The Characteristics Of Offshore Ground Motion And Its Attenuation Relationship

With the continuing construction of offshore engineering, the antiseismicproblem has been receiving increasing attention. We need to know about thecharacteristics of offshore ground motion if we want to estimate the designingparameters of ground motion for offshore structures. This paper focuses on thecharacteristics of seafloor ground motion and its attenuation relationships. The mainwork is as follows:1. We summarize the research work about offshore ground motion at home andabroad, and introduce the regulations of designing parameters of ground motion、thecorresponding research status and the existing problems in specifications of offshorestructures such as oil platform and so on.2. We study the effect of seawater on incident plane P and SV waves of verticalground motions from the perspective of seismology and then analyze the engineeringcharacteristics of seafloor ground motion using the offshore records off the coast ofsouthern California, USA.Our study shows that the affected frequency range of verticalground motionsdue to P wave resonance in the water layer can be measured by theimpedance ratio between seawater and its underlying medium, the impedance ratio islarger, the affected frequency range is more prominent. Vertical ground motions whichconsist of mostly P waves are more easily affected by seawater than those dominatedby SV waves. Under the influence of soft surface soil at seafloor, both horizontal andvertical spectral accelerations of offshore motions are exaggerated at long periods.3. For several different fault types, we study the effect of seawater of differentdepths on the characteristics of seafloor ground motion systematically using thewavenumber integration method of CPS software. On the whole, the effect of seawateron seafloor ground motions has little to do with fault types; the effect of seawater onhorizontal ground motions is very little and can be neglected in engineering, but theeffect of seawater on vertical ground motions is siginificant. In general, the effect ofseawater on seafloor vertical ground motion is as follows: as the depth of seawaterbecomes larger, the waverform of seafloor vertical motion becomes more complicatedand its corresponding PGA becomes smaller; the Fourier spectra of seafloor verticalmotion decreases greatly near the P wave resonance frequency of seawater; the maineffect of seawater on spectral acceleration of seafloor vertical motion falls in shortperiods less than0.1s, as the depth of seawater becomes larger, the values of spectral acceleration become smaller; the effect of seawater on90%energy duration of seafloorvertical motion has something to do with fault types.4. We establish the attenuation relationships of PGA and spectral acceleration forseafloor vertical ground motion with50-meter seawater and without water (on behalfof land) respectly using CPSsoftware. Through comparing the differences betweenthem, we find that: the50-meter seawater has a great effect on the attenuationrelationships of PGA and the spectral acceleration in very short periods (no more than0.04s) of seafloor vertical ground motion, the values of seafloor vertical motion aresmaller than those without water (on half of land) apparently; for spectral accelerationin longer periods, the effect of seawater can be neglected, especially near the period theP wave resonance frequency of seawater corresponds to, the seawater has littleinfluence on the spectral accelerations of seafloor vertical motion.