Study On Pn Wave And Ambient Noise Tomography In The Middle-southern Segment Of Tan-Lu Fault And Its Adjacent Areas

The middle-southern segment of Tan-Lu fault and its adjacent areas(114°E~123°E,28°N ~ 38°N)mainly include the southeastern North China Craton,Dabie-Sulu high-pressure and ultrahigh-pressure(HP-UHP)metamorphic belt,Lower Yangtze fault block area,South China fold system and Jiaoliao fault block.The subduction of the Yangtze plate from south to north,intense collision,compression and exhumation formed the Tan-Lu fault zone and the Dabie and Sulu HP-UHP metamorphic belt,making them natural laboratories for studying intercontinental collision,geodynamics and lithospheric deformation.The destruction of the North China Craton,the more active seismicity and the unique tectonic phenomena of the world's largest and deepest exposed HP-UHP metamorphic belt are largely related to the crust-mantle structure of the region.In addition,there is a close relationship between the movement of material in the crust and mantle,the deformation and tectonic movement in the plate and seismic anisotropy.Therefore,by studying the crust-mantle P and S-wave velocity and their anisotropy in this region,it is helpful to analyze the formation and evolution mechanism of Tan-Lu fault zone and the mechanism of strong earthquakes.Firstly,the Pn phase data from the observation reports of the International Seismic Center,China Seismic Network and Regional Network are collected,and the average Pn wave velocity at the top of the upper mantle in the study area is obtained by the least square fitting method.Then through 9156 Pn seismic phase data and time-term tomography method,the Pn velocity and azimuthal anisotropy distribution in the study area were obtained.The resolution of velocity and azimuthal anisotropy is 1.0°×1.0° and 2.0°×2.0° respectively.Finally,the velocity distributions with and without anisotropy are compared.The correlation between Pn wave velocity and modern tectonic activity is discussed,and the relationship between Pn wave velocity and geothermal flow,epicenter distribution of strong earthquakes is compared.The azimuthal anisotropy of Pn wave is compared with SKS wave splitting.Secondly,by using the vertical component of continuous data,recorded by 261 broad-band stations of the China digital seismic network,Provincial and Municipal network from January 2015 to December 2016,we calculated the inter-station Green functions and measured the group velocity dispersion curves of Rayleigh wave by using the time-frequency analysis method.After passing quality control and strict screening,we finally obtained 15627 group velocity dispersion curves.The group velocity and azimuthal anisotropy distributions of 5~50s are obtained by inversion,and the 3-D S-wave velocity structure in the study area is reconstructed by inversion.The velocity resolution in the core area of the study area reached 0.5°×0.5° and that of the vast majority of the region is 0.75°×0.75°.The resolution scale of azimuthal anisotropy is 1.5°×1.5°.The group velocity distribution of 6 representative periods,S-wave velocity images of 6 representative depths and 7 longitudinal depth profiles in different directions are analyzed.These images reveal that the velocity structure of the crust and upper mantle in the study area has the characteristics of transverse block and vertical stratification.We not only compare the azimuthal anisotropy of surface wave with the SKS wave splitting,but also compare the azimuthal anisotropy characteristics of the similar period obtained by ambient noise method and seismic surface wave method;analyze the correlation between strong earthquake distribution and S-wave velocity;and discuss the P and S-wave velocity structure below the epicenter of Tancheng earthquake.The following scientific understandings are obtained in this paper:? Compared with the average velocity of 7.95 km/s at the top of the upper mantle,the velocity varies in the range of 7.68 km/s to 8.24 km/s.The velocity of Pn wave is inversely related to the topography and geomorphology of the surface.The mountain uplift areas exposed by bedrock,such as Luxi uplift,Sulu-Dabie HP-UHP metamorphic belt,Jiaoliao uplift,Jiangnan orogenic belt,South China fold system,Taihang Mountains and Yanshan Mountains,show P-wave low-speed anomalies,while the topography shows high-speed distribution characteristics under plain or depression basins,such as Subei-south Huanghai Basin,Bohai Bay Basin and Hehuai Basin.? The distribution of Pn wave velocity obtained by considering the influence of anisotropy is in better agreement with geological structure.But,without considering the influence of azimuthal anisotropy,there exists a NNE-SSW weak high-velocity anomaly zone in the Yellow Sea.Most of the strong earthquakes in the Yellow Sea occurred at the edge of the anomaly zone.We speculate that there is a deep and large fault zone cutting through through the Moho.? The distribution of Pn wave velocity along Tan-Lu fault zone has typical sectional characteristics.According to the results of velocity and the characteristics of geological structure,it can be divided into A(36.5°N~40°N),B(34.8°N~36.5°N),C(32.5°N~34.8°N)and D(30°N~32.5°N)from north to south.Among them,section A and C exist as the boundary of high and low speed,while section B and D present the characteristics of low speed distribution.Section D corresponds to Guangji-Jiashan section,while Section B+C corresponds to Jiashan-Weifang section.? The azimuthal anisotropy of Pn wave shows that the direction of fast wave is different in the east and west sides of Tan-Lu fault zone.The dominant direction of Pn wave in North China fault block on the west side of Tan-Lu fault is NW-SE,which is consistent with the direction of plate movement,while the dominant direction on the east side is NE-SW,which may be related to the large-scale shear of Tan-Lu fault.The dominant direction of Pn fast wave along Tan-Lu belt is NW-SE,which is consistent with its spatial distribution.? The horizontal and depth slices of S-wave velocity show that the crust-mantle S-wave velocity structure in the study area is characterized by transverse block and longitudinal stratification.The S-wave velocity distribution in the shallow part of the upper crust is well correlated with surface geology and topography.Basins with thick and loose sedimentary layers,such as Subei-south Huanghai Basin,Hehuai Basin,Bohai Basin and Hefei Basin,are characterized by low-velocity anomalies,while mountain or uplift areas where bedrock is widely exposed,such as Sulu-Dabie HP-UHP metamorphic belt,Jiangnan orogenic belt,Luxi uplift and South China fold system,show high-speed distribution.The middle crustal velocity has a certain inheritance and correlation with the upper crust,but the influence of the surface topography has been weakened.The S-wave velocity near the bottom interface of the lower crust is negatively correlated with the depth of the Moho surface,i.e.the Dabie,Sulu and North China fault blocks with thicker crust show low-velocity distribution,while the lower Yangtze fault blocks with thinner crust and the South China fold system show high-speed distribution.? Rayleigh wave azimuthal anisotropy has obvious spatial difference and significant lateral variation in different periods(depths).Typical regional studies of the North China and Yangtze plates show that from the shallow crust to the upper mantle,the anisotropic strength of the Yangtze plate increases from 1% to 6.5%,while that of the North China block fluctuates from 1.2% to 4.2%.It is interesting to note that the directions of surface wave fast waves in both plates have changed near the periodic T=17s(up and down the Conrad interface).The dominant direction of the Yangtze plate has changed from NW-SE to NNW-SSE,while that of the North China block has changed from NW-SE to NE-SW or near SN.Whether there is a certain correlation between the occurrence of earthquake in the crust and the changes in the direction of surface wave remains to be anylyzed after further work.? The Sulu and Dabie HP-UHP metamorphic belt not only have similar uplift geomorphological features on the surface,but also have the same S-wave velocity distribution structure in the deep crust,that is,the upper crust is relative high-speed and upwardly uplift;The middle crust is asymmetricly thickened;the lower crust bottom interface(Moho)is buried deep and has the shape of concave downward.The Pn wave velocities at the top of the upper mantle concide with each other,showing a low velocity distribution.The very similar P and S wave velocity distribution between the crust and mantle may imply that they were originally integrated and were staggered by the left-lateral translation of the Tan-Lu fault zone in the later stage.? By comparing and studying Pn wave velocity at the top of the upper mantle,S wave velocity and the spatial distribution characteristics of strong earthquakes,we find that most of the strong earthquakes occur in transitional zones of high and low velocity or in the low-speed zone.The stress near the high and low velocity boundaries is easy to concentrate and the stress difference is generated.Low-speed areas can be considered to be prone to ductile deformation and induce earthquakes,which may be more conducive to the transmission and migration of underground energy and material.? By comparing the azimuthal anisotropy obtained by Pn wave at the top of the upper mantle and ambient noise surface wave,it is found that although the two methods inconsistent,they have similar fast wave directions near the Moho surface,indicating that the anisotropic fast wave directions observed by different methods reflect the migration direction of the crust and mantle materials.? The epicenter of the Tancheng M8.5 earthquake in 1668 was located in the transition zone of Pn wave high and low velocity.There are S-wave high-speed bodies in the middle and lower crust below the epicenter,and they are in the shape of flexural uplift.According to the characteristics of S-wave velocity distribution in the crust and the seismic sounding and deep seismic reflection profiles completed by predecessors,we infer that the focal depth of Tancheng earthquake may be about 15 km.