The Influence Of Site Factors On Offshore Ground Motion

In order to determine the influence of site factors such as seabed terrain and site condition on offshore ground motion.In this paper,the seabed models with different thickness of soft-soil overburdens,with sloped terrain,with ocean trench and ridge terrain,and with complex terrain were built by combining with the wave analysis program and the finite element dynamic analysis software.Comparing with the general offshore site and corresponding onshore site models,the acceleration and displacement time histories,transfer functions,W/L ratio spectra and S/G ratio spectra on the seabed surface were analyzed to determine the influence of the site condition and seabed terrain on the offshore ground motion.In addition,the influence of the coupling site factors with step terrain,site angle and adjacent slope height on offshore seismic response are discussed,the main research contents are as follows:(1)The main contents of the research on the characteristics of offshore ground motions in the world are summarized,and the research methods of the site condition and terrain on the offshore ground motion are concluded.And it summarizes the current status of existing research,as well as the remaining problems and gaps in this thesis.(2)The seabed and land models with different thickness of soft-soil overburden are built.The effect of soft-soil overburden and its thickness on offshore ground motion are analyzed.It is found that the silt soft-soil layer can amplify both vertical ground motion of P wave and horizontal ground motion of SV wave,especially for the SV wave.The acceleration and displacement time histories of vertical and horizontal ground motions on the silt soft-soil seabed are obviously amplified,and the amplification is related to the thickness of silt soft-soil layer.(3)It is found that the slope site has a significant amplification effect on the vertical P-wave ground motion by studying the influence of single seabed terrain on the offshore ground motion.And the trench and ridge terrain have obvious amplification effect on P-wave vertical ground motion,the amplification effect of the ridge terrain on the P wave is more obvious than that of trench terrain.Moreover,it is found that the seismic response of the inflexion point is particularly significant.In addition,the seismic response of P wave at different locations of the slope site decreases with the increase of sea water depth.It is also found that the influence of different terrains on the horizontal ground motion of SV wave is very limited.(4)A model of complex seabed site is built to analyze the effect of multiple site factors coupling on offshore ground motions.The ground motion of a complex seabed site can be affected by the seabed terrain,site condition,and water depth etc.Moreover,the water depth and seabed terrain have a coupling effect on the offshore ground motion.The conclusion that the silt soft-soil layer will amplify the offshore ground motion is also applicable in the complex seabed site,but it can be affected by other site factors.In addition,both the seabed surface angle and the slope height of the seabed overburden have an effect on the offshore ground motion.