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Title: Crustal anisotropy from shear-wave splitting of local earthquakes in the Garhwal Lesser Himalaya
Authors: Kanaujia J.
Mitra S.
Gupta, Subhash Chander
Sharma, Mukat Lal
Published in: Geophysical Journal International
Abstract: Crustal anisotropy of the Garhwal Lesser Himalaya has been studied using local earthquake data from the Tehri seismic network. Earthquakes with magnitude (mL) up to 3, which occurred between January 2008 to December 2010, have been used for the shear wave splitting (SWS) analysis. SWS measurements have been done for steeply incident ray paths (ic = 45°) to estimate the anisotropy fast axis orientation (f) and the delay time (°t). A total of 241 waveforms have been analysed, which yielded 209 splitting measurements, and 32 null results. The analysis reveals spatial and depth variation of f and °t, suggesting complex anisotropic structure beneath the Garhwal Lesser Himalaya. The mean °t is estimated to be 0.07 ± 0.065 s with a mean depth normalized °t of 0.005 s km-1. We present the f and Vs per cent anisotropy results by segregating these as a function of depth, for earthquakes originating above and below the Main Himalayan Thrust (MHT); and spatially, for stations located in the Outer Lesser Himalaya (OLH) and the Inner Lesser Himalaya (ILH). Earthquakes above the MHT sample only the Himalayan wedge, while those below the MHT sample both the underthrust Indian crust and the Himalayan wedge. Within the Himalayan wedge, for both OLH and ILH, the mean f is oriented NE-SW, in the direction of maximum horizontal compressive stress axis (SHmax). This anisotropy is possibly due to stress-aligned microcracks controlled by the local stress pattern within the Himalayan wedge. The mean of normalized °t for all events originating within the Himalaya is 0.006 s km-1, which yields a Vs per cent anisotropy of ?2.28 per cent. Assuming a homogeneous distribution of stress-aligned microcracks we compute a crack density of ?0.0228 for the Garhwal Lesser Himalaya. At stations close to the regional fault systems, the mean f is subparallel to the strike of the faults, and the anisotropy, locally, appears to be structure-related. For earthquakes originating below the MHT, in OLH, the mean f orientation matches those from the Himalayan wedge and the normalized °t decreases with depth. This suggests depth localization of the anisotropy, primarily present within the Himalayan wedge. In the ILH, we observe large variations in the mean f orientation and larger values of °t close to the regional fault/thrust systems. This is possibly a composite effect of the structure-related shallow crustal anisotropy and the frozen anisotropy of the underthrusting Indian crust. However, these cannot be segregated in this study. © 2019 The Author(s).
Citation: Geophysical Journal International (2019), 219(3): 2013-2033
Issue Date: 2019
Publisher: Oxford University Press
Keywords: Body waves
Composition and structure of continental crust
Computational seismology
Seismic anisotropy
Seismicity and tectonics
ISSN: 0956540X
Author Scopus IDs: 56755442700
Author Affiliations: Kanaujia, J., Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, West Bengal, India
Mitra, S., Department of Earth Sciences, Indian Institute of Science Education and Research Kolkata, West Bengal, India
Gupta, S.C., Department of Earthquake Engineering, Indian Institute of Technology, Roorkee, Uttarakhand, India
Sharma, M.L., Department of Earthquake Engineering, Indian Institute of Technology, Roorkee, Uttarakhand, India
Funding Details: The data set of the local events used in the study was collected under a project sponsored by the Tehri Hydro Development Corporation (THDC) India Limited. This project is being executed by the Department of Earthquake Engineering (DEE), Indian Institute of Technology (IIT), Roorkee. Under this project, a 12-station digital telemetered seismological network has been deployed in the environs of the Tehri dam in the Garhwal Lesser Himalaya since January 2008. The local earthquake data collected from this network are archived by the DEE on a regular basis. Quarterly seismological bulletins containing phase data of local events (ML ? 1) are provided to the THDC and are available on request from DEE. The waveform data can be requested through email from the THDC. For verification of the results of this study, the authors are willing to provide analysed waveform data on request. Authors thank THDC India Limited for sponsoring the project. Authors are also grateful to all the team members of the telemetry project (Seismic Observatory, DEE, IIT Roorkee) who contributed to the recording and collection of data. JK sincerely acknowledges SERB N-PDF project no PDF/2016/003235 for research fellowship and IISER Kolkata for being her host institution. SM acknowledges Academic Research Funding (ARF) from IISER Kolkata. Authors sincerely acknowledge Siddharth Dey for critical reading of the manuscript and valuable inputs regarding crustal anisotropy studies. Authors are immensely grateful to the two anonymous reviewers for their constructive criticism and detailed comments, which significantly improved the contents and quality of the manuscript. Data preprocessing and part of the analysis was performed using Seismic Analysis Code, version 101.6a (Goldstein et al. 2003) and in MAT-LAB version 9.1.0 (R2016b) (MATLAB 2016). All plots were made using the Generic Mapping Tools version 5.0 (Wessel et al. 2013).
Appears in Collections:Journal Publications [EQ]

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