Multi-Scale Crustal Velocity Structure Revealed By Artificial Sources And Dense Array

Seismic wave can penetrate through the entire Earth and hence provide an effective tool for studying the Earth's interior structure.Although natural earthquakes can generate strong seismic waves,they are often spatially and temporally localized and limited in number,which result in low resolution and hence have limited use for local crustal structure imaging.However,high-performance artificial sources can generate active seismic waves with accurate excitation time and location,with prominent advantage to image high-resolution subsurface structures.In the past few years,we carried out dense-array seismic experiments using artificial sources along the Yangtze River in Anhui,Binchuan in Yunnan and Zhuxi ore deposit in Jiangxi.In this thesis,we used body-wave traveltime tomography(TOMOG3D and simul2000 programs)to study the crustal velocity structure at different scales(regional,local and near-surface)based on artificial sources and dense arrays.Our studies provided a comprehensive evaluation on the imaging capability of artificial sources,laid a foundation for the “underground bright lamp” plan to illuminate the underground structure and solve major geological problems,and provided a basis for seismic disaster assessment and mineral resources exploration.In addition,we conducted preliminary joint inversions of artificial sources and natural earthquakes,and used the airgun source data to verify the reliability of inverted velocity model,which can be used for further in-depth joint inversion.The major achievements and innovations in this thesis are as follow:(1)P-and S-wave velocity structures beneath the southern segment of the Tan-Lu fault using the Anhui Airgun Experiment.We manually picked 4,118 P-wave and 1,906 S-wave first arrival times from the airgun signals and collected 28,957 P-wave and 26,257 S-wave first arrival times from local earthquakes listed in the China Earthquake Network Center(CENC)bulletin,and then imaged the three-dimensional(3D)crustal velocity structures beneath the southern segment of the Tan-Lu fault and surrounding areas using traveltime tomography(TOMOG3D program).Our results demonstrate that the mobile large-volume airgun sources are promising for 3D crustal structure surveys.The Vp and Vs crustal structures are consistent with the local geological settings,and earthquakes are primarily concentrated near faults and are spatially correlated with low-velocity zones.Large velocity contrasts are observed across the Tan-Lu fault zone,which is the main factor controlling local geophysical/geological anomalies.The ultra-high-pressure metamorphic rocks beneath the Qinling-Dabie orogenic belt and ore deposits beneath the Middle-Lower Yangtze River Metallogenic Belt are associated with high Vp,Vs and Vp/Vs bodies,which may be related to large-scale magmatism and mineralization caused by the Mesozoic lithospheric delamination and asthenospheric upwelling.(2)P-wave velocity structure of the Zhuxi ore deposit revealed by artificial sources and dense array.We picked 761,653 P-wave first arrivals recorded by the dense array from 998 artificial-source shots and obtained 3D shallow P-wave velocity images of the Zhuxi ore deposit using the simul2000 program.Our velocity model agrees with local topographic and tectonic structures and shows depth-dependent seismic velocity similar to those obtained in laboratory measurements.The Carboniferous and Permian formations hosting the known ore-bodies are imaged as high velocities,and the locations of high–low velocity boundaries are consistent with the boundaries between the Neoproterozoic and the Carboniferous–Triassic formations.Our results can be used to evaluate the total ore reserves and suggest that seismic tomography provides a useful tool for mineral exploration.(3)Upper crustal Vp and Vp/Vs structures of Binchuan,Yunnan revealed by dense array local seismic tomography.Based on the dense array observation experiment around Binchuan basin for three months in 2017,we picked 11,721 P-wave and 5,475 S-wave first arrivals from 96 small local earthquakes,and performed earthquake relocation and body-wave tomography using the simul2000 program.In addition,we used the airgun source data to verify the reliability of our velocity model.The obtained results show that earthquakes in the Binchuan basin are concentrated at the turning area of the eastern margin fault in the Binchuan basin and show a NE-trending band in the southern edge of the Erhai Lake.The velocity distribution at surface has a good correspondence with the Binchuan basin and the high–low velocity boundaries from 3 km to 9 km correspond well to local faults.Our results also reveal that the dipping angle of the eastern margin fault in the Binchuan basin is large in the northern and southern portions.However,the dipping angle is gentle in the middle portion,reflecting the rotation characteristics of the regional tectonic block.Our relocation and tomographic results indicate that near NS and NNE trending faults are gradually replacing the NW trending faults as the main boundary faults that control the seismic bebaviors in this region.(4)Upper crustal Vp structure and earthquake relocation of Binchuan,Yunnan revealed by airgun dense array.Based on the airgun dense array(G1 database),we picked 48,381 P-wave first arrivals from 2,344 small local earthquakes,and performed earthquake relocation and body-wave tomography using the simul2000 program.We also used the airgun source data to verify the reliability of our velocity model.Our results reveal that the P-wave velocity images are consistent with the high–low velocity anomaly distribution obtained by the dense array in 2017.The earthquakes after relocation are mainly distributed along the NS-NNE trending faults,such as the north section of the strike-slip Qina fault,the turning area of the eastern margin fault in the Binchuan basin,the Baitupo-Yanggongjing fault,and along four NE-trending bands in the southwest of Erhai fault.The earthquakes formed negative flower structure in depth direction,which is common for the tension-torsionall fault system.