| Ground motion estimation is an important step in earthquake engineering,the main purpose of which is to provide input for seismic design.There are two main paths,one is through the attenuation relationship of ground motion,and the other is the composite ground motion,both of which describe the influence of source,propagation medium and site conditions.Among them,the seismic wave propagation in the medium can be expressed in terms of geometric spreading and inelastic attenuation.In this paper,the two areas are taken as the research object,and Sichuan,Xinjiang and Yunnan regions in western China are taken as the research area.The geometric spreading and inelastic attenuation are studied separately according to their physical meanings,and the influencing factors are discussed and the regional parameter values are determined with the help of strong earthquake records.(1)Records of three-component strong earthquakes with magnitudes 3.5 to 5.5,focal depths less than 30km and focal distances less than 300km in the study area from 2007 to 2018 were collected.The data were preprocessed by baseline correction,filtering,S-wave extraction,etc.,and the database was formed according to the principle that each earthquake should be recorded by at least three stations,each station should have at least three records,and the signal-to-noise ratio should be greater than 3.(2)Analysis of influencing factors of geometric spreading parameters.The records in the database were grouped according to Moho buried depth,magnitude of earthquake and focal depth.The H/V spectral ratio method was used to calculate the site amplification factor of the station to eliminate the influence of the site.The attenuation curve of normalized spectral amplitude with focal distance at different frequencies was drawn.The factors affecting the geometrical spreading term parameters are determined.The results show that the Moho buried depth has a great influence on the geometric segment points.When the Moho buried depth increases,the values of R1 and R2 increase.The magnitude,focal depth and frequency(1Hz,2Hz,4Hz,8Hz)have no effect on the geometric segment point.Data quality has a significant effect on geometric segment points.(3)Analysis of influencing factors of inelastic attenuation parameter.According to the geological time,earth heat flow value,surface elevation,Moho buried depth,shear wave velocity of upper crust and seismic activity,the regions are redivided,and the four parameters of Q0,η,κ0,Δσ are determined by genetic algorithm.The results show that the quality factors are affected by elevation,Moho depth,geological time,shear wave velocity distribution in the upper crust,terrestrial heat flux value and seismic activity.The seismic wave attenuation is faster and Q0 value is usually lower in the regions with higher terrestrial heat flow,shallower Moho,lower shear wave velocity in the upper crust and stronger seismic activity.With the increase of geological time from ancient to present and elevation,the value of Q0 decreases gradually.When determining the value of regional parameter Q in ground motion estimation,regions can be divided according to geologic time,elevation,shear wave velocity of upper crust and seismic activity.The eastern part of Sichuan Province and the Tianshan Mountains of Xinjiang Province have high Q values,and the recommended value range is 290 to 500.Yunnan,central and western parts of Sichuan,and Kashi of Xinjiang are areas with low Q values.The recommended value range is 90 to 230.If the latter is further subdivided,the northwestern Yunnan and the central and western Sichuan are the regions with relatively low Q value. |
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