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.

$\bf{Highlights}$

$\bullet$ 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.

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.

$\bf{Highlights}$

$\bullet$ 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.

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.

$\bf{Highlights}$

$\bullet$ 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.

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.

$\bf{Higlights}$

$\bullet$ 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.