High-resolution imaging of fault zone structure and its temporal changes can not only advance our understanding of earthquake physics,but is also critical for better seismic hazard preparation and mitigation.In the pa...High-resolution imaging of fault zone structure and its temporal changes can not only advance our understanding of earthquake physics,but is also critical for better seismic hazard preparation and mitigation.In the past a few years,we deployed multi-scale dense arrays across the Chenghai fault system in Binchuan,Yunnan,China.The first array consisted of 381 intermediate-period three-component seismometers with an average station spacing of~2 km.The array has been deployed in the field for~3 months in 2017 and recorded numerous local and teleseismic earthquakes.Travel time analyses based on teleseismic earthquakes and an airgun source in the region indicated clear signature of low-velocity fault zones in the southern branch of the Chenghai fault system.In 2018 we deployed two other linear arrays using the same instruments with much smaller inter-station spacing,e.g.30-50 m,across the southern branch the Chenghai fault.The profile lengths were 8 and 5 km,respectively.Record sections of the airgun source on the two linear arrays clearly marked a low-velocity zone(LVZ)within the southern array but no such signature in the northern array,suggesting along-strike variation of the LVZ.Although the instruments within our dense arrays had an intermediate frequency band,we demonstrated that they were capable of characterizing crustal structure with techniques commonly applied to broadband signals such as receiver functions.To our best knowledge,this was the first time to have multi-scale across-fault dense arrays with three-component seismometers in such apertures.These results lay out the pavement to comprehensively investigate fault zone structures as well as to derive subsurface structural changes using dense arrays and the airgun source.展开更多
From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associa...From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.展开更多
基金This work is supported by National Key R&D Program of China(2018YFC1503400)China Earthquake Science Experiment Project,CEA(grants no.2018CSES0101,2018CSES0102,2019CSES0107)+1 种基金HKSAR Research Grant Council GRF Grant 14305617,National Science Foun-dation of China(grants 41774071,41974069,41790463,and 41674058)Chen Yong Academician Workstation of Yunnan Province in China(2014IC007).
文摘High-resolution imaging of fault zone structure and its temporal changes can not only advance our understanding of earthquake physics,but is also critical for better seismic hazard preparation and mitigation.In the past a few years,we deployed multi-scale dense arrays across the Chenghai fault system in Binchuan,Yunnan,China.The first array consisted of 381 intermediate-period three-component seismometers with an average station spacing of~2 km.The array has been deployed in the field for~3 months in 2017 and recorded numerous local and teleseismic earthquakes.Travel time analyses based on teleseismic earthquakes and an airgun source in the region indicated clear signature of low-velocity fault zones in the southern branch of the Chenghai fault system.In 2018 we deployed two other linear arrays using the same instruments with much smaller inter-station spacing,e.g.30-50 m,across the southern branch the Chenghai fault.The profile lengths were 8 and 5 km,respectively.Record sections of the airgun source on the two linear arrays clearly marked a low-velocity zone(LVZ)within the southern array but no such signature in the northern array,suggesting along-strike variation of the LVZ.Although the instruments within our dense arrays had an intermediate frequency band,we demonstrated that they were capable of characterizing crustal structure with techniques commonly applied to broadband signals such as receiver functions.To our best knowledge,this was the first time to have multi-scale across-fault dense arrays with three-component seismometers in such apertures.These results lay out the pavement to comprehensively investigate fault zone structures as well as to derive subsurface structural changes using dense arrays and the airgun source.
基金supported by the National Natural Science Foundation of China (Grant Nos. 91014006, 90914005 & 41474073)
文摘From the 1960 s to 1970 s, North China has been hit by a series of large earthquakes. During the past half century,geophysicists have carried out numerous surveys of the crustal and upper mantle structure, and associated studies in North China.They have made significant progress on several key issues in the geosciences, such as the crustal and upper mantle structure and the seismogenic environment of strong earthquakes. Deep seismic profiling results indicate a complex tectonic setting in the strong earthquake areas of North China, where a listric normal fault and a low-angle detachment in the upper crust coexist with a high-angle deep fault passing through the lower crust to the Moho beneath the hypocenter. Seismic tomography images reveal that most of the large earthquakes occurred in the transition between the high-and low-velocity zones, and the Tangshan earthquake area is characterized by a low-velocity anomaly in the middle-lower crust. Comprehensive analysis of geophysical data identified that the deep seismogenic environment in the North China extensional tectonic region is generally characterized by a low-velocity anomalous belt beneath the hypocenter, inconsistency of the deep and shallow structures in the crust, a steep crustalal-scale fault,relative lower velocities in the uppermost mantle, and local Moho uplift, etc. This indicates that the lithospheric structure of North China has strong heterogeneities. Geologically, the North China region had been a stable craton named the North China Craton or in brief the NCC, containing crustal rocks as old as ～3.8 Ga. The present-day strong seismic activity and the lower velocity of the lower crust in the NCC are much different from typical stable cratons around the world. These findings provide significant evidence for the destruction of the NCC. Although deep seismic profiling and seismic tomography have greatly enhanced knowledge about the deep-seated structure and seismogenic environment, some fundamental issues still remain and require further work.
