Using continuously operating Global Positioning Stations in the Pacific Northwest of the United States, over 100 station-station baseline length changes were determined along seven West-East transects, two North-South...Using continuously operating Global Positioning Stations in the Pacific Northwest of the United States, over 100 station-station baseline length changes were determined along seven West-East transects, two North-South transects and in three localized areas to determine both the average annual strains over the past several years, and the variation in strain over the central Cascadia convergent margin. The North-South transects (composed of multiple baselines) show shortening. Along West-East transects some baselines show shortening and others extension. The direction of the principle strains calculated for two areas 100 km from the deformation front are close to per-pendicular to the deformation front. The North-South strains are 10?8 a?1, which is an order-of-magnitude less than the West-East strains (10?7 a?1). Along several West-East transects, the magnitude of the strain increases away from the deformation front. All West-East transects showed a change in strain 250 km inland from deformation front.展开更多
The Deep-towed Acoustics and Geophysics System (DTAGS) is a high frequency (220-820 Hz) multichannel seismic system towed about 300 m above seafloor.Compared to the conventional surface-towed seismic system,the DTAGS ...The Deep-towed Acoustics and Geophysics System (DTAGS) is a high frequency (220-820 Hz) multichannel seismic system towed about 300 m above seafloor.Compared to the conventional surface-towed seismic system,the DTAGS system is characterized by its shorter wavelength (<6 m),smaller Fresnel zone,and greater sampling in wavenumber space,so it has unique advantages in distinguishing fine sedimentary layers and geological structures.Given the near-bottom configuration and wide high-frequency bandwidth,the precise source and hydrophone positioning is the basement of subsequent seismic imaging and velocity analysis,and thus the quality of array geometry inversion is the key of DTAGS data processing.In the application of exploration for marine gas hydrate on mid-slope of northern Cascadia margin,the DTAGS system has shown high vertical and lateral resolution images of the sedimentary and structural features of the Cucumber Ridge (a carbonate mound) and Bullseye Vent (a cold vent),and provided abundant information for the evaluation of gas hydrate concentration and the mechanism of fluid flow that controls the formation and distribution of gas hydrate.展开更多
Modern horizontal strain (2006-2016) measured along 56 new and 108 previously published GPS station baselines are used to establish the length (800 km) and width (300 - 400 km) of the central Cascadia convergent margi...Modern horizontal strain (2006-2016) measured along 56 new and 108 previously published GPS station baselines are used to establish the length (800 km) and width (300 - 400 km) of the central Cascadia convergent margin seismogenic structure. Across-margin (west-east) annual rates of shortening range from 10﹣9 a﹣1 at the eastern (landward) limit of the central Cascadia seismogenic structure to 10﹣7 a﹣1 along the western onshore portion of the interplate zone. Relatively high shortening strain rates (10﹣8 a﹣1 to 10﹣7 a﹣1) are also measured in western transects from the northern (Explorer plate) and southern (Gorda plate) segments of the convergent margin, demonstrating that the full length of the margin (1300 km length) is currently capable of sustaining and/or initiating a major great earthquake. Vertical GPS velocities are averaged over the last decade at 321 stations to map patterns of uplift (0 - 5 mm yr﹣1) and subsidence (0 - 9 mm yr﹣1) relative to the study area mean. Along-margin belts of relative uplift and subsidence, respectively, are approximately associated with Coast Ranges and the Cascade volcanic arc. However, the vertical velocity data are locally heterogeneous, demonstrating patchy “anomalies” within the larger along-margin belts. A large coastal subsidence anomaly occurs in southwest Washington where the modern short-term trend is reversed from the long-term (~200 yr) tidal marsh record of coastal uplift since the last co-seismic subsidence event (AD1700). The modern vertical displacements represent a late stage of the current inter-seismic interval. If the horizontal strain is considered largely or fully elastic, extrapolating the modern strain rates over the last 100 years show the accumulated strains would be similar in magnitude to the observed co-seismic strains resulting from the Tōhoku, Japan, Mw 9.0 earthquake in 2011. We believe that the central Cascadia seismogenic structure has accumulated sufficient elastic strain energy, during the last 300 years, to yield a Mw 9.0 earthquake from a rupture of at least one-half (400 km) of its length.展开更多
The comparative study on natural gas hydrate accumulation models between active and passive continental margins as well as their controlling factors is of great significance to the guidance of natural gas hydrate expl...The comparative study on natural gas hydrate accumulation models between active and passive continental margins as well as their controlling factors is of great significance to the guidance of natural gas hydrate exploration.Based on the data and research results of international typical active continental margin hydrate accumulation areas such as the Cascadia margin of the Northeast Pacific,the Nankai trough,etc.and passive continental margin areas like the Blake Ridge,the models of the gas hydrate accumulation system are summarized and numerically simulated,and a preliminary comparison of active and passive continental margin reservoir accumulation models was also carried out.The following results were obtained.(1)The active continental margin provides a driving force and channel for vertical gas migration,which induces deep free gas and in-situ biogas to migrate along the fault.The migration channels are mainly faults,fractures and slumps produced by subductioneaccretion.(2)Coarse-grained turbidity sediments such as silt and sandy silt have good porosity and permeability.Moreover,the sediment thickness on the accretionary wedge is large,which provides a good storage space for hydrate accumulation.(3)Numerical simulations of the Blake Ridge,and Niger Delta hydrate accumulation show that the passive continental margin lacks the lateral stress caused by the subduction zone compared with the active continental margin.However,due to the plastic materials in the thick sedimentary layer,high-pressure fluids and volcanic activities outside the continental margin,vertical accretion and tensile stress are generated and the accumulation rate of diffusion-type hydrates mainly depends on the methane supply rate.(4)Organic matter content,gas production rate,geothermal gradient and sedimentation rate at the passive continental margin have different effects on the spatial distribution of hydrate content.Mud volcanoes or diapir structures provide an ideal place for the formation and occurrence of hydrates.展开更多
文摘Using continuously operating Global Positioning Stations in the Pacific Northwest of the United States, over 100 station-station baseline length changes were determined along seven West-East transects, two North-South transects and in three localized areas to determine both the average annual strains over the past several years, and the variation in strain over the central Cascadia convergent margin. The North-South transects (composed of multiple baselines) show shortening. Along West-East transects some baselines show shortening and others extension. The direction of the principle strains calculated for two areas 100 km from the deformation front are close to per-pendicular to the deformation front. The North-South strains are 10?8 a?1, which is an order-of-magnitude less than the West-East strains (10?7 a?1). Along several West-East transects, the magnitude of the strain increases away from the deformation front. All West-East transects showed a change in strain 250 km inland from deformation front.
基金supported by National Natural Science Foundation of China (Grant Nos. 40830423 and 40904029)Scientific Research Foundation for the Returned Overseas Chinese Scholars,Ministry of Education of China
文摘The Deep-towed Acoustics and Geophysics System (DTAGS) is a high frequency (220-820 Hz) multichannel seismic system towed about 300 m above seafloor.Compared to the conventional surface-towed seismic system,the DTAGS system is characterized by its shorter wavelength (<6 m),smaller Fresnel zone,and greater sampling in wavenumber space,so it has unique advantages in distinguishing fine sedimentary layers and geological structures.Given the near-bottom configuration and wide high-frequency bandwidth,the precise source and hydrophone positioning is the basement of subsequent seismic imaging and velocity analysis,and thus the quality of array geometry inversion is the key of DTAGS data processing.In the application of exploration for marine gas hydrate on mid-slope of northern Cascadia margin,the DTAGS system has shown high vertical and lateral resolution images of the sedimentary and structural features of the Cucumber Ridge (a carbonate mound) and Bullseye Vent (a cold vent),and provided abundant information for the evaluation of gas hydrate concentration and the mechanism of fluid flow that controls the formation and distribution of gas hydrate.
文摘Modern horizontal strain (2006-2016) measured along 56 new and 108 previously published GPS station baselines are used to establish the length (800 km) and width (300 - 400 km) of the central Cascadia convergent margin seismogenic structure. Across-margin (west-east) annual rates of shortening range from 10﹣9 a﹣1 at the eastern (landward) limit of the central Cascadia seismogenic structure to 10﹣7 a﹣1 along the western onshore portion of the interplate zone. Relatively high shortening strain rates (10﹣8 a﹣1 to 10﹣7 a﹣1) are also measured in western transects from the northern (Explorer plate) and southern (Gorda plate) segments of the convergent margin, demonstrating that the full length of the margin (1300 km length) is currently capable of sustaining and/or initiating a major great earthquake. Vertical GPS velocities are averaged over the last decade at 321 stations to map patterns of uplift (0 - 5 mm yr﹣1) and subsidence (0 - 9 mm yr﹣1) relative to the study area mean. Along-margin belts of relative uplift and subsidence, respectively, are approximately associated with Coast Ranges and the Cascade volcanic arc. However, the vertical velocity data are locally heterogeneous, demonstrating patchy “anomalies” within the larger along-margin belts. A large coastal subsidence anomaly occurs in southwest Washington where the modern short-term trend is reversed from the long-term (~200 yr) tidal marsh record of coastal uplift since the last co-seismic subsidence event (AD1700). The modern vertical displacements represent a late stage of the current inter-seismic interval. If the horizontal strain is considered largely or fully elastic, extrapolating the modern strain rates over the last 100 years show the accumulated strains would be similar in magnitude to the observed co-seismic strains resulting from the Tōhoku, Japan, Mw 9.0 earthquake in 2011. We believe that the central Cascadia seismogenic structure has accumulated sufficient elastic strain energy, during the last 300 years, to yield a Mw 9.0 earthquake from a rupture of at least one-half (400 km) of its length.
基金Project supported by the National Key R&D Program Project“Hydrate Trial Production,Environmental Monitoring and Comprehensive Evaluation and Application Demonstration”(No.:2017YFC0307600)National Natural Science Foundation of China Project(No.91858208)National Marine Geology Special Project“Gas Hydrate Accumulation Mechanism Research”(No.GZH201100306).
文摘The comparative study on natural gas hydrate accumulation models between active and passive continental margins as well as their controlling factors is of great significance to the guidance of natural gas hydrate exploration.Based on the data and research results of international typical active continental margin hydrate accumulation areas such as the Cascadia margin of the Northeast Pacific,the Nankai trough,etc.and passive continental margin areas like the Blake Ridge,the models of the gas hydrate accumulation system are summarized and numerically simulated,and a preliminary comparison of active and passive continental margin reservoir accumulation models was also carried out.The following results were obtained.(1)The active continental margin provides a driving force and channel for vertical gas migration,which induces deep free gas and in-situ biogas to migrate along the fault.The migration channels are mainly faults,fractures and slumps produced by subductioneaccretion.(2)Coarse-grained turbidity sediments such as silt and sandy silt have good porosity and permeability.Moreover,the sediment thickness on the accretionary wedge is large,which provides a good storage space for hydrate accumulation.(3)Numerical simulations of the Blake Ridge,and Niger Delta hydrate accumulation show that the passive continental margin lacks the lateral stress caused by the subduction zone compared with the active continental margin.However,due to the plastic materials in the thick sedimentary layer,high-pressure fluids and volcanic activities outside the continental margin,vertical accretion and tensile stress are generated and the accumulation rate of diffusion-type hydrates mainly depends on the methane supply rate.(4)Organic matter content,gas production rate,geothermal gradient and sedimentation rate at the passive continental margin have different effects on the spatial distribution of hydrate content.Mud volcanoes or diapir structures provide an ideal place for the formation and occurrence of hydrates.
