Attenuation Characteristics Of Offshore Ground Motion And Its Intensity Measure

With the development of global marine resources,seismic fortification of marine engineering is becoming more and more important,but the research on marine earthquake enginnering is limited at present.The seismic zones in the South China Sea and the Bohai Sea in China are highly active and frequent earthquakes occur in this region,therefore,the seismic fortification of marine engineering structures is very essential.The seismic design of marine structures such as offshore oil platforms,subsea tunnels and cross-sea bridges requires the use of offshore ground motions as inputing motions,and the study on offshore ground motions can provide a reference for the zoning of offshore areas and the seismic design of marine engineering structures.This paper focuses on the characteristics of the offshore ground motion and its attenuation relationships,and the main work is as follows:1.We collected the ground motion records of the offshore stations of the Sagami Bay in Japan,and the nearby onshore ground motion records are also collected as a comparison database.All records were classified according to the focal mechanism and the location of the source,and these records were processed by baseline correction and filtering to obtain a database for establishing ground motion prediction equations for the offshore sites near Sagami Bay.2.The prediction equation of horizontal and vertical 5%-damping elastic response spectra for offshore ground motions and the prediction model of vertical-to-horizontal(V/H)response spectral ratio for the Sagami Bay region in Japan were proposed.To compare the difference in response spectrum and V/H spectral ratio between offshore and onshore ground motions,this paper presents the prediction model of the response spectra and V/H ratio of onshore motions nearby the offshore stations.The differences of attenuation characteristics between offshore and onshore ground motions for the Sagami Bay region is presented.3.The horizontal and vertical offshore ground motions with prominent long-period characteristics were identified,the dynamic amplification factor spectrum of long-period offshore ground motions was compared with the design spectrum in the seismic design standard and the correlation between the offshore ground motion intensity indexes PGA,PGV,PGD and the maximum deformation demand of elastoplastic single degree of freedom(SDOF)systems with various strength reduction factors was studied.PGD is suggested as the seismic analysis intensity measure for offshore far-filed long-period ground motions.