Reconstruction Of Empirical Green's Functions Based On Coda Interferometry

Seismic interferometry,which has been developed rapidly in the past 20 years,is a revolutionary approach of reconstructing Empirical Green's Functions(EGFs).Actually,it uses continuous ambient seismic noise or coda to extract EGFs.Here,we retrieve EGFs from coda of large earthquake in local and global scales and confirm some core phases by scanning ray parameters.However,we need to stack large numbers of cross-correlations of station-pairs to get stable EGFs with high Signal-to-Noise Ratios(SNRs).So,through synthetic and real cross-correlations,we systematically explore how source factors(dip angles,azimuth and strike and distances)influence extracting EGFs(mainly PKIKP2,PKIIKP2 and Rayleigh waves).Our results infer the average SNRs of PKIKP2 firstly increase then decrease with increase of the dip angles,and reach the maximum at the middle dip angle(?45°).This trend may be caused by the fact that the major energy contribution of extracted PKIKP2 is from the radial direction,while radiated energy along the radial direction from dip-slip earthquakes is the largest when dip angle reaches about 45°,which matches well with the analysis of the stationary point spatial distributionFurthermore,the SNRs of retrieved Rayleigh waves significantly depend on the azimuth while is not influenced by the strike.Coda interferometry can extract clear Rayleigh waves when epicenters locate near the great circle arc of the station-pairs and it's consistent with the stationary points of surface waves.However,PKIKP2 and PKIIKP2 with high SNRs can be reconstructed when the epicenters are far away from the great circle arc of the station-pairs and the strike is approximately parallel to line-shaped array.Calculating the radiated energy from sources,to some extent,it indicates that above setting of the azimuth and the strike is fit for retrieving two core phases.Lastly,the distribution of epicenters can influence the reconstruction of PKIKP2.Namely,the smaller the acute angle between the ray path of the PKIKP2 and the radial direction of the earthquake,as well as the smaller distance,the higher SNRs of PKIKP2 can really be acquired.Our results,indeed,can filter out some adverse events and efficiently extract EGFs of core phases and Rayleigh waves.