• Sankar Kumar Nath

      Articles written in Journal of Earth System Science

    • Seismic hazard scenario and attenuation model of the Garhwal Himalaya using near-field synthesis from weak motion seismometry

      Sankar Kumar Nath Khemraj Shukla Madhav Vyas

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      In this paper, we present a seismic hazard scenario for the Garhwal region of the north-western Himalayan range, in terms of the horizontal Peak Ground Acceleration. The scenario earthquake of moment magnitude $M_w$ 8.5 has a 10% exceedance probability over the next 50 years. These estimates, the first for the region, were calculated through a stepwise process based on:

      An estimation of the Maximum Credible Earthquake from the seismicity of the region and Global Seismic Hazard Assessment Program considerations, and

      four seismotectonic parameters abstracted from near field weak-motion data recorded at five stations installed in Chamoli District of the Garhwal region in the aftermath of the 1999 Chamoli earthquake. The latter include:

      The frequency dependent power law for the shear wave quality factor, $Q_S$,

      the site amplification at each station using horizontal-to-vertical-spectral ratio and generalized inversion technique,

      source parameters of various events recorded by the array and application of the resulting relations between the scalar seismic moment $M_0$ (dyne-cm) and moment magnitude Mw and the corner frequency, $f_c$ (Hz) and moment magnitude $M_w$ to simulate spectral acceleration due to higher magnitude events corresponding to the estimated Maximum Credible Earthquake, and

      regional and site specific local spectral attenuation relations at different geometrically central frequencies in the low, moderate and high frequency bands.

    • Earthquake hazard in northeast India – A seismic microzonation approach with typical case studies from Sikkim Himalaya and Guwahati city

      Sankar Kumar Nath Kiran Kumar Singh Thingbaijam Abhishek Raj

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      A comprehensive analytical as well as numerical treatment of seismological, geological, geomorphological and geotechnical concepts has been implemented through microzonation projects in the northeast Indian provinces of Sikkim Himalaya and Guwahati city, representing cases of contrasting geological backgrounds – a hilly terrain and a predominantly alluvial basin respectively. The estimated maximum earthquakes in the underlying seismic source zones, demarcated in the broad northeast Indian region, implicates scenario earthquakes of $M_W$ 8.3 and 8.7 to the respective study regions for deterministic seismic hazard assessments. The microzonation approach as undertaken in the present analyses involves multi-criteria seismic hazard evaluation through thematic integration of contributing factors. The geomorphological themes for Sikkim Himalaya include surface geology, soil cover, slope, rock outcrop and landslide integrated to achieve geological hazard distribution. Seismological themes, namely surface consistent peak ground acceleration and predominant frequency were, thereafter, overlaid on and added with the geological hazard distribution to obtain the seismic hazard microzonation map of the Sikkim Himalaya. On the other hand, the microzonation study of Guwahati city accounts for eight themes – geological and geomorphological, basement or bedrock, landuse, landslide, factor of safety for soil stability, shear wave velocity, predominant frequency, and surface consistent peak ground acceleration. The five broad qualitative hazard classifications – ‘low’, ‘moderate’, ‘high’, ‘moderate high’ and ‘very high’ could be applied in both the cases, albeit with different implications to peak ground acceleration variations. These developed hazard maps offer better representation of the local specific seismic hazard variation in the terrain.

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