Near-surface site characterization at Quriyat City, Sultanate of Oman using HVSR and MASW techniques

It is well known that the effects of near-surface structures on ground shaking represent an important factor in earthquake ground motion amplification; in particular, soft sedimentary cover could strongly amplify the seismic motion. The parameters of the soft layer that controls the amplification most are the fundamental frequency and the shear-wave velocity. The frequency band of the ground motion that is affected by such factors depends on the thickness and the velocity of the sedimentary layers. In order to investigate and characterize the near-surface structures beneath Quriyat coastal city in the Sultanate of Oman, Nakamura’s technique which was characterized by the horizontal-to-vertical spectral ratio (HVSR) of the ambient noise measurements was used to estimate the fundamental frequency of the soft soil at 70 sites. The shear-wave velocity is an important tool in the design of buildings for site-specific conditions and is a much more diagnostic tool for engineering properties. The multichannel analysis of surface waves (MASW) was carried out at 40 sites at Quriyat City in order to evaluate the near-surface shear-wave velocities. The fundamental frequencies were found to be in the range of 2.0 Hz (occupying the coastal soft soil area) and 23.3 Hz (occupying the area close to the hard rock). The average shear-wave velocities down to 30 m depth (Vs₃₀) ranged from 225 m/s (class D according to NEHRP standard for site classification and covering the coastal area) to 1058 m/s (class B covering the area in vicinity of hard rock). The results demonstrate that the HVSR of ambient vibrations were in a good agreement with the values of V_S30. Areas of lower V_S30 values exhibit lower fundamental frequency and cover areas with considerable soft soil thickness and vice versa.