Mapping sediment thickness of Islamabad city using empirical relationships: Implications for seismic hazard assessment

摘要

Soft sediments make an important component of the subsurface lithology, especially in areas underlain by river/stream basins. Occupying a position directly above the bedrock up to the land surface, these soft sediments can range in thickness from few centimeters to hundreds of meters. They carry a specialnuisance in seismic hazards, as they serve as a source of seismic amplification that may enhance the seismic shaking of many folds. Determination of the thickness of the soft sediments is therefore crucial in seismic hazard analysis. A number of studies in recent years have demonstrated that frequency andamplitude spectrum obtained from the noise measurements during the recording of natural seismicitycan be used to obtain thickness of soft sediments covering the bedrock. Nakamura (1989) presented atechnique to determine such spectrum using ratio of horizontal to vertical components of the Rayleighwaves. The present study is based on an extensive set of microtremor measurements carried out in theIslamabad city, Pakistan. Fundamental frequencies were obtained from weak motion sensors and TrominoEngy Plus instruments to show that the correlation is clearly valid for a wide range of sediment thickness.A simple formula was derived for the investigated area to determine directly the thickness of sedimentsfrom the main peaks in the H/V spectrum for seismometer and Tromino data separately. A comparisonis made between sediment thicknesses derived from empirical relations developed in this study withthose given in literature to demonstrate a positive correlation. The correlation of instrumental resonantfrequencies with calculated resonant frequencies (theoretical) suggests that the relation derived from thenoise measurements mostly depends on the velocity depth function of the shear wave. The fundamentalfrequency of the main peak of spectral ratio of H/V using the both instruments correlates well withthe thickness of sediments at the site obtained from the borehole data. It is found out that there is awide variation in soft-sediment thickness in the Islamabad area, but as a general rule, soft sedimentsare thicker adjacent to stream courses compared to the areas intervening the streams. The distributionof sediments in the studied area is illustrated by means of cross sections constructed from results ofthe microtremor analyses and available borehole data, which provides a visual distribution of the softsediments underlying the Islamabad city.