Seismicity, Velocity Structure And Attenuation of North-Western Himalaya

Agrawal, Himanshu (2020) Seismicity, Velocity Structure And Attenuation of North-Western Himalaya. Masters thesis, Indian Institute of Science Education and Research Kolkata.

Text (MS dissertation of Himanshu Agrawal (15MS087)) 15MS087.pdf - Submitted Version Restricted to Repository staff only Download (31MB)

Abstract

We investigate the crustal structure of North-West (NW) Himalaya through earthquake relocation and computation of seismic velocity and attenuation tomography. Data for the analysis comprise 900 local earthquakes with magnitude in the range of 1-4, recorded by the Jammu and Kashmir Seismological Network (JAKSNET) deployed in NW Himalaya. These earthquakes are relocated using the probabilistic non-linear approach, and the best set of 214 events are selected with precise locations for 3D velocity tomography. The minimum 1D velocity model is obtained through simultaneous inversion of hypocentral and velocity parameters which is further extended to obtain 3D models of Vp, Vs and Vp/Vs. We use these local earthquake waveform data to compute seismic attenuation for body waves (characterised by the quality factor Q) and combine it through a tomography formulation to obtain lateral variations in Q. We measure source-receiver, 1-D path averaged Q from the spectra, using a non-linear least-squares procedure to determine an attenuation parameter (t*) simultaneously with source parameters at frequencies between 1 and 20 Hz. In this study, Q value for a particular trace is quantified using the arrival time and t* value using the back-projection method of Xie and Mitchell (1990). The seismicity distribution, body wave velocities and attenuation structures reveal detailed crustal structure ofNWHimalaya. The shallow seismicity is concentrated in a band between the MCT and the Kishtwar Window. This marks the frictionally locked to creep zone on the MHT and the region of stress accumulation within the Himalayan wedge. The deeper seismicity reveals that the underthrusting Indian crust beneath NW Himalaya is seismogenic. The asymmetric foreland basin sedimentary layers are characterised by low velocity and low Q. The Kishtwar window is underlain by marginally lower velocities associated with Lesser Himalayan rocks and is embedded within higher velocities of the Higher Himalayan crystallines. The Kashmir Basin is characterised by a shallow high velocity layer, high Q, and increasing Vp/Vs, which is indicative of a mafic crustal underplating.

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