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One-dimensional regional shear velocity structure from joint inversion of fundamental mode group velocity dispersion measurements of Love and Rayleigh waves: application to the Uttarakhand Himalaya

Gupta Abhishek Kumar, Mandal Prantik, Srinagesh D., Tiwari Anil, Sain Kalachand, Paul Ajay

Acta Geophysica

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2023

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Vol. 71, no. 6

2619--2632

EN

Between 2017 and 2019, the CSIR-NGRI, Hyderabad, Telangana, established a broad-band seismic-network with fifty-five 3-component broadband 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 twenty-one (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 (6-73 s) and Rayleigh waves (at 6.55-73 s) period, and then, we finally invert these dispersion curves to compute the final 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 Vs) 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 to 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 to 4.69 km per sec. The Moho-depth is calculated to be 45 km deep below the K-G Himalaya, and the shear-velocity (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 fluids 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 underthrusting Indian plate.

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Ekspedycja IODP 360 : pierwszy etap odwiertu do płaszcza Ziemi

Ciążela J., Dick H. J. B., Mac-Leod Ch., Blum P.

Przegląd Geologiczny

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2016

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Vol. 64, nr 11

889--895

EN

The aim of this paper is to provide a report on the IODP expedition 360 to the Polish geoscientific community. Expedition 360 to the Atlantis Bank along the Southwest Indian Ridge was Leg 1 of the SloMo Project. The primary objective of the SloMo Project is to test competing hypotheses on the nature of the Moho at the slow-spreading oceanic lithosphere. Based on a seismic survey and geologic mapping, the Moho beneath Atlantis Bank is believed to represent a serpentinization front, and not an igneous boundary between gabbro andperidotite. Expedition 360 started on November 30,2015 in Colombo (Sri Lanka), and ended on January 30,2015 in Port Louis (Mauritius). Hole U1473A was drilled 790 m through massive gabbro. Core recovery ranges from 44 to 96% towards the bottom of the hole, where excellent drilling conditions occurred. This deepest single-leg basement hole drilled into ocean crust is in overall good condition andcan be re-entered at Leg 2. For the first time, a Polish nominee has been selectedfor the scientific party of an oceanic IODP expedition. The mantle drilling project raised much attention in the Polish media. One hundred rock samples have been collected to investigate in Poland.

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Topography of the Moho and earth crust structure beneath the East Vietnam Sea from 3D inversion of gravity field data

Nguyen N.T., Nguyen T.H.

Acta Geophysica

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2013

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Vol. 61, no. 2

357--384

EN

The Moho depth, crustal thickness and fault systems of the East Vietnam Sea (EVS) are determined by 3D interpretation of satellite grav- ity. The Moho depth is calculated by 3D Parker inversion from residual gravity anomaly that is obtained by removing the gravity effects of sea- floor and Pre-Cenozoic sediment basement topographies from the free air anomaly. The 3D inversion solution is constrained by power density spectrum of gravity an omaly and seismic data. The calculated Moho depths in the EVS vary from 30-31 km near the coast to 9 km in the Central Basin. A map of the lithosphere extension factor in the Cenozoic is constructed from Moho and Pre-Cenozoic sediment basement depths. The fault systems constructed by the maximum horizontal gradient approach include NE-SW, NW-SE, and N-S oriented faults. Based on the interpretation results, the EVS is sub-divided into five structural zones which demonstrated the different characteristics of the crustal structure.

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A method of estimating the Moho density contrast with a tentative application of EGM08 and CRUST2.0

Sjoberg L., Bagherbandi M.

Acta Geophysica

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2011

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Vol. 59, no. 3

502-525

EN

Based on Vening Meinez-Moritz global inverse isostatic problem, the Moho density contrast is formulated as that of finding a solution of a Fredholm integral equation of the first kind. We present solutions to this equation by combining global models of gravity (EGM08), topography (DTM2006) and seismic crust (CRUST2.0) to a resolution of 2° x 2°. The test computations yielded Moho density contrasts ranging from 81.5 kg/m3 (in Pacific) to 988 kg/m3 (Tibet), with averages of 678 ± 78 and 334 ± 108 kg/m3 for continental and oceanic regions, respectively, and a global average of 448 ± 187 kg/m3. Estimated Moho depths range from 8 to 75 km with continental and oceanic averages of 36.6 ± 5.3 km and 12.9 ± 5.8 km, respectively, and a global average of 21 ± 12.5 km. This article has its emphasis on the new theory, while significant corrections to computational results are expected in a forthcoming study, where the isostatic gravity anomaly will be reduced for several disturbing signals.