• Koushik Biswas

      Articles written in Journal of Earth System Science

    • Evidence for a fluid flow triggered spatio-temporal migration of seismicity in the 2001 M$^w$ 7.7 Bhuj earthquake region, Gujarat, India, during 2001–2013

      Prantik Mandal Manish Kumar Koushik Biswas

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      We studied the variations in spatial and temporal clustering of earthquake activity (during 2001–2013) in the Kachchh seismic zone, Gujarat, India, by precisely relocating 3478 events using a joint hypocentral determination (JHD) relocation technique, and high-quality arrival times of 21032 P- and 20870 S-waves.Temporal disposition of estimated station corrections of P- and S-waves suggests that the fluid flow in the causative fault zone of the 2001 Bhuj mainshock increased during 2001–2010, while it reduced during 2011–2013, due to the healing process associated with the perturbed Kachchh fault zone. We also estimated the isotropic seismic diffusivities from epicentral growth patterns, which are found to bemuch lower than those observed for reservoir-induced seismicity sites in the world. Finally, we analysed the spatial and temporal evolution of this earthquake sequence by solving the diffusion equation of pore-pressure relaxation caused by co- and post-seismic stress changes associated with earthquakes. The value of the isotropic diffusivity is estimated to be 100 m2/s for the Kachchh rift zone. This gives a higher permeability (after a lapse time of 14 years from the occurrence of the 2001 Bhuj mainshock) in comparison to those observed for other intraplate regions in the world. Our results suggest that the observed spatio-temporal migration of seismicity is consistent with the shallow (meteoric water circulationat 0–10 km depths) and deeper (metamorphic fluid and volatile CO2 circulation at 10–40 km depths) fluid flows in the permeable and fractured causative fault zone of the 2001 Bhuj earthquake.

    • Source characteristics of the upper mantle 21 May, 2014 Bay of Bengal earthquake of $M_{w}$5.9

      Prantik Mandal Koushik Biswas Akhileshwar Prasad

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      We measure source parameters for the 21 May, 2014 Bay of Bengal earthquake through inversion modeling of S-wave displacement spectra from radial–transverse–vertical (RTZ) components recorded at ten broadband stations in the eastern Indian shield. The average source parameters are estimated using estimates from seven near stations (within epicentral distances $\leq$ 500 km). The average seismic moment and source radius are determined to be 1.0$\times$ 10$^{18}$ N-m and 829 m, respectively, while average stress drop is found to be 76.5 MPa. The mean corner frequency and moment magnitude are calculated to be 1.6 $\pm$ 0.1 and 5.9 $\pm$ 0.2 Hz, respectively. We also estimated mean radiated energy and apparent stress, which are found to be 6.1$\times$10$^{13}$ joules and 1.8 MPa, respectively. We observe that mean $E_{s}$/$M_{o}$ estimate of 5.5 $\times$ 10$^{-5}$ is found to be larger than the global average for oceanic strike-slip events. This observation along with large stress drop and apparent stress estimates explains the observed remarkably felt intensity data of the 2014 event. The full waveform moment tensor inversion of the band-passed (0.03–0.12 Hz) broadband displacement data suggests the best fit for the multiple point sources on a plane located at 65 km depth, with a moment magnitude 6.4, and a focal mechanism with strike 318$^{\circ}$, dip 87$^{\circ}$, and rake 34$^{\circ}$.

    • Estimation of coda Q for the eastern Indian craton

      Koushik Biswas Prantik Mandal Prosanta Kumar Khan

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      We herein present new frequency-dependent coda-Q ($Q_{\rm{c}}$) relations ($Q_{\rm{c}}$=$Q_{0}f ^{n}$) (frequency ranges between 2 and 18 Hz) for three regions of the eastern Indian craton (EIC), viz., the Singhbhum Odisha craton (SOC) and the Eastern Ghat mobile belt (EGMB), comprising the Mahanadi basin and the Chotanagpur granitic gneissic terrain (CGGT). The frequency-dependent coda-$Q_{\rm{c}}$ relations are obtained through the single backscattering model for coda waves ($Q_{\rm{c}}$) of local earthquakes which are recorded on 15 three-component broadband seismograph stations in the regions. In this work, we pay special attention to test the lapse time ($t_{\rm{L}}$) dependency of coda-Q ($Q_{\rm{c}}$) estimates for the three regions. Lapse time signifies the sample area of the coda wave of the study region. Generally, the sample area increases with lapse time. To test the lapse time ($t_{\rm{L}}$) dependency, nine different lapse time windows ($t_{\rm{L}}$) from 10 to 90 s with 10 s interval are considered. On the ground of estimated poor correlation coefficients, only six lapse time windows ($t_{\rm{L}}$) from 40 to 90 s with 10 s interval are considered. Our results suggest more heterogeneity in EGMB than that of the SOC and CGGT region. Estimates of $Q_{0}$ and n for the three regions of EIC (SOC, EGMB and CGGT) are found to be consistent with the results of $Q_{0}$ and n for mildly active less heterogeneous seismic zones in different parts of the world. By assuming entirely intrinsic attenuation characteristics, actual hazard parameters, i.e., extinction distance and anelastic attenuation coefficients are also computed for the three regions. The extinction distance ($L_{\rm{e}}$) provides an idea of the distribution of scatterers in the lithosphere and anelastic attenuation coefficients signify the anelasticity of the medium, i.e., fluid movement and grain distribution. The estimate of extinction distance and attenuation coefficients suggests that for all three study regions, the upper mantle is relatively less heterogeneous and attenuation below 110–126 km depth is also comparatively lower. Coda Q indicates the degree of fracture and heterogeneity in the lithosphere related to seismicity. A higher estimate of $Q_{0}$ values in the Archaean SOC region and the Proterozoic CGGT region is found when compared with that of the sedimentary-rich EGMB. It can be inferred that seismically less active cratons in general comprise high $Q_{0}$ values, whereas the sedimentary-rich EGMB is more attenuative, characterised by a low coda $Q_{0}$ value. Moreover, it is found that the estimated $Q_{0}$ values for CGGT region are a little bit higher than that for the SOC region. This can be explained as a comparatively less disturbed and less heterogonous land mass that is present in the CGGT region as compared to the SOC region, which comprises different minerals, ore bodies, fault scarps and shear zones. The developed $Q_{\rm{c}}$ relation for the EIC region could be useful for the study of hazards and ground motion prediction.

    • Correction to: Estimation of coda Q for the eastern Indian craton

      Koushik Biswas Prantik Mandal Prosanta Kumar Khan

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