Abstract
Abstract. Seismic microzonation is a process of estimating site-specific effects due to an earthquake on urban centers for its disaster mitigation and management. The state of West Bengal, located in the western foreland of the Assam–Arakan Orogenic Belt, the Himalayan foothills and Surma Valley, has been struck by several devastating earthquakes in the past, indicating the need for a seismotectonic review of the province, especially in light of probable seismic threat to its capital city of Kolkata, which is a major industrial and commercial hub in the eastern and northeastern region of India. A synoptic probabilistic seismic hazard model of Kolkata is initially generated at engineering bedrock (Vs30 ~ 760 m s−1) considering 33 polygonal seismogenic sources at two hypocentral depth ranges, 0–25 and 25–70 km; 158 tectonic sources; appropriate seismicity modeling; 14 ground motion prediction equations for three seismotectonic provinces, viz. the east-central Himalaya, the Bengal Basin and Northeast India selected through suitability testing; and appropriate weighting in a logic tree framework. Site classification of Kolkata performed following in-depth geophysical and geotechnical investigations places the city in D1, D2, D3 and E classes. Probabilistic seismic hazard assessment at a surface-consistent level – i.e., the local seismic hazard related to site amplification performed by propagating the bedrock ground motion with 10% probability of exceedance in 50 years through a 1-D sediment column using an equivalent linear analysis – predicts a peak ground acceleration (PGA) range from 0.176 to 0.253 g in the city. A deterministic liquefaction scenario in terms of spatial distribution of liquefaction potential index corresponding to surface PGA distribution places 50% of the city in the possible liquefiable zone. A multicriteria seismic hazard microzonation framework is proposed for judicious integration of multiple themes, namely PGA at the surface, liquefaction potential index, NEHRP soil site class, sediment class, geomorphology and ground water table in a fuzzy protocol in the geographical information system by adopting an analytical hierarchal process. The resulting high-resolution surface consistent hazard, liquefaction and microzonation maps are expected to play vital roles in earthquake-related disaster mitigation and management of the city of Kolkata.
Highlights
Natural disasters inflicted by earthquakes cannot be prevented, nor is there any possibility in the near future for accurate and socially useful short-term prediction for an impending earthquake
The input time series obtained by inverse Fourier transform of the pseudo-spectral acceleration computed for 10 % probability of exceedance in 50 years at engineering bedrock is used to model the nonlinear behavior of a 30 m soil column at 350 borehole locations with a precise 1-D Vs profile in order to generate site amplification and compute surface consistent peak ground acceleration (PGA) and PSA at each location
The hazard themes pertaining to the study region which are materialized as thematic layers on the GIS platform are (i) peak ground acceleration (PGA) at the surface, (ii) the liquefaction potential index (LPI), (iii) National Earthquake Hazard Reduction Program (NEHRP) soil site class (SC), (iv) sediment class (SEC), (v) geomorphology (GM), and (vi) ground water table (GWT)
Summary
Natural disasters inflicted by earthquakes cannot be prevented, nor is there any possibility in the near future for accurate and socially useful short-term prediction for an impending earthquake. The prevailing seismic zoning map of India (BIS, 2002) prepared based on the peak ground acceleration (PGA) induced by the maximum considered earthquake (MCE) further constrained by the geologic and seismotectonic considerations, scaling it down to the design basis earthquake (DBE) for urban codal provisions, places the entire city of Kolkata at the boundary between zone III and IV, with an equivalent PGA range of 0.16–0.24 g As per this zonation scheme, zone V is the highest expected level of seismic hazard, with a PGA level of 0.36 g and MM intensity of VIII (and above), while seismic zone IV corresponds to 0.24 g PGA, adhering to MM intensity VII. Seismic zone III corresponds to 0.16 g PGA and MM intensity VI, whereas zone II is associated with the lowest level of hazard, with a PGA level of 0.10 g and MM intensity of V (or less)
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