The Properties Of Ps Converted Seismic Waves And Applications In The Determination Of Shallow Shear Wave Velocity Structure

Seismic converted waves are powerful tools for detecting of the velocity structures of the crust and mantle,and both the teleseismic and local seismic converted waves are widely used.The teleseismic P-wave receiver function,which mainly uses the converted waves of crust and the mantle,has been rapidly developed and has become an important method for the obtaining of crust-mantle velocity structures.The receiver function is based on the assumption of plane wave incidence and that the incident waves are considered as a single seismic phase with the same ray parameters as the direct P wave.However,at a certain epicentral distances,the core-mantle boundary reflected P-waves(such as PcP phase),whose ray parameters are significantly different from the direct P-waves,enter into the receiver function extraction window,which may cause deviations in the extracted receiver function.The calculation results show that the PcP reduces the amplitudes of the receiver functions.The PcP excited by the deep dip-slip earthquakes has a great influence on the receiver functions,and the maximum receiver function amplitude can be reduced by 20%.However,The PcP excited by the strike-slip earthquake is weak and has little impact on the receiver functions.The velocity of the crust and mantle inverted from the contaminated receiver functions is reduced to some extent.The stacking method of receiver function can effectively suppress the artifacts caused by the PcP phase,but still can not completely remove its effects on the amplitude of the receiver functions.Teleseismic earthquakes have large magnitudes,long durations,large wavelengths,and low resolution for thin layers.Local earthquakes are of small magnitudes,short duration,small wavelength and high resolution,thus ideal for detecting fine structures.In the local earthquake waveforms,the converted phase Ps generated at the bedrock interface carries information about the velocity structure of the sedimentary layer and can he used to constrain the shallow shear wave velocity.In this paper,with the assumption that the sedimentary layer is linear gradient,a method of the measuring of shear wave velocity structure of the sediment is developed,which is based on the amplitude ratio and travel-time difference between the direct P-wave and the local seismic converted wave Ps.Theoretical tests show that the sedimentary structure can be recovered well by above method is of high accuracy.The measurement results of real data show that the S-wave velocity at the top of the sedimentary layer is consistent with the velocity of the Quaternary loose sediments in this area.The distribution of the thickness of the sedimentary layer is consistent with the regional tectonic strike,which is comparable to the results of artificial seismic exploration.This method is simple and effective and can be used for the detection of shallow shear wave velocity structures.