Near-surface shear wave velocity estimation and V s 30 mapping for Dhaka City, Bangladesh

Abstract

Dhaka, the capital city of Bangladesh, is one of the seismically vulnerable cities in the world due to its location close to the convergent boundary between the Eurasian and Indian Plates, unplanned urbanization, non-engineered construction practice, high population density, and weak emergency response system. The city is developed on an elevated Pleistocene terrace with surrounding Holocene floodplains. The terrace consists of the Pleistocene clayey soils, and the floodplains are composed of alluvial sandy and clayey soils. The average shear wave velocity of the near-surface soils up to a depth of 30 m (V s 30 ) is required for Dhaka City to estimate the amplification factors of seismic waves for site-specific seismic hazard analysis. Therefore, the V s 30 has been estimated in the city using downhole seismic (DS), multichannel analysis of surface waves (MASW), and small-scale microtremor measurement (SSMM) methods, and empirical correlation between the shear wave velocity (Vs) and standard penetration test blow count (SPT-N). The V s 30 has also been predicted using the relationship between the V s 30 and Holocene soil thickness. The V s 30 results of the DS, MASW, SSMM, and SPT-N vary from 127 to 320 m/s. The V s 30 values that are predicted based on the Holocene soil thickness vary from 145 to 260 m/s. Then, a V s 30 map has been prepared for Dhaka City using the V s 30 values that are predicted from the Holocene soil thickness at a grid of 30 m. The near-surface soils of the city are classified based on the V s 30 as site classes D (stiff soils) and E (soft soils) according to the NEHRP (National Earthquake Hazards Reduction Program, the USA) and as subsoil classes C and D according to the Eurocode 8.