New methods to improve the assessment of shear wave velocities and seismic hazard parameters in Jeddah city, western Saudi Arabia

Abstract

Seventy-five geotechnical boreholes were drilled in Jeddah city to identify the soil characteristics and calculate the shear wave velocity. The depth of boreholes extends up to 40 m in some sites, where the standard penetration test (SPT) was assigned at each 1.5 m depth interval. The groundwater table fluctuated between 0.5 and 9 m below the ground level. In addition, the multichannel analysis of surface waves (MASW) method was applied at the same borehole locations to estimate the shear wave velocity of the subsurface layers. 1D shear velocity profiles were processed using SeisImager software. The calculated shear wave velocity at the mid-point of each surveying line was compared with that of the borehole because the mid-point of the surveying line coincides with the borehole location. In addition, horizontal–vertical-spectral ratio inversion (INV_HVSR) of microtremor measurements recorded at the borehole locations represents a third method to estimate the shear wave velocity of subsurface layers. Results of these three methods are then compared and, finally, the average shear wave velocity values, up to a depth of 30 m, were assessed for Jeddah city. According to Vs (30) values, the soil classes for Jeddah city can be classified into three classes as “site class D,” “site class C,” and “site class B,” according to the National Earthquake Hazard Reduction Program (NEHRP). These results should be added to the Saudi Building Code (SBC) to improve the requirements for the design of earthquake-resistant structures in Jeddah city.