Abstract
Microtremor horizontal-to-vertical spectral ratio (HVSR) method has been conducted at 33 sites in Ahud Rufeidah urban expansion zone in order to assess the fundamental frequency and the corresponding amplitude of the sediments. Clear HVSR peaks have been identified in the majority of the surveyed area. The eastern and northern parts of the area are characterized by two well-separated peaks which indicate distinct shallow and deep impedance contrasts. The frequency map of sediments shows a distribution in the range of 0.86-3.13 Hz. The observed frequencies can be related to the total thickness of Quaternary alluvial sediments (sand, gravel, and shale) deposited over the gneiss bedrock. Lower resonance frequencies are attained at sites in the northern part, while the higher values are attained at sites in the southern part. The amplitudes of HVSR peaks are in the range 3-15. In general, the higher peak amplitudes are identified at lower frequencies. Since low fundamental frequencies are related to bedrock, this can be an indication of high-impedance contrast between alluvial sediments and gneiss bedrock. The results of this study represent one step for seismic hazard assessment and risk mitigation of this urban area where great damage can be attained in case of strong earthquakes. Hence, these results should be taken into consideration before establishing the new urban constructions in the area of study.