Study On The Characteristics Of Active Source Seismic Wave Phase

The recognition of seismic phase characteristics plays an important role in the development of seismology and the study of earth structure.The arrival time,amplitude and seismic phase contain abundant information of underground medium on the propagation path of seismic wave.From natural earthquakes to active source earthquakes such as explosives,shock-control source and air guns,people have never stopped studying seismic phases.Qilian mountain active airgun source is located in the hinterland of Qilian Mountain on the northern margin of Qinghai-Tibet Plateau,and its waveform data contain abundant seismic information.The identification of the active source phase of the Qilian Mountain not only can better understand the underground layered structure and velocity distribution in the area,but also provide reference for the identification of the active source phase of the airgun in the future.This paper investigates the literature data of the Qilian Mountains station coverage area,using the method of full-space model construction.The method of stripping is used for ray tracing,the ray tracing program needs manual operation to calculate the model.Each station calculates at least 600 models and chooses the best model.By comparing the simulation results of phase arrival time of 40 stations with the actual phase arrival time,the following main results are obtained:1.At dozens of stations such as zdy18,zdy27,zdy35,and zdy38,a better correspondence is obtained,which can clearly identify the Pg,the reflected wave P1 from the bottom of the upper crust,the reflected waves P2 and P3 from the inner and bottom of the middle crust and the reflected wave Pm of the Moho surface.The absolute value of the time difference between the theoretical simulated phase and the actual phase is usually less than 0.2 s.2.Most of the stations in the study area are distributed along the Qilian Mountains in the northwest-southeast direction.The velocity structure shows:(1)The velocity of shallow deposits is about 5.0-5.8 km/s,the depth of crystallization base is about 6 km,and the velocity of basement surface is about 5.8 km/s.(2)The depth at the bottom of the upper crust is about 16 km,and the average layer velocity is about 5.7-6.2 km/s.(3)The middle crust is divided into two layers.The depth of the upper layer is between 24 and 30 km,and that of the lower layer is about 40 km.The average velocity of the upper layer is between 6.0 and 6.7 km/s.(4)The depth of Moho surface is 52-56 km,the Qilian Mountains area is very deep,and the Hexi Corridor is a little shallow.The results are consistent with previous studies.3.The Qilianshan active source phase is rich and complex,and the P-wave sequence of each station has higher seismic phase identification,and the S-wave phase intensity is weak,the reflected wave with stronger energy is the main phase of analysis in this paper.The first arrival wave phase in the station sequence is often superimposed with the reflected wave phase at the top of the upper crust,which makes the amplitude of the first arrival phase significantly enhanced,and makes the P1 phase identification more difficult.The above research results provide a reference for the analysis of airgun source data,and have a deeper understanding of the study of velocity changes or stress changes in different depth ranges,which is of great significance for advancing the scientific exploration of earthquake prediction.