Study On Seismic Characteristics And Stochastic Finite-Fault Method Of Offshore And Inshore In Sagami Bay,Japan

With the strategic conception of "21st century Maritime Silk Road" put forward,the study of offshore ground motion and ocean engineering aseismic is paid more and more attention.However,due to the scarcity of offshore ground motion observation data,the study of offshore ground motion design,simulation and sea site characteristics is still in its infancy in China.Therefore,in this paper,abundant records of offshore and seafloor observed strong earthquakes in Sagami Bay,Japan are used to carry out work by statistical analysis and stochastic finite-fault simulation based on the characteristics of path duration and local site effect of offshore and offshore ground motions.The main contents are as follows:1.This study systematically summarize the research status of seismic simulation methods,duration models and seismic characteristics in offshore areas.Expounding the reasonableness of using the stochastic finite-fault method,the necessity of calculating the path duration model and the importance of studying the characteristics of offshore ground motion.2.This study introduce the method of determining parameters of stochastic finite-fault method in source,propagation path and site effect.Calculate the key simulation parameters such as path duration model,crustal amplification Horizontal-to-Vertical Spectral Ratio(HVSR)and high-frequency attenuation parameters of offshore stations.3.Use the 70%energy duration model,90%energy duration model or duration model obtained by picking up S-wave arrival time in this paper and use the stochastic finite fault method to simulate the Fourier spectrum.The comparison of three duration models shows that the residual between the Fourier spectrum simulated by the duration model obtained in this study and the observed value is the smallest in the whole frequency band.The duration model obtained in this study,the duration model in North America and the duration model in Lushan were simulated and obtain their Fourier spectra.The comparison of residuals of 25 stations shows that the duation model in this study is closest to the observed value in the middle and high frequency(3-10Hz).The results show that the proposed method can improve the accuracy of the stochastic finite fault method in middle and high frequency.4.The parameter determination method and inshore site amplification calculated from borehole data are used to simulate 19 inshore stations in Sagami Bay and get response spectrum.Except for a few stations affected by near-field effect and complex propagation path,the simulation results of other stations correspond well with the observed values.This result proves the accuracy of the parameter determination method in this study.5.Use the site amplification calculated by spectral ratio method to simulate 6 offshore stations in Sagami Bay and get response spectrum.The results show that the simulated value is too large,which is particularly obvious in low and middle frequency bands(0.1-2Hz).Because sea water has a certain suppression effect on vertical ground motion,and the suppression phenomenon is enhanced with the increase of water depth,the horizontal/vertical Fourier spectrum ratio increases,and the use of HVSR as the site amplification of offshore stations will lead the simulation result get larger in the frequency band affected by vertical suppression.6.The parameter determination method in this study is used to simulate the six offshore stations in the Sagami Bay without using site amplification and obtain their response spectra.The simulated results are smaller than the observed values in the whole frequency band(0.1-30Hz),and the error increases in the high frequency part,indicating that site conditions have a more obvious effect on the high frequency.Since the accuracy of the set of parameters was verified in the simulation of 19 inshore stations in the Sagami Bay,the residual between the simulated values and the observed values was considered to be caused by site conditions.Compared the simulated residual with the HVSR of the offshore station,the curves of the simulated values are similar,and the residual values can reflect the site conditions of the offshore station to a certain extent.Through the analysis of residual curves and topographic slope,it is considered that KNG201,KNG203 and KNG205 stations are soft site conditions.KNG202,KNG204 and KNG206 stations are hard site conditions.The critical topographic slope of soft and hard sites is between 0.02 and 0.08.