Site response assessment at the city of Al Khobar, eastern Saudi Arabia, from microtremor and borehole data

Abstract

Al Khobar City is affected by distant earthquakes from the Zagros fold-fault belt, which is part of the subduction zone between the Arabian and Eurasian plates. These earthquakes have generated substantial site effects on the sedimentary layers, which in turn significantly influence earthquake ground motions in the area. Mapping of site response using microtremor measurements compared with geological and borehole data from Al Khobar City is the main objective of this work. The resonance frequency and the corresponding horizontal-to-vertical (H/V) ratio values have been calculated using the Nakamura technique on data from seismograph instruments deployed at 113 sites in Al Khobar City for different time periods. The recording length was about 1 h with a sampling frequency of 100 Hz. Most of the measured sites present three peaks of resonance frequency; the first peak ranges from 0.33 to 1.03 Hz, the second peak ranges from 1.03 to 1.23 Hz, while the third one ranges from 1.23 to 1.73 Hz. Tests have been conducted to ensure that these peaks are of natural origin The northern zones of Al Khobar City have lower resonance frequency values, indicating great sediment thickness. In contrast, the southern parts of the city have higher resonance frequency values, suggesting shallow bedrock depths. Furthermore, 29 geotechnical boreholes have been drilled to different depths in Al Khobar City. Standard penetration test (SPT) data has been corrected and used to calculate the resonance frequency at their locations. The borehole results showed that the resonance frequency values range from 0.27 to 1.95 Hz. These results are well correlated with those from the microtremor measurements. Accordingly, the first peak has been interpreted as being due to the impedance contrast between the limestone bedrock and the overlying sediments, while the third peak originates from a boundary between the uppermost surface layer and the underlying sediments.