Theoretical modeling of seafloor spatial earthquake ground motion with random fluctuation sea surface

Abstract

With the growing number of offshore structures, the damage degree and probability of offshore structures under earthquake increases. However, the influence of the fluctuating seawater layer on the propagation of seafloor ground motion is generally ignored in seismic design of offshore structures or ocean ground motion researching. Numerous studies have revealed that the spatial earthquake ground motion on the seafloor is notably affected by the saturation in the soil and the seawater layer overlaying the seabed. A new seawater transfer function to estimate the effect of random fluctuations in the sea surface on the behavior of ground motion was proposed in this paper. The seawater transfer function was derived by incorporating the reflection coefficient of the seawater layer and the scattering coefficient corresponding to the random fluctuation of the sea surface. The reflection of the P wave in the seawater layer between the lower medium and upper medium was considered by a reflection coefficient. The scattering coefficient was utilized to facilitate the estimation of the P wave attenuation due to random sea surface fluctuations. Moreover, the effects of the soil layer and soil saturation under the sea on ground motion are also considered. Taking the above factors into consideration, the ground motions affected by random fluctuations in the sea surface can be synthesized. A water layer transfer function proposed by Crouse and Quilter is used to validate the proposed seawater transfer function. To demonstrate the effect of the new seawater transfer function on ground motion, three example cases are presented. Comparisons among these three case results show that the new seawater transfer function exhibits attenuation at relatively high frequencies. Furthermore, the higher a sea wave is, the more severe the attenuation in the seawater transfer function is. The results also suggest that ground motion energy can be greatly influenced by random fluctuations in the sea surface. Lastly, the simulation method presented in this paper, which considers the fluctuating sea surface, offers a valuable reference for the study of ocean ground motion and presents a more rational approach to seismic simulations for offshore structure research.