| Tomography is an effective method to study the velocity structure inside the earth.Body wave tomography and surface wave tomography have developed rapidly in recent years.The seismic surface wave propagates along the earth's surface,and the dispersion curve of medium and long period can be extracted from the seismic surface wave.The cross correlation technique can be used to extract the dispersion curves of medium and short period Rayleigh wave group velocity from the noise data,so as to study the crustal scale S-wave velocity structure.Body wave data can obtain better results for areas with better ray coverage.The P-wave and S-wave velocity structure models with higher accuracy can be obtained by using joint inversion of body wave and surface wave data.Yinchuan Basin is located in the compound intersection of Tibetan Plateau,Alxa and Ordos block in China,which is affected by complex tectonic stress.A series of fault zones are developed in the Yinchuan Basin and its boundary.Therefore,this paper will use the data of the Himalayan II stations located in the study area,our Yinchuan station array and the fixed stations in the study area to carry out joint inversion,and obtain the P wave and S wave velocity structure models within the range of 0-10 km in this area.The main research contents and results are as follows:(1)The seismic phase reports obtained by the artificial intelligence method were relocated using the Hyposat localization method.In the event relocation,we only used the nearest 4 stations distributed within the event radius,and finally obtained a total of2,154 events,including 42,738 P wave phase and 31,946 S wave phase.The crosscorrelation calculation was carried out on the ambient noise data recorded by the station,and then the Rayleigh wave group velocity dispersion curves were extracted.The initial extracted dispersion curves were analyzed by clustering and combined with the terrain for quality control,and finally 2,736 dispersion curves were extracted.These dispersion data were used as surface wave data of joint inversion.The quality control of dispersion curve provides high quality surface wave data for joint inversion and ensures the accuracy of inversion results.(2)Velocity model of Yinchuan area.In this paper,the P-wave and S-wave velocity structure models in the 0-10 km depth range in the study area are obtained by using the joint inversion method of body wave and surface wave,and the results are compared with the results of double difference tomography and direct inversion of surface wave.It is found that the joint inversion has a better effect.Yinchuan Basin is a Cenozoic fault basin,with the eastern piedmont Helan Mountain fault zone on its west boundary,the Yellow River fault zone on its east boundary,the Niushou section fault zone on its south boundary,the Zongbeili-Zhengyiguan fault on its north boundary,and two concealed faults on the interior of the basin,namely Luhuatai fault and Yinchuan fault.The results show that the low-velocity distribution profile of Yinchuan Basin is in good agreement with the tectonic distribution characteristics of Yinchuan Basin and its surrounding faults due to its thick sedimentary layers.The eastern part of the adjacent Alxa block in Yinchuan Basin shows the distribution characteristics of weak high velocity and weak low velocity,while the western part of Ordos Basin shows the distribution characteristics of weak low velocity,which is consistent with the sedimentary layer distributed in the shallow part of the block.In the west of Yinchuan Basin is the Helan Mountain.The imaging results show that the mountainous area has obvious characteristics of high wave velocity,and the low speed on both sides of the mountainous area is in sharp contrast with its own high-speed characteristics.The sag and stretch of the Yinchuan Basin are formed by the NE-SW compression and NW-SE tensile stress,which are mainly driven by the expansion and extension of the Tibetan Plateau in northeast direction. |
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