Understanding the viscoelastic structure of subduction zones is essential for assessing seismic hazards and understanding subduction-zone dynamics.However,the influence of lateral variations in elastic upper-plate thi...Understanding the viscoelastic structure of subduction zones is essential for assessing seismic hazards and understanding subduction-zone dynamics.However,the influence of lateral variations in elastic upper-plate thickness(H_(c))remains poorly constrained and is often overlooked.In this study,we use two-dimensional forward viscoelastic earthquake-cycle models to fit both horizontal and vertical Global Navigation Satellite System(GNSS)observations.We identify a clear trade-off between locking depth(D)and H_(c)in both components.To resolve this ambiguity,we incorporate constraints from thermal models and tremor distributions along the Cascadia Subduction Zone.As a novel result extending beyond previous kinematic models,our results reveal a systematic northward increase in H_(c)from~20 km to~30 km.This trend correlates with increasing oceanic plate age and likely reflects variations in the subaccretion and wedge-cooling processes along the trench-parallel direction.In contrast,D remains relatively uniform at~10 km,consistent with previous findings.These results demonstrate the robustness of our approach for simultaneously constraining H_(c)and D,and they suggest it may be applied to other subduction zones.Lateral variations in H_(c)significantly affect crust deformation and should not be ignored in earthquake-cycle models.Accounting for these heterogeneities improves estimates of H_(c)and D and enhances our understanding of megathrust locking,seismic hazard potential,and the physical conditions controlling episodic tremor and slip events.展开更多
A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the centra...A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems.展开更多
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
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 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
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
The area near the Cape Mendocino earthquake of 1992, magnitude 6.9, was the site of many moderate to large shocks during the previous decades. It and the Honeydew event of 1991, however, are distinguished from most ea...The area near the Cape Mendocino earthquake of 1992, magnitude 6.9, was the site of many moderate to large shocks during the previous decades. It and the Honeydew event of 1991, however, are distinguished from most earthquakes in the region by their thrust-fault mechanisms. The magnitude of the 1991 shock was also unusually large for the preceding decades, Mw 6.1. The mechanisms of most other large events involved strike-slip faulting. The 1992 mainshock occurred in a volume of space characterized by few decadal forerunning earthquakes of moderate to large size. Most of those forerunners took place on the periphery of that volume. The presence of that zone suggests that it broke previously in a large to great earthquake. Precise locations indicate that slip in the 1991 and 1992 earthquakes occurred on faults dipping shallowly to the NE and ENE. They likely took place within the North American plate above the subduction plate boundary. Their implications for earthquake forecasting using sparse precursors are discussed.展开更多
本文采用近年来美国地震灾害研究取得的最新进展,对美国西雅图市中心某拟建高层建筑地震危险性分析展开优化研究,以期达到在满足美国土木工程师协会抗震设计规范(ASCE 7-10)的前提下,合理降低建筑物抗震设计费用之目的。研究首先采用ASC...本文采用近年来美国地震灾害研究取得的最新进展,对美国西雅图市中心某拟建高层建筑地震危险性分析展开优化研究,以期达到在满足美国土木工程师协会抗震设计规范(ASCE 7-10)的前提下,合理降低建筑物抗震设计费用之目的。研究首先采用ASCE 7-10推荐的方法 2分析基于具体场地参数条件下的风险系数;对于地震动最大方向系数,采用最新的Shashi and Baker 2014模型进行优化;由于现有的卡斯卡迪亚俯冲带地震动衰减模型缺乏对盆地放大效应的考虑,研究采用基于2014 NGA West2的最新潜源地震动衰减模型盆地放大效应系数进行优化考虑。研究结果显示,优化后的地震反应谱在中短周期范围内与非优化的地震反应谱基本一致,但是在长周期范围内,进行优化后的反应谱值一般要比不优化的反应谱值小20%到30%。在建筑物的第一卓越周期7秒左右,这个差别是25%。可见采用优化后的地震反应谱将会达到安全合理节约建筑物抗震设计费用的目的。展开更多
基金supported by the National Key R&D Program of China(Grant No.2023YFF0803200)the National Natural Science Foundation of China(Grant No.42288201).
文摘Understanding the viscoelastic structure of subduction zones is essential for assessing seismic hazards and understanding subduction-zone dynamics.However,the influence of lateral variations in elastic upper-plate thickness(H_(c))remains poorly constrained and is often overlooked.In this study,we use two-dimensional forward viscoelastic earthquake-cycle models to fit both horizontal and vertical Global Navigation Satellite System(GNSS)observations.We identify a clear trade-off between locking depth(D)and H_(c)in both components.To resolve this ambiguity,we incorporate constraints from thermal models and tremor distributions along the Cascadia Subduction Zone.As a novel result extending beyond previous kinematic models,our results reveal a systematic northward increase in H_(c)from~20 km to~30 km.This trend correlates with increasing oceanic plate age and likely reflects variations in the subaccretion and wedge-cooling processes along the trench-parallel direction.In contrast,D remains relatively uniform at~10 km,consistent with previous findings.These results demonstrate the robustness of our approach for simultaneously constraining H_(c)and D,and they suggest it may be applied to other subduction zones.Lateral variations in H_(c)significantly affect crust deformation and should not be ignored in earthquake-cycle models.Accounting for these heterogeneities improves estimates of H_(c)and D and enhances our understanding of megathrust locking,seismic hazard potential,and the physical conditions controlling episodic tremor and slip events.
文摘A search of Willamette River cutbanks was conducted for the presence of late Holocene paleoli-quefaction records in the Willamette forearc valley, located 175 ± 25 km landward from the buried trench in the central Cascadia subduction zone. Eight cutbank sites are reported that show evidence of large-scale fluidization features (≥10 cm width) including clastic sand dikes and intruded sand sills in Holocene overbank mud deposits. The targeted alluvial sequences, and hosted paleoliquefaction records, are of late Holocene age, as based on radiocarbon dating, flood silt thickness (≤4 m thickness), and minimal consolidation of dike sand (~1.5 ± 0.5 kg·cm-2 unconfined compressive strength). Two of the paleoliquefaction sites, which are separated by 150 km distance, overlap in age (175 - 500 yr BP) with the last great megathrust rupture (Mw 8.5 - 9.0) in the Cascadia margin, dated at AD 1700. The scarcity of exposed late Holocene paleoliquefaction sites in the Willamette River cutbanks motivated subsurface searches for thick basal sand deposits and overlying fluidization features, using floodplain geomorphological analyses, ground penetrating radar, and remote pole-camera scans of deep trench walls (3 - 4 m depth). The onset of large-scale fluidization features occurred in overbank mud deposits (2 - 3 m thickness) above unconsolidated sand bodies (≥2 m thickness) with unconfined compressive strengths of ~1.5 ± 0.5 kg·cm-2. We recommend geomorphically-targeted subsurface explorations rather than traditional cutbank searches for evidence of coseismic paleoliquefaction in high-gradient river valley systems.
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.
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.
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.
文摘The area near the Cape Mendocino earthquake of 1992, magnitude 6.9, was the site of many moderate to large shocks during the previous decades. It and the Honeydew event of 1991, however, are distinguished from most earthquakes in the region by their thrust-fault mechanisms. The magnitude of the 1991 shock was also unusually large for the preceding decades, Mw 6.1. The mechanisms of most other large events involved strike-slip faulting. The 1992 mainshock occurred in a volume of space characterized by few decadal forerunning earthquakes of moderate to large size. Most of those forerunners took place on the periphery of that volume. The presence of that zone suggests that it broke previously in a large to great earthquake. Precise locations indicate that slip in the 1991 and 1992 earthquakes occurred on faults dipping shallowly to the NE and ENE. They likely took place within the North American plate above the subduction plate boundary. Their implications for earthquake forecasting using sparse precursors are discussed.
文摘本文采用近年来美国地震灾害研究取得的最新进展,对美国西雅图市中心某拟建高层建筑地震危险性分析展开优化研究,以期达到在满足美国土木工程师协会抗震设计规范(ASCE 7-10)的前提下,合理降低建筑物抗震设计费用之目的。研究首先采用ASCE 7-10推荐的方法 2分析基于具体场地参数条件下的风险系数;对于地震动最大方向系数,采用最新的Shashi and Baker 2014模型进行优化;由于现有的卡斯卡迪亚俯冲带地震动衰减模型缺乏对盆地放大效应的考虑,研究采用基于2014 NGA West2的最新潜源地震动衰减模型盆地放大效应系数进行优化考虑。研究结果显示,优化后的地震反应谱在中短周期范围内与非优化的地震反应谱基本一致,但是在长周期范围内,进行优化后的反应谱值一般要比不优化的反应谱值小20%到30%。在建筑物的第一卓越周期7秒左右,这个差别是25%。可见采用优化后的地震反应谱将会达到安全合理节约建筑物抗震设计费用的目的。
