Applicability of H/V microtremor technique for site response analysis in Dhaka city

Abstract

The degree of damage during earthquakes strongly depends on dynamic characteristics of buildings as well as amplification of seismic waves in soils. Among the other approaches, microtremor is, perhaps, the easiest and cheapest way to understand the dynamic characteristics of soil as well as structural element. The purpose of this research is to apply Horizontal to Vertical Spectral ratio (H/V) Microtremor Technique for Site Response Analysis within Dhaka city. Non-reference microtremor measurements have been carried out in one hundred and thirty two selected locations within BUET campus and fourty five locations in and around the Dhaka city. The amplitude ratio (H/V) of horizontal to vertical spectra has been used to estimate predominant frequency and amplification. For spectral analysis three low noise segments of 20.48s each of the recordings have been taken at 100 Hz instrumental sampling. Fast Fourier Transformation (FFT) has been applied on the time domain records. Then, non-reference spectral ratio (H/V) technique has been applied. The resultant of two horizontal components of H/V ratio have been digitally filtered using suitable average smoothing point. After calculating three sets of the H/V ratios at selected location, the mean H/V ratio has been plotted with half standard deviation in order to assess the deviation of microtremor data at different time instants. The stability of microtremor data has been checked in fourteen different locations in various time instants (Morning, Evening and Night) within BUET campus. In this research resultant of H/V ratio data for five different filtering options (Rectangular, Hanning, Hamming, Welch and Blackman) have been carried out to select the most suitable one. Soil model has been developed in the investigated points using the programme SHAKE. Standard Penetration Test (SPT) has been carried out at twenty seven locations and field Shear Wave Velocity data has been collected in the investigated locations. Existing correlation between Shear Wave Velocity and SPT-N has been used in order to estimate Shear Wave Velocity at twenty eight other locations. These Shear Wave Velocity data has been used as input for the programme SHAKE. Damage assessment of structures using resonance criteria and damage assessment of soils using Nakamura’s Vulnerability Index (Kg) has been executed in the investigated locations. From the stability of microtremor data, it can be said that non reference Horizontal to Vertical spectral (H/V) ratio provides stable data to estimate predominant frequency and amplification of soil without source effect. Fourier Spectrum along East-West and North-South provides better result to estimate predominant frequency of building. Root multiplication of H/V ratio along East-West and North-South directions shows reliable resultant for the estimation of dynamic properties of soil. The mean peak H/V ratio data in rectangular window for filtering show similar result to other four windows. The peak of H/V ratio data decreases if higher average smoothing points is applied. Out of 132 locations within BUET campus the overall mean predominant frequency and mean peak H/V ratio are 1.39 Hz and 2.09. Predominant frequency for most of the locations lie between 1.0 Hz and 1.99 Hz. Out of 45 microtremor observation locations in and around the Dhaka city, the mean predominant frequency varies from 0.48 Hz to 3.65 Hz. On the other hand, the mean Horizontal to Vertical spectral ratio varies from 1.08 to 4.78. From the comparision of microtremor H/V ratio and Transfer Function obtained from the programme SHAKE, four types of characteristics curves have been observed. Among 42 locations thirty five Transfer function have higher frequency compared to microtremor and one has lower predominant frequency. For six sites two curves have identical frequency. For two sites, no similarity of predominant frequency between microtremor and transfer function have been observed. Microtremor consists of either shear-wave, or surface-wave or a mixture of both waves. Transfer Function obtained from SHAKE is basically shear-wave. Sometimes there will be some similarity between microtremor H/V ratio and Transfer function, but sometimes dissimilarity may also exist. Out of 45 buildings considered, four buildings have been identified as vulnerable due to resonance criteria within BUET campus. Most of the buildings predominant frequency is not close to the predominant frequency of site soil. The seismic Vulnerability Index (Kg) for 45 sites varies between 0.45 and 31.85. Ten sites have been identified as having moderate vulnerability of soil layers to deform.