Crustal Structure By Ambient Noise Tomography And Receiver Function Methods Beneath The Anyuan Coal Mine And Its Adjacent Areas,Jiangxi Province

The Anyuan Coal Mine is located in Pingxiang City,Jiangxi Province.It is situated in western Pingxiang-Leping(Pingle)Depression of the Qinhang metallogenic belt,which is the collisional junction of the Yangtze block and the Cathaysia block.Due to complex tectonic stress metamorphism,there are lots of nappe structures developed in this area.In order to study the structure of the Earth’s inner crust beneath the Anyuan coal mine and its adjacent areas,two kinds of passive source detection techniques based on dense array are applied successively to obtain the high-resolution 3-D velocity structure in the shallow subsurface and the velocity structure below the stations in deep crust.Firstly,the ambient noise imaging method is applied to cross-correlate the continuous ambient noise data,recorded by 141 sets of short-period seismographs and 13 sets of broadband seismographs in the study area.After extracting Rayleigh wave group velocity dispersion curves from 0.5s to 5s period,direct inversion of surface wave dispersion based on ray tracing is used to obtain3-D S-wave velocity structures at different depths from the surface to about 3km underground in the study area.Then,the high-quality P-wave receiver function waveform is extracted by using the receiver function method,and the crustal thickness and Poisson’s ratio characteristics under the 12 broadband instruments are obtained by using the H-κ-c stacking method based on harmonic fitting correction.The crust-mantle 1-D S-wave velocity structure image and discontinuity surface information below the stations are obtained by using the time-domain linear inversion method.The main results are as follows:(1)The Fine 3-D velocity structure below the shallow part of the study area shows that there are obvious low-velocity anomalies in the shallow surface of coal mining areas and fault zones influenced by sedimentary structure in the Pingle Depression and coal mining activities.An obvious low-velocity anomaly zone is formed by the Anyuan fault and the Wangkeng fault,which may even extend to the depth of about 3km.The analysis shows that the shallow S-wave low-velocity anomaly above 1.2km depth is caused by the sedimentary environment of coal mine and coal mining activities,while the S-wave low-velocity anomaly below 1.2km depth is the comprehensive result of fault medium in the fault zone,oil and gas storage.S-wave velocity difference characteristics near the Anyuan fault indicate that the Anyuan fault interface tends to northwest dip,extending to about 3km underground.According to the shallow 1km vertical S-wave velocity anomaly images,it is speculated that the maximum mining depth of the Anyuan Coal Mine reaches approximately 0.7km underground.(2)The crustal thickness of the Anyuan coal mine and its adjacent areas ranges from 32 to34.8km,with an average thickness of 33 km,which is consistent with the crustal thickness results in South China obtained by previous studies using receiver function method.The internal crustal medium Poisson’s ratio is relatively low,with an average dominant range from 0.22 to 0.25.It indicates that the inland block of the study area is relatively stable The crustal structure below the station can be roughly divided into three layers.The first layer is mainly composed of sedimentary rocks within a depth of about 1.5km,with a low velocity.The second layer is about 10～15km depth,with the average velocity about 3.2km/s,which is inferred to be the velocity interface discontinuity surface inside the crust.The third layer interface is the boundary surface between the crust and the mantle inside the Earth,the Moho interface,with an average depth of about 33～35km.The velocity fluctuation at the interface changes significantly,with an upper surface S-wave velocity from 3.4 to 3.6km /s and the bottom one about 4km/s.