Copyright
IIT Kharagpur
vi ABSTRACT
An updated seismic hazard model for India is imperative in the light of the ongoing rapid urbanization, and burgeoning population in the country. Rigorous formulations towards the model based on state-of-the-art methodologies are, accordingly, envisaged.
First and foremost, an earthquake catalog for the country is compiled; uniform magnitude scaling is implemented in terms of moment magnitude MW using region-specific relations connecting different magnitude types. The main-shock events are segregated through seismicity declustering based on a data-oriented windowing approach and application of Omori’s decay rate law. The temporal variations in the data completeness for the catalog are as well examined at sub-regional level. Seismogenic sources across the study region are investigated using the earthquake catalog (including historical records), focal mechanism database, fault map, fault-slip rates, and findings of palaeoseismic investigations. Four seismogenic layers segregating hypocentral depth ranges (in km):
0−25, 25−70, 70−180, and 180−300, respectively are considered to obtain 172 areal zones, and smoothened gridded seismicity. In order to derive nation-wide seismic site conditions, the correlations between 30 m averaged soil-column shear-wave velocity VS30 and topographic gradients are appraised with published site classification maps from different cities. The assessment indicates that ‘firm rock’ site condition is pragmatic for the regional hazard computations. Suites of ground motion prediction equations (GMPEs) are selected and ranked based on efficacy tests performed using macro-seismic intensity data. Reasonable conformity between the GMPEs developed for tectonically active shallow crust across the globe is noted while the equations for the intraplate regions cluster into two groups with those of lower ranks catering to higher ground motions.
Earthquakes in the subduction zones are also observed to have regional implications. A logic-tree framework is formulated for probabilistic seismic hazard analysis, which comprises of 16 GMPEs, and two types of source models. The final deliverables include spatial distributions of PGA, and 5%-damped pseudo-absolute-acceleration spectra at spectral periods of 0.2 sec and 1 sec, respectively. The computations are performed 10%
and 2% probability of exceedance in 50 years, respectively. The preliminary probabilistic seismic hazard model reveals considerably higher seismic hazard prevailing in the country compared to those reported earlier.
Keywords: Earthquake catalog; Ground motion prediction equations; Probabilistic seismic hazard analysis; Seismogenic source zone; Seismicity declustering; Seismicity models; Site characterization; India
