排序方式： 共有78条查询结果，搜索用时 15 毫秒
31.
The Cretaceous Kusandong Tuff, Korea, is a thin (1–5 m thick) but laterally extensive (~ 200 km) silicic ignimbrite emplaced in a fluviolacustrine basin adjacent to a continental volcanic arc. The tuff has been used as an excellent key bed because of its great lateral continuity and unique lithology, characterized by the virtual absence of juvenile clasts and an abundance of quartz and feldspar crystals (up to 55–73 vol.%). The tuff is mostly massive and ungraded and locally shows crude internal layering, basal inverse grading and near-top normal grading of crystals, either erosional or non-erosional lower surfaces, and flat-lying to imbricated grain fabrics. Fragile intraformational clasts of mudstone and tuff are also included. These features provide only ambiguous information on the properties of the responsible pyroclastic density currents: i.e. whether they were dense and laminar or dilute and turbulent. The overall lateral continuity and sheet-like geometry of the tuff suggests, however, that the transport system of the currents was highly expanded, dilute, and turbulent. A plug-flow or slab-flow model cannot explain the origin of crude internal layering, imbricated grain fabrics, and the high crystal content, which is most likely the result of vigorous sorting processes within a dilute and turbulent current. Features indicative of deposition from a dense and laminar transporting medium are locally present, suggesting that a dense and laminar depositional system could develop locally at the base of the dilute and turbulent transport system. The virtual absence of juvenile clasts in the tuff is interpreted to be due to rapid ascent, sudden decompression, and full fragmentation of silicic magma into fine glass shards and crystals. Scarcity of basement-derived accidental components together with the absence of pumiceous fallout deposits beneath the tuff is interpreted to be due to shallow-level fragmentation of magma followed by immediate generation of pyroclastic density currents from shallow-level blasts at the onset of eruption. The eruption occurred through multiple vent sites in a short period of time, producing a seemingly single but actually composite ignimbrite unit. Such an eruption was probably possible because of a regional tectonic event within the basin or in its vicinity. It is proposed that a composite ignimbrite with the characteristics of the Kusandong Tuff can be an exemplary product of syntectonic volcanism that can provide an insight into the interpretation of structural and stratigraphic evolution of a sedimentary basin. 相似文献
32.
Hye-Jin Moon Han-Joon Kim Chung-Ho Kim Seonghoon Moon Su-hwan Lee Jin Soo Kim Chung-Kyun Jeon Gwang Hoon Lee Sang Hoon Lee YoungSook Baek Hyeong-Tae Jou 《Journal of Oceanography》2017,73(5):701-709
The Yellow Sea Bottom Cold Water (YSBCW) is a large cold water mass lying in the deep part of the Yellow Sea during the warm season. We acquired multichannel seismic (MCS) data using an air gun source to image the structure of the YSBCW. The MCS data recorded reflections from sea water. The recognition of these reflections was confirmed by finite-difference seismic modeling in the frequency-domain. The seismic section from MCS data enabled discrimination of water masses distinctly separated by reflecting horizons. The structure of the water masses is fairly consistent with temperature-depth variations obtained using expandable bathythermograph (XBT) casts. The YSBCW is imaged as the lowermost water mass, maximally 40 m thick, that extends as a lens-like form along the sea bottom under the warm mixed layer. The correlation of XBT measurements and the seismic section indicates a rapid decrease in temperature from around 11 to 8 °C in the uppermost part of the YSBCW. A transition zone between the mixed layer and the underlying YSBCW is also defined. This transition zone has fairly uniform thickness of 14–18 m and marks an interval of rapid temperature drop, indicating vigorous thermal mixing. Our study demonstrates that MCS profiling is a useful and reliable tool for imaging fine structures in the shallow Yellow Sea. 相似文献
33.
Seismic fragility of lightly reinforced concrete frames with masonry infills is assessed through numerical simulations considering uncertainty in ground motion and building materials. To achieve this aim, a numerical model of the components is developed, a rational approach to proportion and locate individual struts in the equivalent three‐strut model is proposed, and an explicit nonlinear column shear response model accounting for the infill–column interaction and soft‐story mechanism is employed. The proposed numerical model is used to (1) generate probabilistic seismic demand models accounting for a wide range of ground motion intensities with different frequency content and (2) determine limit state models obtained from nonlinear pushover analysis and incremental dynamic analysis. Using the demand and limit state model, fragility curves for the masonry‐infilled frames are developed to investigate the impact of various infill properties on the frame vulnerability. It is observed that the beneficial effect of the masonry infill diminishes at more severe limit states because of the interaction with the boundary frame. In some cases, this effect almost vanishes or switches to an adverse effect beyond a threshold of ground motion intensities. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
34.
Framework of aftershock fragility assessment–case studies: older California reinforced concrete building frames 
下载免费PDF全文
下载免费PDF全文 Current seismic design codes and damage estimation tools neglect the influence of successive events on structures. However, recent earthquakes have demonstrated that structures damaged during an initial event (mainshock) are more vulnerable to severe damage and collapse during a subsequent event (aftershock). This increased vulnerability to damage translates to increased likelihood of loss of use, property, and life. Thus, a reliable risk assessment tool is required that characterizes the risk of the undamaged structure subjected to an initial event and the risk of the damaged structure under subsequent events. In this paper, a framework for development of aftershock fragilities is presented; these aftershock fragilities define the likelihood that a building damaged during a mainshock will exhibit a given damage state following one or more aftershocks. Thus, the framework provides a method for characterizing the risk associated with damage accumulation in the structure. The framework includes the following: (i) creation of a numerical model of the structure; (ii) characterization of building damage states; (iii) generation of a suite of mainshock–aftershocks; (iv) mainshock–aftershock analyses; and (v) development of aftershock fragility curves using probabilistic aftershock demand models, defined as a linear regression of aftershock demand–intensity pairs in a logarithmic space, and damage‐state prediction models. The framework is not limited to a specific structure type but requires numerical models defining structural response and linking structural response with damage. In the current study, non‐ductile RC frames (low‐rise, mid‐rise, and high‐rise) are selected as case studies for the application of the framework. Copyright © 2015 John Wiley & Sons, Ltd. 相似文献
35.
Alexander E. Yankovsky Raymond Torres Legna M. Torres-Garcia Kyungho Jeon 《Estuaries and Coasts》2012,35(6):1500-1509
This study was conducted in the freshwater reach of the Santee River approximately 55?±?4?km from the mouth, a transition zone from a fluvial to an estuarine tidal regime. The dataset comprises bathymetric surveys, current profile and bottom pressure measurements at two locations, and time series of discharge. Our data indicate that the transition zone is characterized by strong tidal dissipation and distinctive channel geometry. Tidal dissipation is evident in the rapid decrease of the M2 amplitude to the mean along-channel velocity ratio from 2.1 to 0.9 over a ??6-km distance. Channel cross-sectional area in the transition zone converges at a higher rate than both upstream and downstream while channel depth reveals threefold variations in the form of adjacent shoals and deeps. We hypothesize that the enhanced tidal dissipation is at the same time a cause and a result of strongly convergent bathymetry in the transition zone. 相似文献
36.
Pan-Mook Lee Seok-Won Hong Yong-Kon Lim Chong-Moo Lee Bong-Hwan Jeon Jong-Won Park 《Oceanic Engineering, IEEE Journal of》1999,24(3):388-395
This paper presents a discrete-time quasi-sliding mode controller for an autonomous underwater vehicle (AUV) in the presence of parameter uncertainties and a long sampling interval. The AUV, named VORAM, is used as a model for the verification of the proposed control algorithm. Simulations of depth control and contouring control are performed for a numerical model of the AUV with full nonlinear equations of motion to verify the effectiveness of the proposed control schemes when the vehicle has a long sampling interval. By using the discrete-time quasi-sliding mode control law, experiments on depth control of the AUV are performed in a towing tank. The controller makes the system stable in the presence of system uncertainties and even external disturbances without any observer nor any predictor producing high rate estimates of vehicle states. As the sampling interval becomes large, the effectiveness of the proposed control law is more prominent when compared with the conventional sliding mode controller 相似文献
37.
Port selection factors by shipping lines: Different perspectives between trunk liners and feeder service providers 总被引：2，自引：0，他引：2
The objective is to identify the factors affecting shipping companies’ port choice based on a survey to a sample of shipping companies. Six factors were considered relatively important: local cargo volume; terminal handling charge; berth availability; port location; transshipment volume and feeder network. Exploratory factor and confirmatory factor analyses identified five port choice categories, i.e. advancement/convenience of port; physical/operational ability of port; operational condition of shipping lines; marketability; and port charge. A comparison between the main trunk and feeder service providers indicated that the former face more intense competition than the latter. Moreover, the main haul shipping lines are more sensitive to port cost factors. 相似文献
38.
Chulhee Kim S.-L. Kim Y.-B. Jeon C.-H. Kim A. Gilmore 《Astrophysics and Space Science》2007,312(1-2):41-46
New differential time-series observations of KZ Hya were secured using the V filters, and 28 new times of light maximum were
identified. We collected 157 times of light maximum from the literature, unpublished data and open database, and proceeded
to investigate the variations of the O–C values for light maxima with the total of 185 times of light maximum. A quasi-sinusoidal
character of the O–C diagram was confirmed, and this was interpreted as a light travelling time effect due to a companion
moving in an eccentric (e=0.23) orbit with a period of about 24.77 years. The periods of KZ Hya were investigated and nine frequencies were determined
using the Fourier decomposition method. All nine were harmonic frequencies and no other frequency was identified. 相似文献
39.
New differential time-series observations of the distant halo SX Phe variable BQ Psc are reported. We have determined 19 new
times of maximum light. The observed minus calculated (O–C) analysis indicates that the period has been increasing with time.
We also applied a Fourier decomposition to the data in order to determine the component frequencies of the synthetic light
curve. The multiperiodicity of this variable star was not confirmed.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
40.
The ADCP records obtained at about 18°N, 135°E show the southern branch of the Subtropical Countercurrent (STCC). The sea surface heights (SSH) show that there is a tendency to increase and decrease in the south/north of STCC, respectively. So the variability of SSH ultimately contributes to the strengthening of STCC through geostrophic balance. The southern STCC branch distinctly persists from winter to spring. Since 2005, the southern STCC exists almost throughout the year, and the STCC is clearly stronger to the east of 145°E. Anticyclonic and cyclonic eddies exist seemingly as bands around 17.5°N and 20.5°N, respectively. The STCC flowing eastward, which is formed by the geostrophic balance, is maintained with the interaction between geostrophic currents and anticyclonic-cyclonic eddies. The rotating eddies exert an additional driving force to maintain the eastward flow of STCC, and then the STCC reveals a meandering movement due to the interaction with the eddies. The trajectories of surface drifters together with the altimeter data analysis in June 2009 dictate the variability of the STCC induced by the interaction between eddies and the eastward flow. These results suggest that the southern STCC slowly changes from an intra-seasonal event an annual one with time duration of over 21 months. 相似文献