We relocate the spatial distribution of its aftershocks. The relocation database is obtained the devastating 12 May 2008 Wenchuan earthquake and from 89 stations deployed by the China Earthquake Administration, includ...We relocate the spatial distribution of its aftershocks. The relocation database is obtained the devastating 12 May 2008 Wenchuan earthquake and from 89 stations deployed by the China Earthquake Administration, including 54 525 seismograms from 1 376 local earthquakes over Ms3.5 between 12 May 2008 and 3 August 2008. The cross-correlation technique used in this paper has greatly improved the relocation precision by giving much more accurate P-wave differential travel-time measurements than those obtained from routinely picked phase onsets. At the same time, we pick P-wave polarity observations of the Wenchuan earthquake series （hereafter referred to as WES） from 1023 stations in China and 59 IRIS （Incorporated Research Institutions of Seismology） stations. Then, employing a newly developed program CHNYTX, we obtain 83 well-determined focal mechanism solutions （hereafter referred to as FMSs）. Based on spatial distribution and FMSs of the WES, we draw following conclusions： （1） The region near the main shock exhibits a buried low-angle northwest-dipping seismic zone with the main shock at its upper end and two conjugated seismic zones dipping southeast with roughly equal dip-angle; （2） The compressional directions of all kinds of FMSs of the WES are subhorizontal, which reflects the dominant stress in this area is eompressional; （3） The principal compressional direction of the regional stress around Wenchuan is roughly perpendicular to the strike of Beichuan-Yingxiu fault, while around Qingchuan it is roughly parallel to the strike of Qingehuan fault. In intermediate part of the Longmenshan area, the principal compressional direction of the stress should be in-between; （4） The possibly existed molten materials in the lower crust of Songpan-Garze terrain have small contribution to the local stress state in Longmenshan area. The listric geometries of the Longmenshan faults most probably resulted from subhorizontal compression along NW-SE direction in history.展开更多
Abstract The 2008 Wenchuan earthquake, a major intraplate earthquake with Mw 7.9, occurred on the slowly deforming Longmenshan fault. To better understand the causes of this devastating earthquake, we need knowledge o...Abstract The 2008 Wenchuan earthquake, a major intraplate earthquake with Mw 7.9, occurred on the slowly deforming Longmenshan fault. To better understand the causes of this devastating earthquake, we need knowledge of the regional stress field and the underlying geodynamic processes. Here, we determine focal mechanism solutions （FMSs） of the 2008 Wenchuan earthquake sequence （WES） using both P-wave first-motion polarity data and SH/P amplitude ratio （AR） data. As P-wave polarities are more reliable information, they are given priority over SH/ PAR, the latter of which are used only when the former has loose constraint on the FMSs. We collect data from three categories： （1） permanent stations deployed by the China Earthquake Administration （CEA）; （2） the Western Sichuan Passive Seismic Array （WSPSA） deployed by Institute of Geology, CEA; （3） global stations from Incorporated Research Institutions for Seismology. Finally, 129 events with magnitude over Ms 4.0 in the 2008 WES are identified to have well-constrained FMSs. Among them, 83 are well constrained by P-wave polarities only as shown by Cai et al. （Earthq Sci 24（1）：115-125,2011）, and the rest of which are newly constrained by incorporating SH/P AR. Based on the spatial distribution and FMSs of the WES, we draw following conclusions： （1） the principle compressional directions of most FMSs of the WES are subhorizontal, generally in agreement with the conclusion given by Cai et al. （2011） but with a few modifications that the compressional directions are WNW-ESE around Wenchuan and ENE-WSW around Qingchuan, respectively. The subhorizontal compressional direction along the Longmenshan fault from SW to NE seems to have a leftlateral rotation, which agrees well with regional stress field inverted by former researchers （e.g., Xu et al., Acta Seismol Sin 30（5）, 1987; Acta Geophys Sin 32（6）, 1989; Cui et al., Seismol Geol 27（2）：234-242, 2005）; （2） the FMSs of the events not only reflected the regional stress state of the Longmenshan region, but also were obviously controlled by the faults to some extent, which was pointed out by Cai et al. （2011） and Yi et al. （Chin J Geophys 55（4）：1213-1227, 2012）; （3） while the 2008 Wenchuan earthquake and some of its strong aftershocks released most of the elastic energy accumulated on the Longmen- shan fault, some other aftershocks seem to occur just for releasing the elastic energy promptly created by the 2008 Wenchuan earthquake and some of its strong aftershocks. （4） Our results further suggest that the Longmenshan fault from Wenchuan to Beichuan was nearly fully destroyed by the 2008 Wenchuan earthquake and accordingly propose that there is less probability for great earthquakes in the middle part of the Longmenshan fault in the near future, although there might be a barrier to the southwest of Wenchuan and it is needed to pay some attention on it in the near future.展开更多
The Tethyan evolution depicts the continuous process of landmasses separating from the Gondwana continent in the south,drifting northwards,and subsequently colliding with the continents in the north over the past 500 ...The Tethyan evolution depicts the continuous process of landmasses separating from the Gondwana continent in the south,drifting northwards,and subsequently colliding with the continents in the north over the past 500 million years.In this process,the Tethyan oceans that formed between the landmass and the southern or northern continents underwent growth,evolution,and eventual closure with the early Cenozoic India-Eurasia collision.However,the Tethyan lithosphere did not disappear but rather continued to evolve after entering into the deep Earth.The current position,morphology,and volume of the subducted Tethyan oceanic slabs in the deep mantle record the latest moment of this continuous evolution,providing critical constraints for Tethyan studies.This paper summarizes and analyzes the results of global-scale whole-mantle seismic tomography in the past nearly two decades,revealing a northwest-southeast seismically high-velocity anomaly,which is linearly distributed at depths of 1000–2000 km beneath the Tethyan realm and referred to as the Tethyan anomaly.By searching for an optimal linear combination of previous global seismic tomographic models to best match the known subducted slabs in the upper mantle,we observe that the Tethyan anomaly extends approximately 8700 km in length and 2600 km in width,exhibiting a parallel structure with northern and southern branches.Combining geological records of oceanic subduction initiation and previous geodynamic studies,this study suggests that the main body of the Tethyan anomaly represents the remnants of the subducted Neo-Tethyan oceanic slabs,which subducted from the Late Jurassic to the early Cenozoic.The northern branch consists of subducted slabs from the Neo-Tethys beneath the southern margin of Eurasia,while the southern branch likely reflects the intra-oceanic subducted slabs of Neo-Tethys during the Cretaceous.The western portion of the Tethyan anomaly may reflect remnants of Paleo-Tethys,while the eastern portion,towards India and the Bay of Bengal,shows signs of subduction towards the core-mantle boundary.Finally,this study discusses the future prospects of whole-mantle seismic tomographic studies focusing on the Tethyan realm.展开更多
基金supported by the Wenchuan Fault Scientific Drilling Program(WFSD),the Key Projects in the National Science and Technology Pillar Program during the Eleventh Five-year Plan Period under grant No. 2008BAC38B02-4the National Natural Science Foundation of China under grant No. 40821062
文摘We relocate the spatial distribution of its aftershocks. The relocation database is obtained the devastating 12 May 2008 Wenchuan earthquake and from 89 stations deployed by the China Earthquake Administration, including 54 525 seismograms from 1 376 local earthquakes over Ms3.5 between 12 May 2008 and 3 August 2008. The cross-correlation technique used in this paper has greatly improved the relocation precision by giving much more accurate P-wave differential travel-time measurements than those obtained from routinely picked phase onsets. At the same time, we pick P-wave polarity observations of the Wenchuan earthquake series （hereafter referred to as WES） from 1023 stations in China and 59 IRIS （Incorporated Research Institutions of Seismology） stations. Then, employing a newly developed program CHNYTX, we obtain 83 well-determined focal mechanism solutions （hereafter referred to as FMSs）. Based on spatial distribution and FMSs of the WES, we draw following conclusions： （1） The region near the main shock exhibits a buried low-angle northwest-dipping seismic zone with the main shock at its upper end and two conjugated seismic zones dipping southeast with roughly equal dip-angle; （2） The compressional directions of all kinds of FMSs of the WES are subhorizontal, which reflects the dominant stress in this area is eompressional; （3） The principal compressional direction of the regional stress around Wenchuan is roughly perpendicular to the strike of Beichuan-Yingxiu fault, while around Qingchuan it is roughly parallel to the strike of Qingehuan fault. In intermediate part of the Longmenshan area, the principal compressional direction of the stress should be in-between; （4） The possibly existed molten materials in the lower crust of Songpan-Garze terrain have small contribution to the local stress state in Longmenshan area. The listric geometries of the Longmenshan faults most probably resulted from subhorizontal compression along NW-SE direction in history.
基金supported by the Wenchuan Fault Scientific Drilling Program(WFSD)
文摘Abstract The 2008 Wenchuan earthquake, a major intraplate earthquake with Mw 7.9, occurred on the slowly deforming Longmenshan fault. To better understand the causes of this devastating earthquake, we need knowledge of the regional stress field and the underlying geodynamic processes. Here, we determine focal mechanism solutions （FMSs） of the 2008 Wenchuan earthquake sequence （WES） using both P-wave first-motion polarity data and SH/P amplitude ratio （AR） data. As P-wave polarities are more reliable information, they are given priority over SH/ PAR, the latter of which are used only when the former has loose constraint on the FMSs. We collect data from three categories： （1） permanent stations deployed by the China Earthquake Administration （CEA）; （2） the Western Sichuan Passive Seismic Array （WSPSA） deployed by Institute of Geology, CEA; （3） global stations from Incorporated Research Institutions for Seismology. Finally, 129 events with magnitude over Ms 4.0 in the 2008 WES are identified to have well-constrained FMSs. Among them, 83 are well constrained by P-wave polarities only as shown by Cai et al. （Earthq Sci 24（1）：115-125,2011）, and the rest of which are newly constrained by incorporating SH/P AR. Based on the spatial distribution and FMSs of the WES, we draw following conclusions： （1） the principle compressional directions of most FMSs of the WES are subhorizontal, generally in agreement with the conclusion given by Cai et al. （2011） but with a few modifications that the compressional directions are WNW-ESE around Wenchuan and ENE-WSW around Qingchuan, respectively. The subhorizontal compressional direction along the Longmenshan fault from SW to NE seems to have a leftlateral rotation, which agrees well with regional stress field inverted by former researchers （e.g., Xu et al., Acta Seismol Sin 30（5）, 1987; Acta Geophys Sin 32（6）, 1989; Cui et al., Seismol Geol 27（2）：234-242, 2005）; （2） the FMSs of the events not only reflected the regional stress state of the Longmenshan region, but also were obviously controlled by the faults to some extent, which was pointed out by Cai et al. （2011） and Yi et al. （Chin J Geophys 55（4）：1213-1227, 2012）; （3） while the 2008 Wenchuan earthquake and some of its strong aftershocks released most of the elastic energy accumulated on the Longmen- shan fault, some other aftershocks seem to occur just for releasing the elastic energy promptly created by the 2008 Wenchuan earthquake and some of its strong aftershocks. （4） Our results further suggest that the Longmenshan fault from Wenchuan to Beichuan was nearly fully destroyed by the 2008 Wenchuan earthquake and accordingly propose that there is less probability for great earthquakes in the middle part of the Longmenshan fault in the near future, although there might be a barrier to the southwest of Wenchuan and it is needed to pay some attention on it in the near future.
基金supported by the National Natural Science Foundation of China (Grant Nos.92155307,U1901602,42274058,42174106)the Shenzhen Science and Technology Program (Grant No.KQTD20170810111725321)the Guangdong Provincial Key Laboratory of Geophysical High-resolution Imaging Technology (Grant No.2022B1212010002)。
文摘The Tethyan evolution depicts the continuous process of landmasses separating from the Gondwana continent in the south,drifting northwards,and subsequently colliding with the continents in the north over the past 500 million years.In this process,the Tethyan oceans that formed between the landmass and the southern or northern continents underwent growth,evolution,and eventual closure with the early Cenozoic India-Eurasia collision.However,the Tethyan lithosphere did not disappear but rather continued to evolve after entering into the deep Earth.The current position,morphology,and volume of the subducted Tethyan oceanic slabs in the deep mantle record the latest moment of this continuous evolution,providing critical constraints for Tethyan studies.This paper summarizes and analyzes the results of global-scale whole-mantle seismic tomography in the past nearly two decades,revealing a northwest-southeast seismically high-velocity anomaly,which is linearly distributed at depths of 1000–2000 km beneath the Tethyan realm and referred to as the Tethyan anomaly.By searching for an optimal linear combination of previous global seismic tomographic models to best match the known subducted slabs in the upper mantle,we observe that the Tethyan anomaly extends approximately 8700 km in length and 2600 km in width,exhibiting a parallel structure with northern and southern branches.Combining geological records of oceanic subduction initiation and previous geodynamic studies,this study suggests that the main body of the Tethyan anomaly represents the remnants of the subducted Neo-Tethyan oceanic slabs,which subducted from the Late Jurassic to the early Cenozoic.The northern branch consists of subducted slabs from the Neo-Tethys beneath the southern margin of Eurasia,while the southern branch likely reflects the intra-oceanic subducted slabs of Neo-Tethys during the Cretaceous.The western portion of the Tethyan anomaly may reflect remnants of Paleo-Tethys,while the eastern portion,towards India and the Bay of Bengal,shows signs of subduction towards the core-mantle boundary.Finally,this study discusses the future prospects of whole-mantle seismic tomographic studies focusing on the Tethyan realm.
