Using seismic data recorded by Yunnan Telemetry Seismic Network from January 1, 2000 to December 31, 2003, the dominant polarization directions of fast shear-waves are obtained at 10 digital seismic stations by SAM te...Using seismic data recorded by Yunnan Telemetry Seismic Network from January 1, 2000 to December 31, 2003, the dominant polarization directions of fast shear-waves are obtained at 10 digital seismic stations by SAM technique, a systematic analysis method on shear-wave splitting, in this study. The results show that dominant directions of polarizations of fast shear-waves at most stations are mainly at nearly N-S or NNW direction in Yunnan. The dominant polarization directions of fast shear-waves at stations located on the active faults are consistent with the strike of active faults, directions of regional principal compressive strains measured from GPS data, and basically consistent with regional principal compressive stress. Only a few of stations.show complicated polarization pattern of fast shear-waves, or are not consistent with the strike of active faults and the directions of principal GPS compressive strains, which are always located at junction of several faults. The result reflects complicated fault distribution and stress field. The dominant polarization direction of fast shear-wave indicates the direction of the in-situ maximum principal compressive stress is controlled by multiple tectonic aspects such as the regional stress field and faults.展开更多
Systematic analyses of seismic data recorded by the Yunnan regional seismograph network reveal significant crustal and upper mantle anisotropy. Splitting of the S phase of local earthquakes and teleseismic SKS, PKS, a...Systematic analyses of seismic data recorded by the Yunnan regional seismograph network reveal significant crustal and upper mantle anisotropy. Splitting of the S phase of local earthquakes and teleseismic SKS, PKS, and SKKS phases indicates time-delays from 1.60 ms/km to 2.30 ms/km in the crust, and from 0.55 s to 1.65 s in the upper mantle which corresponds to an The polarization orientations of fast shear waves in direction, and the mantle anisotropy has a nearly styles and mechanisms exist between the crust and anisotropic layer with a thickness about between 55 165 km. the crust are complicated with a predominantly north-south west-east direction. Our results show different deformation upper mantle.展开更多
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
Seismic anisotropy and its main features along the convergent boundary between Africa and Iberia are detected through the analysis of teleseismic shear-wave splitting. Waveform data generated by 95 teleseismic events ...Seismic anisotropy and its main features along the convergent boundary between Africa and Iberia are detected through the analysis of teleseismic shear-wave splitting. Waveform data generated by 95 teleseismic events recorded at 17 broadband stations deployed in the western Mediterranean region are used in the present study. Although the station coverage is not uniform in the Iberian Peninsula and north- west Africa, significant variations in the fast polarization directions and delay times are observed at stations located at different tectonic domains. Fast polarization directions are oriented predominantly NW-SE at most stations which are close to the plate boundary and in central Iberia; being consistent with the absolute plate motion in the region. In the northern part of the Iberian Peninsula, fast velocity direc- tions are oriented nearly E--W; coincident with previous results. Few stations located slightly north of the plate boundary and to the southeast of lberia show E--W to NE-SW fast velocity directions, which may be related to the Alpine Orogeny and the extension direction in lberia. Delay times vary significantly between 0.2 and 1.9 s for individual measurements, reflecting a highly anisotropic structure beneath the recording stations. The relative motion between Africa and lberia represents the main reason for the observed NW-SE orientations of the fast velocity directions. However, different causes of anisotropy have also to be considered to explain the wide range of the splitting pattern observed in the western Mediterranean region. Many geophysical observations such as the low Pn velocity, lower lithospheric Q values, higher heat flow and the presence of high conductive features support the mantle flow in the western Mediterranean, which may contribute and even modify the splitting pattern beneath the studied region.展开更多
This paper investigates the propagation of horizontally polarised shear waves due to a point source in a magnetoelastic self-reinforced layer lying over a heterogeneous self-reinforced half-space. The heterogeneity is...This paper investigates the propagation of horizontally polarised shear waves due to a point source in a magnetoelastic self-reinforced layer lying over a heterogeneous self-reinforced half-space. The heterogeneity is caused by consideration of quadratic variation in rigidity. The methodology employed combines an efficient derivation for Green’s functions based on algebraic transformations with the perturbation approach. Dispersion equation has been obtained in the closed form. The dispersion curves are compared for different values of magnetoelastic coupling parameters and inhomogeneity parameters. Also, the comparative study is being made through graphs to find the effect of reinforcement over the reinforced-free case on the phase velocity. It is observed that the dispersion equation is in assertion with the classical Love-type wave equation in the absence of reinforcement, magnetic field and heterogeneity. Moreover, some important peculiarities have been observed in graphs.展开更多
We investigated the upper mantle anisotropy beneath China by applying teleseismic shear wave splitting measurements at 119 seismic stations from CDSN and GSN//RIS networks in China. The splitting observations are char...We investigated the upper mantle anisotropy beneath China by applying teleseismic shear wave splitting measurements at 119 seismic stations from CDSN and GSN//RIS networks in China. The splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains, including the Tianshan orogenic belt, Tibetan plateau, the Yangtze craton, the North China craton and northeastern region. In western China （Tianshan orogenic belt and Tibetan plateau）, fast polarization directions of split SKS waves coincide strikingly well with the dominating trend of deformational crustal features and delay times range from 0.5 s to 1.6 s. While in eastern China, seismic anisotropy deduced from shear wave splitting reveals a homogeneous NW-SE trending structure, almost perpendicular to the strike of large-scale surface structures. The observed delay times of 1.5 s to more than 2 s favor consistent mantle flow over large mantle thicknesses Based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks, we interpret the splitting results in terms of tectonic fabric within the upper mantle. Since the lithosphere is less than 100 km thick beneath eastern China and the observed fast directions are subparallel to the trend of the absolute plate motion （APM） of Eurasian plate, we propose that the asthenosphere may mainly contribute to the anisotropic effects beneath eastern China. However, the upper mantle anisotropy beneath western China may have developed more recently in the subcrustal lithosphere because of rather small delay times and thick lithosphere. We also use the opportunity of the dense geodetic measurements available in China to discuss the coupling between the crust and mantle. In the Eura- sia-fixed reference frame, GPS and shear wave splitting both depict a similar trend beneath eastern China, suggesting a lithospheric block ＂escaping＂ toward the east that could orient olivine [001 ] axis in the upper mantle. There is a strong cou- pling between the crust and the mantle in eastern China. A different behavior is observed in western China. The GPS vectors trend NS-NE in Tibet and NW in Tianshan, close to the regional compression direction, whereas the fast directions trend EW in Tibet and NE in Tianshan, suggesting a tectonic regime close to a mode of axial shortening, generating the development of EW-trending foliation in Tibet and NE-trending foliation in Tianshan at depth. The crust and mantle deform independently in western China.展开更多
A seismic hazard was assessed related to site effects at Abbadia San Salvatore, central Italy, on the Mt. Amiata slopes, an ancient volcanic area characterized by residual soils(thick layers of loose to dense sands or...A seismic hazard was assessed related to site effects at Abbadia San Salvatore, central Italy, on the Mt. Amiata slopes, an ancient volcanic area characterized by residual soils(thick layers of loose to dense sands originated from weathering of the trachydacitic lavas). The seismic ground amplification and soil liquefaction related to these layers were recognized as the major seismic hazards for the area.Geological, geophysical, and geotechnical surveys were carried out on the volcanic rocks. The Horizontal-to-Vertical Spectral Ratio(HVSR) analysis of 252 noise measurements and 29 shear-wave velocity models of the subsoil allowed a seismic microzonation of the studied area, distinguished by thick weathered volcanic sands and shear-wave impedance contrast with respect to the seismic bedrock(volcanic bedrock). The differentiation of classified zones allowed recognition of areas characterized by residual(almost undisturbed) soils from those with soils probably affected by flowing water. The analysis of hazards revealed that peak acceleration by seismic amplification of ground motion exceeded the value set by the national rules(0.175 g) in a restricted area of the zone characterized by the most perturbed soils(Zone D);the potential occurrence of soil liquefaction was also greater in this zone. Finally, the study showed potential high hazards due to site effects of the volcanic mountainous area characterized by residual soils as opposed to an alluvial plain formed by volcanic debris where these effects have generally been more recognized.展开更多
基金National Natural Science Foundation of China (40274011).
文摘Using seismic data recorded by Yunnan Telemetry Seismic Network from January 1, 2000 to December 31, 2003, the dominant polarization directions of fast shear-waves are obtained at 10 digital seismic stations by SAM technique, a systematic analysis method on shear-wave splitting, in this study. The results show that dominant directions of polarizations of fast shear-waves at most stations are mainly at nearly N-S or NNW direction in Yunnan. The dominant polarization directions of fast shear-waves at stations located on the active faults are consistent with the strike of active faults, directions of regional principal compressive strains measured from GPS data, and basically consistent with regional principal compressive stress. Only a few of stations.show complicated polarization pattern of fast shear-waves, or are not consistent with the strike of active faults and the directions of principal GPS compressive strains, which are always located at junction of several faults. The result reflects complicated fault distribution and stress field. The dominant polarization direction of fast shear-wave indicates the direction of the in-situ maximum principal compressive stress is controlled by multiple tectonic aspects such as the regional stress field and faults.
基金supported by National NaturalScience Foundation of China Project(No.41174042)China National Special Fund for Earthquake Scientific Research in Public Interest(No.201008001)Basic Research Project of Institute of Earthquake Science,CEA(No.2009-21)
文摘Systematic analyses of seismic data recorded by the Yunnan regional seismograph network reveal significant crustal and upper mantle anisotropy. Splitting of the S phase of local earthquakes and teleseismic SKS, PKS, and SKKS phases indicates time-delays from 1.60 ms/km to 2.30 ms/km in the crust, and from 0.55 s to 1.65 s in the upper mantle which corresponds to an The polarization orientations of fast shear waves in direction, and the mantle anisotropy has a nearly styles and mechanisms exist between the crust and anisotropic layer with a thickness about between 55 165 km. the crust are complicated with a predominantly north-south west-east direction. Our results show different deformation upper mantle.
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.
文摘Seismic anisotropy and its main features along the convergent boundary between Africa and Iberia are detected through the analysis of teleseismic shear-wave splitting. Waveform data generated by 95 teleseismic events recorded at 17 broadband stations deployed in the western Mediterranean region are used in the present study. Although the station coverage is not uniform in the Iberian Peninsula and north- west Africa, significant variations in the fast polarization directions and delay times are observed at stations located at different tectonic domains. Fast polarization directions are oriented predominantly NW-SE at most stations which are close to the plate boundary and in central Iberia; being consistent with the absolute plate motion in the region. In the northern part of the Iberian Peninsula, fast velocity direc- tions are oriented nearly E--W; coincident with previous results. Few stations located slightly north of the plate boundary and to the southeast of lberia show E--W to NE-SW fast velocity directions, which may be related to the Alpine Orogeny and the extension direction in lberia. Delay times vary significantly between 0.2 and 1.9 s for individual measurements, reflecting a highly anisotropic structure beneath the recording stations. The relative motion between Africa and lberia represents the main reason for the observed NW-SE orientations of the fast velocity directions. However, different causes of anisotropy have also to be considered to explain the wide range of the splitting pattern observed in the western Mediterranean region. Many geophysical observations such as the low Pn velocity, lower lithospheric Q values, higher heat flow and the presence of high conductive features support the mantle flow in the western Mediterranean, which may contribute and even modify the splitting pattern beneath the studied region.
文摘This paper investigates the propagation of horizontally polarised shear waves due to a point source in a magnetoelastic self-reinforced layer lying over a heterogeneous self-reinforced half-space. The heterogeneity is caused by consideration of quadratic variation in rigidity. The methodology employed combines an efficient derivation for Green’s functions based on algebraic transformations with the perturbation approach. Dispersion equation has been obtained in the closed form. The dispersion curves are compared for different values of magnetoelastic coupling parameters and inhomogeneity parameters. Also, the comparative study is being made through graphs to find the effect of reinforcement over the reinforced-free case on the phase velocity. It is observed that the dispersion equation is in assertion with the classical Love-type wave equation in the absence of reinforcement, magnetic field and heterogeneity. Moreover, some important peculiarities have been observed in graphs.
基金supported by National Natural Science Foundation of China(Nos.40520120222,40125011,and 40521002)and Peking University
文摘We investigated the upper mantle anisotropy beneath China by applying teleseismic shear wave splitting measurements at 119 seismic stations from CDSN and GSN//RIS networks in China. The splitting observations are characterized by apparent diversity of anisotropy pattern in adjacent tectonic domains, including the Tianshan orogenic belt, Tibetan plateau, the Yangtze craton, the North China craton and northeastern region. In western China （Tianshan orogenic belt and Tibetan plateau）, fast polarization directions of split SKS waves coincide strikingly well with the dominating trend of deformational crustal features and delay times range from 0.5 s to 1.6 s. While in eastern China, seismic anisotropy deduced from shear wave splitting reveals a homogeneous NW-SE trending structure, almost perpendicular to the strike of large-scale surface structures. The observed delay times of 1.5 s to more than 2 s favor consistent mantle flow over large mantle thicknesses Based upon the straightforward relationships between seismic anisotropy and the development of lattice preferred orientation of mineral in upper mantle rocks, we interpret the splitting results in terms of tectonic fabric within the upper mantle. Since the lithosphere is less than 100 km thick beneath eastern China and the observed fast directions are subparallel to the trend of the absolute plate motion （APM） of Eurasian plate, we propose that the asthenosphere may mainly contribute to the anisotropic effects beneath eastern China. However, the upper mantle anisotropy beneath western China may have developed more recently in the subcrustal lithosphere because of rather small delay times and thick lithosphere. We also use the opportunity of the dense geodetic measurements available in China to discuss the coupling between the crust and mantle. In the Eura- sia-fixed reference frame, GPS and shear wave splitting both depict a similar trend beneath eastern China, suggesting a lithospheric block ＂escaping＂ toward the east that could orient olivine [001 ] axis in the upper mantle. There is a strong cou- pling between the crust and the mantle in eastern China. A different behavior is observed in western China. The GPS vectors trend NS-NE in Tibet and NW in Tianshan, close to the regional compression direction, whereas the fast directions trend EW in Tibet and NE in Tianshan, suggesting a tectonic regime close to a mode of axial shortening, generating the development of EW-trending foliation in Tibet and NE-trending foliation in Tianshan at depth. The crust and mantle deform independently in western China.
基金funded by the Regione Toscana (Regional Government of Tuscany), on behalf of the Dipartimento di Protezione Civile (National Civil Protection Department)the Unione dei Comuni Amiata Val d’Orcia (Union of the Municipalities of Amiata Val d’Orcia)
文摘A seismic hazard was assessed related to site effects at Abbadia San Salvatore, central Italy, on the Mt. Amiata slopes, an ancient volcanic area characterized by residual soils(thick layers of loose to dense sands originated from weathering of the trachydacitic lavas). The seismic ground amplification and soil liquefaction related to these layers were recognized as the major seismic hazards for the area.Geological, geophysical, and geotechnical surveys were carried out on the volcanic rocks. The Horizontal-to-Vertical Spectral Ratio(HVSR) analysis of 252 noise measurements and 29 shear-wave velocity models of the subsoil allowed a seismic microzonation of the studied area, distinguished by thick weathered volcanic sands and shear-wave impedance contrast with respect to the seismic bedrock(volcanic bedrock). The differentiation of classified zones allowed recognition of areas characterized by residual(almost undisturbed) soils from those with soils probably affected by flowing water. The analysis of hazards revealed that peak acceleration by seismic amplification of ground motion exceeded the value set by the national rules(0.175 g) in a restricted area of the zone characterized by the most perturbed soils(Zone D);the potential occurrence of soil liquefaction was also greater in this zone. Finally, the study showed potential high hazards due to site effects of the volcanic mountainous area characterized by residual soils as opposed to an alluvial plain formed by volcanic debris where these effects have generally been more recognized.
