Site effect evaluation for Yanbu City urban expansion zones, western Saudi Arabia, using microtremor analysis

Abstract

Recently, Yanbu City has been greatly growing; great developmental projects, urban community settlements, and petrochemical industries have been established. Historical and instrumental earthquakes were felt by this city, such as the earthquake on 19 May 2009 which was a moderate earthquake (moment magnitude (M w) 5.7) whose ground motions have affected some building structures. Yanbu City has been divided by a grid of points separated by about 500 m, and microtremor measurements have been conducted at 141 measuring sites. The acquired data have been processed using worldwide Geopsy software package to calculate the fundamental frequency peaks and their corresponding amplification factors. The natural origin of these peaks has been confirmed. Based on fundamental frequency (f 0), Yanbu City has been classified into three zones—from 0.13 to 1.0 Hz in the first zone, from 1.0 to 4.0 Hz in the second zone, and from 4.0 to 7.6 Hz in the third zone. The coastal zone of Yanbu illustrates smaller values of f 0 that reflect great thickness of soft sediments while the eastern zone presents an opposite phenomenon. Accordingly, high-rise buildings in the coastal zone will be affected greatly by low frequencies originating from distant earthquakes. Most of Yanbu City has f 0 values in the range of 1.0–4.0 Hz. Furthermore, bedrock ground motion could amplify as much as three times. This indicates that one- to three-story buildings in Yanbu City are vulnerable to hazardous resonant shaking from local and near earthquakes.