Neo-deterministic studies for seismic hazard assessment of Bangladesh

Abstract

In this study, a seismic hazard map of Bangladesh has been prepared following neo-deterministic seismic hazard assessment (NDSHA) method. NDSHA is a deterministic approach based on the computation of synthetic seismograms. The input data consisted of structural models, seismogenic source zones, focal mechanisms and earthquake catalogues. There are many probabilistic hazard maps available for Bangladesh, including the seismic zoning map in Bangladesh National Building Code (BNBC). However this study focuses on preparing the seismic hazard map following neo-deterministic methods. With the knowledge of physical process of earthquake generation and wave propagation in anelastic media, realistic strong ground motion modeling was done. Synthetic seismograms at a frequency of 1 Hz were generated at a regular grid of 0.2˚×0.2˚ by the modal summation technique. The ground motion parameters, expressed in terms of maximum displacement ( ), maximum velocity ( ), and peak ground acceleration ( ), were extracted from the synthetic signals and mapped on a regular grid over the studied region. The NDSHA estimated values of were compared with estimates of PGA values using different attenuation laws following both probabilistic (PSHA) and deterministic (DSHA) approach. The NDSHA method in the present study estimated the values of less seismically active zone of Bangladesh at south-west i.e. Khulna region, the central parts of the country including the capital city Dhaka, and the areas near the India –Bangladesh border region at north and north-east, and the values were found in the range of 0.01-0.1 , 0.1-0.2 and 0.2-0.63 respectively. The epicentral areas of the great Indian earthquake 1897 and Srimangal earthquake 1918 represented the maximum hazard with values reaching 0.4–0.63 , exceeding the value of 0.36 as per updated BNBC. The peak displacement and velocity in the same region was estimated as 15–30 cm and 30–60 cms−1 respectively. Moreover, values of the rupture areas of 1762 Arakan earthquake at Chittagong was found up to 0.3-0.4 surpassing the value of BNBC i.e. 0.28 . Comparison of the results with probabilistic map having 2% probability of exceedence in 50 years (return period 2475 years) showed that, NDSHA estimated higher values in seismically more active areas and lower values in areas with less ground shaking. As the NDSHA was found to estimate larger values for south-east and north-east parts of the country exceeding the BNBC values by up to 1.43-1.75 times, to ensure public safety during a probable major earthquake hazard, the BNBC values for this region should be carefully revised again after critical evaluation.