Comparison between weak and strong ground motions at selected locations of Bangladesh

Abstract

Microtremor recordings for weak ground motion and Earthquake recordings for strong ground motions are the easiest and cheapest way to understand the dynamic characteristics of soil. The purpose of this research is to apply Horizontal to Vertical Spectral ratio (H/V) of Microtremor and Earthquakes at six locations namely Bogra, Natore, Jamuna Bridge east at Sirajganj, Police staff college at Mirpur, Haji camp at Ashkona and BUET campus to assess site amplification of those sites. For spectral analysis of Microtremor, five low noise segments each having window of 41s (data recordings) have considered 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 five sets of the H/V ratios at selected location, the H/V ratio has been plotted with respect to their frequency at selected locations. Similarly for Earthquakes, ground motion records of Sikim earthquake 2006 and 2011, Bengal earthquake 2006, Assam earthquake 2010 and Nepal earthquake 2015 have been used. These accelerometers provide data in North-South, East-West and Up-Down directions. Then Fourier spectrum ratio (Horizontal versus Vertical) for various free field stations is computed. Comparing between H/V ratio of Microtremor and Earthquake recordings, it has been observed that the peak H/V ratio of earthquake moved into left side or right side and slightly upwards or downwards with respect to microtremor. From these result, it can be said that although amplitude values of the ratios are close, the predominant frequency for the three cases differs slightly. The reason of this difference is that microtremor and earthquake consists of different types of waves, but the H/V ratio is based on shear-wave only. Damage assessment of soils using Nakamura’s Vulnerability Index (Kg) has been used in the investigated locations. The seismic Vulnerability Index (Kg) at those six locations varies from 6.72 to 20.88 for microtremor observations. The estimated vulnerability Index from earthquakes is always higher than the microtremor due to high input energy. Microtremor vulnerability Index (Kg) values show higher value for reclaimed area which provides evidence that those sites need some kind of soil improvement if they want to sustain earthquake load.