Background Noise Surface Wave For Three-dimensional Velocity Structure Based On The Qilian Mountain Active Source Array

Surface wave imaging,especially noise surface wave imaging,has become an important method to study the crustal velocity structure.The method uses the noise signal as the data source,the noise signal has the influence of the non-uniformity of the seismic location and the seismic distribution,and its resolution mainly depends on the distribution of the station.In the case of rational distribution of the station,the noise surface wave imaging can achieve continuous monitoring of the underground three-dimensional media,which can improve the resolution of the speed structure,especially to improve the time scale resolution,in the seismology has been widely used?In this paper,the Qilian-Haiyuan belt of the Qinghai-Tibet block and the Alashan block is taken as the research area,which is located in the northeastern margin of the Qinghai-Tibet Plateau with strong tectonic activity and frequent seismicity.The study of the S-wave velocity structure in this region is of great significance to the continental dynamic process of the Qinghai-Tibet Plateau.A continuous background noise record of 40 flow stations and 38 fixed stations along the Qilian Mountains fault zone was collected.The data were recorded from August 2014 to December 2015 for 17 months.Taiwan range from more than ten kilometers to more than 700 kilometers,the station basically covers the Qinghai-Tibet block and Alashan plots of the extrusion zone area.Based on the cross-correlation function of all possible station pairs,the law of the noise source energy of different period ranges in different orientations with different seasons is analyzed by normalized amplitude method.The S wave velocity distribution images of the Rayleigh surface wave velocity are extracted from the cross-correlation function.The S-wave velocity distribution in the near-surface to the depth of 40 km is obtained by the direct inversion method of surface wave dispersion based on ray tracing.The main conclusions are as follows:(1)In the summer,0.5 ~ 5s cycle background noise energy direction of the direction of 35 ° ~ 170 ° and 190 ° ~ 250 °,winter was 10 ° ~ 130 ° and 270 ° ~ 350 °,the amplitude of the band in the winter Higher than the summer,may be due to the influence of sediments and strong interference near the station,the extraction of short-period surface wave signal is weak,it is still difficult to determine the noise source of this period,if it is by the waves and the coast,The summer noise source should come mainly from the Pacific and Indian Ocean marine activities,mainly from the Pacific Ocean and the North Atlantic Ocean activities.5 ~ 10 s cycle in the summer background noise energy source advantage direction 110 ° ~ 170 °,winter was 300 ° ~ 350 °,but in 110 ° ~ 150 ° azimuth also exists relatively weak energy distribution,the second microseismic summer Mainly from the South Pacific Ocean activities,the winter mainly from the North Atlantic Ocean activities.10 ~ 20 s cyclein the summer background noise energy source advantage direction 70 ° ~ 150 ° and170 ° ~ 230 °,in the winter are concentrated in the 290 ° ~ 350 ° and 70 ° ~ 130 °position,indicating that the first microseismic noise energy Summer mainly from the Pacific Ocean and the Indian Ocean,the winter mainly from the North Atlantic and the Pacific.That is,the background noise source of 5 ~ 20 s period shows obvious seasonal variation characteristics in time.The cross-correlation function of the surface wave in the 20 ~ 40 s period is smaller than that of the season.It can be considered that the noise origin of this period is different from that of the 5 ~ 20 s period,and the noise source is similar to the uniform distribution.That the long-period noise is formed by the interaction of the atmosphere,the ocean and the seabed near the coast.(2)The lateral crustal velocity structure in the Qilian-Haiyuan belt has horizontal heterogeneity,and the lateral variation of the S-wave velocity is closely related to the distribution of the surface structure.The distribution characteristics of the low-velocity area are mainly affected by the thickness of the sedimentary layer,The S-wave velocity is lower in the thicker sedimentary layer.At the depth of 5 km,the distribution of velocity structure is related to the distribution of surface tectonics,and the thickness of loose sediment layer is obvious.At 18 km depth,the transverse heterogeneity of S wave velocity is obvious,which is different Of the regional.At the same time,the velocity in the depth range of 5 ~ 20 km increases gradually with the depth,and the variation range is 0.2 km / s.At 30 km depth,the distribution of high and low speed in Qilian and Mentouyuan is obvious.Depth,with the depth increases,the impact of shallow sediments gradually disappear,the Qilian Mountains north fault zone,Toledo fault,cold dragon ridge fault zone to the north of the high-speed area,south of the low-speed area,which may be different plots In the deep border of a performance,it may be the Qinghai-Tibet Plateau to the NE movement and deformation caused.(3)In the study area,the velocity of the upper crust and the middle crust increases with the depth,and the S-wave velocity structure with the normal gradient increases in accordance with the velocity variation of the normal continental crust.Based on the analysis of the relationship between S-wave velocity distribution and source-source earthquakes,the results show that the source region is a low-velocity region before the onset of the source earthquake,and the epicentral position is in the obvious region.The use of noise imaging technology to study the low velocity variation of the earthquake breeding process provides a new way for earthquake prediction.