| Ground motion attenuation relationship describes shaking field during an earthquake and provides technical support for seismic hazard assessment of critical engineering site and national/regional seismic zoning.The study and development of regional empirical attenuation relationship mainly depend on abundant observed strong ground motion data.However,in most earthquake countries or regions in the world,except western United States and Japan,there are still not enough strong motion records and then it is inaccessible to develop a regional empirical attenuation relationship directly from statistics.Seismology based attenuation relationship is a solution for the issue in those regions.The NGA-East project of US has contributed to a helpful research,and some problems still need further improvements.An inadequacy in the weighting scheme of attenuation relationships in the NGA-East project is pointed out in this dissertation as not taking into account the important consensus on ground motion that point source model is applicable for estimating motions from small/moderate earthquakes or motions at far field,while finite fault source model for motions at near-field of strong/large quakes.A new weighting scheme is proposed here to consider their own applicable ranges of point and fault source models,in which a weighted mean ground motion PGA from the two source models in the M_w(6.0,7.0),PGA from point source in[5.0,6.0]and from fault source in[7.0,8.5]are combined to develop a PGA hybrid attenuation relationship for Sichuan region.The study of this dissertation consists of mainly two parts:(1)syntheses of ground motions of earthquake with M_w greater than 6.0 by fault source model,which includes to develop scaling laws of parameters in the source model,to build finite-fault source models for 26 magnitudes for regional attenuation study,and to synthesize 30 accelerograms at 192 sites with different distances along strike and dip directions for each source model;(2)to develop a applicable scheme to weight PGAs of ground motions from finite fault source and point source respectively for earthquakes with M_w below 7.0,and to find an appropriate form to express the hybrid attenuation relationship.Firstly,scaling laws of source parameters are developed from the new data published recently.The global parameters of sources from the NGA database are collected to build dataset1(189 earthquakes)and dataset2(204 earthquakes).The empirical and semi-empirical relations between moment magnitude and four parameters,namely,area,width,length of source rupture and average slip on the rupture plane,are developed for three rupture types and three/four magnitude intervals,respectively.The feasibility of the result laws in the Sichuan region is illustrated by a comparison of the results with published corresponding parameter values of nine regional earthquakes.The slip distribution data of 211 earthquakes worldwide are collected from the finite fault source rupture model database,eQuake-RC,and then local parameters of sources are acquired one by one.Then scaling laws of asperity parameters,area,length,average slip,asperity center of main and the other asperity as well as coordinates of rupture starting point,are set up to the corresponding global parameters in three rupture types,for intraplate and subduction zone earthquakes respectively.The results show that discreteness of the relation between local parameter with global parameter is a much smaller than that of those with moment magnitude.Generally,there are multiple asperities on rupture planes of most earthquakes with magnitude above 6.5 while just one asperity on the others.Overlap of the other asperity on main asperity could be avoided by means of developing the scaling laws for center coordinates of the other asperity with considering if the left or right parts the main asperity is on the rupture plane respectively,and the coordinates of the starting point are no longer limited outside of asperity.The applicability of the scaling laws of local parameters in the Sichuan region is illustrated by a comparison of the results of this dissertation with the published corresponding values of local parameter of four earthquakes in this region.Subsequently,general idea and detail steps to build a finite fault source model for development of regional ground motion attenuation relationship are summarized.The rupture dip angle of 70°and rupture type of ALL are selected and an estimation method on upper edge depth of rupture plane is put forward through an analysis on the regional seismo-tectonic condition in Sichuan region and the parametric sensitivity analysis jointly.From the result,the average depth of the regional earthquake hypocenters is about 16 km,which means that the width of the source plane should be less than 32 km.The uncertainties of scaling laws are taken into account by means of truncation normal distributions,and 30 sets of source parameters of each earthquake magnitude are estimated to generate 30 slip distributions.A total of 30 fault source models are built for each of 26 magnitudes in 6.0?M_w?8.5 with interval of 0.1.The formulation on superposition of ground motions from all sub-sources in time domain is improved.The ground motions from the 26 earthquakes at 48 sites along the each of four directions relative to the rupture orientation,which results in the PGA averaged from 30 estimations for each magnitude and all the 192 sites.Then the attenuation relationships along major and minor axes are combined from those in four directions by a comparative argumentation.A two-dimensional tabular form is adopted for ground motion attenuation relationship.In the table,the attenuation relationships in M_w[5.0,6.0]is directly from PGA by point source,and those in M_w[7.0,8.5]from PGA by fault source,while those in M_w(6.0,7.0)is the weighted mean of the two PGA groups with the weighting coefficient of point source decreases linearly from 1.0 to 0.0 and of fault source increases from 0.0 to 1.0 with magnitude increasing respectively.This scheme demonstrate the common view that the motions from small/moderate earthquakes could be estimated from point source models,and those from large earthquake should be estimated by fault source models,and there would be a transition in between.As the result,a hybrid PGA attenuation relationship on rock site applicable for Sichuan region is developed,that means that the final and the key step in overall planning of project to study and apply the seismology based strong ground motion attenuation in China are achieved in this dissertation.Finally,the results are demonstrated by comparing them with a number of regional ground motion data and the attenuation relations adopted for Sichuan region in preparation of the national zoning map.An obvious advantage of the improvement in this dissertation is on the PGA estimation of ground motion from large earthquake at near field.The feasibility of the weighting scheme presented here in engineering practice is demonstrated further from the similar results of seismic hazard assessments at three sites by the suggested and that of logical tree approach,the improvement can save a lot time by repeat computing in the latter. |
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