| Continental dynamics is a new theoretical and practical exploration of continental lithosphere.Continental lithosphere deformation and its dynamics is one of the important research contents of continental dynamics.Continental deformation occurs not only in the continental interior,but also in the continental margin.The comparative study of deformation law and dynamic mechanism between the two is an effective method for further understanding the structure,deformation and dynamic mechanism of continental lithosphere.The NE Tibetan Plateau is located in the continental interior of the collision zone between the Indian plate and the Eurasian plate,whereas California is located in the eastern part of the circum-Pacific seismic belt,which belongs to the continental system of complex subduction tectonic system.The two regions are investigated in this work applying seismic tomography to study the 3-D velocity structure and seismic anisotropy of the crust and upper mantle.On the basis of the present results and previous findings,the relationship between structural heterogeneities and seismogenesis is discussed,and the similarities and differences between the continental deep structure and large earthquake generation,as well as the deep dynamic process beneath the different continental tectonic location are further investigated.To understand the inner continental deformation,we have studied the 3-D P-and S-wave velocity(Vp,Vs)structure,Poisson’s ratio and P-wave anisotropy distribution in the crust and uppermost mantle in and around the source zone of the 2021 Maduo earthquake(M7.3).A total of 11,235 P-and S-wave arrival data recorded at 67 stations in this area are used.Our results show that the main shock of the Maduo earthquake occurred due to the remote effect of the Indian plate extrusion,the rigid block extruded eastward along the Kunlun faults,causing the secondary faults next to the main fault to generate new faults.The main shock of the Maduo earthquake occurred in an anomaly of low Vs and high Poisson’s ratio,reflecting crustal fluids that affected the rupture nucleation.The results of anisotropy show that at 40 km depth in the south of the study area,the fast-velocity direction(FVD)is NW–SE,and the anisotropy is mainly controlled by the collision and compression between India and Eurasia.At 60 km depth under the study region and at 40 km depth under the northern part of the region,the FVDs are NE–SW to N–S,reflecting lower crustal flow.The E–W FVDs at 60 km depth beneath the Qilian mountain range reflect the lower crustal flow that is blocked by the adjacent rigid terrain.The lower crustal flow may lead to crust-mantle decoupling in the study region.The California area is located in the continental margin tectonic zone.The subduction and shear coexist in northern California,while the shear dominates in southern California.Firstly,we applied local seismic tomography to 159,754 P-wave and 101,751 S-wave arrival times of 5,287 local earthquakes to determine 3-D images of Vp,Vs,Poisson’s ratio(ν)and Vp azimuthal anisotropy in the source zone of the 2019 Ridgecrest earthquake in Southern California.Its big foreshock(M6.4)and mainshock(M7.1)took place in a highvelocity(high-V)and low-velocity(low-V)transition belt.The source zone exhibits significant low-Vs and high-ν anomalies in the lower crust and uppermost mantle,probably reflecting crustal fluids associated with upwelling mantle flow.A high consistency between the seismogenic fault of the 2019 Ridgecrest earthquake and a low-Vs and high-ν anomaly suggests that the fluids in the fault zone affected the rupture process.The FVDs of Vp azimuthal anisotropy in the upper crust reflect effects of faults and cracks,whereas mineral alignments affect the FVDs in the low-V parts of the lower crust.Dominant NW–SE and E–W FVDs are revealed in the study area,indicating that the Pacific-North American plate relative motion causes the crustal deformation of the study area.Then,the Mendocino triple junction(MTJ)is the confluence of the Pacific plate,the Golda plate and the North American plate.MTJ is the boundary of a subduction zone in the north and a transform fault zone in the south.The northward migration of MTJ has a great impact on the superficial geomorphology and deep structure of California.We study the deep structure and mantle dynamics in the MTJ area using local and teleseismic tomography.The subducted Golda plate in the north is clearly revealed,and the high-V anomaly about 200 km east of the subduction zone is considered to be the subduction debris under the North American continent during the subduction of the Gorda plate.Another high-V anomaly is revealed at about 400 km depth below the south of the MTJ,which may reflect the subducted debris of an old plate.In addition,a slab window is clearly visible,and the upwelling mantle material through the window may mainly ascend from the asthenosphere beneath North American.Our Vp anisotropy results suggest that complex mantle flows exist under the North American plate.Finally,by comparing the deformation,seismogenic environment and deep dynamic process of continental interior and continental margin,it is found that the earthquakes in continental interior and continental margin are all related to fluid,but the fluid in continental interior is caused by the dehydration reaction of preexisting minerals,while that in continental margin is related to the dehydration of subduction oceanic crust.The continental deformation is indirectly controlled by the remote effect of plate interaction,while the continental deformation is directly related to plate interaction,and the deep dynamic processes of the two different continental positions are related to the plate tectonic system. |
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