Shear-Wave Structure of the South Indian Lithosphere from Rayleigh Wave Phase-Velocity Measurements (original) (raw)
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Shear velocity structure of the laterally heterogeneous uppermost mantle beneath the Indian region
The shear velocity structure of the Indian lithosphere is mapped by inverting regionalized Rayleigh wave group velocities in time periods of 15-60 s. The regionalized maps are used to subdivide the Indian plate into several geologic units and determine the variation of velocity with depth in each unit. The Hedgehog Monte Carlo technique is used to obtain the shear wave velocity structure for each geologic unit, revealing distinct velocity variations in the lower crust and uppermost mantle. The Indian shield has a high-velocity (4.4-4.6 km/s) upper mantle which, however, is slower than other shields in the world. The central Indian platform comprised of Proterozoic basins and cratons is marked by a distinct low-velocity (4.G4.2 km/s) upper mantle. Lower crustal velocities in the Indian lithosphere generally range between 3.8 and 4.0 km/s with the oceanic segments and the sedimentary basins marked by marginally higher and lower velocities, respectively. A remarkable contrast is observed in upper mantle velocities between the northern and eastern convergence fronts of the Indian plate. The South Burma region along the eastern subduction front of the Indian oceanic lithosphere shows significant velocity enhancement in the lower crust and upper mantle. High velocities (~4.8 km/s) are also observed in the upper mantle beneath the Ninetyeast ridge in the northeastern Indian Ocean.
Low shear velocities in the sub-lithospheric mantle beneath the Indian shield?
Journal of Geophysical Research B: Solid Earth, 2013
Ever since its breakup from the Gondwanaland~140 Myr ago, the Indian plate was ravaged by four hot spots. Although the surface manifestations of such deep processes are evident in terms of large igneous provinces like the Deccan and the fast drift of the Indian plate, the modifications to the deep structure remain to be grasped. In this study, we investigate the mantle transition zone (TZ) structure beneath the Indian shield region using 14,000 teleseismic receiver functions from 77 broadband stations sited on diverse geologic terrains. The arrival times of the P-to-s (Ps) conversions from the 410 km discontinuity at most cratonic stations appear to be delayed by~2 s in comparison with the times observed for other Precambrian shield regions like Africa, Australia, and Canada. Such delays in the conversions from the 410 km discontinuity below the Indian shield suggest low shear wave speeds in the lithospheric and sub-lithospheric mantles due to higher temperatures, together with a thinner high velocity lid that contrasts with a thicker one found beneath most Archean cratons. A thin transition zone beneath most of the cratonic stations lends support to the enhanced temperatures within the TZ itself. Also, a further delay of the TZ discontinuities is observed for stations on the southern granulite terrain, which was under the influence of the Marion plume that is responsible for the separation of Madagascar from India. Although the data do not conclusively show evidence for a 520 km discontinuity, an LVL atop the 410 cannot be ruled out beneath certain geological provinces of the Indian shield.
Evidence for a shear velocity discontinuity in the lower mantle beneath India and the Indian Ocean
Physics of the Earth and Planetary Interiors, 1987
SH and sSH seismograms are modeled to determine the shear velocity structure in the D" region beneath India and the Indian Ocean. The signals show waveform complexities similar to those observed in data sampling the D" region beneath Alaska, the Caribbean, and Eurasia (Lay and Helmberger), which have been attributed to a 2.7% shear velocity discontinuity-280 km above the core-mantle boundary. The new data set consists of long-period tangential component recordings at WWSSN stations in Africa, the Middle East, and Europe for 11 intermediate and deep focus Indonesian earthquakes. In the distance range 70-82°the waveforms show an arrival between SH and ScSH with systematic moveout. From 89 to 940 there is a strong distortion of the SH waveforms, indicating the arrival of several phases closely spaced in time. The relative time shifts of similar complexity in the corresponding sSH phases requires a deep mantle origin. The depth dependence and moveout of the interference effects are well-predicted for both SH and sSH phases by a model with a lower mantle discontinuity. Alternative explanations of the interference as resulting from receiver reverberations, SKS contamination, multiple source complexity, or near source multipathing are ruled Out by systematic tests. While it is apparent that lateral variations in the lower mantle velocity structure prevent any single model from fitting all of the data, synthetic waveform modeling (using generalized ray theory and reflectivity) shows that the data can be well-fit by a model with a discontinuity similar in size and depth to that proposed for the previously investigated regions (Lay and Helmberger), but with a negative velocity gradient within the D" layer.
Upper mantle anisotropy inferred from shear wave splitting beneath the Eastern Indian Shield region
Geoscience frontiers, 2018
We estimate the shear wave splitting parameters vis-à-vis the thicknesses of the continental lithosphere beneath the two permanent seismic broadband stations located at Dhanbad (DHN) and Bokaro (BOKR) in the Eastern Indian Shield region. Broadband seismic data of 146 and 131 teleseismic earthquake events recorded at DHN and BOKR stations during 2007e2014 were analyzed for the present measurements. The study is carried out using rotation-correlation and transverse component minimization methods. We retain our "Good", "Fair" and "Null" measurements, and estimate the splitting parameters using 13 "Good" results for DHN and 10 "Good" results for BOKR stations. The average splitting parameters (f, dt) for DHN and BOKR stations are found to be 50.76 AE5.46 and 0.82 AE 0.2 s and 56.30 AE5.07 and 0.95 AE 0.17 s, and the estimated average thicknesses of the anisotropic layers beneath these two stations are w 94 and w109 km, respectively. The measured deviation of azimuth of the fast axis direction (f) from the absolute motion of the Indian plate ranges from w8 to 14. The measured deviation of azimuth of the fast axis direction (f) from the absolute motion of the Indian plate ranges from w8 to 14. The eastward deviation of the fast axis azimuths from absolute plate motion direction is interpreted to be caused by induced outflow from the asthenosphere. Further, the delay time found in the present analysis is close to the global average for continental shield areas, and also coherent with other studies for Indian shield regions. The five "Null" results and the lower delay time of w0.5e0.6 s might be indicating multilayer anisotropy existing in the mantle lithosphere beneath the study area.
Evidence for shear velocity anisotropy in the lowermost mantle beneath the Indian Ocean
Geophysical Research Letters, 2000
Teleseismic recordings (A > 87") of a deep observed for the D" region beneath the central Pacific earthquake beneath the Banda Sea at stations in Tan-[Pulliam and Sen, 1998;; Russel et mania show a difference in the arrival time of the radial al., 19981 where, on average, the shear velocity is low. (Ssv) and transverse component (SsH) S wave rang-Here, I present recordings of the August 30, 1994 ing from l-3 s. Shear velocity anisotropy in the low-Banda Sea earthquake at seismic stations in Tanzania ermost mantle beneath the Indian Ocean is the likely cause of this signal because recordings at the same sta-which provide evidence for the presence of shear velocity anisotropy in D" beneath the Indian Ocean. These data tions of closer-in events (A < 80') in the same source corroborate previous suggestions that anisotropic strucregion do not present a comparable differential travel ture in relatively high shear velocity D" regions yields time. For the Banda Sea event, the SSH signals are positive values of TSSV-SSH and that a strong vertical broader than Ssv signals, suggesting that a discontinu-shear velocity gradient marks its upper boundary [e.g., ity (or strong vertical gradient) in primarily VSH marks Matzel et al., 19961. the sudden onset of transverse isotropy in D" (with a magnitude of 1.4-2.7%) ab.out 350 km above the coremantle boundary. SKKS coda, S-to-p converted phases at the Moho, and upper mantle heterogeneity beneath the stations obscure the onset of Ssv and complicate wave shapes. It is therefore difficult to evaluate whether general anisotropy needs to be invoked into a model of c shear velocity anisotropy. RITSEMA.: SHEAR VELOCITY ANISOTROPY IN D" BENEATH THE INDIAN OCEAN
Crustal shear velocity structure of the south Indian shield
Journal of Geophysical Research, 2003
1] The south Indian shield is a collage of Precambrian terrains gathered around and in part derived from the Archean-age Dharwar craton. We operated seven broadband seismographs on the shield along a N-S corridor from Nanded (NND) to Bangalore (BGL) and used data from these to determine the seismic characteristics of this part of the shield. Surface wave dispersion and receiver function data from these sites and the Geoscope station at Hyderabad (HYB) give the shear wave velocity structure of the crust along this 600 km long transect. Inversion of Rayleigh wave phase velocity measured along the profile shows that the crust has an average thickness of 35 km and consists of a 3.66 km s À1 , 12 km thick layer overlying a 3.81 km s À1 , 23 km thick lower crust. At all sites, the receiver functions are extremely simple, indicating that the crust beneath each site is also simple with no significant intracrustal discontinuities. Joint inversion of the receiver function and surface wave phase velocity data shows the seismic characteristics of this part of the Dharwar crust to be remarkably uniform throughout and that it varies within fairly narrow bounds: crustal thickness (35 ± 2 km), average shear wave speed (3.79 ± 0.09 km s À1 ), and V p /V s ratio (1.746 ± 0.014). There is no evidence for a high velocity basal layer in the receiver function crustal images of the central Dharwar craton, suggesting that there is no seismically distinct layer of mafic cumulates overlying the Moho and implying that the base of the Dharwar crust has remained fairly refractory since its cratonization. Crustal shear velocity structure of the south Indian shield,
Geophysical Journal International, 2006
We present group velocity dispersion results from a study of regional fundamental mode Rayleigh waves propagating across the Indian region. 1-D, path-averaged dispersion measurements have been made for 1001 source–receiver paths and these combined to produce tomographic images between 15 and 45 s period. Because of the dense station coverage in peninsular India, these images have substantially higher lateral resolution for this region than is currently available from global and regional group velocity studies. Testing of the group velocity model shows that the average resolution across the region is about 7.5° for the periods used in this study. The tomographic maps demonstrate that while the Indian shield is characterized by high crustal and uppermost-mantle group velocities, comparatively lower velocities exist beneath the Himalaya due to the thickened crust and beneath the Gangetic plains caused by the mollasse sediments and recent alluvium cover in the Himalayan foredeep. Northeastern India north of the Shillong Plateau also displays higher velocities, similar to the south Indian shield, indicative of colder crust beneath the region. The northern Bay of Bengal shows extremely low velocities due to the thick sediment blanket of the Bengal fan. Likewise, the Katawaz Basin in southern Pakistan shows lower velocities that resemble those seen in the Bay of Bengal. The geometry of the velocity contours south of the Katawaz Basin closely matches the prograding Indus fan in the Arabian Sea. Finally, the Tibetan Plateau has lower group velocities compared to the Indian shield at all periods as a result of the thick crust beneath southern Tibet.
2016
We measure the inter-station Rayleigh and Love wave phase velocities across the northwestern Indian Peninsular shield (NW-IP) through cross-correlation and invert these velocities to evaluate the underneath crust and upper mantle velocity structure down to 400 km. We consider a clus-ter of three stations in the northern tip of the Peninsula and another clus-ter of eight stations in the south. We measure phase velocities along 28 paths for Rayleigh waves and 17 paths for Love waves joining two stations with one from each cluster and using broadband records of earthquakes which lie nearly on the great circle joining the pair of stations. The phase velocities are in the period range of 10 to 275 s for Rayleigh waves and of 10 to 120 s for Love waves. The isotropic model obtained through inver-sion of the phase velocities indicates 199.1 km thick lithosphere with 3-layered crust of thickness 36.3 km; the top two layers have nearly same velocities and both constitute the upper crust with...
The shear-wave velocity gradient at the base of the mantle
Journal of Geophysical Research, 1983
The relative amplitudes and travel have been directed toward obtaining global times of ScS and S phases are utilized to place averages, and the degree of lateral variation in constraints on the shear-wave velocity gradient D" properties remains an open question. above the core-mantle boundary. A previously A conflicting result was found by Mitchell and reported long-period ScSH/SH amplitude ratio Helmberger [1973], who utilized the relative minimum in the distance range 65 ø to 70 ø is shown amplitudes and timing of long-period ScS and S to be a localized feature, apparently produced by phases to constrain the S-wave velocity gradient an amplitude anomaly in the direct S phase, and in D". They found a minimum in the ScSH/SH therefore need not reflect the velocity gradient amplitude ratio near 68 ø , which was attributed to at the base of the mantle. The amplitude ratios low amplitudes of the ScS arrivals. Unable to that are free of this anomaly are consistent with explain this feature by models with negative or calculations for the JB model or models with mild near-zero shear velocity gradients in D", they positive or negative velocity gradients in the proposed models with positive S-wave velocity lowermost 200 km of the mantle. ScSV arrivals gradients above the CMB. These positive are particularly sensitive to the shear velocity gradients extended over 40 to 70 km above the structure just above the core-mantle boundary. core, reaching velocities at the CMB as high as The apparent arrival time of the peak of ScSV is 7.6 to 7.8 km/s. These models can explain the as much as 4 s •reater than that of ScSH in the observed amplitude ratio behavior, as well as an distance range 75 v to 80 ø for Sea of Okhotsk apparent difference observed in the arrival times events recorded in North America. This can be of transversely and radially polarized ScS. explained by interference effects produced by a Mitchell and Helmberger also proposed a low Q$ localized high velocity layer or strong positive zone in D", or finite outer core rigidity, to S wave velocity gradient in the lowermost 20 km explain the baseline of the ScSH/SH amplitude of the mantle. A velocity increase of about 5% ratios. While the majority of their data was for is required to explain the observed shift between deep South American events recorded in North ScSV and ScSH. This thin, high velocity layer America, they did analyze one deep Sea of Okhotsk varies laterally, as it is not observed in event for which the radial and transverse ScS similar data from Argentine events. Refined arrival times were not different, which suggested estimates of the outermost core P velocity lateral variations in the D" velocity structure. structure are obtained by modeling SKS signals in In this paper we extend the analysis of ScS the distance range 75 ø to 85 ø ß and S phases using an enlarged data set in order to understand the discrepancy between the ß SES LHC BOZ ß SCB ON • •RCD AAM• DUG• •GOLFLO• N45*W ß WWSSN Stations ß CSN Stotions X Deep Argentine Events WES SCP
Research Square (Research Square), 2023
Between 2017 and 2019, the CSIR-NGRI, Hyderabad, Telangana, established a broad-band seismic-network with fty-ve 3-component bb seismometers in the Himalayan region of Uttarakhand, India. Out of 55 three component broadband seismic (BBS) networks, we chose 17 for the present study. Using digital waveform data from twentyone (21) regional Indian earthquakes of Mw 5.0-6.2 that were recorded in the 17 broadband seismometer, we compute fundamental mode group-velocity dispersion (FMGVD) characteristics of surface waves (love and Rayleigh waves) and the average one-dimensional regional shear-wave velocity (Vs) structure of the Uttarakhand Himalayan region. First, we compute FMGVD curves for Love waves (6-73sec) and Rayleigh waves (at 6.55-73 sec) period and then, we nally invert these dispersion curves to compute the nal average one dimensional regional crustal & sub-crustal shear-wave velocity (Vs) structure below the Uttarakhand Himalaya. Our best model in Uttarakhand Himalayan region, India, reveals the 8-layered crust with a mid-crustal low velocity layer (MC-LVL) (approximately a drop of 1.5-2.3% in V s) between 8 and 20 km depth in the proximity of MCT (Main Central Thrust). In the upper crustal part (0-20 km depths), our modelling suggests shear velocities (Vs) varies from 3.1-3.9 km/sec while shear velocities (Vs) in the lower crustal part (20-45 km depth) are modelled to be varying from 3.7-4.69 km per sec. The Moho-depth is calculated to be 45 km deep below the K-G Himalaya, and the shearvelocity (Vs) in the sub-crustal sector is 4.69 km/sec. Our estimated mid-crustal low velocity layer (MC-LVL) could be linked to the presence of metamorphic uids in the fractured Main Himalayan Thrust (MHT), resulting from the weakening of the crustal material at the interface between the overriding Eurasian plate and upper-part of the under thrusting Indian plate. Research Highlights Modeled 1-D regional shear velocity below the Indian part of Himalayan region Detected a mid-crustal low velocity representing the main Himalayan thrust(MHT) This low velocity is attributed to the presence of aqueous/metamorphic uids A 45-km thick crust characterizes the region A 4.69 km/s shear velocity characterizes the upper mantle below the region