| Fault zones are closely linked to earthquakes and mineralization,making the study of fault zone structures crucial for mitigating earthquake damages and mineral exploration.Therefore,the detection of fault zone structures is a research hotspot in geophysics.Geophysical explorations,such as gravity,magnetism,electricity,and seismic waves,are commonly used in fault zone studies.Among them,seismic exploration plays a significant role due to the rich information carried by seismic waves.Seismic waves generally have three sources:earthquakes,ambient noise and active sources.Earthquakes contain vast amount of energy and the seismic waves can propagate a long way.Ambient noise data is relatively easy to obtain.The time,location,and frequency of seismic waves generated by active sources can be controlled.These three seismic sources contribute considerably to seismological research.Specifically,active sources such as airguns and methane gaseous sources are environmentally friendly.These sources have great advantages in high-resolution underground detection.The Anninghe fault zone,Changle Zhao’an fault zone,Binhai fault zone,and Tanlu fault zone are important active fault zones in China that have experienced large earthquakes and are still in potential danger of large earthquakes.The Middle-Lower Yangtze River metallogenic belt,a significant mineral resource base in China,is closely related to the Tanlu fault zone and the Changjiang deep fault systems.In this thesis,we perform isotropic and anisotropic tomography using seismic data from active sources(airguns and methane gaseous sources),earthquakes,and ambient noise,and analyze the activities of these fault zones and their control over the surrounding geological structures.There are four study regions in this thesis.(1)We obtain three vertical velocity profiles of the Anninghe fault zone(between Mianning and Xichang)based on the inversion of P-wave first arrival times picked from seismic waves generated by methane gaseous sources.The length and depth of the profiles are about 8 km and 2 km,respectively.(2)We obtain P-and S-wave isotropic and P-wave azimuthally anisotropic velocity models(50 km × 50 km × 4 km)in Pingtan Island using P-and S-wave traveltime data from airguns.(3)We obtain P-wave isotropic and azimuthally anisotropic velocity models(200 km × 200 km × 10 km)in the Middle-Lower Yangtze River metallogenic belt using P-wave traveltime data from airguns.(4)We obtain Pand S-wave isotropic velocity models and Vp/Vs(500 km × 500 km × 40 km)in Middle-Lower Yangtze River region using seismic data from airguns,earthquakes and ambient noise.The shallow crustal structures of fault zones are mainly affected by lithology and state of rocks,which reflect the control of the fault zones.In the velocity profiles of the Anninghe fault zone(between Mianning and Xichang),the seismic velocities of different lithology show significant differences.Sedimentary rocks have low seismic velocities,whereas the igneous rocks and metamorphic rocks have high seismic velocities.The low-velocity anomalies distribute along the west side of the fault zone,which reflect the fracture zones beneath the fault zone.In Pingtan Island,the seismic velocity in sedimentary plains is relatively high since the rocks are hard and solid,whereas the volcanic eroded hills have relatively lower velocities due to fragmentation.Low-velocity anomalies distribute in the northern and southern part of the island,which reflect the influence of the fault zones.Tomography results in Middle-Lower Yangtze River region show that seismic velocities at deeper depths may be affected by more factors.In Hefei Basin,it exhibits low-velocity anomalies in the upper crust due to the thick sediment deposit.The depth of the deposit is larger near the Tanlu fault zone,which reflect the control of the fault zone.The high-velocity anomalies in the middle crust reveal its basement.And in the lower crust,the low-velocity anomalies are probably caused by magmatic activity.In the northeast part of the Dabie orogenic belt,it exhibits high-velocity anomalies in the upper crust and low-velocity anomalies at deeper depths.The high-velocity anomalies in the upper crust may reflect the highpressure/ultrahigh-pressure metamorphic rocks.The low-velocity anomalies in the middle and lower crust may reflect the ductile shear zones and/or brittle fracture zones developed during the exhumation of the high-pressure/ultrahigh-pressure metamorphic rocks.It may also reflect the magmatic activity controlled by Tanlu fault zone.The azimuthal anisotropy of the upper crust is mainly controlled by fault structures.In Pingtan Island,the anisotropic directions are consistent with the two large NEtrending active fault zones(Changle Zhao’an fault zone and Binhai fault zone),whereas in Haitan Strait,the anisotropic directions are the same as the strike of the strait.The magnitude of anisotropy decreases from south to north,which is consistent with the decreasing intensity of fault activities.In the Middle-Lower Yangtze River metallogenic belt and surrounding area,the anisotropic directions are generally NE-SW,consistent with the strike of the fault zones.They are in NW direction near the NW-striking faults.While in areas where tectonic with different strikes are superimposed,the anisotropic directions are more complicated.Fault zones have controlling effects on surrounding geological structures and the distribution of ore deposits.The Tanlu fault zone behaves like a boundary in tomography results.The properties differ significantly between Hefei Basin and the Zhangbaling uplift,and between the Dabie orogenic belt and the Middle-Lower Yangtze River metallogenic belt,which are located on both sides of Tanlu fault zones.In addition,fault zones are major channels for asthenosphere upwelling and control the regional magmatic activities.There is a high-velocity belt in the lower crust and the top of the upper mantle along the Middle-Lower Yangtze River metallogenic belt,which may reflect the magma intrusion along the weak zones of the Changjiang deep fault zone systems.The magma intrusions require pathways,which are mainly provided by fault zones,to reach the near-surface area.Tomographic results with high resolution show the upward uplift of high-velocity anomalies beneath the ore deposits,reflecting the magma upwelling shape and the fault zone structures.Our isotropic and anisotropic tomography in the Mianning-Xichang segment of Anninghe fault zone,Pingtan Island,the Middle-Lower Yangtze River metallogenic belt and the Middle-Lower Yangtze River region further reveal the velocity structure of fault zones from shallow depths to the top of the upper mantle,as well as the activities of these fault zones and their control over crustal deformation and magmatic activities.The high-resolution velocity structures are helpful in mitigating earthquake damages and mineral exploration.The airguns and methane gaseous sources have great advantages in seismic exploration.Joint inversion of data from active and passive sources can take advantage of the complementary strengths of each dataset and provide a better model. |
