave data for periods greater than 75 s are not included in the joint inversion because of their larger uncertainties (> 0:5 km=s) (Figs. 3b,c). Joint Inversion of Rayleigh- and Love-Wave Group Velocity Dispersion Data These dispersion curves for Rayleigh and Love waves are jointly inverted for estimating an average 1D velocity structure using a linearized deterministic inversion approach (Herrmann, 1987, 2004). The crustal part of the initial veloc-

Table 1 Hypocentral Parameters of Regional Events Used for Surface-Wave Group Velocity Dispersion Study Event Number

Date (yyyy/mm/dd)

Origin Time (hh:mm:ss.ss)

Latitude (° N)

Longitude (° E)

Stations

Ms

Focal Depth (km)

1 2 3 4 5 6

2001/01/26 2002/12/19 2002/11/01 2002/11/20 2002/11/20 2007/01/08

03:16:02.00 22:21:35.68 22:09:29.28 21:32:30.89 21:32:30.81 17:21:49.91

23.4 35.30 35.52 35.41 35.41 39.80

70.23 74.49 74.65 74.51 74.51 70.31

7.7 5.0 5.4 6.5 6.5 6.1

23 33 33 33 33 16

7

2007/01/17

23:18:49.8

10.12

58.71

6.2

08

8

2007/05/05

08:51:39.0

34.25

81.97

6.1

09

9

2008/01/09

08:26:45.5

32.29

85.17

6.4

10

10

2008/01/16

11:54:44.0

32.33

85.16

NDI, BHP, BOK, VIS, MNG, and TRV GDM, KNP KNP KNP VND,KNP, RAM BEL, TPM, NPR, BHU, VJP, MTP, GDD, BHA, TAP, NGR, and VGH BEL, TPM, NPR, BHU, VJP, MTP, GDD, BHA, TAP, NGR, and VGH BEL, TPM, NPR, BHU, VJP, MTP, GDD, BHA, TAP, NGR, and VGH BEL, TPM, NPR, BHU, VJP, MTP, GDD, BHA, TAP, NGR, and VGH BEL, TPM, NPR, BHU, VJP, MTP, GDD, BHA, TAP, NGR, and VGH

6.0

09


Figure 4. (a) Initial shear-wave velocity structure (Caen, 1986; Singh et al., 1999), (b) observed and synthetic (using initial velocity structure) Rayleigh-wave group velocity dispersion curves, and (c) observed and synthetic Love-wave group velocity dispersion curves. ity model (Fig. 4a) is designed based on previously published data on the velocity structure of the Indian crust (Table 2; Dube et al., 1973; Bhattacharya, 1981; Singh et al., 1999). Our initial velocity model consists of two crustal layers with a combined thickness of 38.7 km (Singh et al., 1999). The upper crust has velocities of 6.20 and 3:55 km=s, respectively, for P and S waves and a thickness of 13.8 km (Fig. 4a). P-wave velocity of 6:4 km=s and S-wave velocity of 3:85 km=s characterize the lower crustal layer. The above crustal model of Singh et al. (1999) seems to be the better model representing the regional 1D shear velocity structure of the Indian Peninsula, which was obtained through inversion modeling of surface-wave group velocity dispersion data of the 1997 Jabalpur earthquake from different IMD stations that provided good ray coverage of our study area. The lower part of our velocity model (i.e., Moho to a depth of 205 km) is constrained by the available upper mantle shear velocity structure of Caen (1986), which suggests a V S of 4:7 km=s between 38.7 and 155 km depth, a V S of 4:68 km=s between 155 and 175 km,

and a V S of 4:61 km=s between 175 and 205 km. The P-wave velocity is estimated by assuming V P =V S of 1.73. Density (ρ) values are calculated from the P-wave speed, V P ρkg=m3 0:32 × V P km=s 0:77; Berteusen, 1977). The inversion is performed for the S-wave velocity in each layer and the depth of the interfaces. The Poisson ratio in the crust is kept fixed in each layer. The velocities and the thickness of two layers of the crust are allowed to vary freely. To derive V S -depth profiles using dispersion curves, we use the code developed by Herrmann (1987, 2004) to derive the Green’s function of a spherically symmetric Earth model. The dispersion curves are inverted for shear-wave velocities only. Perturbations in P-wave velocity and density in an isotropic medium do not significantly affect the measured group velocities and are, thus, calculated from the obtained V S models using empirical relations. The inversion of the dispersion curves is performed assuming an isotropic model. Model parameters are the shear-wave velocity (V S ) in the two crustal layers, the Moho depth, as well as the V S distribution in the mantle. After choosing an initial model, the inversion process is performed iteratively. Synthetic dispersion curves are computed from the inverted model and compared to the measured curve until the misfit between both curves converges. The misfit Δ is calculated by N 1X Ui − U inv i 2 ; 1 Δ N i1 σi in which Ui is the measured Rayleigh-wave group velocity for period i and U inv i is the Rayleigh-wave group velocity obtained from the inversion. σ i is the standard deviation of Ui. For joint inversion, we include a similar term for the Love-wave group velocity data. An average of these two terms is used as the global misfit. For joint inversions, we apply weights using equation (1) for Rayleigh- and Lovewave group velocities, according to their standard deviations at different periods (σ i ). Modeling of Surface-Wave Group Velocity Dispersion Using Very Fast Simulated Annealing The primary goal of the present study is to estimate a robust 1D shear-wave velocity structure that can explain surface-wave group velocity dispersion. Thus, our optimization algorithm is expected to determine the model that produces

Table 2 Crustal and Upper Mantle Structure of the Indian Peninsula Dube et al. (1973)

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Bhattacharya (1981)

Singh et al. (1999)

This Study

Layer Number

Thick (km)

V S km=s

Thick (km)

V S km=s

Thick (km)

V S km=s

Thick (km)

V S km=s

1 2 3

20.0 18.7 —

3.42 3.92 4.64

20.4 18.3 —

3.53 3.92 4.60

13.8 24.9 —

3.55 3.85 4.65

13.8 24.9 —

3.22 3.62 4.34

2718 synthetic dispersion curves that best fit the observed dispersion curves. For determining the best fit, we use the following normalized objective function as defined by Sen and Stoffa (2013): P 2 jdiobs − disyn jα P e1−P i ; 2 jdobs disyn jα jdiobs − disyn jα in which the sum is taken over all the data points, and the parameter α is equivalent to a norm. Initially, we chose α 1 for the objective function. Because, we know that the objective function corresponding to the surface-wave dispersion is smoothly varying, we chose α 0:5 to introduce more sensitivity. Thus, we employ equal weights to all parts of the normalized objective function (equation 2). Mutually satisfying constraints imposed by the two datasets (i.e., dispersion curves for Rayleigh and Love waves) may constrain a larger subset of model parameters than a single set alone, resulting in a single model that better represents the true structure. Having defined the objective function, we employ very fast simulated annealing (VFSA) for optimization (Pulliam and Sen, 2005; Sen and Stoffa, 2013), see Sen and Stoffa (2013) for a detailed description of the method. VFSA causes no difficulties because we do not require computing derivatives. Because Rayleigh- and Love-wave group velocity dispersion computations are relatively fast, we need less time for forward modeling. Geophysical inverse problems are often associated with nonuniqueness. That is, more than one model can often satisfy the observed data equally well and trade-offs between different model parameters are common (Pulliam and Sen, 2005). It is, therefore, important to categorize the models that are required by the data, rather than simply matching the data. VFSA conducts the search to explore the model space and thus identify the range of models that fit the data efficiently, and the products of multiple such searches provide us the uncertainty in a single, best-fitting solution. We use two important statistical tools in evaluating the reliability of results, which are the posterior probability density (PPD) function, and the parameter correlation matrix. These statistical parameters are estimated by projecting the inverse problem in a Bayesian framework (e.g., Tarantola, 1994; Sen and Stoffa, 2013) and using Gibbs’ sampler (GS) (Pulliam and Sen, 2005; Sen and Stoffa, 2013). The goal of GS is to sample the regions that are the most significant in the sense that the error function is varying rapidly, or many acceptable solutions exist. This significant region is determined by using a Gibbs’ probability distribution (Sen and Stoffa, 2013).

Estimation of Uncertainties The likelihood function e − Em multiplied with a prior probability density function, pm gives the PPD function σmjdobs . The function pm defines the probability of the model m without knowledge of the data. We use a uniform prior within upper and lower bounds for each model parameter. The likelihood function is the data misfit, and its

P. Mandal

choice depends on the distribution of error in the data (Sen and Stoffa, 2013, and references therein). The posterior correlation matrix is the measure of relative trade-off between individual model parameters, which is computed by normalizing the covariance between two model parameters (Sen and Stoffa, 1996). Mathematically, the correlation between ith and jth model parameters is given by their covariances divided by the square root of the product of the covariances of each parameter with itself. Sen and Stoffa (1996) opted for a multiple-VFSA-based approach to sample models from the PPD after examining several individual approaches. In this approach, we fix the number of iterations required to converge and then make several VFSA runs with the same setting of convergence to characterize uncertainty in the model. All these sampled models are used to compute PPD and correlation matrices to characterize uncertainties in the derived results.

Results Fundamental Mode of Rayleigh and Love Waves In our starting model M1, we choose the shear velocity structure of Caen (1986) for the upper mantle and the regional 1D velocity model of Peninsular India (Singh et al., 1999) for the crust assuming V P =V S 1:73 (Fig. 4a). The Moho depth is fixed at 38.7 km. The inversion is performed to a maximum depth of 205 km. For the group velocity dispersion data of Rayleigh waves, the misfit of the starting model is Δ 3:5439; obviously it does not explain the measured dispersion curve (Fig. 4b). After 10 iterations, the misfit decreases to Δ 0:0588 and the resulting dispersion curve lies well within the error bars of the measured group velocity dispersion curve (Fig. 5a). Similarly, for the inversion of Love-wave dispersion values the starting model is again M1, and the related misfit is Δ 1:679 (Fig. 4c). The misfit reduces to 0.0682 after 10 iterations (Fig. 5b). Thus, the resulting dispersion curves lie well within the error bars of the measured group velocity dispersion curves. The joint inversion of Rayleigh- and Love-wave dispersion data led to the final V S -model E1 (Fig. 5c). The model E1 for northwestern India reveals a low-velocity zone (LVZ) starting at 50–70 km depth extending down to 120–140 km. Between the Moho and the upper boundary of the LVZ, we observe a lid with normal mantle values of V S (∼4:34 km=s). The minimum of the V S anomaly is at a depth of 70–80 km with V S 4:20 km=s. The maximum V S perturbation (ΔV S ) reaches −3:2% (Fig. 5c). Figure 5d shows the temperature– depth profile of the KRZ, which has been calculated by assuming a surface heat-flow value of 60 mW=m2 based on the spinel-lherzolite geotherm as suggested by the presence of alkali mantle Xenoliths from Kachchh (Fig. 5d; Mukherjee and Biswas, 1988; Sen et al., 2009; Mandal and Pandey, 2010). This figure also shows the wet and dry peridotite geotherms, which interestingly intersect with the upper and lower bounds of our modeled LVZ in the upper mantle indicating presence of melts.


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Figure 5. (a) Observed and synthetic (using final inverted velocity structure) Rayleigh-wave group velocity dispersion curves, (b) observed and synthetic Love-wave group velocity dispersion curves, (c) final velocity model (shown by the thick black line obtained from this study) and initial starting model of Caen (1986) (shown by the thick gray line), and (d) temperature–depth profile of the KSZ (marked by the solid black line). The solid line marked by WPS represents the wet peridotite geotherm whereas the solid line marked by DPS shows the dry peridotite geotherm. The upper bound of our upper mantle low velocity layer (LVL) cuts the WPS whereas the lower bound of LVL cuts the DPS. ULVZ marks the upper boundary of the LVZ at 60 km depth. LLVZ1 marks the lower boundary of the LVZ at 130 depth. The color version of this figure is available only in the electronic edition.

Model Exploration We check the sensitivity of our Rayleigh- and Lovedispersion data against the high-velocity lid and LVZ structure of the model. For this purpose, we carry out joint inversion using a VFSA method. We define our velocity model with nodes that are 5 km apart; velocities at the nodes are picked randomly from a predefined velocity search space. Once the velocities at the nodes are picked, the velocity function is vertically smoothed using a 3-point running average operator. The smoothed velocity model is then used in forward calculation and objective function evaluation. The main idea of selecting different randomly chosen starting models is to reduce our biases toward the initial velocity models, so that we can verify the existence of an upper

mantle LVZ with a better confidence level. To reduce biases in selecting the starting model from the different available velocity models (Table 2), we select a linear gradient across the crust, which is not influenced by the nonlinear dependency between thickness and seismic velocity within different crustal layers. Further, we know that different combinations of thickness and shear velocity within different crustal layers can give rise to similar dispersion curves. Thus, different randomly chosen starting models using a linear gradient across the crust would probably reduce any inconsistencies significantly. One hundred VFSA inversion runs were made with a different randomly chosen starting model for each run. Final models from these runs are shown in Figure 6. The common feature of these models is the existence of an LVZ. To examine these results further, we plot velocity histograms at 70,

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Figure 6. Crust–mantle models from very fast simulated annealing (VFSA) inversion showing the similar upper bound and lower bounds of upper mantle LVL. Thick line with star symbols marks the final average shear velocity model. The color version of this figure is available only in the electronic edition. 90, 100, 150, and 200 km depths in Figure 7. The figures clearly demonstrate that the data prefer an LVZ in the depth range between 70 and 120 km beyond which the velocity increases. The average model is shown with star symbols in Figure 6, which may be considered as the representative 1D shear velocity model of the region.

Discussion Inversion of dispersion data for both Rayleigh and Love waves shows a low shear-wave velocity zone from 60–70 km to 120–130 km depths underlying the KRZ. The temperature– depth profile of the KSZ (Fig. 5d) suggests that the upper and lower bounds of this LVL cut the wet peridotite and dry peridotite geotherms suggesting presence of melt (Figs. 5c,d and 6). Average Crustal Structure below Northwestern India The obtained crustal structure below northwestern India suggests a two-layer crust with a 13.8 km thick upper crust with V S of 3:22 km=s and a 24.9 km thick lower crust with V S of 3:62 km=s (Fig. 5c). The shear velocity (V S ) in the upper mantle is 4:34 km=s. From Figure 5, we note that our best-fitting model is similar to the model of Singh et al. (1999). The major differences are the slower shear-wave velocities for the crustal and upper mantle layers. In our model, shearwave velocities in upper crust and lower crust are 3.22 and 3:62 km=s, respectively, compared to 3.55 and 3:85 km=s in

the model of Singh et al. (1999). We also found a slower upper mantle beneath northwestern India (i.e., V S 4:34 km=s) as compared to 4:65–4:70 km=s in the model of Singh et al. (1999) and Caen (1986). Table 2 shows a comparison between the existing 1D average shear velocity models for Peninsular India (Dube et al., 1973; Bhattacharya, 1981; Singh et al., 1999) and our model for northwestern India (Figs. 5c and 6). Our model suggests a slower crustal velocity in comparison to all other models of Peninsular India (Dube et al., 1973; Bhattacharya, 1981; Caen, 1986; Singh et al., 1999), which could be attributed to the different propagation paths of surface waves used in our study. However, the ray coverage for our study samples the central part of the low P-wave anomaly extending down to the mantle below northwestern India (Fig. 1a) much better in comparison to other earlier studies of Peninsular India (Dube et al., 1973; Bhattacharya, 1981; Caen, 1986; Singh et al., 1999). The ray coverage of our study enables us to verify the presence of low velocity in the upper mantle beneath northwestern India as delineated by the tomographic study of Kennett and Widiyantoro (1999). Thus, the lower shear velocities in the crust and upper mantle as obtained by our study are probably representing imprints of the Deccan mantle plume as earlier suggested by Kennett and Widiyantoro (1999). Interpretation of Reduction of Shear-Wave Velocity in the Upper Mantle Our study delineates an LVZ (approximately −3% reduction in V S ) starting at 60–70 km depth and extending to a depth of 120–130 km (Figs. 5c and 6). The maximum reduction in V S (approximately −3%) is found at a depth of 70–80 km. Kennett and Widiyantoro (1999) based on regional earthquake tomography, delineated an LVZ of P waves extending from 60– 70 km (approximately −0:25%–0:5% decrease in V P ) to 600 km (approximately −1:5% decrease in V P ) depth underlying northwestern India (Fig. 1b; after fig. 5b of Kennett and Widiyantoro, 1999). Based on inversion of geochemical data for rare earth concentrations, White and McKenzie (1989) have inferred that the top of the melting column rose to within 60–70 km of the surface for the Deccan, which would be compatible with our LVZ (approximately −3% reduction in V S ) (Fig. 6), assuming that thermal diffusion has taken place over the 65 Ma since the Deccan eruptions. Modeling of receiver functions, isotopic ratio of xenoliths, and temperature– depth profile also reveal a similar depth (∼60–70 km) for the LAB beneath northwestern India in general and Kachchh in particular (Pandey and Agrawal, 1999; Sen et al., 2009; Mandal and Pandey, 2010). The central part of low-velocity anomaly as mapped through P-wave tomography (Figs. 2a,b; Kennett and Widiyantoro, 1999) is centered around the Mer-Mundwara and SarnuDendali alkaline magmatism in Rajasthan, where alkaline rocks with a high 3 He=4 He ratio of 14 are reported (Fig. 2a; Basu et al., 1993), representing fossil plume head. This low-velocity anomaly extends further south to Dhandhuka in Saurashtra


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Histograms showing upper bound of upper mantle LVL of final models from VFSA inversion in terms of changes in V S km=s for depth levels (a) 70, (b) 90, (c) 100, (d) 150, and (e) 200 km, respectively. Value of depth level corresponding to each histogram is shown in the right corner of the plot. The color version of this figure is available only in the electronic edition.

Figure 7.

(Fig. 2a), where the picritic basalts (related to initial Deccan plume activity) are found (Krishnamurthy et al., 2000). Further, a large-scale presence of alkaline rocks has been reported from the Kachchh region in the form of plugs, cones, and sheetlike bodies, entrained with mantle xenoliths containing olivine, orthopyroxene, clinopyroxene, and spinel (Desai et al., 2004; Karmalkar et al., 2000, 2008; Sen et al., 2009). Melts for such alkaline rocks are reportedly generated by partial melting of CO2 -rich lherzolite (Sen et al., 2009). Ritter (2005) proposed that at ambient P–T conditions in the uppermost mantle (spinel-lherzolite composition) a 3% V S reduction corresponds to about 90–60 K increase in temperature. Hence, the observed reduction in V S at 70–120 km depth could be attributed to an increase of temperature by 100– 150 K in the upper mantle beneath the KRZ (Fig. 6). Another likely option is the presence of partial melt (CO2 -rich lherzolite) in the uppermost mantle, which has already been proposed by Sen et al. (2009) based on their isotopic ratio study of xenoliths from the Kachchh region. Faul et al. (1994) has shown that small amounts of partial melt, distributed in penny-shaped melt inclusions in the rock fabric, can reduce V S by 3.3% in the presence of 1% melt. Thus, the combined effect of small amount of partial melt (less than 1%) along with 100–150 K excess temperature at 70–120 km depth relative to the adjacent mantle can explain the observed shear velocity reduction (approximately −3%) in the upper mantle

beneath northwest India (Fig. 6). This zone of lowered shearwave velocity might be related to the imprints of fossil Deccan/Reunion plume. This model gets further support from the occurrence of high 3 He=4 He ratio in Rajasthan and picritic basalts in Saurashtra along the plume track beneath northwestern India (Figs. 2a,b).

Conclusions The crustal and upper mantle structures obtained through joint inversion modeling of Rayleigh- and Love-wave group velocity dispersion data suggest a two-layer crust with a 13.8 km thick upper crust with V S of 3:22 km=s and a 24.9 km thick lower crust with V S of 3:62 km=s. V S in the upper mantle beneath northwestern India is found to be 4:34 km=s. Our preferred model suggests slower V S in the crust and upper mantle below northwestern India in comparison to the existing velocity models of Peninsular India. The final velocity model for northwest India reveals the upper boundary of an LVZ (approximately −3% reduction in V S ) at 60–70 km depths. The lower boundary of the LVZ lies in the 120–130 km depth range. The minimum V S of 4:20 km=s is noticed at 70–80 km depth. We feel that the 3% reduction in V S could be attributed to the presence of 100–150 K excess temperature beneath northwestern India at 70–120 km depth relative to the adjacent mantle along with the presence of 1% penny-shaped melt inclusions (probably

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CO2 -rich carbonatite melts) in the upper mantle, which might be related to the imprints of the fossil Deccan/Reunion plume head that might have moved with India since the Deccan volcanism at 65 Ma.

Data and Resources Broadband seismograms of regional earthquakes used in this study were collected as part of the aftershock monitoring efforts after the 2001 M w 7.7 Bhuj earthquake by the National Geophysical Research Institute, Hyderabad, India, using the instruments procured under a Ministry of Earth Sciences, New Delhi, India, sponsored project. Data could be obtained through a request to the Director, Council of Scientific and Industrial Research-National Geophysical Research Institute, Hyderabad 500007, Telangana State, India ([email protected]).

Acknowledgments The author is grateful to the Director, National Geophysical Research Institute (NGRI), Hyderabad, for his kind permission to publish this work. This study is supported by the Council of Scientific and Industrial Research (CSIR) twelfth five-year plan project (Heart) at CSIR-NGRI, Hyderabad and the Ministry of Earth Sciences, Delhi, funded project. The author is thankful to R. B. Herrmann of St. Louis University, Missouri, for providing computer programs used in this study.

References Basu, A. R., P. R. Renne, D. K. Dasgupta, F. Teichmann, and R. J. Poreda (1993). Early and late alkali igneous pulses and a high-3He plume origin for the Deccan flood basalts, Science 261, 902–906. Berteusen, K. A. (1977). Moho depth determinations based on spectral ratio analysis of NORSAR long-period P waves, Phys. Earth Planet In. 31, 313–326. Bhattacharya, S. N. (1981). Observation and inversion of surface wave group velocities across central India, Bull. Seismol. Soc. Am. 71, 1489–1501. Biswas, S. K. (1987). Regional framework, structure and evolution of the western marginal basins of India, Tectonophysics 135, 302–327. Bodin, P., and S. Horton (2004). Source parameters and tectonic implications of aftershocks of the Mw 7.6 Bhuj earthquake of January 26, 2001, Bull. Seismol. Soc. Am. 94, 818–827. Caen, H. L. (1986). Comparison of the upper mantle shear wave velocity structure of the Indian Shield and the Tibetan Plateau and tectonic implications, Geophys. J. Roy. Astron. Soc. 86, 727–749. Campillo, M., S. K. Singh, N. Shapiro, and J. Pacheco (1996). Crustal structure south of Mexican volcanic belt, based on group velocity dispersion, Geofis. Intern. 35, 361–370. Courtillot, V., J. Besse, D. Vandamme, R. Montigny, J. J. Jaeger, and H. Cappetta (1986). Deccan flood basalts at the Cretaceous/Tertiary boundary? Earth Planet Sci. Lett. 80, 361–374. Desai, A. G., A Markwick, O. Vaselli, and H. Downes (2004). Granulite and pyroxenitexenoliths from the Deccan trap: Insight into the nature and composition of the lower lithosphere beneath cratonic India, Lithos 78, 263–290. Dube, R. K., J. C. Bhayana, and H. M. Chaudhury (1973). Crustal structure of the Peninsular India, Pure Appl. Geophys. 109, 1718–1727. Dziewonski, A. S., S. Bloch, and N. Landisman (1969). A technique for the analysis of transient seismic signals, Bull. Seismol. Soc. Am. 59, 427–444. Faul, U. R., D. R. Toomey, and H. S. Waff (1994). Intergranular basaltic melt is distributed in thin, elongated inclusions, Geophys. Res. Lett. 21, 29–32.

Gupta, H. K., T. Harinarayana, M. Kousalya, D. C. Mishra, I. Mohan, N. Purnachandra Rao, P. S. Raju, B. K. Rastogi, P. R. Reddy, and D. Sarkar (2001). Bhuj earthquake of 26 January 2001, J. Geol. Soc. Ind. 57, 275–278. Gupta, M. L., and V. K. Gaur (1984). Surface heat flow and probable evolution of Deccan volcanism, Tectonophysics 105, 309–318. Herrmann, R. B. (1987). Surface wave inversion, in Computer Programs in Seismology, Vol. IV, Saint Louis University, St. Louis, Missouri. Herrmann, R. B. (2004). Computer Programs in Seismology, Department of Earth and Atmospheric Sciences, St. Louis University, St. Louis, Missouri. Kaila, K. L., H. C. Tewari, V. G. Krishna, M. M. Dixit, D. Sarkar, and M. S. Reddy (1990). Deep seismic sounding studies in the north Cambay and Sanchor basins, India, Geophys. J. Int. 103, no. 3, 621–637. Karmalkar, N. R., W. L. Griffin, and S. Y. O’Reilly (2000). Ultramafic xenoliths from Kutch, northwest India: Plume related mantle samples? Int. Geol. Rev. 42, 416–444. Karmalkar, N. R., M. G. Kale, R. A. Duraiswami, and M. Jonalgadda (2008). Magma underplating and storage in the crust-building process beneath the Kutch region, NW India, Curr. Sci. 94, 1582–1588. Kayal, J. R., D. Zhao, O. P. Mishra, R. De, and O. P. Singh (2002). The 2001 Bhuj earthquake: Tomographic evidence for fluids at the hypocenter and its implications for rupture nucleation, Geophys. Res. Lett. 29, 5-1–5-4. Kennett, B. L. N., and S. Widiyantoro (1999). A low seismic wave speed anomaly beneath northwestern India: A seismic signature of a Deccan plume, Earth Planet. Sci. Lett. 165, 145–155. Krishnamurthy, P., K. Gopalan, and J. D. Macdugall (2000). Olivine compositions in picrite basalts and the Deccan volcanic cycle, J. Petrol. 41, 1057–1069. Levshin, A., L. Ratnikova, and J. Berger (1992). Peculiarities of surfacewave propagation across central Asia, Bull. Seismol. Soc. Am. 82, 2464–2493 Liu, L., and M. D. Zoback (1997). Lithospheric strength and intraplate seismicity in the New Madrid seismic zone, Tectonics 16, no. 4, 585–595. Mahoney, J. J., R. A. Duncan, W. Khan, E. Gnos, and G. R. McCormick (2002). Cretaceous volcanic rocks of the South Tethyan suture zone, Pakistan: Implications for the Réunion hotspot and Deccan Traps, Earth Planet. Sci. Lett. 203, 295–310. Mandal, P. (2011). Crustal and lithospheric thinning beneath the seismogenic Kachchh rift zone, Gujarat (India): Its implications toward the generation of the 2001 Bhuj earthquake sequence, J. Asian Earth Sci. 40, 150–161. Mandal, P., and R. K. Chadha (2008). Three-dimensional velocity imaging of the Kachchh seismic zone, Gujarat, India, Tectonophysics 452, 1–16. Mandal, P., and O. P. Pandey (2010). Relocation of aftershocks of the 2001 Bhuj earthquake: A new insight into seismotectonics of the Kachchh seismic zone, Gujarat, India, J. Geodyn. 49, 254–260. Mandal, P., R. K. Chadha, N. Kumar, I. P. Raju, and C. Satyamurty (2007). Source parameters of the deadliest 8th October 2005 Kashmir Earthquake of Mw 7.6, Pure Appl. Geophys. 164, 1963–1983. Mandal, P., B. K. Rastogi, H. V. S. Satyanarayana, M. Kousalya, R. Vijayraghavan, C. Satyamurthy, I. P. Raju, A. N. S. Sarma, and N. Kumar (2004). Characterization of the causative fault system for the 2001 Bhuj earthquake of Mw 7.7, Tectonophysics 378, 105–121. Mishra, D. C., D. V. Chandrasekhar, and B. Singh (2005). Tectonics and crustal structures related to Bhuj earthquake of January 26, 2001: Based on gravity and magnetic surveys constrained from seismic and seismological studies, Tectonophysics 396, 195–207. Mitchell, B. J., L. Cong, and G. Ekstrom (2008). A continent-wide map of 1-Hz Lg coda Q variation across Eurasia and its relation to lithospheric evolution, J. Geophys. Res. 113, no. B04303, 1–19. Mukherjee, A. B., and S. Biswas (1988). Mantle-derived spinel lherzolite xenoliths from the Deccan volcanic province (India): Implications for the thermal structure of the lithosphere underlying the Deccan traps, J. Volcanol. Geotherm. Res. 35, 269–276. Pandey, O. P., and P. K. Agrawal (1999). Lithospheric mantle deformation beneath Indian cratons, J. Geol. 107, 683–692.

Upper Mantle Shear Velocity Structure below Northwestern India Based on Group Velocity Dispersion Pandey, O. P., and J. G. Negi (1995). Geothermal fields of India: A latest update, in Proc. of the World Geothermal Congress (Florence), Florence, Italy, Vol. 1, 163–172. Pulliam, J., and M. K. Sen (2005). Assessing uncertainties in waveform modeling of the crust and upper mantle, in Proc. 27th Seis. Res. Rev.: Ground- Based Nuclear Explosion Monitoring Technologies, Austin, Texas, 152–160. Ritter, J. R. R. (2005). Small scale mantle plumes: Imaging and geodynamic aspects, in Perspectives in Modern Seismology, Lecture notes in Earth Sciences, F. Wenzel (Editor), Springer-Verlag, Heidelberg, Vol. 105, 69–94. Roy, A. B. (2004). The Phanrozoic reconstitution of Indian shield as the aftermath of break-up of the Gondwana land, Gondwana Res. 7, 387–406. Sarkar, D., K. Sain, P. R. Reddy, R. D. Catchings, and W. D. Mooney (2007). Seismic-reflection images of the crust beneath the 2001 M 7:7 Kutch (Bhuj) epicentral region, western India, Geol. Soc. Am. Spec. Pap. 425, 319–327. Sen, G. (2001). Generation of Deccan Trap magmas, Proc. Indian Acad. Sci. (Earth Planet. Sci.) 110, 409–431. Sen, G., M. Bizimis, R. Das, D. K. Paul, A. Ray, and S. Biswas (2009). Deccan plume, lithospheric rifting, and volcanism in Kutch, India, Earth Planet. Sci. Lett. 277, 101–111. Sen, M. K., and P. L. Stoffa (1996). Bayesian inference, Gibbs’ sampler and uncertainty estimation in geophysical inversion, Geophys. Prospect. 44, 313–350. Sen, M. K., and P. L. Stoffa (2013). Global Optimization Methods in Geophysical Inversion, Second Ed., Cambridge University Press, Cambridge, United Kingdom. Shapiro, N. M., and S. K. Singh (1999). A systematic error in estimating surface wave group velocity dispersion curves and a procedure for its correction, Bull. Seismol. Soc. Am. 89, 1138–1142.

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Shapiro, N. M., M. Campillo, A. Paul, S. K. Singh, D. Jongmans, and F. J. Sanchez-Sesma (1997). Surface wave propagation across the Mexican volcanic belt and origin of the long period seismic wave amplification in the valley of Mexico, Geophys. J. Int. 128, 151–166. Sheth, H. C. (2005). Were the Deccan flood basalts derived in part from ancient oceanic crust within the Indian continental lithosphere? Gondwana Res. 8, 109–127. Singh, S. K., R. S. Dattatrayam, N. M. Shapiro, P. Mandal, J. F. Pacheco, and R. K. Midha (1999). Crustal and upper mantle structure of Peninsular India and source parameters of the May 21, 1997, Jabalpur earthquake [Mw 5:8]: Results from a new regional broad-band network, Bull. Seismol. Soc. Am. 89, 1632–1641. Tarantola, A. (1994). Inverse Problem Theory: Methods or Data Fitting and Model Parameter Estimation, Elsevier, Amsterdam, The Netherlands, 600 pp. White, R. S., and D. McKenzie (1989). Magmatism at rift zones: The generation of volcanic continental margins and flood basalts, J. Geophys. Res. 94, 7685–7729.

National Geophysical Research Institute Council of Scientific and Industrial Research Hyderabad, Andhra Pradesh India [email protected]

Manuscript received 3 December 2014; Published Online 1 September 2015