• S MUKHOPADHYAY

      Articles written in Journal of Earth System Science

    • Fine structure of seismotectonics in western Shillong massif, north east India

      S Mukhopadhyay R Chander K N Khattri

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      The epicentral tract of the great Assam earthquake of 1897 of magnitude 8·7 was monitored for about 6 months using an array of portable seismographs. The observed seismicity pattern shows several diversely-oriented linear trends, some of which either encompass or parallel known geological faults. A vast majority of the recorded micro-earthquakes had estimated focal depths between 8–14 km. The maximum estimated depth was 45 km. On the basis of a seismic velocity model for the region reported recently and these depth estimates we suggest that the rupture zone of the great 1897 earthquake had a depth of 11–12 km under the western half of the Shillong massif. Four composite fault plane solutions define the nature of dislocation in three of the seismic zones. Three of them show oblique thrusting while one shows pure dip slip reverse faulting. The fault plane solutions fit into a regional pattern of a belt of earthquakes extending in NW-SE direction across the north eastern corner of the Bengal basin. The maximum principle stress axis is approximately NS for all the solutions in conformity with the inferred direction of the Indian-EuroAsian plate convergence in the eastern Himalaya.

    • Observed variability of the East India Coastal Current on the continental slope during 2009–2018

      S MUKHOPADHYAY D SHANKAR S G APARNA A MUKHERJEE V FERNANDO A KANKONKAR S KHALAP N P SATELKAR M G GAONKAR A P TARI R R KHEDEKAR S GHATKAR

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      We describe the variability of the East India Coastal Current (EICC) during 2009–2018 using data from ADCP (acoustic Doppler current profiler) moorings deployed on the continental slope in the western Bay of Bengal. The four moorings are deployed off Gopalpur ($19.5^{0}\rm{N}$), Visakhapatnam ($\sim 18^{0}\rm{N}$), Kakinada ($\sim 16^{0}\rm{N}$), and Cuddalore ($\sim 12^{0}\rm{N}$) on the Indian east coast. The longer data record allows us to attach a statistically more robust basis to the conclusions drawn by Mukherjee et al. (2014) on the basis of four years (2009–2013) of ADCP data. The data confirm that the seasonal cycle dominates the variability of the EICC. The amplitude of the annual band varies over the time series. In the intra-annual band, the variability switches between the semi-annual and 120-day bands off Gopalpur, Visakhapatnam and Kakinada, but the semi-annual band is stronger than the 120-day band off Cuddalore throughout the time series. Upward phase propagation is common in the seasonal bands, but downward phase propagation is common in the intra-annual band of Cuddalore during the summer and winter monsoons, leading to stronger undercurrents there. Off Cuddalore, even the annual EICC appears as a shallow current. In contrast, the EICC appears as a deep flow of Gopalpur, Visakhapatnam, and Kakinada particularly during the spring inter-monsoon. This deep flow is evident at these locations even in the intraseasonal (30–90-day) band; the longer data set suggests, however, that the intraseasonal variability does not necessarily peak during spring. The annual EICC is coherent along the coast, but it is only the semiannual band that shows a comparable coherence between Kakinada and Cuddalore: in the 120-day and intraseasonal bands, the EICC decorrelates along the coast. Wavelet analysis suggests significant variability at sub-annual periods. The sub-annual EICC exceeds $20 cm s^{-1}$ on many occasions, but it too decorrelates along the coast. The long ADCP record allows us to confirm the dominance of seasonality in the EICC regime in a robust fashion; the data show that the EICC tends to flow in its canonical poleward (equatorward) direction during spring (winter). This dominance of seasonality enhances the predictability of the EICC.

    • Observed variability of the East India Coastal Current on the continental shelf during 2010–2018

      S MUKHOPADHYAY D SHANKAR S G APARNA V FERNANDO A KANKONKAR

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      We describe the variability of the East India Coastal Current (EICC) during 2010–2018 on the outer continental shelf using data from four ADCP (acoustic Doppler current profilers) moorings deployed off Gopalpur ($\sim19.5^{\circ}\rm{N}$), Visakhapatnam ($\sim18^{\circ}\rm{N}$), Kakinada ($\sim16^{\circ}\rm{N}$), and Cuddalore ($\sim12^{\circ}\rm{N}$) on the east coast of India. In general, the shelf EICC mirrors the slope EICC for the annual and semi-annual cycles, but the shelf-slope coherence is weaker and patchy for the 120-day and intraseasonal bands. The seasonal cycle, which consists of the annual, semi-annual, and 120-day bands, dominates the observed variability. The amplitude of the annual cycle varies over the time series. In the intra-annual band, variability tends to switch between the semi-annual and 120-day bands, but the former dominates throughout the time series off Cuddalore, the southernmost location. The EICC appears as a shallow current in all period bands, including the seasonal cycle, off Cuddalore, but even the intraseasonal EICC appears as a deep current at the other three locations. A wavelet analysis shows seasonal variation of the wavelet power in the intraseasonal band, suggesting that the amplitude of intraseasonal variability itself varies with season, but there is no clear seasonal pattern. As on the continental slope, the annual and semi-annual components are coherent along the coast, but alongshore coherence is weak at shorter time scales. Upward phase propagation is evident for the seasonal cycle at all locations, but downward phase propagation, seen on the slope off Cuddalore, is evident on the shelf as well. The 500-day low-pass filtered shelf EICC is not weak and the sub-annual variability is comparable to that on the slope. The long ADCP record allows us to confirm the dominance of seasonality in the EICC regime in a robust fashion; the data show that the EICC tends to flow in its canonical poleward (equatorward) direction during spring (winter). This dominance of seasonality enhances the predictability of the EICC.

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