High Amplification factor for Low Amplitude Ground Motion: Assessment for Delhi

Abhishek Kumar, Harinarayan N.H. and Olympa Baro

Building damages as well as induced effects of earthquake are the consequences of surface ground motion at the site. While due importance is given to the soil beneath the ground, the selection of input motion in site response studies is inadequate. Present work highlights the importance of bedrock motion upon the response of a soil column. A typical borehole up to 30m depth is analyzed for wide range of globally recorded bedrock motions in absence of regional ground motion records. Based on the analyses, it is observed that the bedrock motions with low amplitude produce high amplification factors while high amplitude bedrock motions produce low amplification factor. In seismic microzonation practices of urban centers, the amplitude of bedrock motion and the amplification factor are considered independently for assigning ranks while estimating hazard index. However, from this work, it is found that the amplitude bedrock motion and amplification factor are strongly correlated. Thus, a more combined approach is required while assigning ranks in estimating the hazard index value. In similar way, the correlations between other thematic layers of seismic microzonation practice can be studied in future. Such outcomes will affect the current seismic microzonation practices as well.

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Evolution of watermasses in the upper layers of the Bay of Bengal

Anoopa Prasad C. and Hareesh Kumar P. V.

The evolution of watermasses in the upper layers of Bay of Bengal (BoB) is investigated utilizing monthly mean temperature and salinity profiles. Northern Dilute Watermass (NDW), Transition Watermass (TW) and Southern Bay of Bengal Watermass (SBBW) are prominent in the surface layers. NDW is present during May to January in the northern BoB; its formation and sustenance owes to the freshwater influx. NDW advects along the eastern boundary of BoB during summer monsoon due to the prevailing currents whereas in winter Kelvin wave and southward East India Coastal Current carries this watermass southward upto 15°N along its western boundary. In BoB, TW and SBBW are present throughout the year, but at different depths. At the surface, TW has maximum spatial extent during April-May, but divergence induced by the positive wind stress curl off the east coast of India during the summer monsoon, pumped the SBBW towards the surface and replaces TW. In winter, TW disappears from the central BoB as upwelling due to a cyclonic eddy brings SBBW towards the surface. Turner angle suggest diffusive convection in the region of NDW whereas in TW region this condition exists only during winter and doubly stable situation prevails throughout the year for SBBW.

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