Assessment of seismic vulnerability index of gas network area in Dhaka city using microtremor measurements

Abstract

The aim of this study is to assess seismic vulnerability index (Kg) of gas network area within Dhaka city by utilizing H/V spectral ratio method. Fifty eight locations in and around gas network area of Dhaka city have been selected for microtremor observation. Microtremor data has been collected and analyzed in order to determine predominant frequency and amplification factor according to the guideline of SESAME. Finally, seismic vulnerability index (Kg) of site soil using Nakamura’s technique has been calculated from predominant frequency and amplification factor parameter for the purpose of this research. Microtremor measurements of 46 sites have produced reliable curves and peaks of H/V spectral ratio to determine the site predominant frequency and amplification factor. It is not possible to determine the predominant frequency and amplification factor for 12 sites since unreliable and flat curve. The predominant frequencies of the study area are relatively uniform, ranging from 0.36 to 1.5 Hz. The frequencies of the study area have been grouped ranges of 0.36Hz ≤ f0 ≤ 0.50Hz, 0.51Hz ≤ f0 ≤ 1.0Hz and 1.0Hz < f0 ≤ 1.5Hz. It has been observed a consistent peak in a frequency range 0.36Hz ≤ f0 ≤ 0.50Hz covered the most part of the study area. The main reason for such a small range of results arises from the small differences in the near surface geological conditions of the study area. The observed frequencies can be related to the total thickness of soft sediments, deposited on the hard strata built of Madhupur tract. The low frequencies are obtained to soft soil sites in maximum measurements and the high frequencies are obtained to stiff soil sites in a few measurements. The amplification factor (A0) or peak of H/V spectral ratio is in investigation sites ranging from 2.05 to 12.1. In the majority of the study area, the H/V spectral ratio peak amplitudes are between 2 and 6. Low values are measured for stiff soil sites and high values for soft soil sites. Finally, Seismic vulnerability index (Kg) is calculated from the predominant frequency and amplification factor for 46 sites. Seismic vulnerability index (Kg) is an index indicating the level of vulnerability of a layer of soil to deform at the time of occurrence of earthquakes. Damaging earthquakes will occur when the limits are exceeded due to the shear strain deformation surface layer of soil. The seismic vulnerability index (Kg) for 46 sites varies between 4.41 and 186.14. The low seismic vulnerability index indicates that the areas are very stiff as well as thick sediment deposit. Seismic vulnerability index (Kg) in the study area has been found that the high values are scattered in the soft alluvial deposits area having a high seismic vulnerability indication. Based on the basement ground acceleration, ab = 0.2 g and the bedrock shear wave velocity, vb = 800 m/s, about 17.4% of the study area will face very high damage during earthquake. On the other hand, while ab = 0.2 g and engineering bed rock shear wave velocity, vb = 500 m/s, about 28.3% of the study area will face very high damage during earthquake.