The Xigeda formation is a set of interbreds comprised of halfconsolidated silt,fine sand and clay with clear lamination and total thickness of 300 m.It is dated to be the early Pleistocene and widespread distributed o...The Xigeda formation is a set of interbreds comprised of halfconsolidated silt,fine sand and clay with clear lamination and total thickness of 300 m.It is dated to be the early Pleistocene and widespread distributed on the Quaternary planes of denudation and leveling.This formation has particular tectonic deformation as expressed by deformed belts confined to certain locations,which extend linearly over long distances.It also manifests itself as graben and horst sequences where fault planes are very straight.Various deformation styles,such as horizontal,vertical and oblique dislocations,normal,reverse and thrust faulting resulted in tilts of small blocks in different directions.The clay beds are intruded by underlying sand layers.Parallel ridges and depressions characterize the landscape.These phenomena of deformation are probably the result of intensive vibration of soil bodies during major earthquakes which caused instantaneous shear,compression and extension.The analysis of generation factors for such tectonic deformation provides a new approach for determination of areas with seismic risks,assessment of earthquake intensity and research of earthquake mechanisms.展开更多
In accordance with the requirements of the National Key Technology R＆D Program of the 11th ＂Five-year Plan＂, a densified seismic network consisting of 26 seismic stations was established at the Three Gorges Reservo...In accordance with the requirements of the National Key Technology R＆D Program of the 11th ＂Five-year Plan＂, a densified seismic network consisting of 26 seismic stations was established at the Three Gorges Reservoir area in the section of Hubei Province in March 2009 （21 short-period seismographs, 5 broadband digital seismographs）. From March to December, 2009, a total of 2,995 ML -0. 8 - 2. 9 earthquakes were detected during the trial impounding of the Three Gorges Reservoir （ water level rose from 145 m to 172.8m）. Using the double difference earthquake location algorithm, 2,837 earthquakes were precisely re-located. The results show that the pattern of small local earthquake swarms in the Three Gorges Reservoir area took on a linear distribution or mass-like cluster distribution, the mass-like clusters of events were generally within a distance of 5 km from waterfront, and the linear distribution of the earthquakes could be extended to a distance of 16 km away from the waterfront. In the Hubei section of the Three Gorges Reservoir, earthquakes were mainly concentrated in the northern end of the Xiannvshan and Jiuwanxi faults near the Xiangxihe River, and along the banks of the Yangtze River at the west of Xietan township and the Shenlongxi area on the northern bank in the Badong region, with focal depths less than 10km, and 4km in average. Earthquake frequency in the reservoir region had a positive correlation with reservoir water level fluctuations, indicating that the seismicity belongs to reservoir induced earthquakes. Along the Shenlong River in the reservoir area, earthquakes showed three linear distributions in the northern Badong county, and distributed according to Karst distribution. There are underground rivers in the carbonate strata. When the reservoir was impounded, water permeated into the underground rivers, thus inducing earthquakes. Earthquakes in the areas on the crossriver segment of Xiannvshan fault, the Jiuwanxi fault and at the areas west of Xietan, Shazhen and Xizhen, may be related to the softening of discontinuities, such as the Nukou fault, the Xiannvshan fault, or the bedding joints, which would lead to failure of rock masses, thus, inducing earthquakes. However, convincing conclusions about the triggering mechanism still need further study. Additionally, near the areas south of Wenhua and Yanglin of Zigui county and at Rangkou town east of Badong county, mininginduced earthquakes occurred at the mines nearby, and on the shores of the reservoir are some collapse earthquakes.展开更多
815 earthquakes recorded by 12 seismic stations of the Zipingpu reservoir seismic network in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocati...815 earthquakes recorded by 12 seismic stations of the Zipingpu reservoir seismic network in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocation results show that the earthquakes are concentrated relatively in three zones.The distribution characteristics of focal depth are obviously different among different concentration zones.This means earthquakes in different concentration zones may have different causes.Compared to relocation of earthquakes taking place before the Wenchuan earthquake done by other researchers,the seismic concentration zones in the reservoir area shifted obviously after the Wenchuan earthquake.These variations are related to local stress adjustment in the reservoir area and may also be related to the diffusion depth and range of increased pore pressure caused by rock failure in the course of Wenchuan earthquake.展开更多
文摘The Xigeda formation is a set of interbreds comprised of halfconsolidated silt,fine sand and clay with clear lamination and total thickness of 300 m.It is dated to be the early Pleistocene and widespread distributed on the Quaternary planes of denudation and leveling.This formation has particular tectonic deformation as expressed by deformed belts confined to certain locations,which extend linearly over long distances.It also manifests itself as graben and horst sequences where fault planes are very straight.Various deformation styles,such as horizontal,vertical and oblique dislocations,normal,reverse and thrust faulting resulted in tilts of small blocks in different directions.The clay beds are intruded by underlying sand layers.Parallel ridges and depressions characterize the landscape.These phenomena of deformation are probably the result of intensive vibration of soil bodies during major earthquakes which caused instantaneous shear,compression and extension.The analysis of generation factors for such tectonic deformation provides a new approach for determination of areas with seismic risks,assessment of earthquake intensity and research of earthquake mechanisms.
基金sponsored by the National Key Technology R&D Program (2008BAC38B04),China
文摘In accordance with the requirements of the National Key Technology R＆D Program of the 11th ＂Five-year Plan＂, a densified seismic network consisting of 26 seismic stations was established at the Three Gorges Reservoir area in the section of Hubei Province in March 2009 （21 short-period seismographs, 5 broadband digital seismographs）. From March to December, 2009, a total of 2,995 ML -0. 8 - 2. 9 earthquakes were detected during the trial impounding of the Three Gorges Reservoir （ water level rose from 145 m to 172.8m）. Using the double difference earthquake location algorithm, 2,837 earthquakes were precisely re-located. The results show that the pattern of small local earthquake swarms in the Three Gorges Reservoir area took on a linear distribution or mass-like cluster distribution, the mass-like clusters of events were generally within a distance of 5 km from waterfront, and the linear distribution of the earthquakes could be extended to a distance of 16 km away from the waterfront. In the Hubei section of the Three Gorges Reservoir, earthquakes were mainly concentrated in the northern end of the Xiannvshan and Jiuwanxi faults near the Xiangxihe River, and along the banks of the Yangtze River at the west of Xietan township and the Shenlongxi area on the northern bank in the Badong region, with focal depths less than 10km, and 4km in average. Earthquake frequency in the reservoir region had a positive correlation with reservoir water level fluctuations, indicating that the seismicity belongs to reservoir induced earthquakes. Along the Shenlong River in the reservoir area, earthquakes showed three linear distributions in the northern Badong county, and distributed according to Karst distribution. There are underground rivers in the carbonate strata. When the reservoir was impounded, water permeated into the underground rivers, thus inducing earthquakes. Earthquakes in the areas on the crossriver segment of Xiannvshan fault, the Jiuwanxi fault and at the areas west of Xietan, Shazhen and Xizhen, may be related to the softening of discontinuities, such as the Nukou fault, the Xiannvshan fault, or the bedding joints, which would lead to failure of rock masses, thus, inducing earthquakes. However, convincing conclusions about the triggering mechanism still need further study. Additionally, near the areas south of Wenhua and Yanglin of Zigui county and at Rangkou town east of Badong county, mininginduced earthquakes occurred at the mines nearby, and on the shores of the reservoir are some collapse earthquakes.
基金funded jointly by National Science& Technology Pillar Program (Grant No. 2008BAC38B0401)special fund for basic scientific research of Institute of Geology,CEA (DF-IGCEA060828)
文摘815 earthquakes recorded by 12 seismic stations of the Zipingpu reservoir seismic network in 2009 were relocated using the double difference algorithm to analyze the seismic activity of the Zipingpu reservoir.Relocation results show that the earthquakes are concentrated relatively in three zones.The distribution characteristics of focal depth are obviously different among different concentration zones.This means earthquakes in different concentration zones may have different causes.Compared to relocation of earthquakes taking place before the Wenchuan earthquake done by other researchers,the seismic concentration zones in the reservoir area shifted obviously after the Wenchuan earthquake.These variations are related to local stress adjustment in the reservoir area and may also be related to the diffusion depth and range of increased pore pressure caused by rock failure in the course of Wenchuan earthquake.
