A New Method On Moho Structure Research Which Utilizes The Ratio Of PmP And P_n’s Wave Amplitude

Today’s Moho structure is largely due to the evolution of physical properties、structure and construction in the solid earth’s interior, therefore Moho structure is quite essential in understanding deep process and coupling relations between spheres. This paper looks back the history of Moho structure studies, points out that there is a new idea about making full use of dynamic information of seismic waves. It also points out that the comprehensive utilization of various infomation is the future studying trend. Then, this paper proposes a new approach based on theoretical reflectivity seismograms to gain Moho structure, it utilizes the ratio of PmP and Pn’s wave amplitude and adjusts model to fits the predicted PmP/Pn curve with measured PmP/Pn curve. Numerical simulation and the real data processing results show that, PmP/Pn curve method is quite feasible and effective. Using the ratio of PmP and Pn’s wave amplitude help us get a delicate structures, eliminate influence that the source and instruments induces, it fully mines the dynamic information of seismic waves.Deep seismic reflection reveals that there are three kinds of Moho structure:Single Layer structure、Gradient Layers structure、layers stacked with High and Low Velocities structure. Numerical tests indicate that there exists significant difference among the theoretical PmP/Pn curves of those structures. VD structure and VG structure’s PmP/Pn curve have similar shapes convex shape, but the value of the former is roughly twice that of the latter, and HLV structure’s PmP/Pn curve has concave shape. All shows that this method could effectively work for the identification of three Moho structures and is sensitive to them. PmP/Pn curve’s influencing factors include velocity grades of upper mantle, Moho structure’s thickness, crustal structures’s thickness, as well as crustal structures. The shape of PmP/Pn curves mainly depends on the type of Moho structure. PmP/Pn curve is quite sensitive to the variation of upper mantle’s velocity grades, but it isn’t so sensitive to the variation of Moho structure and crustal structures’s thickness; the change in crustal structures could just influence the value of PmP/Pn curve rather than the shape of it. In addition, noise test suggests that PmP/Pn curve can resist noise to a certain degree, and PmP/Pn curve method require a high signal-to-noise ratio.The real data processing results show that it fully mines the kinematic and dynamic information of seismic waves. Compared with the real data, the unferground structure model produced by this algorithm meets two significant appraisal criterions:the one is same ratio of PmP and Pn’s wave amplitude, the other is the same travel time of main phases. All shows that this method is superior to the conventional travel-time method.