Study On Path Attenuation And Source Characteristics In Ground Motions From Complex Tectonic Region

Kanto is the most densely populated area in Japan,which is located at the boundary of Eurasian plate,Philippine Plate and Pacific plate.The complex geological structure and frequent seismic activities in Kanto area lead to high seismic risk,which is also an ideal place to study the seismic characteristics of complex plate tectonics.The generalized spectral inversion method can effectively separate the source effect,path attenuation and site response from the ground motion data,and it is an effective method for the analysis of regional seismic characteristics.In recent years,with the intensive layout of observation network,the observation ability has been improved,which provides a good prerequisite for the inversion of strong motion data in Japan.In this paper,the generalized spectral inversion method is used to study the path attenuation and source characteristics in the Kanto area and the nearby sea area.The specific contents are as follows:(1)Analyzing the difference of calculation results of path attenuation and source characteristics caused by different spectral inversion methods.18237 ground motion recordings obtained from 591 moderate and small earthquakes in Kanto area and the nearby sea area in recent ten years were processed in time domain and frequency domain,and the path attenuation and source characteristics of ground motions are calculated by using parametric generalized spectrum inversion method(parametric),two-steps nonparametric generalized spectrum inversion method(two-steps)and one-step nonparametric generalized spectrum inversion method(one-step),respectively.The residuals between the observed spectra and the synthetic results from the product of the inverted source spectra,site responses,and path attenuation were computed to verify the reliability of the inversion,the reliability of the results of three methods were ordered by one-step?parametric?two-steps.The results of acceleration source spectrum are basically the same when the frequency is less than 3 Hz.In addition,the value were ordered by parametric?one-step?two-steps.Compared with the M_W value provided by CMT,the M_Wresults obtained by three methods are consistent with the values provided by CMT for earthquakes with M_JMA>5.5.For earthquakes with M_JMA?5.5,M_Wresults are obviously overestimated,the value were ordered by parametric?one-step?two-steps.The results of path attenuation calculated by three methods are quite different and need further discussion.(2)Analyzing the factor affecting the reliability and sensitivity of the inversion results.Discussing the difference of results caused by controlling factors.Proposing reasonable method optimization suggestions.Specifically,there are three aspects:First,analyzing the sensitivity of 12 geometrical spreading function hypotheses to the results of parametric method.After comparing 7 reasonable results with other research,it is found that the results of parametric method are uncertain.When the geometrical spreading function is R^-0.892,the quality factor is 151.04f^0.9902 which is obtained by nonparametric method.Second,discussing the influence of distance division on calculation results.On the premise of satisfying both the physical characteristics of attenuation function and the stability of calculation results,it is suggested 2 km for the records with hypocentral distance less than 60 km,and 1 km for the records with hypocentral distance more than 60 km.Third,analyzing the uncertainty of site response in two-step inversion.It is found that the records obtained from rock sites can reduce the uncertainty of the results.With that,it is further suggested that when the proportion of other data in rock site dataset is less than or equal to 40%,it can not only improve the reliability,but also will not add more uncertainty to the results.(3)Analyzing the difference of ground motion attenuation characteristics of complex tectonic region.The tectonic types of 591 earthquakes are distinguished,and four research regions are divided according to the tectonic types and spatial distribution of earthquake.Using the improved method,four regions are inverted.Analyzing the ground motion attenuation characteristics of regional difference.Region I is all shallow crustal earthquakes with epicenter located on the land of northeast Kanto basin,with the propagation path of ground motion is in the shallow crust.The simple structure leads to a large quality factor value of 92.33f^1.87.Region?is all upper mantle earthquakes with epicenter in Kanto basin,with the propagation path of ground motion is through the upper mantle and crust.The complex structure leads to a small quality factor value of 27.75f^1.08.Region?and region?are mainly shallow crust and upper mantle earthquakes with epicenter located in northeast and east coast of Kanto basin.The quality factors are 52.58f^0.95 and 58.07f^0.89,respectively.Pointing out that there is a strong correlation between path attenuation and hypocentral distance for earthquakes with epicenter located on the land,and the attenuation function curve decreases uniformly with the increase of hypocentral distance.The path attenuation of earthquakes occurred in the offshore performed obvious frequency correlation,and the decline rate of attenuation function curve increases with the increase of hypocentral distance.The attenuation function curves with higher frequency decreased more obviously,which indicates that the seismic attenuation function of sea area contains stronger inelastic attenuation.(4)Analyzing the difference of ground motion source characteristics of complex tectonic region.Based on the acceleration source spectrum results,the grid search method is improved,and the uncertainty of the source parameters is analyzed with f_c as searching variable.The maximum value of searching variable is f_cmax,and the minimum value is f_cmin,and the relative error is defined as the ratio of f_cmax-f_cmin to f_c.When f_c<0.5 Hz or f_c>1.0 Hz,the relative error value is small.When f_cis between 0.5Hz and 1.0 Hz,the relative error value is large,indicating that there is a strong trade-off between f_c and M_win this frequency range.Compared with the M_W value provided by CMT and the M_Wresults obtained by GIT from the whole dataset and the four regional datasets,which proves the rationality and superiority of the regionalization.Analyzing the source characteristic parameters of M₀,f_c,M_w,??,r,E_sin the four research regions,and providing the numerical relationship between the parameters.The results show that there is a positive correlation between the stress drop of earthquakes with epicenter on land and M_W while the correlation between the stress drop and M_W when the earthquakes occurred on land,and a weak correlation between the stress drop and M_Wwhen the earthquakes occurred in the offshore,and the earthquakes basically follow the self similarity.Discussing the ground motion stress drop in sea area,and providing its spatial distribution characteristics.