Calibration of horizontal seismic design spectra of offshore earthquake ground motions by applying improved differential evolution method

Abstract

Design Response Spectrum (DRS) serves as a crucial cornerstone for seismic design, encapsulating the characteristic parameters of the design spectrum. To investigate the offshore design spectrum required for seismic design of offshore structures, improved differential evolution (DE) methods are first used to determine the characteristic parameters of the horizontal design spectra of offshore ground motions. In assessing the stability and precision of the chosen improved DE methods, we scrutinize the applicability of various renowned DE methodologies in the calibration process. Our findings indicate that differential evolution with global and local neighborhoods (GLDE) exhibits superior stability and accuracy compared to other DE methods, rendering it suitable for calibrating the design spectrum to delineate optimal characteristic parameters of the DRS. Subsequently, the properties of the offshore horizontal design spectra are investigated by applying the recommended GLDE to 6756 seismic data collected from three offshore regions. The site conditions of water depth and sedimentary thickness of the ocean-bottom stations were investigated by the finely divided seismic data with classified earthquake types. The ocean-bottom stations are classified as A-D by the water depth and sedimentary thickness, and the offshore DRS for four site classes with different earthquake types are presented in this study. According to the observed offshore response spectra with different water depth groups, it is found that the water depth should be considered in the offshore design spectra mainly due to the differences between buried and unburied seafloor stations, and the spectral values with deep water layer are considerably larger than those with shallow water. Offshore DRSs are compared with onshore DRSs of seismic codes and different peak ground acceleration (PGA) groups are presented in this study.