Journal of Asian Earth Sciences 42 (2011) 1243–1255

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Journal of Asian Earth Sciences

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Anomalous surface wave dispersion and the enigma of ‘‘continental-like’’structure for the Bay of Bengal

Supriyo Mitra a,b,⇑, Keith Priestley b, Charlotte Acton b, Vinod K. Gaur c,d

a Department of Geology and Geophyiscs, Indian Institute of Technology, Kharagpur 721 302, Indiab Department of Earth Sciences, Bullard Laboratories, University of Cambridge, Cambridge CB3 0EZ, UKc Indian Institute of Astrophysics, Bangalore, Indiad Center for Mathematical Modeling and Computer Simulations, Bangalore, India

a r t i c l e i n f o a b s t r a c t

Article history:Received 28 June 2010Received in revised form 29 June 2011Accepted 10 July 2011Available online 23 July 2011

Keywords:Bay of BengalSurface wavesGroup velocityCrustal structure

1367-9120/$ - see front matter � 2011 Elsevier Ltd. Adoi:10.1016/j.jseaes.2011.07.008

⇑ Corresponding author. Present address: IISER KolBCKV Campus Main Office, Mohanpur 741 252, Nadia33 9433155639.

E-mail address: supriyomitra@gmail.com (S. Mitra

Brune and Singh (1986) showed that fundamental mode surface wave group velocities measured acrossthe Bay of Bengal were inconsistent with velocity models of a normal oceanic crust overlain by sedi-ments. They attributed this to a possible transformation of the oceanic crust to ‘‘continental-like’’ crust.We re-visit this unexpected result by first inverting fundamental mode Rayleigh wave group velocity dis-persion across the Bay of Bengal gleaned from a much larger data set than available to Brune and Singh(1986), to better constrain the Vs structure beneath the Bay, and then comparing the Vs results with lab-oratory measured velocities in metamorphosed rocks corresponding to the pressure–temperature condi-tions evaluated at various depths. One-dimensional inversion of averaged dispersion curves calculatedfor seven clusters of event-receiver paths sampling the northern, central and southern Bay has been per-formed. This reveal three distinct southward tapering layers overlying a uniform oceanic crust of �7 kmthickness. The third layer immediately overlying the oceanic crust has increasing Vs and thickness from3.33 km s�1 and 1 km, respectively, in the south to 3.61 km s�1 and 8 km in the north. These match thevelocities of progressively higher grade metapelites, from zeolite through greenschist to amphibolitefacies, suggesting it to be metamorphosed sedimentary rocks, mapped here for the first time. This isan extraordinary feature of the Bay of Bengal crust. Due to loading of these overlying layers, the Mohodeepens from 17 ± 3 km below sea level in southern Bay to �20 ± 3 km in central and 30 ± 3 km in north-ern Bay, roughly paralleling the thickening wedge of meta-sedimentary rocks. Further north approachingcontinental India the Moho aligns with the receiver-function-determined Moho beneath the southeasternBengal Basin. These findings strongly support the view that the appearance of a continental type crust innorthern Bay found by Brune and Singh (1986) is in fact the effect of a wedge of meta-sedimentary andsedimentary rocks overlying a normal oceanic crust.

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1. Introduction

The Bengal Basin and the Bay of Bengal to its south contain thelargest delta-fan complex in the world. The sediments filling theBengal geosyncline are derived from erosion of the Himalaya,mainly at the headwaters of the Ganges and the Brahmaputra riv-ers. Sediments reach thicknesses as great as 22 km in the northernpart of the Bay of Bengal and the deep-sea fan complex stretchesfor a total length of over 3000 km from the continental shelf atabout 22�N to about 5–7�S where the sediments of the distal endof the fan gradually pinch out. This is almost an order of magnitudelarger than other large deep-sea fans.

ll rights reserved.

kata, Mohanpur Campus, PO:, West Bengal, India. Tel.: +91

).

Brune and Singh (1986) found that fundamental mode Rayleighwave group velocities measured across the Bay of Bengal could notbe fit by a model composed of thick sediments overlying normaloceanic crust. They proposed four possible explanations for theirobservation: (i) that the thermal blanketing effect of the sedimentshad caused a rise in temperature resulting in the differentiation ofthe basalt of the oceanic crust, increasing the crustal thickness, (ii)that the change in Moho depth results from an isostatic adjust-ment, possibly due to a phase change boundary which deepensdue to the thermal blanketing effect of the sediments or from apressure perturbations from the collision, (iii) that the collisionhad underthrust a wedge of low-velocity material beneath the oce-anic crust, or (iv) that the increased crustal thickness correspondsto a fortuitous ocean continent transition which happens to occurin the northern Bay of Bengal.

The geodynamical implications of the Brune and Singh (1986)observations and their implications for continental growth are ma-

1244 S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255

jor. If the dispersion observations are correct and cannot be fit by astructure consisting of sediments overlying a ‘‘normal’’ oceaniccrust, they would profoundly affect our understanding of the Mohoand provide new insight for the growth of the continents. Theavailability of seismic data from India has greatly improved sincethe Brune and Singh (1986) study. Accordingly, we designed anexperiment to analyze a more comprehensive data set than thatavailable to Brune and Singh (1986) from seismograms generatedat the Indian subcontinental east coast stations at KGP, HYB, VIZand PALK. Specifically, we calculated fundamental mode Rayleighwave dispersion curves in the 12–100 s period range, along sevenevent-receiver paths similar to those paths along which Bruneand Singh (1986) measured fundamental mode Rayleigh wavegroup velocity. We then model our dispersion data to obtain theshear wave speed structure for these paths. The purpose of thisstudy is not to decipher the details of the lateral variations in struc-ture for the Bengal Basin and the Bay of Bengal but to verify theobservations of Brune and Singh (1986) and, explore explanationsfor the dispersion observations.

2. Data

For this study we use events of magnitude between 4.9 and 6.5,located along the western margin of the Andaman–Sumatra sub-duction zone. Seismograms were recorded at (i) the Indian Insti-tute of Technology, Kharagpur broadband observatory (KGP), (ii)the Cambridge University and Indian Institute of Astrophysics tem-porary station at Vizag (VIZ), (iii) the Geoscope permanent stationHyderabad (HYB) and (iv) the IRIS permanent station at Palkellele(PALK) (Fig. 1). Earthquakes recorded between 1997 and 1999were used from the VIZ station and between 2000 and 2008 forthe other stations. The ray paths between the event and stationpairs sample the Bay of Bengal progressively from north to south.The instrumentation at KGP consists of a STS-2 sensor, QuanterraQ330 data logger and GPS timing. KGP has been in operation since2004. The VIZ instrumentation consist of a CMG-3T sensor, Reftek

PlateBurmese

BengalBasin

PALK

VIZHYB

India

BrahmaputraGanges Shillong

KGP

AndamanNicobarIslands

90E

Rid

ge

BAY of BENGAL

Sumatra

AGT

INDIAN OCEAN

Fig. 1. Map of the Bay of Bengal and surrounding regions with location of stations(black triangles) from which data has been used for this study.

data logger and GPS timing. VIZ was in operation from 1997 to1999. HYB and PALK have STS-1 sensors. Data from HYB and PALKwere obtained from the IRIS Data Management Center.

3. Methodology

Our analysis procedure is sub-divided into four sections. Wefirst decimated the broadband records to 1 sample/s, correct forthe instrument response and applied a 2 pole 2 pass butterworthbandpass filter with corners at 7 and 120 s to remove unwantedsignal and noise. Those seismograms with a signal-to-noise ratiogreater than 2 were kept for analysis. Second, we carry out a Multi-ple Taper Polarization Analysis of the three component data to de-tect the presence of off great circle path arrivals for the Rayleighwaves in the frequency band of 10 and 100 s. We next apply a Mul-tiple Filter Analysis to measure the group velocity in the 12–100 speriod band for Rayleigh waves arriving close to the great circlepath. Ray paths sampling similar regions are clustered to formaverage group velocity dispersion curves representative of thenorthern, central and southern Bay of Bengal. Finally, we invertthe group velocity dispersion measurements to obtain shear wavevelocity structure of the crust and upper mantle beneath the north,central and southern Bay of Bengal.

3.1. Multiple taper polarization analysis

Refraction and scattering of surface waves by lateral heteroge-neity in the crust and upper mantle can introduce substantial er-rors in group arrival times which in turn results in erroneousgroup velocity values. Surface waves propagating through thesuper-thick heterogeneous sediments of the Bay of Bengal andacross the continent–ocean boundary may suffer interference fromrefracted wave packets at nearby frequencies. This multipathed en-ergy must be avoided while measuring surface wave dispersions.Frequency dependent polarization measurements of Rayleighwaves map the deviation from great circle arc path and the tilt inthe elliptical particle motion from the vertical through a range offrequencies. The theoretical framework behind such measure-ments is given in Park et al. (1987a,b), Lerner-Lam and Park(1989), Laske et al. (1994), and Laske (1995).

We perform multiple taper polarization analysis on three com-ponent data using the algorithm of Laske et al. (1994). Fig. 2ashows an example of three component seismogram recorded atHYB from an event near the northern tip of Sumatra. Fig. 2b–eshow the polarization analysis for this event. Rayleigh and Lovewave arrivals are marked R1 and G1 on the waveforms (Fig. 2a).The Rayleigh wave is windowed on the vertical component andthe multiple taper polarization analysis performed on the threecomponent data within this window. The eigenvalues (Fig. 2b)for the polarization ellipse satisfies the criteria k1� k2 > k3 indicat-ing a well-defined polarization. The 3D ellipticity (Fig. 2c) is closeto 0.33, showing that the particle motion is retrograde ellipticalas expected for Rayleigh waves. In the ideal case of a Rayleigh wavewith no contamination the tilt of the polarization ellipse should bevertical (180�). Our result for this event satisfies this criteria be-tween frequencies 0.01 and 0.08 Hz (Fig. 2d). For the same fre-quency range, the deviation of the polarization ellipse from thetheoretical radial direction indicates the Rayleigh wave energy ar-rives along the radial direction (Fig. 2e). These data are typical ofthe events we have chosen for the dispersion measurement. Thesame analysis was done for all 53 – three component seismogramsrecorded at KGP, HYB and VIZ and 31 seismograms within ±10� ofthe great circle azimuth, ±30� for the tilt and �0.33 ellipticity werekept for the dispersion analysis. The frequency range in which thedata satisfied these criteria was considered reliable for the disper-

Time in seconds

BHZ

BHR

BHT

R1(a)

G1

(b) (c)

(e)(d)

Fig. 2. The MTPA for (a) three component seismogram recorded at HYB for an aftershock of the Andaman–Sumatra earthquake. R1 and G1 are the Rayleigh and Love waves,respectively, (b–e) are results of the MTPA plotted vs. frequency: (b) three eigenvalues k1� k2 > k3, (c) ellipticity of the polarization ellipse, (d) tilt of the polarization ellipse,(e) horizontal azimuth of the major axis of the polarization ellipse.

S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255 1245

sion curve. The horizontal component data for PALK stations werenot available at the IRIS Data management Center and thereforecould not be tested for polarization. The selection of the PALK datais based on signal-to-noise ratio discussed in the next section.

These stringent criteria for choosing the seismograms for anal-ysis reduced the overall number of dispersion measurements weobtain but it is important for the possible variation we are examin-ing in the north–south structure of the Bay of Bengal crust.

3.2. Rayleigh wave group velocity dispersion measurements

Rayleigh wave group velocities are determined by the MultipleFilter Analysis (MFA) algorithm of Herrmann and Ammon (2004)which is an implementation of the Frequency Time Analysis meth-od of Levshin et al. (1972, 1992). The vertical component seismo-grams are filtered using narrow-band Gaussian filters and theenvelope is used to determine the Rayleigh wave group arrivaltime as a function of frequency. Group velocity is computed fromthe epicentral distance and the group travel time. Earthquake loca-tions and origin times were taken from the Engdahl et al. (1998)catalogue and its updates (Engdahl, written communications). Agroup velocity vs. period diagram for the vertical component isconstructed and displayed as a frequency-time amplitude plot.

The group velocity curve is picked following the amplitude ridgewhere the signal is well observed.

An example of the MFA analysis sequence is shown for seismo-grams of the Andaman–Sumatra aftershock on the 4th of January2005 recorded at PALK (Fig. 3a). The broadband vertical component(Fig. 3b) is used to determine the fundamental mode Rayleighwave group velocity dispersion for the period range of 12–100 s.A comparison of the signal and noise spectra (Fig. 3c) show thatthe signal-to-noise ratio is greater than 2 for the chosen periodrange. An initial MFA diagram generated from the vertical compo-nent seismogram is used to pick the dispersed surface wave en-ergy. This is then used to filter the seismogram and removerandom and signal generated noises (e.g. multipathed energy).The filtered seismogram is then used to perform a more accuratemeasurements of the group velocity (Fig. 3d). The same procedurewas applied to all of the events to measure fundamental mode Ray-leigh wave dispersion for (i) 11 source-receiver paths to KGP, (ii)10 for HYB, (iii) 10 for VIZ, and (iv) 65 for PALK.

3.3. Error weighted average dispersion curves

The Burmese arc and the Andaman–Sumatra subduction zonespan a length of �2000 km. Dispersion curves for similar propaga-

PALK

India

(a)

(e) (f)

(b)

(c)

Time (sec)

Signal

Signal

Noise

Time Period (sec)

Time Period (sec)10 100G

roup

vel

ocit

y (k

m/s

ec)

0.1

2

4

6

X 1

0−4

76543 21X 10+2

0

0.01 1

2.4

3.2

4.0

(d) 4.8

1.6

Frequency (Hz)

Am

plit

ude

Fig. 3. An example of the dispersion analysis (a) map showing the raypath between the 4th January 2005 Andaman–Sumatra aftershock and PALK, (b) the vertical componentseismogram for this event recorded at PALK. The Rayleigh wave signal used for computing the dispersion curve is plotted in bold. (c) Comparison of the amplitude spectra ofthe Rayleigh wave signal (solid line) with the pre-signal noise (dotted line). (d) MFA diagram showing the fundamental mode Rayleigh wave dispersion curve calculated fromthe seismogram in (b), (e) raypaths sampling the central Bay of Bengal for which the individual dispersion curves are averaged by error weighting. (f) plot of individual anderror weighted average dispersion curve with ±1 standard deviation bounds plotted as error bars on the average curve.

1246 S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255

tion paths are grouped into seven clusters. Fig. 3e shows one suchcluster. The individual dispersion values in each cluster are aver-aged by error weighting to obtain a representative curve for thesampled region (Fig. 3f).

The error in group velocity observation occurs due to errors insource parameters (location and origin time) and error in pickingthe group arrival time. Since the Gaussian filter has a longer impulseresponse at low frequencies, a reading error is associated with a pos-sible misplaced maximum of the envelope. The number computedassumes that the group travel time can be mis-measured by one fil-ter period. These errors are used as weights in the calculation of theaverage dispersion curve for each cluster. The larger the error in thedispersion measurement the lower is the weight associated with itwhile calculating the average curve. The error weighted averagegroup velocity from n dispersion curves in a cluster is calculated as:

Vjew ¼

Xn

i¼1

Vji=Ej

i

� � Xn

i¼1

1=Eji

� �,

where Vjew is the error weighted group velocity for the jth period, Vi

is the group velocity from ith dispersion curve for the jth period,and Ej

i is the associated group velocity error. We then computethe ±1 standard deviation bounds for the average group velocity

at every period (Fig. 3f). These bounds are used to constrain theshear wave velocity structure from inversion of the average groupvelocity dispersion data for each cluster. The dispersion curves foreach region are shown in Figs. 4–6.

3.4. Inversion of dispersion data

Rayleigh wave dispersion is primarily sensitive to the shearwave velocity structure. The short-period dispersion data are sen-sitive to the ocean water depth, sedimentary layers and the uppercrust, while the longer periods are sensitive to the lower crust andupper mantle structure. The average Rayleigh wave group velocitydispersion curves obtained through clustering (Figs. 4–6) are in-verted to obtain 1-D shear wave velocity structure beneath thoseparts of the Bay of Bengal sampled by these paths (Figs. 7–9). Weconstructed a starting velocity model (e.g., gray model in Fig. 7b)consisting of thinly parametrized homogeneous, horizontal layersof fixed equal thickness (1 km in the crust and 5 km in the uppermantle). The average depth of the water along each cluster is ob-tained from ETOPO Global Relief Model (Amante and Eakins,2009) and is fixed in the inversion. Although surface wave disper-sion is not sensitive to interfaces, but to vertical averages of shear

52−110 Ma

> 110 Ma

20−52 Ma

52−110 Ma

> 110 Ma

20−52 Ma

(a)

(b)

Cluster − S2 (PALK)

Path A

Cluster − S1 (PALK)

Path A

Fig. 4. Plots of Rayleigh wave dispersion curves for ray paths sampling the southern Bay of Bengal (ray path map in inset) and comparison with Brune and Singh (1986)dispersion data for southeast path A. Ocean age dependent Rayleigh wave dispersion curves of Nishimura and Forsyth (1988) have been modified by (i) replacing the topsedimentary layer with that from path A of Brune and Singh (1986) – dashed line curves and (ii) replacing the entire crustal structure by path A of Brune and Singh (1986) –bold line curves.

S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255 1247

wave velocities, the starting model is parameterized as layers forthe ease of calculation of group velocities. Moreover the choice of1 km thickness of the crustal layers is to represent velocity gradi-ents in the final model rather than to resolve the structure at thescale of a kilometer. The choice for the starting model is guidedby the similarity of the paths to those of Brune and Singh (1986).First we invert the group velocities using this thinly parametrizedmodel to obtain a smooth velocity model (minimum number ofinterfaces) which fits the observed data within ±1 SD bounds. Wethen simplify this model by grouping adjacent layers with similarwave speeds and re-invert the dispersion data to obtain a velocitymodel with minimum number of layers that fit the observed groupvelocity data. We plot the dispersion data fit (e.g., Fig. 7a) for boththe minimum interface model and the minimum layer model (e.g.,Fig. 7b) representing end members in crustal structure possible forthe real Earth. Dispersion curves from all the seven clusters aremodeled similarly and the results are given in Section 5.

We explore the sensitivity of the dispersion data only to theMoho depth through forward modeling. Decreasing the Mohodepth by 3 km causes the calculated dispersion curve to overesti-mate the observed dispersion between periods 10 and 20 s (e.g.,dotted gray line in Fig. 9a); whereas by increasing the Moho depth

by 3 km the dispersion is underestimated between periods 25 and35 s (e.g., bold gray line in Fig. 9a). Similar Moho depth tests havebeen performed for all the seven clusters. The Moho depth boundsare found to be ±3 km in all these cases.

Finally we compare our 1D velocity models with fundamentalmode Rayleigh wave group velocity tomography results of Actonet al. (2010) who constructed Rayleigh wave group velocity mapsfor the period range 10–70 s from 4054 paths crossing India, theBay of Bengal and Tibet. The Acton et al. (2010) study provide addi-tional constraints on the Bay of Bengal structure from long(>2000 km) paths crossing the region but has a greater propensityfor smearing the velocity results. The resolution of tomographicmaps in the Bay of Bengal region is estimated to be 5� (Actonet al., 2010). Group velocity dispersion curves for an approximatelysouth–north profile (3�N, 84�E to 24�N, 92�E – shown as dashedblack line in Fig. 1) across the Bay of Bengal are extracted from thesetomographic maps and modeled for the shear wave velocity struc-ture in an identical manner to the dispersion data we measure herefor paths crossing the Bay of Bengal. The 2D profile from invertingthe Acton et al. (2010) dispersion is plotted along with the 1D mod-els obtained from inversion of clustered path average measure-ments in Fig. 10.

20−52 Ma

> 110 Ma

52−110 Ma

20−52 Ma

> 110 Ma

52−110 Ma

20−52 Ma

> 110 Ma

52−110 Ma

(a)

(b)

(c)

Cluster − C1 (HYB)

Path B

Cluster − C2 (VIZ)

Path B

Path B

Cluster − C3 (PALK)

Fig. 5. Plots of Rayleigh wave dispersion curves for ray paths sampling the central Bay of Bengal (ray path map in inset) and comparison with Brune and Singh (1986)dispersion data for east–west path B. Rayleigh wave dispersion curves for ocean age from Nishimura and Forsyth (1988) have been modified in the same way as in Fig. 4 usingthe model of path B from Brune and Singh (1986).

1248 S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255

4. Rayleigh wave dispersion for the Bay of Bengal

We discuss the dispersion characteristics by subdividing theBay of Bengal into three regions approximately comparable to

those of Brune and Singh (1986) (Figs. 4–6). We compare the dis-persion characteristics for each path cluster with the dispersiondata from Brune and Singh (1986) for the same region and withdispersion curves calculated for ocean floor of the appropriate

> 110 Ma

52−110 Ma

20−52 Ma

52−110 Ma20−52 Ma

> 110 Ma

(a)

(b)

Cluster − N1 (KGP)

Path D

Cluster − N2 (PALK)

Path D

Fig. 6. Plots of Rayleigh wave dispersion curves for ray paths sampling the northern Bay of Bengal (ray path map in inset) and comparison with Brune and Singh (1986)dispersion data for north–south path D. Rayleigh wave dispersion curves for ocean age from Nishimura and Forsyth (1988) have been modified in the same way as in Fig. 4using the model of path D from Brune and Singh (1986).

S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255 1249

age (Nishimura and Forsyth, 1988) loaded with sediment. Thesouthern Bay of Bengal is sampled by the two clusters of raypaths from events located along the southern segment of theAndaman–Sumatra subduction zone, recorded at PALK (Fig. 4 in-sets). The ray path for the two clusters S1 and S2 successivelysample southward and are closest to path-A of Brune and Singh(1986). The dispersion curves for both S1 and S2 are measuredbetween 14 and 100 s periods (Fig. 4a and b) and match theBrune and Singh (1986) path-A group velocity measurementsonly below <20 s period. At longer periods (50–100 s) the ob-served dispersion curves are similar to the dispersion curve for52–110 Ma oceanic lithosphere (Fig. 4).

The central Bay of Bengal is sampled by: (i) all events recordedat HYB (Fig. 5a – inset – path C1), (ii) all events recorded at VIZ(Fig. 5b – inset – path C2), and (iii) events from the central sectionof the arc recorded at PALK (Fig. 5c – inset – path C3). These pathsclusters are similar to path-B of Brune and Singh (1986) except thatC1 traverses a part of the Indian continental crust to HYB. The dis-persion curves for C1, C2 and C3 are between 17 and 100 s, 13 and95 s, and 12 and 95 s periods, respectively (Fig. 5), and are identi-cal, except between 20 and 30 s period. The C1 and C2 curves aremarginally slower than the path-B group velocity data of Bruneand Singh (1986) between 20 and 40 s, while the C3 curve is mar-

ginally faster than the Brune and Singh (1986) path-B data up to25 s. The C1 and C2 clusters match the dispersion curve for 52–110 Ma oceanic lithosphere at periods longer than 40 s while theyare slower than the dispersion curves for oceanic lithosphere be-tween 20 and 40 s period. The C3 curve almost matches the oceaniclithosphere dispersion curve with modified crustal structure(dashed line in Fig. 5c).

The northern Bay of Bengal is sampled by events from the And-aman-Nicobar region along a SE–NW trending path to KGP (Fig. 6a– inset – path N1) and by event located southwest of the Nagathrust along a NE–SW trending path to PALK (Fig. 6b – inset – pathN2). Clusters N1 and N2 share a common propagation path acrossthe northern Bay of Bengal, but only N1 samples the northern Baywhile N2 averages over part of the central Bay of Bengal. Hence, inour discussions cluster N1 which is similar to path D of Brune andSingh (1986) represents the northern Bay of Bengal. The dispersioncurves for N1 and N2 are between 12 and 85 s and 16 and 100 speriods, respectively (Fig. 6a and b). The N1 and N2 dispersioncurves are similar at longer periods but differ at shorter periods(<30 s period). The Brune and Singh (1986) group velocities forpath-D are closer to the N1 dispersion and are significantly slowerthan the N2 dispersion. This is understandable as path-D to Shil-long (SHL) on the Meghalaya Plateau traverses at least 300 km of

(a)

(c)

(b)

(d)

Cluster S1

Cluster S2

Fig. 7. Result of Rayleigh wave group velocity dispersion inversion for the southern Bay of Bengal. (a) and (c) synthetic dispersion curves (bold line) calculated for theminimum interface and minimum layer models in (b) and (d), respectively. Fit to the dispersion measurements are estimated by ± 1 standard deviation bounds (error bars in(a) and (c)) taken from the data. The starting model of Brune and Singh (1986) is plotted as dotted gray line. Sensitivity test of the Moho depth is performed on the finalinverted model (black line in (a)). The gray bold lines in (b) are ±3 km change in the Moho depths. The synthetics (gray bold and dashed lines in (a)) computed from these twomodels do not fit the observed dispersion between 25 and 35 s period.

1250 S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255

continental crust. The dispersion curve for 52–110 Ma oceanic lith-osphere with the sediment and crust replaced by Brune and Singh(1986) model corresponds to the observed dispersion for N1 (boldline in Fig. 6a). The N2 dispersion data is similar to the dispersioncurve for 52–110 Ma oceanic lithosphere with only the sedimentstructure replaced by Brune and Singh (1986) path-D model(dashed line in Fig. 6b).

5. Shear wave velocity structure of the Bay of Bengal

The averaged Rayleigh wave group velocity curve for each of thecluster was inverted to obtain 1D shear wave velocity structure.Following this the 1D models are compared with the shear wavevelocity profile obtained by inversion of group velocity tomo-graphic maps of Acton et al. (2010) to better understand the grad-ual variation of the structure from north to south.

5.1. Southern Bay of Bengal

The paths for clusters S1 and S2 which sample the southern Bayof Bengal (Fig. 4 insets) have very similar dispersion curves. Inver-sion results for cluster S1 reveal a 5 km thick upper layer with po-sitive velocity gradient. The Vs at the top of the layer is 0.68 km s�1

and increases to 2.54 km s�1 at the base of the layer. This is under-lain by a zone of �1.5 km thickness with Vs of 3.10 and 3.35 km s�1,respectively. Beneath this lies a �7 km-thick layer with a Vs of3.95 km s�1. The Moho is at a depth of �18 ± 3 km with a sub-Moho shear wave velocity of 4.51 km s�1 (Fig. 7b, Table 1). Theinversion result of the S2 cluster is almost the same except for amarginally thinner layer 3, resulting in a slightly shallower Mohobut forward modelling indicates this difference is not significant(Fig. 7d, Table 1). For both models the upper mantle lid has a posi-tive velocity gradient and is 60 km thick with an average Vs of4.56 km s�1. Below this there is a LVZ with average Vs of 4.5 km s�1.

5.2. Central Bay of Bengal

The paths for clusters C1, C2 and C3 sample the central Bay ofBengal (Fig. 5 insets). The inversion results for the C1 and C2 clus-ters are almost identical and show a �6 km thick upper layer witha positive velocity gradient. The Vs at the top is 0.5 km s�1 increas-ing to 2.5 km s�1 at the base of the layer. Below this are two layerseach �3 km thick with Vs of 3.12 and 3.58 km s�1, respectively.This is underlain by a �7 km-thick layer with Vs of 4.03 km s�1.The Moho is at a depth of �21 ± 3 km with a sub-Moho shear wavevelocity of 4.46 km s�1 and a positive velocity gradient in the

(a)

(c)

(b)

(d)

(f)

(e)

Cluster C1

Cluster C2

Cluster C3

Fig. 8. Result of Rayleigh wave group velocity dispersion inversion for the central Bay of Bengal. Figure format same as Fig. 7.

S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255 1251

uppermost mantle. The upper mantle lid is �70 km thick with anaverage Vs of 4.65 km s�1 (Fig. 8b and d, Table 2). This is underlainby a LVZ with average Vs of 4.36 km s�1.

Inversion results of cluster C3 gives a �5.5 km thick upper layerwith Vs varying from 0.74 km s�1 at the top to 2.64 km s�1 at thebottom of the layer. This is underlain by two layers of �2.5 kmand �2 km thickness with Vs of 3.24 and 3.67 km s�1, respectively.Below this is �7 km-thick layer with average Vs of 4.01 km s�1. TheMoho is at a depth of �20 ± 3 km with a sub-Moho shear wavevelocity of 4.47 km s�1 and a positive velocity gradient in the

uppermost mantle. The upper mantle lid is �63 km thick and isunderlain by an LVZ with average Vs of 4.49 km s�1 (Fig. 8f,Table 2).

5.3. Northern Bay of Bengal

The paths for cluster N1 sample the northern Bay of Bengal(Fig. 6a inset). These paths are primarily oceanic except for �8%continental path from the eastern coast of India to KGP. Inversionof the dispersion data results in a upper layer of increasing shear

(a)

(b)

(c)

(d)

Cluster N1

Cluster N2

Fig. 9. Results of inversion of Rayleigh wave dispersion data for the northern Bay of Bengal. Figure format same as Fig. 7.

1252 S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255

wave velocity from 0.41 km s�1 below the sea bed to 2.35 km s�1 atthe base of the �4.5 km-thick layer. Beneath this are two layers of�10 km and �7 km thickness with Vs of 3.20 and 3.61 km s�1,respectively. This is underlain by a �7 km-thick 4.03 km s�1 layerhaving characteristics of the oceanic crust. The Moho is at a depthof �30 ± 3 km with a sub-Moho shear wave wave velocity of4.59 km s�1. The upper mantle lid is �70 km thick with a positivevelocity gradient and an average Vs of 4.67 km s�1 (Fig. 9b, Table 3).This overlies a low velocity zone (LVZ) with average Vs of4.26 km s�1.

Cluster N2 paths traverse a larger N–S section of the Bay of Ben-gal (Fig. 6b inset) compared to the N1 cluster and they have highergroup velocities at shorter periods (12–30 s). The inversion resultfor cluster N2 shows a �2.5 km thick upper layer with an averageVs of 2.39 km s�1 above a �2 km thick layer of Vs 3.34 km s�1. Thisis underlain by two layers of �10 km and �8.5 km thickness withVs of 3.69 and 3.82 km s�1, respectively. Below this is �7 km-thicklayer with Vs of 3.96 km s�1. The Moho is at a depth of �32 ± 3 kmwith a sub-Moho shear wave velocity of 4.5 km s�1. The uppermantle lid thickness and average velocities are similar to that ofthe N1 model and lay above a LVZ with average Vs of 4.36 km s�1

(Fig. 9d, Table 3).

6. Discussions

The shear wave structure for the Bay of Bengal obtainedthrough inversion of fundamental mode Rayleigh wave group

velocity dispersion data can be parametrized by four layers be-neath the sea bed: (i) a 4–6 km-thick uppermost layer (red layersin Fig. 10) with increasing shear wave speed from extremely lowat the sea bed to a maximum Vs of 2.64 km s�1 at its base, (ii) alayer increasing in thickness towards the north from �1 km inthe southern Bay (S2) to �10 km beneath the northern Bay (N1)(orange to yellow layer in Fig. 10) and velocity varying between3.1 and 3.24 km s�1, (iii) a layer of increasing thickness and Vs from�1 km and 3.35 km s�1 in the south to �8 km and 3.62 km s�1 inthe north (white to intermediate blue layer in Fig. 10), and (iv) alayer of near uniform thickness of �7 km across the entire regionwith velocities varying between 3.96 and 4.04 km s�1 (deep bluelayer in Fig. 10). The Moho lies at a depth of �17.5 km beneaththe �3 km deep ocean floor in the southern Bay of Bengal (S2)and dips northwards to a depth of �30.5 km beneath the northernpart of the Bay (N1). The uncertainty in the Moho depth is ±3 km.

We should point out that the inversion of surface wave disper-sion data such as that presented here does not yield a uniquevelocity model. This four layered model for the crust of the Bayof Bengal is a simplification. Our goal here is to demonstrate thatthe anomalous dispersion observed by Brune and Singh (1986) iscorrect, but that it can be explained by simple geologic processesresulting from the great volume of sediment in the Bengal fanoverlying normal oceanic crust. The observed dispersion does notrequire a large scale modification of the underlying oceanic crust.

The increase in wave velocity in sediments is controlled by thedecrease in sediment porosity with depth of burial. Considering a60% porosity at the sea bed and a simple exponential decay of

?

Moho

?

Unconsolidated sediments

Consolidated sediments

ZF

GF

AF

Sedimentary rocks

Sediments (LM97)

Moho (Curray82)

Moho (BS86)

Oceanic crust

Metasedimentary rocks

South North

Dep

th (

km)

Distance (km)

AGT

S2 S1 C3 N2 N1C2 C1

Fig. 10. Crustal Profile across the Bay of Bengal (from 3�N, 83�E to 24�N, 92�E – dashed line on Fig. 1) with the 1D shear wave velocity models from each cluster, plotted attheir respective position of intersection of the profile line, on top of the 2D tomographic profile taken from Acton et al. (2010). All ray paths are approximately E–W with theexception of N2 which samples the Bay from north east to south west and is therefore not representative of the structure between N1 and C1. The ±3 km uncertainty in theMoho depth are plotted as vertical line in the Moho interface of the 1D models. The AGT station is on the Bengal Basin and the Moho depth beneath the station is taken fromreceiver function modeling done by Mitra et al. (2006). Sediment thickness and Moho depths from previous studies are over-plotted and are labeled as follows: BS86 – Bruneand Singh (1986), LM97 – Laske and Masters (1997), Curray82 – Curray et al. (1982). Isograd (red dotted) lines marking increasing metamorphic grade with depth is labeledAF – amphibolite, GF – greenschist and ZF – zeolite facies. The top sediment layer and Moho from this study shows a close match with the Brune and Singh (1986) results. (Forinterpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255 1253

porosity with depth (Sclater and Christie, 1980), the compressionalwave velocity increases from 1.5 km s�1 at the sea bed to a satura-tion velocity of 6 km s�1 at a depth of 6 km where the porosity be-

Table 1Southern Bay of Bengal minimum layer models.

S1 – PALK S2 – PALK

Thickness (km) Vs (km s�1) Thickness (km) Vs (km s�1)

3.1963 0.0000 3.3242 0.00001.0000 0.6879 1.0000 0.82291.0000 1.1545 1.0000 1.26441.5000 2.0610 1.5000 2.04851.5000 2.5479 1.5000 2.54371.5000 3.1017 1.0000 3.11251.5000 3.3504 1.0000 3.37787.0000 3.9560 7.0000 3.998828.0000 4.5100 28.0000 4.441435.0000 4.6193 35.0000 4.65691 4.5356 1 4.5227

Table 2Central Bay of Bengal minimum layer models.

C1 – HYB C2 – VIZ

Thickness (km) Vs (km s�1) Thickness (km)

2.3180 0.0000 2.48750.2500 0.5584 0.25000.7500 1.0019 0.75002.7500 1.6779 2.75002.0000 2.5126 2.00003.0000 3.1240 3.00003.0000 3.5824 2.00007.0000 4.0358 7.000033.0200 4.4630 33.020040.3000 4.7988 40.00001 4.3644 10.0000– – 10.0000– – 1

comes negligible (Athy, 1930). The shear velocity gradient in thetop 6 kms of our inverted models from all the path clusters are con-sistent with such a rate in increase of velocity with burial and

C3 – PALK

Vs (km s�1) Thickness (km) Vs (km s�1)

0.0000 2.8978 0.00000.4918 1.0000 0.74580.9218 1.0000 1.09741.5520 1.5000 1.98872.5027 2.0000 2.64433.1256 2.5000 3.24753.5923 2.0000 3.67174.0463 7.0000 4.01194.4267 28.0000 4.47144.7621 35.0000 4.66174.6315 1 4.49804.5481 – –4.4525 – –

Table 3Northern Bay of Bengal minimum layer models.

N1 – KGP N2 – PALK

Thickness (km) Vs (km s�1) Thickness (km) Vs (km s�1)

2.0000 0.0000 2.4102 0.00000.5000 0.4127 0.5000 1.49552.0000 1.5868 2.0000 2.62852.0000 2.3537 2.0000 3.349310.0000 3.2057 10.0000 3.69067.0000 3.6199 8.5000 3.82987.0000 4.0317 7.0000 3.968230.0000 4.5981 70.0000 4.497040.0000 4.7041 1 4.36051 4.2629 – –

1254 S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255

compaction (decay in porosity). Layer 2 has an average velocity of3.15 km s�1 which is similar to that of a typical lithified sedimen-tary rock. Layer 3 has velocities higher than those of sedimentaryrocks but lower than that of a typical oceanic crust. Moreover, itsaverage layer velocity increases with increasing thickness anddepth of burial from south to north. We interpret this layer to becomposed of meta-sedimentary rocks and examine the degree ofits burial metamorphism under the sedimentary load by comput-ing the pressure and temperature condition at its top. The shearwave velocity of this meta-sedimentary layer obtained from inver-sion of our dispersion data is compared with laboratory measure-ments on comparably metamorphosed rocks (Christensen, 1996).The pressure at the top of this layer is computed using the thick-ness and density of the overlying water and rock column. The onlyavailable measurement of a temperature gradient in the region isfrom the northern Indian ocean (717C hole of the 116 ODP leg)which is 39.5 ± 0.04�/km (ODP-Report, 1989; Cochran et al.,1988). If we consider this to be representative for the Bay of Bengaland extrapolate it linearly to crustal depths, we obtain a lowerbound on the temperature at any given depth. The actual effectsof a variable sediment deposition rate over the oceanic crust andof thermal conductivities of sedimentary rocks would be to elevatethe thermal gradient, both because of the radioactive heating with-in the thick pile of sediments as well as its blanketing effect.

As a first estimate therefore we use the only available data oftemperature gradient from the borehole measurement and com-pute the P-T condition in the south, central and northern Bay ofBengal. The top of the metasedimentary layer beneath S2 is at adepth of �6 km beneath the ocean bottom and will thus be at apressure of �0.18 GPa and temperature of �237 � C. These valuesfall on the boundary between the zeolite and greenschist faciesmetamorphism and laboratory measurements of Vs for pelitic rocksunder this P-T condition yields a velocity of �3.3 km s�1 (Christen-sen, 1996) which is similar to our inverted velocity for layer 3. Forthe central Bay of Bengal (C3) the top of the metasedimentary rockcolumn is at �7.5 km beneath the ocean floor and hence will be at�296 �C and 0.20 GPa. This is in the greenschist facies P-T field. Thelaboratory measured velocity for meta-pelites in green-schist fa-cies is 3.57 km s�1 closely matching our inverted velocity for layer3 beneath the central Bay of Bengal. In the northern Bay (N1) thetop of the metasedimentary rocks is at a depth of �14.5 km andhence is at �573 �C and 0.42 GPa. This falls in the amphibolite fa-cies P-T condition. Laboratory measurement of shear wave velocityfor meta-pelites under amphibolite facies condition is 3.61 km s�1

which is again in good agreement with our inverted models forlayer 3 in the northern Bay of Bengal.

As stated above these are lower bounds on temperature andhence the sediments beneath the Bay would most likely haveundergone load metamorphism at least to zeolite facies in thesouth, greenschist facies in the central and amphibolite facies inthe north in response to increasing thickness of sediment accumu-lation in the Bengal fan. Beneath these metasedimentary rocks liesthe average oceanic crust of uniform thickness of �7 km across theentire Bay of Bengal. The wedge of metasedimentary rock sand-wiched between the sedimentary rocks and the underlying oceaniccrust is a unique feature in the structure of the Bay of Bengal(Fig. 10). The sediment loading on the oceanic crust will possiblyresult in metamorphism of the basalts to the amphibolite to gran-ulite facies (south to north in the Bay) with seismic velocitiesaround 4.0 km s�1. This is in agreement with the oceanic crustalvelocities in our models (3.96–4.04 km s�1) represented by layer4. The oceanic crust has been flexed down with increasing sedi-ment load northwards and the Moho dips gently in the south-cen-tral section, roughly paralleling the tapered wedge of thesedimentary rocks, and thereafter, dip more steeply on approach-ing continental India (Fig. 10).

The total thickness of the crust, with an uncertainty of ± 3 km, issimilar to Brune and Singh (1986)’s observations, but much thickerthan that obtained from reflection and refraction data (Currayet al., 1982) (Fig. 10). We believe that the Moho inferred by Currayet al. (1982) was a strong reflector of compressional waves withinthe crust possibly the metasediment–oceanic crust boundary.Moreover, shear wave velocity models from this study do not re-veal the discontinuity related to the Eocene hiatus in sedimenta-tion between pre- and post-India-Asia collision reported byCurray (1991, 1994). The average shear wave velocity (excludingthe top 2 km of uncompacted sediments) is 3.51 km s�1 through-out the crustal section from south to north and is closer to the con-tinental average than oceanic.

In light of these findings, the observation of Brune and Singh(1986) of a thicker continental type crust in the northern Bay is re-solved as a mistaken appearance of what indeed is a normal oceaniccrust overlain by a thick layer of metasedimentary rocks that theoverlying sediment load is expected to produce on the basis ofour estimates of thermal gradients in the region. Further north, be-neath the Bangladesh plains, the total sedimentary thickness hadbeen reported to be �18 km (Hiller, 1988) and receiver function in-verted models from station AGT on the Bangladesh Plains revealed acrustal thickness of 36 km (Mitra et al., 2006). Both these valuesconform well with extrapolated sediment thickness and Mohodepth from our models of the northern Bay of Bengal.

The oceanic crust is underlain by an upper mantle lid withthickness varying from 60 to 70 km which overlies a low velocityzone (LVZ). The average Vs in the lid is 4.56–4.67 km s�1 (southto north) with a positive velocity gradient. Our dispersion dataare not very sensitive to the total thickness of the LVZ but the aver-age Vs in the LVZ is 4.25 km s�1.

7. Conclusions

To resolve the issue of the anomalous crust proposed for thenorthern Bay of Bengal, we explore the variation in crustal struc-ture of the Bay of Bengal using fundamental mode Rayleigh wavegroup velocity dispersion data between 12 and 100 s period. Wegroup each of the dispersion curves into seven clusters which sam-ple different parts of the Bay of Bengal similar to the paths sampledby the data of Brune and Singh (1986), and inverted these for shearwave velocity structure of the crust and upper mantle. These dis-persion curves reveal a gradual change in the dispersion character-istics from south to north in the Bay. The final models thusobtained for the three zones have been interpreted as follows:

(i) The Bay of Bengal crust can be characterized by four layers,three southward tapering layers of sediments, sedimentaryrocks and metasedimentary rocks that overlie a uniform oce-anic crust. The uppermost layer consists of �4–6 km of sed-iments throughout the section and has a positive velocitygradient which is consistent with a layer having an exponen-tial decay in porosity with compaction and overpressurewith burial in a basin. A second layer is consistent with alayer of sedimentary rocks with thickness increasing from1 km in the south to 10 km in the north, and has a shearwave velocity that corresponds closely to a completely com-pacted sedimentary rocks devoid of porosity. The third layerwhich has a shear wave velocity increasing from 3.33 km s�1

in the southern Bay to 3.61 km s�1 in the northern, is inter-preted as metamorphosed sedimentary rocks with increas-ing thickness from 1 km in the south to 8 km in the north.The shear wave velocities from south to north match the lab-oratory measured values of increasing grade of metamor-phism for metapelites from zeolite through greenschist to

S. Mitra et al. / Journal of Asian Earth Sciences 42 (2011) 1243–1255 1255

amphibolite facies. This is underlain by oceanic crustthroughout the Bay of Bengal with thickness of �7 km, typ-ical for oceanic crusts.

(ii) In consequence of the varying thickness of the sediments,sedimentary and metasedimentary rocks overlying the oce-anic crust, the Moho beneath the �3 km deep ocean bed inthe southern Bay lies �17.5 km below sea level and isdepressed to �20 km below sea level in the central and�30.5 km in the northern Bay. The existence of the thick pileof sedimentary and metasedimentary layer overlying theoceanic crust, explains the ‘‘continental-like’’ seismic struc-ture of the northern Bay of Bengal observed by Brune andSingh (1986).

(iii) Longer period dispersion data constrains the base of theupper mantle lid, deepening from �80 km beneath thesouthern Bay to a little over 100 km south of continentalIndia. This is broadly consistent with its globally average dis-position beneath oceans and lends credence to the hypothe-sis that the observed anomalous structure of the Bay ofBengal crust has been produced by metamorphism of thedeeply buried sedimentary rocks in the basin by sedimentload of the prodigious Bengal fan that thickens from�7 km in the south (at 3� latitude) to �21.5 km in the north-ern Bay of Bengal.

Acknowledgements

We thank S.K. Nath for providing us data from the IIT, Kharag-pur Broadband observatory, established under the Department ofScience and Technology project for seismotectonic study of theBengal Basin. We acknowledge receipt of HYB and PALK data fromthe Data Management Center at IRIS. Data preprocessing and partof the analysis is performed using Seismic Analysis Code 2000, ver-sion 100 (Goldstein et al., 2003). Dispersion measurements andinversion are performed using the ‘‘Computer Programs in Seis-mology: Surface waves, Receiver Functions and Crustal Structure(version 3.30)’’ of Herrmann and Ammon (2004). All plots are madeusing the Generic Mapping Tools version 4.0 (www.soest.hawaii.e-du/gmt; Wessel and Smith (1998)). SM thanks the British Councilfor the UKIERI fellowship (2008) during which this work was car-ried out.

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