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Seismic anisotropy of upper mantle in eastern China

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Science China Earth Sciences 2009年第6期52卷 774-783页

作者： CHANG LiJun WANG ChunYong ding zhifeng Institute of Geophysics China Earthquake AdministrationBeijing 100081China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 65 seismic stations which respectively involved in the permanent and temporary broadband seismograph networks deployed in eastern China, the SKS fast-wave direction and the delay time between the fast and slow shear waves at each station were determined by use of SC method and the stacking analysis method, and then the image of upper mantle anisotropy in eastern China was acquired. In the study region, from south to north, the fast-wave polarization directions are basically EW in South China, gradually clockwise rotate to NWW-SEE in North China, then to NW-SE in Northeast China. The delay time falls into the interval [0.41 s, 1.52 s]. Anisotropic characteristics in eastern China indicate that the upper mantle anisotropy is possibly caused by both the collision between the Indian and Eurasian Plates and the subduction from the Pacific and Philippine Sea Plates to the Eurasian Plate. The collision between two plates made the crust of western China thickening and uplifting and the material eastwards extruding, and then caused the upper mantle flow eastwards and southeastwards. The subduction of Pacific Plate and Philippine Sea Plate has resulted in the lithosphere and the asthenosphere deformation in eastern China, and made the alignment of upper mantle peridotite lattice parallel to the deformation direction. The fast-wave polarization direction is consistent with the direction of lithosphere extension and the GPS velocity direction, implying that the crust-upper mantle deformation is possibly a vertically coherent deformation.

关键词： eastern China upper mantle anisotropy SKS wave fast-wave direction lithosphere deformation

Seismic anisotropy of upper mantle in Sichuan and adjacent regions

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Science China Earth Sciences 2008年第12期51卷 1683-1693页

作者： CHANG LiJun WANG ChunYong ding zhifeng Institute of Geophysics China Earthquake AdministrationBeijing 100081China

Based on the polarization analysis of teleseismic SKS waveform data recorded at 94 broadband seis-mic stations in Sichuan and adjacent regions, the SKS fast-wave direction and the delay time between the fast and slow shear waves were determined at each station using the grid searching method of minimum transverse energy and the stacking analysis method, and the image of upper mantle anisot-ropy was acquired. The fast-wave polarization directions are mainly NW-SE in the study area, NWW-SEE to its northeast and NS to its west. The delay time falls into the interval [0.47 s, 1.68 s]. The spatial variation of the fast-wave directions is similar to the variation of GPS velocity directions. The anisotropic image indicates that the regional tectonic stress field has resulted in deformation and flow of upper mantle material, and made the alignment of upper mantle peridotite lattice parallel to the di-rection of material deformation. The crust-upper mantle deformation in Sichuan and adjacent regions accords with the mode of vertically coherent deformation. In the eastern Tibetan Plateau, the crustal material was extruded to east or southeast due to SE traction force of the upper mantle material. The extrusion might be obstructed by a rigid block under the Sichuan Basin and the crust has been de-formed. After a long-term accumulation of tectonic strain energy, the accumulative energy suddenly released in Yingxiu town of the Longmenshan region, and Wenchuan MS8.0 earthquake occurred.

关键词： upper mantle anisotropy SKS wave fast-wave polarization direction lithospheric deformation Wenchuan MS8.0 earthquake

Upper mantle anisotropy in the Ordos Block and its margins

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Science China Earth Sciences 2011年第6期54卷 888-900页

作者： CHANG LiJun WANG ChunYong ding zhifeng Institute of Geophysics China Earthquake Administration Beijing 100081 China

Based on the polarization analysis of teleseismic data,SKS (SKKS) fast-wave directions and delay times between fast and slow shear waves were determined for each of the 111 seismic stations from both permanent and temporary broadband seismograph networks deployed in the Ordos Block and its *** the Silver and Chan and stacking analysis methods were *** this way,an image of upper mantle anisotropy in the Ordos Block and its margins was *** the western and northern margins of the Ordos Block,the fast-wave directions are consistently *** fast-wave directions are mainly NWW-SEE and EW in the southern margin of the Ordos *** the eastern margin of the Ordos Block,the fast-wave directions are generally EW,although some run NEE-SWW or *** the Ordos Block,the fast-wave directions trend near N-S in the north,but switch to near EW in the *** delay time between fast and slow waves falls into the interval 0.48-1.50 s,and the average delay time at the stations in the Ordos Block is less than that in its *** suggest that the anisotropy of the stable Ordos Block is mainly caused by "fossil" anisotropy frozen in the ancient North China *** NE-trending push of the northeastern margin of the Tibetan Plateau has caused NW-SE-trending lithospheric extension in the western and northern margins of the Ordos Block,and made the upper mantle flow *** in turn has resulted in the alignment of the upper mantle peridotite lattice with the direction of material *** the southern margin of the Ordos Block,the collision between the North China and Yangtze blocks resulted in the fast-wave direction running parallel to the collision boundary and the Qinling *** this with the APM and velocity structure of the Qinling Orogen,we propose that eastward-directed asthenospheric-mantle channel flow may have occurred beneath the Qinling *** the eastern margin of the Ordos Block,the complex anisotropic charac

关键词： Ordos Block upper mantle anisotropy SKS wave fast-wave direction lithosphere deformation

Bird guild loss and its determinants on subtropical land-bridge islands, China

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Chinese Birds 2015年第2期6卷 99-106页

作者： zhifeng ding Kenneth James Feeley Huijian Hu Ping ding The Key Laboratory of Conservation Biology for Endangered Wildlife of the Ministry of Education College of Life Sciences Zhejiang UniversityZijin’gang Campus Guangdong Entomological Institute & South China Institute of Endangered Animals Department of Biological Sciences Florida International University Center for Tropical Plant Conservation Fairchild Tropical Botanic Garden

Background: The guild concept is useful for understanding the community structure in a land-bridge island system,but most fragmentation studies have focused only on the importance of island area and isolation, other island attributes such as perimeter-area ratio(PAR) were overlooked or ***: We have adopted a guild approach to investigate the impacts of island attributes on bird guild richness on a set of 41 recently isolated land-bridge islands in the Thousand Island Lake(TIL), ***: We found insectivores had the largest number of species(34 species), fol owed by understory foraging guilds(28 species), omnivores(27 species) and canopy guilds(25 species). Furthermore, our data showed that migrants and residents responded equal y to island area, insectivores and understory guilds were sensitive to island area but omnivores and canopy guilds were not very sensitive. Most guild richness was determined by island area, except for omnivores and canopy ***: Although PAR or habitat diversity found to be important for bird species richness, our results highlight the importance of island area in maintaining bird diversity in fragmented island systems.

关键词： Bird guilds Island area Land-bridge islands PAR Thousand Island Lake z values

Upper-mantle velocity structures beneath the Tibetan Plateau and surrounding areas inferred from triplicated P waveforms

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Earth and Planetary Physics 2019年第5期3卷 444-458页

作者： RiSheng Chu LuPei Zhu zhifeng ding State Key Laboratory of Geodesy and Earth’s Dynamics Institute of Geodesy and GeophysicsWuhan 430077China Department of Earth and Atmospheric Sciences Saint Louis UniversitySt LouisMO 63108USA Institute of Geophysics and Geomatics China University of GeosciencesWuhan 430074China Institute of Geophysics China Earthquake AdministrationBeijing 100083China

P-wave waveforms in the distance range between 12°and 30°were analyzed to investigate upper-mantle P velocity structures beneath the Tibetan Plateau and surrounding *** waveform data from 504 earthquakes with magnitudes larger than 5.0 between 1990 and 2005 that occurred within 30°from the center of the Plateau were *** divided the study area into 6 regions and modeled upper-mantle-distance P waveforms with turning points beneath each region *** results show that the uppermantle P-wave velocity structures beneath India,the Himalayas,and the Lhasa Terrane are similar and contain a high-velocity lid about 250 km *** upper-mantle velocities down to 200 km beneath the Qiangtang Terrane,the Tarim Basin,and especially the Songpan-GarzêTerrane are lower than those in the *** 410-km discontinuity beneath these two terranes is elevated by about 20 *** anomalies are found in the transition zone below 500 km under the Lhasa and Qiangtang *** results suggest that the Tibetan Plateau was generated by thrusting of the Indian mantle lithosphere under the southern part of *** of the thickened Eurasian mantle lithosphere were delaminated;they are now sitting in the transition zone beneath southern Tibet and atop of the 410-km discontinuity underneath northern Tibet.

关键词： Tibetan Plateau upper mantle structure triplication waveform modelling

Effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves

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Earthquake Science 2018年第4期31卷 175-186页

作者： Laiyu Lu Kaiming Wang zhifeng ding Institute of Geophysics China Earthquake Administration

With the development of the dense array,the surface wave velocity and azimuthal anisotropy under the array can be directly obtained by beamforming the noise cross-correlation functions(NCFs). However, the retrieval of the Green’s function by cross-correlating the seismic noise requires that the noise source has a uniform distribution. For the case with uneven noise source, the azimuthal dependence on the sources in the expression for the spatial coherence function, which corresponds to the NCF in the time domain,has the same form as the azimuthal dependence of the surface wave velocity in weakly anisotropic media. Therefore, the uneven noise source will affect the surface wave anisotropy extraction. In this study, three passive seismic methods, i.e.,beamforming, SPAC(spatial autocorrelation), and NCF, are compared to demonstrate that an uneven source distribution and uneven station distribution have equivalent effects on the outcome from each method. A beamforming method is proposed to directly extract the velocity and azimuthal anisotropy of surface waves. The effect of uneven noise source and/or station distribution on estimating the azimuth anisotropy of surface waves was investigated using data from the ChinArray Phase Ⅱ. A method for correcting the apparent anisotropy in beamforming results caused by an uneven station distribution is suggested.

关键词： beamforming uneven seismic noise source uneven station distribution azimuthal anisotropy

Deep Seismogenic Environment in the Southern Section of the Longmenshan Fault Zone on the Eastern Margin of the Tibetan Plateau and Lushan M_s 7.0 Earthquake

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Acta Geologica Sinica(English Edition) 2015年第2期89卷 519-530页

作者： LI Dahu ding zhifeng ZHAN Yan WU Pingping YE Qingdong Institute of Geophysics China Earthquake Administration Earthquake Administration of Sichuan Province Institute of Geology China Earthquake Administration

The 2,026 earthquake events registered by the Sichuan regional digital seismic network and mobile seismic array after the April 20th, 2013 Lushan earthquake and 28,188 pieces of data were selected to determine direct P waves arrival times. We applied the tomographic method to inverse the characteristics of the velocity structure for the three-dimensional （3D） P wave in the mid-upper crust of the seismic source region of the Lushan earthquake. The imaging results were combined with the apparent magnetization inversion and magnetotelluric （MT） sounding retest data to comprehensively study the causes of the deep seismogenic environment in the southern section of the Longmenshan fault zone and explore the formation of the Lushan earthquake. Research has shown that there are obvious differences in velocity structure and magnetic distribution between the southern and northern sections of the Longmenshan fault zone. The epicenter of the Lushan earthquake is located near the boundary of the high and low-velocity anomalies and favorable for a high-velocity section. Moreover, at the epicenter of the Lushan earthquake located on the magnetic dome boundary of Ya＇an, the development of high velocity and magnetic solid medium favors the accumulation and release of strain energy. Low- velocity anomalies are distributed underneath the are of seismogenic origin, The inversion results of the MT retest data after the April 20th Lushan earthquake also indicate that there a high-conductor anomaly occurs under the area of seismogenic origin of the Lushan earthquake, Therefore, we speculated that the presence of a high-conductivity anomaly and low-velocity anomaly underneath the seismogenic body of the Lushan earthquake could be related to the existence of fluids. The role of fluids caused the weakening of the seismogenic layer inside the mid-upper crust and resulted in a seismogenic fault that was prone to rupture and pIayed a triggering role in the Lushan earthquake.

关键词： The Longmenshan fault zone Lushan earthquake the three-dimensional velocitystructure the apparent magnetization inversion magnetotelluric sounding

Upper crustal anisotropy observed around the Longmenshan fault in the 2013 M_S7.0 Lushan earthquake region

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Earthquake Science 2018年第4期31卷 187-198页

作者： Lijun Chang zhifeng ding Chunyong Wang Institute of Geophysics China Earthquake Administration

Based on the shear wave splitting analysis of the seismic recordings at 17 temporary stations and three permanent stations, we measured the shear wave splitting parameters(i.e., the polarization direction of fast shear wave and the time delay of slow wave) to perform a systematic analysis of the crustal seismic anisotropy around the Longmenshan fault in the 2013 M7.0 Lushan earthquake region. We observed apparent spatio-temporal characteristics in the shear wave splitting parameters. The spatial distribution of fast polarization directions showed a clear partitioning in the characteristics from northwest to southeast in the focal region,which changed from NW-SE to NE-SW. In the northwest of the focal region, the fast polarization direction was oriented to NW-SE, which was parallel to the maximum horizontal compressive stress direction. However, the NE-SW fast polarization direction in the southeast of the focal region was parallel to the Longmenshan fault strike. For station BAX on the Central fault in the middle of the focal region, the distribution of fast polarization directions showed a bimodal pattern, with one dominant in the NE-SW direction and the other in the NW-SE direction. With regard to the temporal variation, the time delays were large in the initial stage after the mainshock but then gradually decreased over time and tended to be stable in the later period. This indicated that stress in the focal region increased to a maximum when the main shock occurred, with the stress release caused by the mainshock and aftershock activity, and the stress gradually decreased after a period of time. The scatter of fast polarization directions was large after the main shock, but over time the scatter gradually decreased, indicating that the Lushan earthquake caused a large perturbation in the local stress field. As the stress gradually decreased and was adjusted by the aftershock activity, the perturbation gradually weakened.

关键词： Lushan earthquake crustal anisotropy crustal stress fast polarization direction time delay

Focal depths for moderate-sized aftershocks of the Wenchuan M_S8.0 earthquake and their implications

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Science China Earth Sciences 2008年第12期51卷 1694-1702页

作者： ZHANG RuiQing WU QingJu LI YongHua ding zhifeng ZENG RongSheng Institute of Geophysics China Earthquake AdministrationBeijing 100081China

Sliding-window cross-correlation method is firstly adopted to identify sPn phase, and to constrain focal depth from regional seismograms, by measuring the time separation between sPn and Pn phases. We present the focal depths of the 17 moderate-sized aftershocks (MS≥5.0) of the Wenchuan MS8.0 earth-quake, using the data recorded by the regional seismic broadband networks of Shaanxi, Qinghai, Gansu, Yunnan and Sichuan. Our results show focal depths of aftershocks range from 8 to 20 km, and tend to cluster at two average depths, separate at 32.5°N, i.e., 11 km to the south and 17 km to the north, indicating that these aftershocks are origin of upper-to-middle crust. Combined with other results, we suggest that the Longmenshan fault is not a through-going crustal fault and the Pingwu-Qingchuan fault may be not the northward extension of the Longmenshan thrust fault.

关键词： Wenchuan MS8.0 earthquake aftershocks sPn phase