Analysis Of Offshore Ground Motion Characteristics And Site Influence

With the rapid development of coastal economy and the strategic needs of marine development,the construction of a large number of marine engineering projects and cross-sea traffic engineering have entered a climax.Following it is how to guarantee the seismic safety of the engineering structure in the face of the complex geological environment of the sea area,the lack of historical earthquake loss data and the reference seismic design specifications.China is located between the circum-Pacific Seismic Belt and the Eurasian Seismic Belt.The geological and tectonic activities in the sea area,especially the plate subduction zone near the country are very active due to the motion squeezing between plates.Relative to the terrestrial area,the offshore seismic data is more scarce.And there are obvious differences between the plate margin/interplate earthquakes in the subduction zone and the intraplate earthquakes,the oceanic crust and the continental crust,and the local site conditions of the ocean and land.Therefore,there are still many problems to be solved urgently in the engineering characteristics of offshore ground motion and the determination of the ground motion for the seismic design of marine engineering.The research in this paper is based on offshore ground motion data from the United States and Japan.The engineering characteristic of the offshore ground motion is analyzed.And the characteristic of the ground motion of offshore and land is compared.Referring to the land site classification standards,the category of typical offshore sites is recommended,the nonlinearity of the offshore site seismic response is studied.A prediction model of offshore ground motions,which comprehensively considers the sea source,propagation path and special site conditions,is established to provide reliable design ground motion parameters for seismic fortification of major marine projects under complex seismic geological structures.The main content and research results are as follows:1.Review the worldwide development and research status of strong seismic characteristics,offshore engineering design ground motion parameters,offshore site effects and strong ground motion simulation methods.The relevant regulations on the construction of offshore strong seismic observatory networks and the seismic design codes for offshore engineering are summarized.2.The analysis of offshore ground motion based on the wavelet transform and the Hilbert-Huang transform method shows that the frequency domain energy of the ground motion is mainly concentrated in the low frequency band,and there are periodic energy release phenomena in some offshore seismic events.The horizontal and vertical components of the ground motion have similar marginal spectrum shapes,showing the characteristics of pulse distribution.The frequency-domain energy statistics also reveals that the offshore ground motion are rich in medium and long period components.3.Based on the offshore seismic data,considering the source,epicentral distance,site and other factors to analyze the characteristics of the ground motion,study the difference between the offshore and land ground motion.According to the location of epicenter,the offshore ground motion is divided into the ground motion caused by earthquakes in the sea and land.After grading according to magnitude and epicentral distance,it is found that there is no obvious regularity in the response spectrum of the ground motion in the sea and land within the same magnitude and epicentral distance.For offshore ground motions,the shape of the response spectrum of the EW and NS components corresponding to each epicenter interval is consistent and the intensity of the vertical component is an order of magnitude smaller than the horizontal component.The dynamic amplification coefficient spectrum of the selected offshore ground motion is calculated and compared with the commonly used standard spectrum in the mainland of the country.The results show that there is a great difference between the actual offshore ground motion strength and the ground motion parameter designed by the land specification.4.Statistically calculate the observation data of offshore ground motion,study the influence of the vibration and the nonlinear characteristics of the site under strong motion.The average shear wave velocity is obtained from the site soil layer data of the land station,and the site category of the land station is given based on the horizontal and vertical spectral ratio(HVSR)method.Three typical marine site magnification factors are calculated and used to classify the site category.The change of the epicentral distance has almost no effect on the peak period of the HVSR curve of the sea station site,and there is a strong correlation between the PGA and the characteristic period of the HVSR curve,the peak period of the HVSR curve becomes larger as the input PGA increases.the nonlinear effect of the offshore site under strong ground motion is revealed,and the site nonlinear parameter DNL and PNL are calculated.Based on the comparison results of the normative standards and the sea and land sites,the recommended values of shear wave velocity the marine sites are given.The reference value and error range of the high-frequency attenuationparameter(?)of the horizontal and vertical components of the Japanese offshore area are given through calculation.5.The stochastic finite fault method is introduced,and the ground motion prediction model is established for the offshore source,crustal medium and site conditions.The simulation and observation results of acceleration time history,PGA,pseudo-acceleration response spectrum,fourier spectrum,and other ground motion parameters of three earthquakes in the Japanese sea area,offshore and land areas are comparatively studied.It is proved that with the suitable method of ground motion simulation,the effective simulation of offshore strong ground motion parameters can be realized.