Broadband Seismic Ambient Noise Tomography In Typical Regions Of Northeast China

The methodology of ambient noise tomography to research on crustal structure is that extracting the Green's Functions between seismic stations from long-time cross-correlation calculation of ambient noise recorded by these two station pairs,obtaining characteristics of surface wave dispersion,and inverting for surface wave tomography.The broadband seismography is essential equipment when we use the ambient noise tomography to detect the accurate underground structure.Firstly,ambient noise data,observed in South China at 10 broad-band seismographs developed by Jilin University,are calculated by the cross correlation and power spectral density,and the corresponding results were all analyzed.The analysis results are as follows: the strong asymmetries in amplitude of cross-correlation results imply dominant sources toward the southwest of the deployment;diurnal strong variation characteristics of ambient noise's power spectral density in high frequency component accord well with day activity regularity of human beings;while noise's power spectral density in low frequency component are consistent with each station,and this is to be expected as a result of an effect of the collision between sea waves and continental coasts..This laid the foundations for algorithm research of deep exploration that uses the data recorded by self-developed seismograph.Crustal structures beneath Jinlin Province,Liaoning Province and Bohai Sea using seismic stations from national seismic network and regional seismic network based on ambient noise tomography are studied.The crustal structure beneath China's Jingling Province and Liaoning Province based on ambient noise tomography are conducted,and the crustal structure beneath China's Liaoning Province and Bohai Sea based on ambient noise tomography are conducted again.According to the distribution characteristics of broadband seismic stations and uniformity of their ray distribution,the both two regions we studied cover Liaoning Province.Particularly,the imaging results of volcanic areas,geothermal areas,earthquake prone area and special geological area are analyzed and interpreted and inferred.We imaged the cruststructure beneath Jilin Province and Liaoning Province in China with fundamental mode Rayleigh waves recorded by 61 broad-band stations deployed in the region from January 2013 to December 2013.The resulting phase velocity maps show clear lateral variations and a good correlation between phase maps in short periods and known geological features.Subsequently,the phase-dispersion curves extracted from each cell of the 2D phase velocity maps were inverted to determine the 3D SV wave velocity structures of the crust.Then,the SV-wave velocity structures inverted from Rayleigh wave dispersion maps with a depth down to 42 km beneath the Tianchi Volcano,the Jinlongdingzi Volcano,and the Dunmi Fault crossing the Jingbohu Volcano were presented.The results are summarized below: a ring-like low-velocity anomaly is distributed from the surface down to approximately 12 km under the Tianchi Crater;the results also demonstrate the existence of a low-velocity layer under the Longgang-Jinlongdingzi Volcano with the centre of 126.5°E,42°N in the middle crust along the WE direction.Because shear wave velocities are sensitive to high temperatures,the special geological setting is also taken into account.We believe that all the above-mentioned low velocities were caused by geothermal anomalies.The shallow earthquakes in 2010-2015 years in southern Liaoning occurred in the shallow crustal,and there is a correspondence with the southeastern slope of the upper mantle uplift belt.The velocity structure of the crust beneath Liaoning Province and the Bohai Sea in China was also imaged using three-component time series for the 12-month span between January and December 2013 recorded at 73 regional broad-band stations.The resulting Rayleigh wave and Love wave phase velocity maps generally show good correlations with major geological and tectonic units in the study region.Next,phase dispersion curves of the Rayleigh wave and the Love wave extracted from each cell of the 2D Rayleigh wave and Love wave phase velocity maps,respectively,were inverted simultaneously to determine the 3D shear wave velocity structures.And geological implications of the 3D shear wave velocity structures and relationship to the time-space distribution of earthquake swarms were discussed.Themain results are as follows: the earthquake swarms tend to concentrate in the high-low velocity boundaries in the upper crust coinciding with fault zones in the Haicheng region and the Tangshan region,and they occur along these predominant faults or along the extension traces of these faults;the lower crust downward to the uppermost mantle are featured by high velocities,with even a high-velocity thinner layer existing at the bottom of the lower crust near Moho,in the central and northern the Bohai Sea along the Tanlu Fault.These phenomena could be caused by the instrusion of mantle material ascending along the Tanlu Fault.The processing of broadband seismic data recorded by self-developed broadband seismograph deployed in South China tends to be a preliminary exploration and studies for self-developed broadband seismograph being applied to broadband array and its broadband record being processed by ambient noise tomography and other related method,and pave the way for the further related data processing.The analysis results of ambient noise tomography in this paper provide reference for geothermal exploration,provide basic phenomena for predicting earthquake occurrence regularity,and strengthen the understanding of special geology such as the geology in Bohai region.