| Seismic ambient noise method is a new seismological method developed rapidly in recent years.The ambient noise is extracted from long-term continuous waveform records of seismic stations and then the cross-correlation function is calculated between stations.The cross-correlation functions can be approximated as empirical Green's functions between stations.As a result,people gradually realize that ambient noise contains abundant information of underground structures.By measuring the travel time migration of cross-correlation function on different time scales,the crustal velocity structure of different periods can be obtained,which can reveal the distribution characteristics of underground media very well.In this thesis,we study the Mw7.1 earthquake in Mexico in 2017 by using the cross-correlation method of ambient noise of conventional double stations.The traveltime migration of cross-correlation function is extracted by Stretching method and MWCS method respectively.We calculate two years relative seismic velocity changes,found that the wave velocity is relatively stable as a whole,but the occurrence of earthquakes will cause sudden changes of wave velocity and shows inconsistent characteristics in different depths.Although both methods can observe this phenomenon,the results of MWCS are more jittery than Stretching method.In order to refine the results and obtain more detailed information of underground media,this paper tries to improve the signal-to-noise ratio of one-day cross-correlation function by using new superposition method and adaptive filtering respectively.It is found that phaseweighted stack PWS and time-frequency phase-weighted stack tf-pws,wavelet transform and wave-atom transform can effectively improve the cross-correlation signal-to-noise ratio and make the original wave velocity change result smoother.The results show that the Mexican earthquake caused about 0.04% wave velocity rise in the depth range of 2-10 km below the stations.In order to monitor the complex geological structures with frequent earthquakes in Sichuan area,the variation of relative velocity of underground media in Lushan and Kangding earthquakes is also studied by using the ambient noise correlation of 12 stations in the central part of the region for two and a half years.The average amplitude of wave velocity obtained is relatively stable,and the variation of co-seismic wave velocity caused by two earthquakes can be seen.Relatively speaking,the rupture amplitude of Lushan Earthquake is larger,which can cause the wave velocity of most stations to rise in the range.Kangding Earthquake has less influence,only a small range of stations can observe the wave velocity decline,and there is no obvious distribution characteristics.Although the accuracy of the results is not as high as that of the Mexican double stations,the time resolution of the two earthquakes can be effectively improved by using the method of PWS and waveatom transformation,which reflects the medium changes of the crust in a short time.From the results of the earthquake day,the Lushan earthquake caused a short-time increase of 0.15%,while the Kangding earthquake caused a decrease of 0.05%.Based on the research results of this paper and other types of data,the mechanism of wave velocity variation in medium is further discussed.The variation of wave velocity is discussed from stress and strain,shallow surface damage,aftershock distribution and fracture healing.It is considered that the main reason of wave velocity variation is the shallow crustal structure damage in the focal area caused by earthquakes or strong aftershocks,which changes the physical properties of rocks. |
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