Articles written in Journal of Earth System Science

    • Spatial variation of the aftershock activity across the Kachchh Rift Basin and its seismotectonic implications

      A P Singh O P Mishra Dinesh Kumar Santosh Kumar R B S Yadav

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      We analyzed 3365 relocated aftershocks with magnitude of completeness (Mc) ≥ 1.7 that occurred in the Kachchh Rift Basin (KRB) between August 2006 and December 2010. The analysis of the new aftershock catalogue has led to improved understanding of the subsurface structure and of the aftershock behaviour. We characterized aftershock behaviour in terms of 𝑎-value, 𝑏-value, spatial fractal dimension ($D_s$), and slip ratio (ratio of the slip that occurred on the primary fault and that of the total slip). The estimated 𝑏-value is 1.05, which indicates that the earthquake occurred due to active tectonics in the region. The three dimensional 𝑏-value mapping shows that a high 𝑏-value region is sandwiched around the 2001 Bhuj mainshock hypocenter at depths of 20–25 km between two low 𝑏-value zones above and below this depth range. The $D_s$-value was estimated from the double-logarithmic plot of the correlation integral and distance between hypocenters, and is found to be 2.64 ± 0.01, which indicates random spatial distribution beneath the source zone in a two-dimensional plane associated with fluid-filled fractures. A slip ratio of about 0.23 reveals that more slip occurred on secondary fault systems in and around the 2001 Bhuj earhquake (Mw 7.6) source zone in KRB.

    • GIS-based prediction of groundwater fluoride contamination zones in Telangana, India

      Kiran Kumar Reddy S Dinesh Kumar Sahadevan Harish Gupta Dontireddy Venkat Reddy

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      Groundwater is the only perennial water resource available to rural communities, especially in semi-arid regions. This study aims to provide an overview of fluoride-contaminated groundwater in the Telangana, India, by predicting potentially affected areas. The prevalence of endemic fluorosis in different parts of Telangana has been widely reported. Therefore, it is necessary to demarcate the fluoride-affected areas to adopt the remedial measures. In this context, the available information on related environmental variables such as geological settings, hydro-morphological inputs, climatic information and soil properties have been integrated as thematic layers in an ArcGIS environment. The thematic layers and their features were assigned with suitable weights, which were normalised using the analytic hierarchy process to obtain final ranks and the weighted overlay analysis method was carried out to delineate the potential fluoride contamination (PFC) zones. The entire state was classified into four broad categories, i.e., very high (17.6%), high (15.8%), medium (32.7%) and low (33.9%), in terms of groundwater fluoride enrichment. A comparison of the output map and the reported data indicates that the PFC zone model could explain 68.7% of fluoride variation. This study is the first such attempt to offer a regional-scale PFC zone for an entire state and offers a first-hand insight into the severity of fluoride contamination.

    • Integration of geophysics and petrography for identifying the aquifer and the rock type: A case study from Giddalur, Andhra Pradesh, India


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      A comprehensive geophysical and petrological study was carried out at Giddalur area in Prakasam district, Andhra Pradesh, which is geologically a highly deformed area and is difficult to delineate the aquifer zone(s). The task was to find out the exact rock type in which aquifer is concealed as well as to delineate the aquifer zone, which can yield sufficient quantity of water. The resistivity models derived from geophysical dataset were interpreted in terms of hydrogeology and the results revealed substantial resistivity contrast of the geological formations within the study area. We have delineated two major groundwater potential zones based on this study. These zones were tapped at different depths in diverse rock types. Drilled hand specimens (rock cuttings) were not adequate, so these specimens were petrographically studied to reveal the exact contact zones of the rock type. On integration of the geophysical and the petrographic results, it was illustrated that two aquifer zones were struck at a depth of 92 and 122 m between shale-phyllite and phyllite-quartzite, respectively. These findings were correlated, which matched with the lithology of the drilled borehole. This integrated approach will be helpful in strategy for groundwater assessment as well as prospecting groundwater resources in different geological terrain.

    • Estimation of Coda Q for northeast India using nonlinear regression


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      Classical logarithmic linearization of the single backscattering model to estimate the coda Q ($Q_{c}$) leads to biased results in the presence of low signal-to-noise ratio. Non-linear regression using the Levenberg–Marquardt (L–M) method has been proposed to estimate the $Q_{c}$ in the frequency range of 3–24 Hz on local earthquakes recorded in northeast region of India. Results of both classical log-linear and non-linear approaches to the single backscattering model are compared. On datasets with good signal-to-noise ratio both the approaches lead to almost the same results. However, for datasets having low signal-to-noise ratio, we found that the log-linear technique estimates are biased. Results demonstrate that the log-linear approach overestimates the $Q_{c}$ in comparison to the non-linear approach. Frequency dependence parameter ‘${\eta}$’ for the L–M method is slightly higher than the conventional log-linear approach. The $Q_{c}$ variation with lapse-time is also studied with both the approaches. The biased results of log-linear approach were observed at different frequencies at all lapse times. Significantly lower $Q_{0}$ (1 Hz) estimates of non-linear approach indicated more heterogeneous lithosphere than the log-linear approach.


      $\bullet$ A novel approach to the single scattering model is proposed wherein nonlinear regression is performed to estimate coda $Q_{c}$.

      $\bullet$ Coda attenuation and its variation with lapse time is analysed (3–24 Hz) from local earthquakes of North East India

      $\bullet$ For datasets having a low signal-to-noise ratio the estimates of the conventional log-linear regression were biased.

      $\bullet$ Significantly lower $Q_{0}$ estimates of the non-linear approach indicated a more heterogeneous lithosphere than a log-linear approach.

    • The impacts of climate change and post bleaching effects on the octocoral communities of Gulf of Mannar, India


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      The objective of the present study is to assess the status of the octocoral communities, and the extent of bleaching they suffered in 2016 (in line with the third Global Bleaching Event). Bleaching survey was carried out in the permanently fixed study sites in the 21 island of Gulf of Mannar. A focused study was carried out in Kariyachalli Island to monitor post-bleaching effects on octocorals. The overall octocoral cover in 2015 was 1.59±0.29%, and octocoral genera such as Sinularia, Sarcophyton, Lobophytum and Subergorgia were observed predominantly in GoM (Gulf of Mannar). Temperature reached a maximum of 32.5°C (March) during 2016 bleaching. The extent of bleaching in 2016 was 1.05±0.65% and the overall octocoral cover was reduced to 1.16±0.22%. Multiple paired t-test results indicate a significant variation in the octocoral covers between 2015 and 2016 (t = 3.173, p${\le}$0.01). Sinularia, Xenia, and Subergorgia were the most affected genera during this bleaching episode. The percentage bleaching in octocorals was the highest in Vembar group of islands (1.9±0.62%) and the continuous monitoring studies reveal the increase in the octocorals diversity from 1.16±0.22% to 1.34±0.21% in 2018 indicating the recovery pattern of octocorals in GoM. Post-bleaching surveys carried out in Kariyachalli Island revealed the occurrence of tissue degradation and algal overgrowth following the bleaching event. However, this study concludes that octocorals in GoM could adapt and Courish in the context of changing climatic conditions. However, more studies on octocoral ecology and biology are warranted.


      $\bullet$ Percentage cover of the octocorals in Gulf of Mannar was assessed with a total of 16 octocoral genera was recorded during the study.

      $\bullet$ The octocoral cover decreased from 1.59±0.29% (2015) to 1.16±0.22% (2016) and then again increased to 1.34% (2018); with the extent of bleaching of 1.05±0.65% (2016).

      $\bullet$ Sinularia, Xenia and Subergorgia were the most affected genera during this bleaching episode of octocorals.

      $\bullet$ Unusual tulf algal overgrowth with tissue degradation was witnessed in the octocoral genera Sinularia sp.

    • Characterization of shear wave attenuation and site effects in the Garhwal Himalaya, India from inversion of strong motion records


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      The frequency-dependent shear-wave quality factor (Q_\beta(f )) and site amplification are computed for the seismically and tectonically active Garhwal Himalaya. The inversion technique of strong motion data is applied to obtain Q_\beta(f ) and site effect at each recording station. The strong motion data of 82 earthquakes recorded in the Garhwal region is used for the present inversion algorithm. The comparison of site effects obtained by the present inversion scheme and well developed H/V technique (H/V is the ratio of Fourier spectra horizontal to vertical components) shows that site effects computed through the inversion technique have close resemblance with these estimates from the H/V technique. Both horizontal components are used to establish the frequency-dependent Q_\beta(f ) relations at each station. The values of ‘Q_o’ and ‘n’ at different stations vary from 92 to 112 and 0.9 to 1.1, respectively. The close resemblance of obtained Q_\beta(f ) relations at different stations suggest, the presence of almost similar type of lithology, i.e., hard rock at these stations. A regional quality factor relationship of form, Q_\beta(f ) = (102 ± 3.9)f^(1.0±0.1) is established for the Garhwal Himalaya based on modelled Q_\beta values of each station. This relationship reveals low Q_o value (${\le}$200) and high n value (${\ge}$0.8) for the Garhwal Himalaya, which correspond to tectonically and seismically active region.


      $\bullet$ The frequency-dependent shear-wave quality factor and site amplification are computed simultaneously for the Garhwal region, NW Himalaya.

      $\bullet$ A regional quality factor relationship of form, Qβ(f) = (102 ± 3.9)f(1.0±0.1) is established for the Garhwal Himalaya.

      $\bullet$ The acceleration records corrected from the obtained site effect are used to develop attenuation relations at each recording station.

      $\bullet$ The close resemblance of obtained Qβ(f) relations and the geology has been observed for the study region.

    • Evaluating the seismic hazard to Sikkim region of Himalaya using simulated accelerograms


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      The 2011 Sikkim earthquake (M 6.9) of the Himalaya has shown that a moderate size earthquake may also cause severe damage to the region. Simulated accelerograms using a modified hybrid technique have been generated at spatially distributed 702 points for a moderate size (M 6.9) earthquake in the Sikkim region of Himalaya to evaluate the seismic hazard in the region. The peak ground acceleration (PGA) values and response spectra have been derived from the simulated accelerograms. The synthetic PGA values, corresponding modified mercalli (MM) intensity values and response spectral acceleration values have been contoured to present the scenario hazard maps of the region. A PGA value of 400 cm/s$^2$ has been estimated in the region near the epicentre of the earthquake. The response spectral acceleration values for five periods corresponding to the natural period (T) of single-storey buildings, double-storey buildings, 3–4 storey buildings, tall buildings and high-rise buildings have been obtained. The maximum values of response spectral accelerations estimated in the centre of meizoseismal zone are 1200 cm/s$^2$ (T = 0.1 s), 1800 cm/s$^2$ (T = 0.2 s), 900 cm/s$^2$ (T = 0.5 s), 800 cm/s$^2$ (T = 1.0 s) and 80 cm/s$^2$ (T = 6 s). The decay curves show that PGA values in the west direction are lower as compared to the values in other directions at similar distances. Also, it has been observed that the PGA and spectral acceleration values decay slowly in the west direction compared to other directions. The scenario hazard maps present here are expected to be useful for mitigating seismic hazards from the region as these maps indicate the damage potential due to future moderate size earthquakes in the region.


      $\bullet$ The application of a modified hybrid technique has been presented to evaluate the seismic hazard based on simulating accelerograms for a moderate size earthquake in the Sikkim region of Himalaya.

      $\bullet$ The synthetic accelerograms have been generated at 702 points distributed spatially on a grid.

      $\bullet$ The scenario hazard maps showing the spatial distribution of PGA values, MM intensity values and response spectral acceleration values at different periods.

    • Ocean state forecasting during VSCS Ockhi and a note on what we learned from its characteristics: A forecasting perspective


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      Tropical Cyclone Ockhi was an intense cyclone, with a peculiar and long track, in the Arabian Sea in 2017. It caused severe damage to coastal infrastructure and death of 282 people. Indian National Centre for Ocean Information Services (INCOIS) issued the Joint INCOIS-IMD (India Meteorological Department) bulletins on the Ocean State Forecasts (OSF) and alerts/warnings during Ockhi. Validation of the OSF from INCOIS using buoys reveals that the forecasts were in good agreement with the observations [average correlation 0.9, RMSE ${\le}$0.8 m (for larger waves), and scatter index ${\le}$25%]. Climatological analysis of Genesis Potential Index (GPI) suggests that the southeast Arabian Sea, where the TC-Ockhi was intensified, had all the favourable conditions for intensification during November/December. Moreover, it was found that four days before the genesis of Ockhi, the environmental vorticity and relative humidity were more favourable for the cyclogenesis compared to vertical wind shear and potential intensity. The intensification rate was rapid as experienced by earlier cyclones in this region. Also, the cyclone track closely matched the background tropospheric winds. The present study suggests that the forecasters should look into the background dynamic and thermodynamic conditions extensively in addition to multi-model guidance to better predict the genesis, intensity and track of the cyclones.


      $\bullet$ In the Arabian Sea, during the TC-Ockhi, the forecasts of wave parameters from the model forced with bias-corrected ECMWF winds resulted in very good agreement with observations.

      $\bullet$ Climatologically, TC-Ockhi region has large potential for the genesis and intensification of TC due to an enhanced low-level cyclonic vorticity and the reduction in vertical wind shear.

      $\bullet$ During the TC-Ockhi period, low-level vorticity and mid-tropospheric relative humidity were the dominant contributing factors, which lead to an enhanced GPI in the Arabian Sea.

      $\bullet$ TC-Ockhi also had rapid intensification in a similar fashion the earlier cyclones in this region behaved.

      $\bullet$ There is no abnormality also in the TC-Ockhi track, as the TC-Ockhi track matches well with the background tropospheric flow.

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