When propagating through anisotropic rocks in the crust, shear-waves split into faster and slower components with almost orthogonal polarizations. For nearly vertical propagation the polarization of fast shear- wave ...When propagating through anisotropic rocks in the crust, shear-waves split into faster and slower components with almost orthogonal polarizations. For nearly vertical propagation the polarization of fast shear- wave （PFS） is parallel to both the strike of the cracks and the direction of maximum horizontal stress, therefore it is possible to use PFS to study stress in the crust. This study discusses several examples in which PFS is applied to deduce the compressive stress in North China, Longmenshan fault zone of east edge of Tibetan plateau and Yunnan zone of southeast edge of Tibetan plateau, also discusses temporal variations of PFS orientations of 1999 Xiuyan earthquake sequences of northeastern China. The results are consistent to those of other independent traditional stress measurements. There is a bridge between crustal PFS and the crustal principal compressive stress although there are many unclear disturbance sources. This study suggests the PFS results could be used to deduce regional and in situ principal compressive stress in the crust only if there are enough seismic stations and enough data. At least, PFS is a useful choice in the zone where there are a large number of dense seismic stations.展开更多
We applied the g CAP algorithm to determine 239 focal mechanism solutions 3：0≤MW≤ 6：0） with records of dense Chin Array stations deployed in Yunnan,and then inverted 686 focal mechanisms（including 447 previous r...We applied the g CAP algorithm to determine 239 focal mechanism solutions 3：0≤MW≤ 6：0） with records of dense Chin Array stations deployed in Yunnan,and then inverted 686 focal mechanisms（including 447 previous results） for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum（r1） and minimum（r3） principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations（i.e., maximum and minimum horizontal compressive stress axes： S H and S h, respectively） accordingly and found an abrupt change near ＊26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress（i.e., maximum strain axis） S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further toNE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.展开更多
The heat flow in crust and the thermal stress generated by the flow play a very important role in earthquake occurrence. Different crustal structure has different effect on heat distribution and associated thermal str...The heat flow in crust and the thermal stress generated by the flow play a very important role in earthquake occurrence. Different crustal structure has different effect on heat distribution and associated thermal stress. In all of typical seismogenic crustal structure models, including the bulge of Moho surface, the deep-large fault in the mid-lower crust, low-velocity and high-conductive layer in the middle crust, and the typical crustal structure in mid-upper crust, the thermal stress produced by deep heat disturbance may move up to the mid-upper crest. This leads to upper brittle part of crust break and hence, strong earthquakes. This result is constructive in enhancing our understanding of the role of deep fieat flow in curst in development of earthquake and its generation, as well as the generation mechanism of the shallow flowing fluid.展开更多
The upper crustal anisotropy of Yunnan area, SE margin of Tibetan Plateau, is investigated by measuring the shear wave splitting of local earthquakes. The mean value of the measured delay times is 0.054 s and far less...The upper crustal anisotropy of Yunnan area, SE margin of Tibetan Plateau, is investigated by measuring the shear wave splitting of local earthquakes. The mean value of the measured delay times is 0.054 s and far less than that from Pms splitting analysis, indicating that the crustal anisotropy is contributed mostly from mid-lower crust. The fast polarization directions are mostly sub-parallel to the maximum horizontal compression directions while the stations near fault zones show fault-parallel fast polarization directions, suggesting both stress and geological structure contribute to the upper crust anisotropy.Comparing fast polarization directions from shear wave splitting of local earthquakes and Pms, large angle differences are shown at most stations, implying different anisotropy properties between upper and mid-lower crust. However, in southwestern Yunnan, the fast polarization directions of Pms and Swave splitting are nearly parallel, and the stress and surface strain rate directions show strong correlation, which may indicate that the surface and deep crust deformations can be explained by the same mechanism and the surface deformation can represent the deformation of the whole crust. Therefore,the high correlation between surface strain and mantle deformation in this area suggests the mechanical coupling between crust and mantle in southwestern Yunnan. In the rest region of Yunnan, the crustmantle coupling mechanisms are supported by the lack of significant crustal anisotropy with Ne S fast polarization directions from Pms splitting. Therefore, we conclude that the crust and upper mantle are coupled in Yunnan, SE margin of Tibetan Plateau.展开更多
基金supported by International Science and Technology Cooperation Program of China(2010DFB20190)National Natural Science Foundation of China(41040034 and 41174042)the support by basic research project of Institute of Earthquake Science,China Earthquake Administration(2009IES0211)
文摘When propagating through anisotropic rocks in the crust, shear-waves split into faster and slower components with almost orthogonal polarizations. For nearly vertical propagation the polarization of fast shear- wave （PFS） is parallel to both the strike of the cracks and the direction of maximum horizontal stress, therefore it is possible to use PFS to study stress in the crust. This study discusses several examples in which PFS is applied to deduce the compressive stress in North China, Longmenshan fault zone of east edge of Tibetan plateau and Yunnan zone of southeast edge of Tibetan plateau, also discusses temporal variations of PFS orientations of 1999 Xiuyan earthquake sequences of northeastern China. The results are consistent to those of other independent traditional stress measurements. There is a bridge between crustal PFS and the crustal principal compressive stress although there are many unclear disturbance sources. This study suggests the PFS results could be used to deduce regional and in situ principal compressive stress in the crust only if there are enough seismic stations and enough data. At least, PFS is a useful choice in the zone where there are a large number of dense seismic stations.
基金supported by the National Natural Science Foundations of China (No.41204040)China National Special Fund for Earthquake Scientific Research in Public Interest (Nos.201008001, 201308011)Most figures were made using GMT (Wessel et al.2013)
文摘We applied the g CAP algorithm to determine 239 focal mechanism solutions 3：0≤MW≤ 6：0） with records of dense Chin Array stations deployed in Yunnan,and then inverted 686 focal mechanisms（including 447 previous results） for the regional crustal stress field with a damped linear inversion. The results indicate dominantly strike-slip environment in Yunnan as both the maximum（r1） and minimum（r3） principal stress axes are sub-horizontal. We further calculated the horizontal stress orientations（i.e., maximum and minimum horizontal compressive stress axes： S H and S h, respectively） accordingly and found an abrupt change near ＊26°N. To the north, S H aligns NW-SE to nearly E-W while S h aligns nearly N-S. In contrast, to the south, both S H and S h rotate laterally and show dominantly fan-shaped patterns. The minimum horizontal stress（i.e., maximum strain axis） S h rotates from NW-SE to the west of Tengchong volcano gradually to nearly E-W in west Yunnan, and further toNE-SW in the South China block in the east. The crustal strain field is consistent with the upper mantle strain field indicated by shear-wave splitting observations in Yunnan but not in other regions. Therefore, the crust and upper mantle in Yunnan are coupled and suffering vertically coherent pure-shear deformation in the lithosphere.
基金National Eleventh Five-year Science and Technology Development Foundation (2006BAC01B02-03-01).
文摘The heat flow in crust and the thermal stress generated by the flow play a very important role in earthquake occurrence. Different crustal structure has different effect on heat distribution and associated thermal stress. In all of typical seismogenic crustal structure models, including the bulge of Moho surface, the deep-large fault in the mid-lower crust, low-velocity and high-conductive layer in the middle crust, and the typical crustal structure in mid-upper crust, the thermal stress produced by deep heat disturbance may move up to the mid-upper crest. This leads to upper brittle part of crust break and hence, strong earthquakes. This result is constructive in enhancing our understanding of the role of deep fieat flow in curst in development of earthquake and its generation, as well as the generation mechanism of the shallow flowing fluid.
基金supported by the National 973 Project of China (No.2013CB733303)the open fund of Key Laboratory of Geospace Environment and Geodesy,Ministry of Education (No.15-02-07)
文摘The upper crustal anisotropy of Yunnan area, SE margin of Tibetan Plateau, is investigated by measuring the shear wave splitting of local earthquakes. The mean value of the measured delay times is 0.054 s and far less than that from Pms splitting analysis, indicating that the crustal anisotropy is contributed mostly from mid-lower crust. The fast polarization directions are mostly sub-parallel to the maximum horizontal compression directions while the stations near fault zones show fault-parallel fast polarization directions, suggesting both stress and geological structure contribute to the upper crust anisotropy.Comparing fast polarization directions from shear wave splitting of local earthquakes and Pms, large angle differences are shown at most stations, implying different anisotropy properties between upper and mid-lower crust. However, in southwestern Yunnan, the fast polarization directions of Pms and Swave splitting are nearly parallel, and the stress and surface strain rate directions show strong correlation, which may indicate that the surface and deep crust deformations can be explained by the same mechanism and the surface deformation can represent the deformation of the whole crust. Therefore,the high correlation between surface strain and mantle deformation in this area suggests the mechanical coupling between crust and mantle in southwestern Yunnan. In the rest region of Yunnan, the crustmantle coupling mechanisms are supported by the lack of significant crustal anisotropy with Ne S fast polarization directions from Pms splitting. Therefore, we conclude that the crust and upper mantle are coupled in Yunnan, SE margin of Tibetan Plateau.
