Shear wave velocity structure beneath North-Western Himalaya and adjoining areas (original) (raw)
Related papers
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
A 1D velocity model of the Tehri region in the Garhwal Himalaya is estimated from the travel-time inversion of 145 well-located local events having 1177 P and 1090 S arrivals. The velocity model consists of six layers up to 24 km depth, with P- and S-wave velocities ranging from 4.42 to 6:78 km=s and 2.41 to 3.71 km=s, respectively. The depth of the Moho, estimated using travel-time curves of crustal phases, is about 46 km. A low-velocity layer deciphered between 12 and 14 km depths is ascribed to fractured basement thrust representing the upper surface of the Indian plate. Using the proposed velocity model, 1457 events are relocated. About 70% of the locatable events occur in the Inner Lesser Himalaya between the Main Central thrust (MCT) and the Srinagar thrust. The postulated depth of the basement thrust in the vicinity of the MCT is about 10–12 km. The depth distribution of events delineates the geometry of the seismically active Main Himalayan thrust (MHT) below a 300-km-long segment of the MCT. The MHT is composed of two shallow-dipping fracture zones that seem to represent seismically active thrust zones dipping in opposite directions. Two seismicity zones, at 10 and 15 km depths with a 5 km vertical separation, define a flat-ramp-flat type structure of the MHT in the vicinity of the MCT. The postulated front of the underthrusting Indian plate is at a depth of about 15– 18 km. The lower-flat seismicity zone bifurcates into two, indicating further slicing of the lower-flat zone. The postulated thickness of the brittle part of the underthrusting Indian crust is about 20 km in the vicinity of the MCT.
In order to understand present day earthquake kinematics at the Indian plate boundary, we analyse seismic broadband data recorded between 2007 and 2015 by the regional network in the Garhwal-Kumaun region, northwest Himalaya. We first estimate a local 1-D velocity model for the computation of reliable Green's functions , based on 2837 P-wave and 2680 S-wave arrivals from 251 well located earthquakes. The resulting 1-D crustal structure yields a 4-layer velocity model down to the depths of 20 km. A fifth homogeneous layer extends down to 46 km, constraining the Moho using travel-time distance curve method. We then employ a multistep moment tensor (MT) inversion algorithm to infer seismic moment tensors of 11 moderate earthquakes with M w magnitude in the range 4.0–5.0. The method provides a fast MT inversion for future monitoring of local seis-micity, since Green's functions database has been prepared. To further support the moment tensor solutions, we additionally model P phase beams at seismic arrays at teleseismic distances. The MT inversion result reveals the presence of dominant thrust fault kinematics persisting along the Himalayan belt. Shallow low and high angle thrust faulting is the dominating mechanism in the Garhwal-Kumaun Himalaya. The centroid depths for these moderate earthquakes are shallow between 1 and 12 km. The beam modeling result confirm hypocentral depth estimates between 1 and 7 km. The updated seismicity, constrained source mechanism and depth results indicate typical setting of duplexes above the mid crustal ramp where slip is confirmed along out-of-sequence thrusting. The involvement of Tons thrust sheet in out-of-sequence thrusting indicate Tons thrust to be the principal active thrust at shallow depth in the Himalayan region. Our results thus support the critical taper wedge theory, where we infer the microseismicity cluster as a result of intense activity within the Lesser Himalayan Du-plex (LHD) system.
Journal of the Geological Society of India, 2020
During 2014-16, a semi-permanent network of four 3component broadband seismographs was operational in the Rajasthan craton and Aravalli mobile belt in the NW Indian shield. The reliable and accurate broadband data from 16 selected regional Indian earthquakes of M w 5.5-7.8 from this seismic network enabled to estimate the group velocity dispersion characteristics and onedimensional regional shear velocity structure of northern India, covering the region below north India between the entire Himalaya (from the Pakistan Himalaya in west to the Burmese arc in the east) and Rajasthan (including Aravalli mobile belt). First, Rayleigh-(at 7-87s) and Love-(at 7-82s) wave group velocity dispersion curves were measured and then these curves were inverted to estimate the crustal and upper mantle structure below north India. It is observed that group velocities are of variable nature within the region. This could be attributed to the complex crust-mantle structure in the study region resulted from the magmatism episodes associated with the Proterozoic collision, 65 Ma Deccan volcanism and the Himalayan collision. The best model in the study region reveals a two-layered crust, with a 15-km thick upper-crust (UC) of average shear velocity (V s) of 3.12 km/s and a 25-km thick lower-crust(LC) of average V s of 3.44 km/sec. The modeling detects a drop in V s (~1-2%) at 79-120 km depths, underlying north India, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 79 km depth. A geothermal gradient extrapolated from the surface heat flow (~74 mW/m 2) shows that such a gradient would intercept CO 2-bearing mantle peridotite solidus at 100 km depth, and thus could signal the presence of small amounts of partially melted magma below 100 km depth. Therefore, this 1-2% drop in V s could be attributed to the presence of carbonatite melts in the upper mantle related to magmatic episode of 65 Ma Deccan plume activity as also suggested by existing geological and seismological evidence.
Journal of Asian Earth Sciences, 2014
We investigated the seismic shear-wave velocity structure of the crust beneath nine broadband seismological stations of the Shillong-Mikir plateau and its adjoining region using teleseismic P-wave receiver function analysis. The inverted shear wave velocity models show 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto34-38 km thick crust beneath the Shillong Plateau which increases to 34−38kmthickcrustbeneaththeShillongPlateauwhichincreasesto37-38 km beneath the Brahmaputra valley and 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin46-48 km beneath the Himalayan foredeep region. The gradual increase of crustal thickness from the Shillong Plateau to Himalayan foredeep region is consistent with the underthrusting of Indian Plate beyond the surface collision boundary. A strong azimuthal variation is observed beneath SHL station. The modeling of receiver functions of teleseismic earthquakes arriving the SHL station from NE backazimuth (BAZ) shows a high velocity zone within depth range 2-8 km along with a low velocity zone within 46−48kmbeneaththeHimalayanforedeepregion.ThegradualincreaseofcrustalthicknessfromtheShillongPlateautoHimalayanforedeepregionisconsistentwiththeunderthrustingofIndianPlatebeyondthesurfacecollisionboundary.AstrongazimuthalvariationisobservedbeneathSHLstation.ThemodelingofreceiverfunctionsofteleseismicearthquakesarrivingtheSHLstationfromNEbackazimuth(BAZ)showsahighvelocityzonewithindepthrange2−8kmalongwithalowvelocityzonewithin8-13 km. In contrast, inversion of receiver functions from SE BAZ shows high velocity zone in the upper crust within depth range 10−18kmandlowvelocityzonewithin10-18 km and low velocity zone within 10−18kmandlowvelocityzonewithin18-36 km. The critical examination of ray piercing points at the depth of Moho shows that the rays from SE BAZ pierce mostly the southeast part of the plateau near Dauki fault zone. This observation suggests the effect of underthrusting Bengal sediments and the underlying oceanic crust in the south of the plateau facilitated by the EW-NE striking Dauki fault dipping 30 0 toward northwest.
Scientific Reports
The possibility of a major earthquake like 2015 Gorkha–Nepal or even greater is anticipated in the Garhwal–Kumaun region in the Central Seismic Gap of the NW Himalaya. The interseismic strain-rate from GPS derived crustal velocities show multifaceted strain-rate pattern in the region and are classified into four different strain-rate zones. Besides compressional, we identified two NE–SW orienting low strain rate (~ 20 nstrain/a) zones; namely, the Ramganga-Baijro and the Nainital-Almora, where large earthquakes can occur. These zones have surface locking widths of ~ 72 and ~ 75 km respectively from the Frontal to the Outer Lesser Himalaya, where no significant surface rupture and associated large earthquakes were observed for the last 100 years. However, strain reducing extensional deformation zone that appears sandwiched between the low strain-rate zones pose uncertainties on the occurences of large earthquakes in the locked zone. Nevertheless, such zone acts as a conduit to transf...
Lithosphere, 2022
The optimum 1D velocity model is calculated for the Kinnaur sector of the NW Himalaya utilizing the arrival time information of the local earthquakes (137 no.) recorded with 12 broadband seismic network within the azimuthal gap of ≤180°. This optimum 1D velocity model is a five-layer model and ranges from the surface to 90 km in the shallow mantle. P velocity varies from 5.5 km/s to 8.6 km/s in the crust and upper mantle, and S-wave velocity varies between 3.2 km/s and 4.9 km/s for the same range. When we relocated the earthquakes with the Joint Hypocenter Determination program incorporating the optimum 1D velocity model, it resulted in a lower RMS residual error of 0.23 s for the hypocenter locations compared to initial hypo71 locations. A total of 1274 P and 1272 S arrival times were utilized to compute station delays. We observed positive variations in P-station delays from -0.19 s below the PULG station to 0.11 s below the SRHN station. Similarly, for S-station delays, we observ...
Shear wave splitting and crustal anisotropy in the Eastern Ladakh-Karakoram zone, northwest Himalaya
Journal of Asian Earth Sciences, 2017
Seismic anisotropy of the crust beneath the eastern Ladakh-Karakoram zone has been studied by shear wave splitting analysis of S-waves of local earthquakes and P-to-S or Ps converted phases originated at the crust-mantle boundary. The splitting parameters (Φ and δt), derived from S-wave of local earthquakes with shallow focal depths, reveal complex nature of anisotropy with NW-SE and NE oriented Fast Polarization directions (FPD) in the upper ~22 km of the crust. The observed anisotropy in the upper crust may be attributed to combined effects of existing tectonic features as well as regional tectonic stress. The maximum delay time of fast and slow waves in the upper crust is ~0.3 s. The Ps splitting analysis shows more consistent FPDs compared to S-wave splitting. The FPDs are parallel or sub parallel to the Karakoram fault (KF) and other NW-SE trending tectonic features existing in the region. The strength of anisotropy estimated for the whole crust is higher (maximum delay time δt: 0.75 s) in comparison to the upper crust. This indicates that the dominant source of anisotropy in the Trans-Himalayan crust is confined within the middle and lower crustal depths. The predominant NW-SE trending FPDs consistently observed in the upper crust as well as in the middle and lower crust near the KF zone support the fact that the KF is a crustal-scale fault which extends at least up to the lower crust. Dextral shearing of the KF creates shear fabric and preferential alignment of mineral grains along the strike of the fault, resulting in the observed FPDs. A Similar observation in the Indus Suture Zone (ISZ) also suggests crustal scale deformation owing to the India-Asia collision.
Journal of Asian Earth Sciences, 2017
In the past three years, a semi-permanent network of fifteen 3-component broadband seismographs has become operational in the eastern Indian shield region occupying the Archean ($2.5-3.6 Ga) Singhbhum-Odisha craton (SOC) and the Proterozoic ($1.0-2.5 Ga) Chotanagpur Granitic Gneissic terrane (CGGT). The reliable and accurate broadband data for the recent 2015 Nepal earthquake sequence from 10 broadband stations of this network enabled us to estimate the group velocity dispersion characteristics and one-dimensional regional shear velocity structure of the region. First, we measure fundamental mode Rayleigh-and Love-wave group velocity dispersion curves in the period range of 7-70 s and then invert these curves to estimate the crustal and upper mantle structure below the eastern Indian craton (EIC). We observe that group velocities of Rayleigh and Love waves in SOC are relatively high in comparison to those of CGGT. This could be attributed to a relatively mafic-rich crust-mantle structure in SOC resulting from two episodes of magmatism associated with the 1.6 Ga Dalma and 117MaRajmahalvolcanisms.ThebestmodelfortheEICfromthepresentstudyisfoundtobeatwo−layeredcrust,witha14−kmthickupper−crust(UC)ofaverageshearvelocity(Vs)of3.0km/sanda26−kmthicklower−crust(LC)ofaverageVsof3.6km/s.ThepresentstudydetectsasharpdropinVs(117 Ma Rajmahal volcanisms. The best model for the EIC from the present study is found to be a two-layered crust, with a 14-km thick upper-crust (UC) of average shear velocity (V s) of 3.0 km/s and a 26-km thick lower-crust (LC) of average V s of 3.6 km/s. The present study detects a sharp drop in V s (117MaRajmahalvolcanisms.ThebestmodelfortheEICfromthepresentstudyisfoundtobeatwo−layeredcrust,witha14−kmthickupper−crust(UC)ofaverageshearvelocity(Vs)of3.0km/sanda26−kmthicklower−crust(LC)ofaverageVsof3.6km/s.ThepresentstudydetectsasharpdropinVs(À2 to 3%) at 120-260 km depths, underlying the EIC, representing the probable seismic lithosphere-asthenosphere boundary (LAB) at 120 km depth. Such sharp fall in Vs below the LAB indicates a partially molten layer. Further, a geothermal gradient extrapolated from the surface heat flow shows that such a gradient would intercept the wet basalt solidus at 88-103 km depths, suggesting a 88-103 km thick thermal lithosphere below the EIC. This could also signal the presence of small amounts of partial melts. Thus, this 2-3% drop in V s could be attributed to the presence of partial melts in the upper mantle related to the earlier volcanic episodes viz. back-arc volcanism associated with the Archean/Proterozoic subduction, 1.6 Ga Dalma volcanism, and $117 Ma Rajmahal volcanism. The main result of our modeling provides evidences for the absence of Keel or thick lithosphere below the EIC.