Site effect and microzonation of the Jizan coastal area, southwestern Saudi Arabia, for earthquake hazard assessment based on the geotechnical borehole data

Abstract

Ninety-one geotechnical boreholes were drilled throughout Jizan City and the subsoil profiles were modeled. The SPT-N values were corrected to the N60 parameter. Then, the calculated shear wave velocity for 30 m depth (Vs30) varies between 196.086 and 589.931 m/s. Moreover, the site soil was classified into two of soil classes detected. Thereafter, the site response with SMSIM software is assigned at all borehole locations using Vs30, predominant frequency, and corresponding amplitude amplification factor. The estimated amplification factor varied between 1.6 and 18.2, reflecting the significant difference in the thicknesses of soil. The values of high amplification extend in a north–south strip to the west indicating very soft/loose sediments with a considerable thickness. Higher fundamental frequency values extend due north along the coast until the Ash-Shati and Ar-Rawabi districts verifying the compacted sediments. The peak ground acceleration (PGA) has been estimated for 91 borehole sites at bedrock level in the Jizan area using Mw 6.25 earthquake magnitude. This PGA was used to generate an ArcGIS contour map. The values of the PGA for the bedrock vary between 4.8 and 10.7 cm/s2. Moreover, PGA is computed at the ground surface level, where it varies between 7.7 and 32.0 cm/s2. Finally, the ground motion response spectra were also estimated with 5% critical damping for the locations of all of the boreholes. The pseudo-spectral acceleration (PSA) and its corresponding frequency/period were calculated for the 91 borehole locations. The values of the maximum PSA ranged between 17 and 103 cm/s2, covering the western zone of the Jizan study area. Furthermore, the PSAs were computed for 1.5, 3, 5, 8, and 10 Hz at 91 locations, and then contoured using the ArcGIS package. This frequency range from 1.5 to 10 Hz represents the range in the natural frequencies for tall to low-rise buildings.