Study On Seismic Phase Characteristics Of Seismic Waves Of Active Source In Qilian Mountains,Gansu

Seismic phase contains abundant information of underground medium and source.Seismic phase analysis is an important means to study the velocity structure of the earth.Both natural and man-made earthquakes can trigger seismic waves containing a variety of seismic phases,which contribute greatly to"lighting up"the earth's interior.Air-gun active source is the largest non-explosive artificial seismic source at present,and has the characteristics of good repeatability,high accuracy,long propagation distance,green and environmental protection.Qilian Mountains air-gun source actively motivate platform is over the reservoir in Zhangye,the observation stations located in the Hexi corridor,Qilian Mountains north rim region,the region's earthquake have occurred many times in nearly two hundred years,tectonic activity is frequent.The study of active source phase in the Qilian Mountains can help improve the excitation conditions of active source in the Qilian Mountains,promote the study of underground structure imaging,underground medium stress variation and travel time variation,improve the ability of microseismic identification,etc.,and provide scientific basis for earthquake prediction.This paper selects 2017 Qilian Mountains air-gun source actively waveform data,from the year average superposition signal picked up in the real seismic phase,then using SEIS83 program for ray tracing,calculation theory travel time,then take the reality and theory when residual minimum model for optimal velocity structure model,using"skinning method"to calculate a total of 183415 models,The conclusions are as follows:1.A few stations of the Qilian Mountains mobile station can identify the reflected seismic phase P₁ from the upper crust,while most stations can clearly identify the reflected seismic phase P_g,the reflected seismic phase P₂ from the upper crust,the reflected seismic phase P₃ from the middle crust,the reflected seismic phase P₄ from the internal layers of the lower crust and the reflected seismic phase P_m from the Moho plane.For different epicentral distances,the source of P_g phase is different.When the epicentral distance is less than 90 km,P_g phase comes from the shallow sedimentary cap rock.When the epicenter distance is greater than 90 km,the P_g phase comes from the upper crust.When the epicenter is more than 150 km away,the first phase can be received.The average error of 97.1%is within 0.20 s,which shows that the fitting effect is very good.2.The Hexi Corridor,the direction of the faults in Qilian Mountains north rim-north east to southwest,deftly divided for the center with reference station,east and west two average comprehensive model,compare the two model results show that the east side of interface depth deeper,west shallow depth,lower layer within the P-wave velocity on the east side,the west high speed.It is in high agreement with the results of predecessors.3.The average comprehensive model of the eastern area shows that the thickness of the shallow sedimentary layer is 5.24km,and the P-wave velocity in the layer is5.48?5.60km/s.The bottom of the upper crust is located at 18.53 km,and the velocity is 5.73?6.10 km/s.The bottom of the middle crust is located at 31.06 km and the velocity is 6.21?6.32 km/s.There is a stratification in the lower crust,located at 41.81km with a velocity of 6.41?6.51 km/s.The Moho surface is located at 56.06 km with a velocity of 6.61?6.71 km/s.4.The average composite model of the western region shows that the average thickness of the shallow sediments is 4.89km and the P-wave velocity is 5.49-5.61km/s;The bottom of the upper crust is located at 17.69 km,and the velocity is 5.84?6.17km/s.The bottom of the middle crust is located at 28.39 km and the velocity is6.27?6.38 km/s.There is a stratification in the lower crust,located at 43.24 km,and the P-wave velocity within the stratification is 6.49?6.59 km/s.The Moho surface is located at 53.61km,and the velocity within the layer is 6.73-6.81km/s.The results of this paper are helpful for future research on complex seismic phase identification and structural imaging of active source in this region,and can better explore the spatial and temporal four-dimensional stress variation in the Qilian Mountains.