• BHOOPENDRA SINGH

      Articles written in Journal of Earth System Science

    • Modelling of earthquake locations and source parameters in Kachchh region to understand genesis of earthquakes

      BHOOPENDRA SINGH PRANTIK MANDAL

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      Modelling of earthquake source locations and parameters infers seismogenesis of earthquakes. In this study, we modelled the earthquake source locations through hypocenter location algorithm using the difference in arrival time of P and S waves and source parameters through the Levenberg–Marquardt nonlinear inversion method using S-wave spectra. A total of 340 aftershocks of 2001 Bhuj mainshock (1.8$\leq M_{w}$ <4.3), which have occurred in Kachchh, Gujarat, India from January 2014 to January 2015, are located in this study. Out of 340 aftershocks, digital waveforms of 78 aftershocks (2.2$\leq M_{w}$ <3.9) are used for estimation of the earthquake source parameters. The results obtained from earthquake locations show two clusters of seismicity along the Kachchh Mainland Fault (KMF) and North Wagad Fault (NWF) and three felt events ($M_{w} =\geq 3.0$); one along the Katrol Hill Fault (KHF) ($M_{w} = 3.3$), two along the Banni Fault (BF) ($M_{w} = 3.0; 3.2$). The generation of these three felt events is attributed to the triggering mechanisms caused by the migration of Cuids or the stress pulse generated by the 20 MPa stress drop of the $M_{w} 7.7$ Bhuj earthquake. A marked concentration of events is noticed in 15–30 km depth range, which could be attributed to the presence of a mafic intrusive body, resulting in stress build-up for earthquake generation in this region. The results of source parameters; seismic moment ($M_{0}$), source radius ($r$) and stress drop ($\Delta\sigma$) vary from $1.86 \times 10^{12} \rm{to 3.2 \times 10^{15} N m}$, 146–262 m and 0.04–5.73 MPa, respectively. The maximum stress drop value is estimated to be 5.73 MPa at 24 km depth for the largest studied event of $M_{w} 3.9$. Large stress drops are confined to the 8–33 km depth range, which indicates the probable existence of the base of the seismogenic layer in this depth range. This observed large stress drops could be attributed to stresses induced by crustal maBc intrusive bodies and the presence of aqueous fluids in the lower crust below the region.

    • Upper mantle seismic anisotropy beneath the Kachchh rift zone, Gujarat, India, from shear wave splitting analysis

      BHOOPENDRA SINGH PRANTIK MANDAL

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      Unravelling the anisotropic behaviour of the upper mantle helps to shed light on its present and past deformation processes. In this study, we attempt to explore the seismic anisotropy prevailing within the upper mantle beneath the Kachchh rift zone through shear wave splitting analysis. We have measured the splitting parameters (e.g., fast axis orientation ($\Phi$) and delay time ($\delta t$)) using SKS/SKKS core refracted phases from 112 teleseismic events recorded at NGRI network in the Kachchh region, during 2006–2009 and 2013–2016. The ‘$\Phi$’ and ‘$\delta t$’ estimates vary from $\rm{N}34^{\circ}$ to $\rm{N73^{\circ}E}$ and 0.80 to 1.5 s, respectively. The average vector means of ‘$\Phi$’ and ‘$\delta t$’ for all the stations are found to be $\rm{N(58 \pm 10)^{\circ}E}$ and $(0.99 \pm 0.19)\,\rm{s}$, respectively. Measurements of 59 good SKS/SKKS splitting parameters from 112 earthquakes reveal that the upper mantle is highly anisotropic beneath the Kachchh rift zone with an average fast axis orientation of $\rm{N(58 \pm 10)^{\circ}E}$, which is deviated nearly ($\sim\rm{N18^{\circ}E}$) from the absolute plate motion (APM) direction ($\rm{N40^{\circ}E}$) of the Indian plate in a no-net-rotation reference frame. This deviation of fast axis orientation from APM direction may be attributed to the effect of Kachchh rift zone as well as the presence of structural imprints of the 65 Ma Deccan mantle plume in the study region. And the average delay time of $(0.99 \pm 0.19)\,\rm{s}$ is consistent with the global average (1 s) for continents. Furthermore, the modelled seismic layer thicknesses reveal that anisotropic sources beneath study region are associated with both the lithospheric deformation processes (e.g., 184 Ma African rifting, 88 Ma Madagascar rifting, 65 Ma Deccan mantle plume) as well as asthenospheric flows.

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