A number of aftershocks of the May 10th 1997, Zirkuh (Ghaen-Birjand) destructive earthquake have been used to investigate the anisotropy in the upper crust by observing shear wave splitting. Particle motion diagram an...A number of aftershocks of the May 10th 1997, Zirkuh (Ghaen-Birjand) destructive earthquake have been used to investigate the anisotropy in the upper crust by observing shear wave splitting. Particle motion diagram and aspect ratio methods were used as two different approaches to obtain splitting parameters. Clear shear wave splitting was observed on the records of the selected aftershocks, indicating that the media in the region was highly anisotropic. By using particle motion method, the direction of fast shear wave was found 22°N±19°E, while the delay time between the fast and slow shear waves was obtained to be (65±16) ms. By aspect ratio method, the direction of fast shear wave was determined to be 35°N±18°E and the delay time between fast and slow shear waves was found to be (49±10) ms. For a simple horizontal layer with a thickness about 5 km and uniformly distributed anisotropy, a stress aligned cracks model was used and the result was interpreted in terms of vertical aligned cracks in the direction of N22°E, having a density about 0.01. It is assumed that cracks are fluid-filled since they are located in the upper crust. Finally, by using Hudson cracks model for three crack densities 0.005, 0.01, 0.03, the velocity curves of shear wave were plotted as a function of angle between the symmetrical axis of cracks and the azimuth of source to receiver. It was concluded that when shear wave was polarized parallel to the crack surface, the velocity was uniform, but the velocity curve varied clearly if shear wave was polarized perpendicular to the crack surface.展开更多
The Xiaojiang faults,striking north-tosouth(NS),and the Honghe faults,striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern par...The Xiaojiang faults,striking north-tosouth(NS),and the Honghe faults,striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern part of the Sichuan-Yunnan rhombic block(SYB).It is also a crucial zone for material escaping from the Tibetan Plateau(TP)due to the collision between the Indian Plate and the Eurasian Plate.In December 2017,the Institute of Earthquake Forecasting of the China Earthquake Administration(CEA)deployed a linear temporary seismic broadband array,the Honghe-Xiaojiang temporary Seismic Array(HX Array),across first-order block boundaries in the southern SYB.By using the waveform data of small earthquakes recorded by stations in the HX Array across Xiaojiang faults from 2017 to 2019,and by permanent seismic stations of the China National Earthquake Networks from 2012 to 2019,this paper adopts the systematic analysis method of shear-wave splitting(SWS),SAM method,to obtain preliminary results for seismic anisotropy in the upper crust.The study area can be divided into two subzones according to the spatial distribution of the directions of polarization of the fast shear-wave(PFS)at the stations:the northern zone(zone A,where the HX Array is located)and the southern zone(zone B,to the south of the HX Array).The results show that the directions of the PFS at stations in zone A were highly consistent,dominant in the NE direction,correlated with the in-situ principal compressive stress,and were seemingly unaffected by the Xiaojiang faults.The directions of the PFS as recorded at stations in zone B were more complicated,and were dominant in the NS direction parallel to that of the regional principal compressive stress.This suggests the joint influence of complex tectonics and regional stress in this narrow wedge area.By referring to the azimuthal anisotropy derived from seismic ambient noise in the southeast margin of the TP,the NS direction of the PFS in the middle and lower crust,and its EW direction in the upper mantle,this paper concludes that azimuthal anisotropy in the upper crust differed from that in the lower crust in the south segment of Xiaojiang faults,at least beneath the observation area,and azimuthal anisotropy in the crust was different from that in the upper mantle.The results support the pattern of deformation of ductile flow in the lower crust,and the decoupling between the upper and lower crusts as well as that between the crust and the mantle in the study area.The crustal directions of the PFS appeared to be independent of the Xiaojiang faults,suggesting that the influence of the South China block on the SYB passed through the Xiaojiang faults to the Yimen region.The results of this study indicate that anisotropic studies based on data on the dense temporary seismic array can yield clearer tectonic information,and reveal the complex spatial distribution of stress and deformation in the upper crust of the south segment of Xiaojiang faults.展开更多
On 25 April 2015,an M_w 7.8 earthquake occurred on the Main Himalaya Thrust fault with a dip angle of^7° about77 km northwest of Kathmandu,Nepal.This Nepal Gorkha event is the largest one on the Himalayan thrust ...On 25 April 2015,an M_w 7.8 earthquake occurred on the Main Himalaya Thrust fault with a dip angle of^7° about77 km northwest of Kathmandu,Nepal.This Nepal Gorkha event is the largest one on the Himalayan thrust belt since 1950.Here we use the compressive sensing method in the frequency domain to track the seismic radiation and rupture process of this event using teleseismic P waves recorded by array stations in North America.We also compute the distribution of static shear stress changes on the fault plane from a coseismic slip model.Our results indicate a dominant east-southeastward unilateral rupture process from the epicenter with an average rupture speed of ~3 km s^(-1).Coseismic radiation of this earthquake shows clear frequency-dependent features.The lower frequency(0.05-0.3 Hz) radiation mainly originates from large coseismic slip regions with negative coseismic shear stress changes.In comparison,higher frequency(0.3-0.6 Hz) radiation appears to be from the down-dip part around the margin of large slip areas,which has been loaded and presents positive coseismic shear stress changes.We propose an asperity model to interpret this Nepal earthquake sequence and compare the frequency-dependent coseismic radiation with that in subduction zones.Such frequency-dependent radiation indicates the depth-varying frictional properties on the plate interface of the Nepal section in the main Himalaya thrust system,similar to previous findings in oceanic subduction zones.Our findings provide further evidence of the spatial correlation between changes of static stress status on the fault plane and the observed frequency-dependent coseismic radiation during large earthquakes.Our results show that the frequency-dependent coseismic radiation is not only found for megathrust earthquakes in the oceanic subduction environment,but also holds true for thrust events in the continental collision zone.展开更多
文摘A number of aftershocks of the May 10th 1997, Zirkuh (Ghaen-Birjand) destructive earthquake have been used to investigate the anisotropy in the upper crust by observing shear wave splitting. Particle motion diagram and aspect ratio methods were used as two different approaches to obtain splitting parameters. Clear shear wave splitting was observed on the records of the selected aftershocks, indicating that the media in the region was highly anisotropic. By using particle motion method, the direction of fast shear wave was found 22°N±19°E, while the delay time between the fast and slow shear waves was obtained to be (65±16) ms. By aspect ratio method, the direction of fast shear wave was determined to be 35°N±18°E and the delay time between fast and slow shear waves was found to be (49±10) ms. For a simple horizontal layer with a thickness about 5 km and uniformly distributed anisotropy, a stress aligned cracks model was used and the result was interpreted in terms of vertical aligned cracks in the direction of N22°E, having a density about 0.01. It is assumed that cracks are fluid-filled since they are located in the upper crust. Finally, by using Hudson cracks model for three crack densities 0.005, 0.01, 0.03, the velocity curves of shear wave were plotted as a function of angle between the symmetrical axis of cracks and the azimuth of source to receiver. It was concluded that when shear wave was polarized parallel to the crack surface, the velocity was uniform, but the velocity curve varied clearly if shear wave was polarized perpendicular to the crack surface.
基金This work was supported by the National Natural Science Foundation of China(No.41730212)the National Key R&D Project of China(No.2017YFC1500304)the Basic Research Project of Institute of Earthquake Science,CEA(No.2017IES010202).
文摘The Xiaojiang faults,striking north-tosouth(NS),and the Honghe faults,striking north-to-west(NW),are first-order block boundaries that intersect to form a concentrated stress zone at an acute angle in the southern part of the Sichuan-Yunnan rhombic block(SYB).It is also a crucial zone for material escaping from the Tibetan Plateau(TP)due to the collision between the Indian Plate and the Eurasian Plate.In December 2017,the Institute of Earthquake Forecasting of the China Earthquake Administration(CEA)deployed a linear temporary seismic broadband array,the Honghe-Xiaojiang temporary Seismic Array(HX Array),across first-order block boundaries in the southern SYB.By using the waveform data of small earthquakes recorded by stations in the HX Array across Xiaojiang faults from 2017 to 2019,and by permanent seismic stations of the China National Earthquake Networks from 2012 to 2019,this paper adopts the systematic analysis method of shear-wave splitting(SWS),SAM method,to obtain preliminary results for seismic anisotropy in the upper crust.The study area can be divided into two subzones according to the spatial distribution of the directions of polarization of the fast shear-wave(PFS)at the stations:the northern zone(zone A,where the HX Array is located)and the southern zone(zone B,to the south of the HX Array).The results show that the directions of the PFS at stations in zone A were highly consistent,dominant in the NE direction,correlated with the in-situ principal compressive stress,and were seemingly unaffected by the Xiaojiang faults.The directions of the PFS as recorded at stations in zone B were more complicated,and were dominant in the NS direction parallel to that of the regional principal compressive stress.This suggests the joint influence of complex tectonics and regional stress in this narrow wedge area.By referring to the azimuthal anisotropy derived from seismic ambient noise in the southeast margin of the TP,the NS direction of the PFS in the middle and lower crust,and its EW direction in the upper mantle,this paper concludes that azimuthal anisotropy in the upper crust differed from that in the lower crust in the south segment of Xiaojiang faults,at least beneath the observation area,and azimuthal anisotropy in the crust was different from that in the upper mantle.The results support the pattern of deformation of ductile flow in the lower crust,and the decoupling between the upper and lower crusts as well as that between the crust and the mantle in the study area.The crustal directions of the PFS appeared to be independent of the Xiaojiang faults,suggesting that the influence of the South China block on the SYB passed through the Xiaojiang faults to the Yimen region.The results of this study indicate that anisotropic studies based on data on the dense temporary seismic array can yield clearer tectonic information,and reveal the complex spatial distribution of stress and deformation in the upper crust of the south segment of Xiaojiang faults.
基金supported by National Natural Science Foundation of China(Grant Nos.41374055,41225010)Chinese University of Hong Kong Direct Grant for Research(Grant No. 3132771)+1 种基金HKSAR Research Grant Council ECS(Grant No.2191093) and GRF(Grant No.2130509)the Fundamental Research Funds for the Central Universities(Grant No.WK2080000053)
文摘On 25 April 2015,an M_w 7.8 earthquake occurred on the Main Himalaya Thrust fault with a dip angle of^7° about77 km northwest of Kathmandu,Nepal.This Nepal Gorkha event is the largest one on the Himalayan thrust belt since 1950.Here we use the compressive sensing method in the frequency domain to track the seismic radiation and rupture process of this event using teleseismic P waves recorded by array stations in North America.We also compute the distribution of static shear stress changes on the fault plane from a coseismic slip model.Our results indicate a dominant east-southeastward unilateral rupture process from the epicenter with an average rupture speed of ~3 km s^(-1).Coseismic radiation of this earthquake shows clear frequency-dependent features.The lower frequency(0.05-0.3 Hz) radiation mainly originates from large coseismic slip regions with negative coseismic shear stress changes.In comparison,higher frequency(0.3-0.6 Hz) radiation appears to be from the down-dip part around the margin of large slip areas,which has been loaded and presents positive coseismic shear stress changes.We propose an asperity model to interpret this Nepal earthquake sequence and compare the frequency-dependent coseismic radiation with that in subduction zones.Such frequency-dependent radiation indicates the depth-varying frictional properties on the plate interface of the Nepal section in the main Himalaya thrust system,similar to previous findings in oceanic subduction zones.Our findings provide further evidence of the spatial correlation between changes of static stress status on the fault plane and the observed frequency-dependent coseismic radiation during large earthquakes.Our results show that the frequency-dependent coseismic radiation is not only found for megathrust earthquakes in the oceanic subduction environment,but also holds true for thrust events in the continental collision zone.
