Ambient noise measurements in Rawalpindi-Islamabad, twin cities of Pakistan: a step towards site response analysis to mitigate impact of natural hazard.

Earthquakes are one among the highly dangerous geological hazards. The damage from earthquakes is greatly influenced by the local geological conditions. The history of Pakistan is full of severe earthquakes that caused the mass destruction at national scale. Major earthquake-affected areas in Pakistan are mainly concentrated in the north and western sections of the country. Site response analysis is an initial step towards seismic risk assessment. The present study aims at local site effects by acquiring data along 105 sites of Rawalpindi-Islamabad, the twin cities of federal capital of Pakistan, by applying H/ V spectral ratio method. Out of the data acquired for 105 sites, data for 88 sites were considered reliable after observing reliability conditions determined by SESAME guidelines devised in 2004. Fundamental frequency f of soft sediments, amplitude A of corresponding H/ V spectral ratios, thickness of soft sediments residing on the bedrock and soil vulnerability index K over 88 reliable data sites within the study area were estimated and analysed. The results show that the fundamental frequency of the sediments reflects variation and lies within a wide range between 0.6 and 14.4 Hz. Similarly, amplification factor A up to 5.5 was observed at a site. Overburden thickness of soft sediments H also indicates high variation and has been calculated in the range of 1.7-316.5 m. As far as K is concerned, it is also highly variable and is lying in the range of 0.30-62.7. These results will help in designing a policy to mitigate the impact of seismic hazard in the study area. [ABSTRACT FROM AUTHOR]