核幔边界反射P波对接收函数影响的研究

Study on the effects of core-mantle boundary reflected P-wave on receiver functions

远震P波接收函数是探测壳幔结构的有效方法,它采用平面波入射假定,将入射波视为具有相同射线参数的单一震相.然而,在一定震中距时,核幔边界反射P波(如PcP震相)会进入接收函数提取窗口,其射线参数与直达P波相差较大,可能导致提取的接收函数存在偏差.本文理论测试了不同震源机制,震中距以及震源深度等情况下核幔边界反射P波对远震P波接收函数的影响,并结合实际观测资料进行对比分析.测试结果显示,与走滑断层产生的PcP震相对接收函数影响较小不同,倾滑断层产生的PcP震相会使接收函数出现虚假震相并降低直达P波及后续震相的振幅,从而干扰壳幔间断面(如410 km和660 km间断面)的识别,并导致反演的台站下方S波速度降低达到5%.进一步分析倾滑型地震对接收函数波形影响发现,当震源深度小于300 km时,PcP震相导致接收函数直达P波振幅要低于4%.在震源深度在300~450 km、震中距为50°~60°、断层倾角在大约30°~60°,以及震源深度大于450 km、震中距为50°~60°、断层倾角大约25°~65°情况下影响可达20%.接收函数叠加方法可以有效压制结果中PcP震相导致的虚假信号,但是仍然无法完全去除PcP对接收函数振幅的影响,叠加后直达P波振幅降低仍然能够达到4%.本文结果表明,在计算接收函数时,剔除掉产生较强PcP的特定震源机制、震中距和震源深度的地震事件有助于反演精确的壳幔结构.

Teleseismic P-wave receiver function is a powerful tool for detecting crust and mantle structure.The plane wave incident is assumed that the incident wave is regarded as a mono-phase with the same ray parameter.However,the reflected P-wave(such as the PcP phase)of the core-mantle boundary will arrive at the receiver function extraction window at a certain epicentral distance.The difference of ray parameters between the PcP and the direct P-wave may lead to the deviations of the extracted receiver functions.This results in violation of the mono-phase hypothesis.In this paper,the effects of core-mantle boundary reflected phases on teleseismic P-wave receiver functions are analyzed under different focal mechanisms,epicenter distances and focal depths and compared with observed data.It shows that the strong PcP excited by dip-slip fault will decrease the amplitudes of direct P wave and subsequent phases,as well as artifacts will arise in the receiver functions,which is different from the weak PcP excited by strike-slip fault.Thereby they will interfere with the identification of crust-mantle discontinuities and result in a 5%reduction in inversed S-wave velocity under the station.The further study shows that the amplitude reduction of receiver function is less than 4%when the focal depth is less than 300 km.The effects on receiver function up to 20%when the focal depth is 300~450 km,the epicenter distance is 50°~60°,the fault dip is in the range of about 30°~60°,and the focal depth is more than 450 km,the epicenter distance is 50°~60°,the dip is about 25~o^65°.Although the stacking of receiver functions can suppress the artifacts result from PcP,but still cannot completely remove the influence of amplitude of receiver function.After the stacking,the amplitude of direct P can still be reduced by 4%.In the calculation of the receiver function,the removal of events with specific focal mechanisms,epicentral distances and focal depths that excite strong PcP phase can help to invert accurate crustal and mantle structure.