Bounding of near-fault ground motion based on radiated seismic energy with a consideration of fault frictional mechanisms

The energy radiated as seismic waves strongly depends on the fault rupture process associated with rupture speed and dynamic frictional mechanisms involved in the fault slip motion.Following McGarr and Fletcher approach,we derived a physics-based relationship of the weighted average fault slip velocity vs apparent stress,rupture speed and static stress drop based on a dynamic circular fault model.The resultant function can be approximately used to bound near-fault ground motion and seismic energy associated with near-fault coseismic deformation.Fault frictional overshoot and undershoot mechanisms governed by a simple slip-weakening constitutive relation are included in our consideration by using dynamic rupture models named as M-and D-models and proposed by Madariaga(1976) and Boatwright.We applied the above function to the 2008 great Wenchuan earthquake and the 1999 Jiji(Chi-Chi) earthquake to infer the near-fault ground motion called slip weighted average particle velocity and obtained that such model-dependent prediction of weighted average ground velocities is consistent to the results derived from the near-fault strong motion observations.Moreover,we compared our results with the results by McGarr and Fletcher approach,and we found that the values of the weighted average particle velocities we obtained for these two earthquakes are generally smaller and closer to the values by direct integration of strong motion recordings of the near-fault particle velocity waveform data.In other words,if this result comes to be true,it would be a straightforward way used to constrain the near-fault ground motion or to estimate source parameters such as rupture speed,static and dynamic stress drops.     

The impacts of pyroclastic surges on buildings at the eruption of the Soufrière Hills volcano, Montserrat

We investigated the impacts on buildings of three pyroclastic surges that struck three separate villages on 25 June, 21 September and 26 December, 1997, during the course of the andesitic dome building eruption of the Soufrière Hills Volcano, Montserrat, which began on 18 July, 1995. A detailed analysis of the building damage of the 26 December event was used to compare the findings on the flow and behaviour of dilute pyroclastic density currents (PDCs) with the classical reports of PDCs from historical eruptions of similar size. The main characteristics of the PDC, as inferred from the building damage, were the lateral loading and directionality of the current; the impacts corresponded to the dynamic pressure of the PDC, with a relatively slow rate of rise and without the peak overpressure or a shock front associated with explosive blast; and the entrainment of missiles and ground materials which greatly added to the destructiveness of the PDC. The high temperature of the ash, causing the rapid ignition of furniture and other combustibles, was a major cause of damage even where the dynamic pressure was low at the periphery of the current. The vulnerability of buildings lay in the openings, mainly windows, which allowed the current to enter the building envelope, and in the flammable contents, as well as the lack of resistance to the intense heat and dynamic pressure of some types of vernacular building construction, such as wooden chattel houses, rubble masonry walls and galvanised steel-sheet roofs. Marked variability in the level of damage due to dynamic pressure (in a range 1–5 kPa, or more) was evident throughout most of the impact area, except for the zone of total loss, and this was attributable to the effects of topography and sheltering, and projectiles, and probably localised variations in current velocity and density. A marked velocity gradient existed from the outer part to the central axis of the PDC, where buildings and vegetation were razed to the ground. The gradient correlated with the impacts due to lateral loading and heat transfer, as well as the size of the projectiles, whilst the temperature of the ash in the undiluted PDC was probably uniform across the impact area. The main hazard characteristics of the PDCs were very consistent with those described by other authors in the classic eruptions of Pelée (1902), Lamington (1951) and St Helens (1980), despite differences in the eruptive styles and scales. We devised for the first time a building damage scale for dynamic pressure which can be used in research and in future volcanic emergencies for modelling PDCs and making informed judgements on their potential impacts. Editorial responsibility: T. Druitt     

Numerical modeling of scour and deposition around permeable cylindrical structures

Flow past wall-mounted cylindrical structures is commonly encountered in natural rivers where piers of bridge crossings or vegetation stalks are common within channels.In the current study,the influence of cylindrical structures on flow/bathymetric alterations for three different permeabilities is explored via two-dimensional numerical modeling.In model construction processes,the structure permeability is varied with the surface void ratio along the perimeter of the cylinder,i.e.the density of emergent and submerged solid elements is used to delineate the cylinder boundaries.The validation of this model is guaranteed through careful comparison with experimental data obtained for similar hydrodynamic conditions and cylinder properties.The validated model then is applied to investigate flow properties and scour and deposition patterns with structure permeabilities of 0.0,0.38,and 0.62.Simulated results show that a permeable structure has less impeding effects on flow than a solid cylinder.The wake velocity reduction decreases 38% with a 63% increase in the structure permeability due to increasing intensity of the bleeding flow through surface voids,causing less flow contraction and diversion,lower turbulent kinetic energy,and lower lee-side scour around the permeable structure and less deposition downstream under live-bed conditions.     

Numerical simulation and analysis of the mean coastal circulation off California

A two-dimensional numerical model is applied to a coastal ocean wherein alongshore elevation and density gradients, normally calculated by a three-dimensional model, are instead supplied by climatologically averaged data for the California Current System between 25 and 40°N. Surface wind stress is also obtained from climatological data. Both surface and bottom boundary layers are resolved in the model calculations; a second moment turbulence closure submodel supplies vertical diffusivities. Near steady state solutions are possible when surface buoyancy flux is imposed at the surface.Model results are as follows: Southward wind stress produces a broad equatorward current with an embedded coastal jet in accordance with previous studies. Positive wind stress curl reduces the jet current and produces a poleward undercurrent which then surfaces as the curl is increased. The jet currents are reduced and poleward flow increases as bottom steepness increases; to a lesser extent, inclusion of the beta effect has a similar effect. The existence of near bottom, poleward or equatorward flow is explained rather simply in terms of the bottom stress resulting from the alongshore balance of surface wind stress and vertically integrated pressure gradient, the latter involving the alongshore surface elevation and density gradient. A further finding is that the upwelling circulation associated with wind stress is confined to the top 200 to 300 m of the ocean along the California coast.     

Spreading of Stratified Currents in Lowland Reservoirs

The results of comprehensive hydrophysical studies of gradient and density-induced gravitational currents with thermal stratification in valley reservoirs are considered. The lengthwise and vertical development of currents as well as their evolution is considered. Local swells are detected in the near-bed nepheloid layer and clouds of suspended materials are found to be ejected from the density-induced current near abrupt changes in the bed relief. The near-bed density-induced current is found to strongly interact with the drift current in the presence of inner wave and variable-direction wind. The measured vertical profiles of velocity in the sections along the axis of the current and the longitudinal distributions of the average flow velocity and suspended material concentration are found to agree with model calculations.     

远场地震波辐射能计算以及对近断层地面运动的约束

从圆盘断层模型出发,根据地震波能量表象定理推导出了滑移弱化过程中远场S-波辐射能量表达式,并同已有的动力学模型作了比较.结果表明,得到的模型能量值或视应力的取值强烈地依赖于断层上的动态、静态应力降和破裂传播速度,而破裂速度则对应了断层带模型中断层破坏过程所耗散的能量.动摩擦应力上调和应力下调力学机制在能量求解中得到了充...     

Numerical investigation of the factors influencing the vertical profiles of current,salinity, and SSC within a turbidity maximum zone

The flow-sediment interaction plays a considerable role on the vertical (internal) profiles of current,salinity and suspended sediment concentration (SSC) within a turbidity maximum zone (TMZ).Numerical modeling provides valuable insights into the complex estuarine physical processes.By combining numerical modeling with field observations,the influencing factors of fine sediment dynamics within the TMZ of Yangtze Estuary have been explored in this study.Firstly,during the neap tide,the measured data present that the current is too weak to break the density stratification,and the vertical flow structure is effectively altered.Secondly,a three-dimensional numerical model based on the Delft3D has been developed and a range of numerical sensitivity analyses were carried out to distinguish the dominant mechanisms and physical processes responsible for the phenomena observed from the measurement data.The numerical investigation highlights the following findings.(1) The vertical profile of currents within the TMZ is largely affected by saltwater intrusion,especially during lower currents when the baroclinic pressure gradient can significantly reshape the local vertical profiles of velocity.(2) The baroclinic effects are primarily determined by the stratification of salinity.(3) In addition to salinity,SSC also influences the local density stratification when its contribution to fluid density is comparable to that of salinity.(4) The settling velocity determines the overall sediment distribution and vertical profiles of the SSC in the water column.The SSC-dependent settling velocity (including the flocculation-induced acceleration and hindered settling deceleration phases) affects the longitudinal movement of the sediments.(5) The vertical profiles of current,salinity and SSC within the TMZ are highly associated with the turbulence determined by the model.The approach to modulate the vertical eddy viscosity in the model,based on the empirical dependency between Rig and Prt,may lead to a numerical instability in the stratified flow.In order to improve the stratification of SSC,additional turbulence damping effect is suggested to be implemented in the model.     

Determining drag coefficients and their application in modelling of turbulent flow with submerged vegetation

Vegetation is a key aspect of water resources and ecology in natural rivers, floodplains and irrigation channels. The hydraulic resistance of the water flow is greatly changed when submerged vegetation is present. Three kinds of drag coefficients, i.e., the drag coefficient for an isolated cylinder, the bulk drag coefficient of an array of cylinders and the vertically distributed or local drag coefficient, have been commonly used as parameters to represent the vegetation drag force. In this paper, a comprehensive experimental study of submerged stems in an open channel flow is presented. Empirical formulae for the three drag coefficients were obtained based on our experimental results and on data from previous studies. A two-layer model was developed to solve the mean momentum equation, which was used to evaluate the vertical mean velocity profile with each of the drag coefficients. By comparing the velocity distribution model predictions and the measurement results, we found that the model with the drag coefficient for an isolated cylinder and the local drag coefficient was good fit. In addition, the model with the bulk drag coefficient gave much larger velocity values than measurements, but it could be improved by adding the bed friction effect and making choice of the depth-averaged velocity within the canopy layer.     

A model for the coast-effect

A model for the coast-effect of geomagnetism is presented, in which the horizontal magnetic field induces currents in a circuit including a thin finite ocean. The currents flow horizontally across the ocean, vertically down into the earth, back through the deep interior of the earth, and vertically up to the ocean to complete the current loop. The upper layers of the earth are given non-zero conductivity, allowing the possibility of such current loops.A two-dimensional model involving such currents has been worked out analytically, and it is found that a significant induced magnetic field at the seafloor can be obtained with a reasonable conductivity in the earth's upper layers. A three-dimensional model has also been worked out numerically. It is found that the induced vertical component of magnetic field is of comparable magnitude to the horizontal component induced normal to the coast, whereas the horizontal component parallel to the coast is small. These relations are required to explain the observation of Parkinson arrows.     

Gravity currents over a rigid and emergent vegetated slope

Lock-exchange experiments are conducted to investigate the effects of emergent vegetation on gravity currents flowing down a slope. Rigid and emergent cylinders are used to represent vegetation such as reeds in aquatic environments. The results show that the head of gravity currents without cylinders forms a semi-elliptic shape, similar to the flat bed case. For the slope-induced gravity, however, the head of gravity currents gradually grows and accelerates. A steeper slope without cylinders causes the evident entrainment and subsequent energy dissipation of gravity currents with ambient fluid. When the cylinder population becomes denser, developments of the head become slower, and the semi-elliptic head can be only seen at the very front end. As the density reaches 6.9%, the interface between the saltwater and freshwater performs an inclined straight line, corresponding to the pervious theoretical derivation. The gravity current event could go through two different routes under the similar toe speeds. The current head without cylinders grows along the downslope path, and the enrollment and mixing with ambient fluid could occur at the edge of the head; while the currents within dense cylinders (∼6.9%) has a nearly constant thickness and a smaller and more streamlined head at the very front end. As the cylinder density increases, the front speed of gravity currents transforms from acceleration to deceleration phases. The experimental results reveal that the head would accelerate over the downslope course if the cylinder density is less than 2%.     

Numerical prediction on the scour burial of cylinder object freely resting on the sandy seabed in the East China Sea using the DRAMBUIE model

After experiencing 8-day combined tidal current, circulation and wave actions, scour depth surrounding cylinder object freely resting on sandy seabed in the East China Sea （ECS） in January is numerically predicted using the DRAMBUIE model designed for scour burial, which has been widely used and verified by in-situ experiments. During the period of numerical integration, the value of time t is generally variable at every time step via the special time-stepped approach developed by this paper to eliminate the time error. The tidal current velocity, wave orbital velocity and the depth-averaged circulation in the ECS have been obtained by numerical simulations with Estuarine Coastal and Ocean Model （ECOM）, Simulating Waves Nearshore （SWAN） model and Regional Ocean Modeling System （ROMS） model respectively. The control experiment and several idealized test cases on influential factors in scour depth reveal that the dominant hydrodynamic factor is tidal current in the ECS under normal weather conditions, and the impacts of shelf circulation and wave motion on local scour almost can be ignored with an exception of the Kuroshio area where the high-speed mainstream of Kuroshio flows. It is also indicated that in sandy sediments, the distribution of scour depth nearly follows the pattern of tidal currents, while the secondary influencing factor on scour depth appears to be grain size of sandy sediment in the ECS. Numerical tests on sediment grain size further testify that much finer sand is more easily scoured, and an increasing trend for scour depth with reduction of grain size is displayed due to imposed resistance of larger sized particles. Three aspects explored by this paper, including the empirical equations in the Defense Research Agency Mine Burial Environment （DRAMBUIE） model, the accuracy of inputs and infill process can severely affect the prediction of scour depth surrounding cylinder objects freely resting on sandy seabed in the ECS.     

Marker-and-cell and Chorin finite difference modeling for fluid flow in a single fracture

It is important to set up a detailed dynamic model of the fluid flow through fractures for understanding many fluid processes in Earth sciences. Numerical simulation is a popular tool for exploring these processes. The objective of this study is to understand fluid flow in fractures. Contrary to the conventional macro-scale modeling approach, micro-scale simulation is carried out. The Navior-Stokes equation solver was developed by a staggered marker-and-cell and the Chorin pressure iterating finite difference approach. We analyze the effects of the Reynolds number and the frequency of pressure fluctuations on flow mainly through visualization. A significant result is that the effect of pressure fluctuation-induced fluid flow can be observed in a broader frequency range. The peak velocity shifts along the spatial axis depending upon the frequency of the pressure fluctuation. An effective frequency band of the pressure fluctuation was identified which dominates dynamic behavior of the flow. Another major finding is that there exits a critical frequency of the pressure fluctuation which controls approximately the flow dynamic behavior. We conclude that it is only possible to estimate the flow behavior from pressure fluctuation, if effective frequency range is properly accounted for.     

Quasi-stationary flow structure in turbidity currents

A turbidity current is a particle-laden current driven by density differences due to suspended sediment particles. Turbidity currents can transport large amounts of sediment down slopes over great distances, and play a significant role in fluvial, lake and submarine systems. To better understand the sediment transport process, the flow system of an experimentally produced turbidity current in an inclined flume was investigated using video processing. We observed that the current progresses with constant frontal velocity and maintains an unchanged global interface geometry. In addition, the spatio-temporal profiles of the inner mean and turbulence velocity obtained by ultrasound velocity profiler (UVP) showed that similar distributions were maintained, with low dissipation. The results indicate that the turbidity current progressed in a quasi-stationary state, which enabled long-distance sediment transport. To understand the mechanisms behind the quasi-stationary flow, we analyzed the forces acting on the turbidity current. We found that under particular densities of suspended particles, the gravitational force is balanced by the viscous forces along the slope direction. We conclude that this specific force balance induces the quasi-stationary flow structure, enabling the long-distance transport of a substantial amount of sediment downstream with low dissipation.     

基于卫星测高交叉点的海洋表面地转流速度

在流体静力平衡状态下,海洋Coriolis力和压力梯度平衡就形成地转流,世界上大多数海流都近似为地转流.本文利用卫星测高交叉点方法计算海洋表面地转流速度,分析了利用测高交叉点计算地转流速度的不确定性,上升和下降弧段的海面倾斜在分辨率50 km上可以达到10^-7量级,才可能获得优于10 cm/s的地转流速度.在低纬度或者纬度接近卫星轨道倾角的地区,由交叉点方法计算的地转流速度精度低于中纬度地区.以中国台湾东部黑潮为试验区,利用最新的中国台湾周边海域大地水准面模型参考场计算高精度的大地水准面高,利用TOPEX/Poseidon和Jason-1的GDR数据(2002~2005年)计算海面高,然后计算交叉点的动力高,确定交叉点的地转流速度,结果与中国台湾NCOR(National Center for Ocean Research)的流速基本一致.     

Wave radiation stress

There are differences in the literature concerning the vertically dependent equations that couple currents and waves. In this paper, currents are purposely omitted until the end. Isolating waves from currents allows one to focus on two main topics: an explanation of Stokes drift with apparent mean vorticity obtained from an otherwise irrotational flow and the determination of vertically dependent wave radiation stress which, when vertically integrated, conforms to that obtained by Longuet-Higgins and Stewart (1964) and Phillips (1977) nearly 50 years ago and, more recently, by Smith (2006). Discussion begins with the simple case of nonlinear flow beneath a stationary wavy wall.     

Transverse structure of tidal and residual flow and sediment concentration in estuaries

An analytical and a numerical model are used to understand the response of velocity and sediment distributions over Gaussian-shaped estuarine cross-sections to changes in tidal forcing and water depth. The estuaries considered here are characterized by strong mixing and a relatively weak along-channel density gradient. It is also examined under what conditions the fast, two-dimensional analytical flow model yields results that agree with those obtained with the more complex three-dimensional numerical model. The analytical model reproduces and explains the main velocity and sediment characteristics in large parts of the parameter space considered (average tidal velocity amplitude, 0.1–1 m s^− 1 and maximum water depth, 10–60 m). Its skills are lower for along-channel residual flows if nonlinearities are moderate to high (strong tides in deep estuaries) and for transverse flows and residual sediment concentrations if the Ekman number is small (weak tides in deep estuaries). An important new aspect of the analytical model is the incorporation of tidal variations in the across-channel density gradient, causing a double circulation pattern in the transverse flow during slack tides. The gradient also leads to a new tidally rectified residual flow component via net advection of along-channel tidal momentum by the density-induced transverse tidal flow. The component features landward currents in the channel and seaward currents over the slopes and is particularly effective in deeper water. It acts jointly with components induced by horizontal density differences, Coriolis-induced tidal rectification and Stokes discharge, resulting in different along-channel residual flow regimes. The residual across-channel density gradient is crucial for the residual transverse circulation and for the residual sediment concentration. The clockwise density-induced circulation traps sediment in the fresher water over the left slope (looking up-estuary in the northern hemisphere). Model results are largely consistent with available field data of well-mixed estuaries.     

Along-shelf evolution and sea levels across the continental slope

If wind-stress or a horizontal oceanic density gradient acts over an ocean basin with an adjacent continental shelf and slope, sea-surface slopes and currents are set up along the shelf and slope with a return flow in the ocean. The currents evolve from zero at blocked ends of the shelf and basin. Such evolution is essentially barotropic (even for baroclinic forcing) and is relevant to all flow adjustments after longshore changes of depth profile or forcing. The distance over which this evolution takes place is investigated analytically for simple geometries, and numerically for a range of shelf, slope and ocean widths, shelf/ocean depth ratios, frictional decay rates and oscillatory frequencies. A close correspondence is found with the decay distance (group velocity x decay time) for a lowest mode continental shelf wave, often exceeding 1000 km. This correspondence is used to interpret some published model calculations of shelf and slope currents or return flows resulting from wind-stress or alongshore pressure gradients.Where a slope current is evolving, coastal sea levels do not follow oceanic levels. Implications for coastal/oceanic level differences are discussed. Oceanic sea-level features of shorter scale than the above 1000 km (say) do not penetrate fully to the coast. However, coastal sea levels averaged around small islands without broad shelves well represent surrounding oceanic levels.     

高频GNSS实时地震学与地震预警研究现状

为实现从注重灾后救助向注重灾前预防转变,如何提高地震灾害监测预警和风险防范能力成为我们关注的重点.本文给出了国际上GNSS位移记录、强震动加速度记录、测震速度记录在地震预警中的应用现状,并总结了各自的特点,归纳出围绕高频GNSS地震学在震级与破裂过程实时反演中的几个需要进一步研究的关键问题：（1）引入北斗系统,基于高频GNSS（GPS/BDS）双系统的实时位移解算方法来提高实时单站位移解算精度,使实时解算精度达到厘米级;（2）开展强震仪加速度记录基线偏移校正研究,弥补地震近场GNSS站密度不足问题;（3）强震仪加速度记录与GNSS位移记录特点不同,开展强震仪加速度数据与GNSS位移数据实时融合处理研究,快速获得包含丰富地震形变和速率的波形数据;（4）测震学方法可快速估算震级,但是在强震发生时会出现震级饱和现象,造成震级估算偏低.需要开展基于GNSS位移时间序列的多种方法相结合的实时震级估算方法研究,通过与地震学方法比较和结合,来得到精度高、计算快的震级估值算法;（5）基于高频GNSS、断层初始模型快速选取、断层尺度、参数自适应调整是快速判断断层破裂方向的基础,在断层破裂过程自适应准实时反演算法方面需要进一步加强.通过国内外研究现状调研、分析,表明基于高频GNSS地震学的震级快速确定、震源破裂过程准实时反演算法的发展将对我国地震预警系统从"二网融合"到"三网融合"提供坚实的技术支撑.     

Spatial variations of flow structure over estuarine hollows

Observations of the flow field over an elongated hollow (bathymetric depression) in the lower Chesapeake Bay showed tidally asymmetric distributions. Current speed increased over the landward side of the hole during flood tides and decreased in the deepest part of the hollow during ebb tides. A simple conceptual analysis indicated that the presence of a horizontal density gradient can generate the asymmetric spatial variations of flow structure depending on the sign of the horizontal density gradient. When water density decreases downstream, the velocity increases over the downstream edge of the hollow. Conversely when water density increases downstream, the flow decreases over the hollow more than a case without a horizontal density gradient. The conceptual analysis is confirmed by numerical experiments of simplified hollows in steady open channel flows and of an idealized tidal estuary. These hollows also alter the local current field of tidally averaged estuarine exchange flows. The residual depth-averaged currents over a hollow show a two-cell circulation when Coriolis forcing is neglected and an asymmetric two-cell circulation, with a stronger cyclonic eddy, when Coriolis forcing is included.     

Instantaneous dynamic pressure effects on the behaviour of lithic boulders in pyroclastic flows: the Abrigo Ignimbrite,Tenerife, Canary Islands

A method for estimating the instantaneous dynamic pressure near the base of ancient pyroclastic flows, using large lithic boulders from the late Pleistocene Abrigo Ignimbrite, is proposed here. The minimum instantaneous dynamic pressure is obtained by determining the minimum aerodynamic drag force exerted by a pyroclastic flow onto a stationary boulder that will allow the boulder to overcome static friction with the underlying substrate, and move within the flow. Consideration is given to the properties of the boulder (shape, roughness, size, density and orientation relative to the flow), substrate (type and hill slope angle), boulder-substrate interface (looseness of boulder, coefficient of static friction) and flow (coefficient of aerodynamic drag). Nineteen boulders from massive, lithic-rich ignimbrite deposits at two localities on Tenerife were assessed in this study. Minimum dynamic pressures required for Abrigo pyroclastic flows to move these boulders ranged from 5 to 38 kPa, which are comparable to dynamic pressures previously calculated from observations of the damage caused by recent pyroclastic flows. Considering the maximum possible range in flow density, the derived minimum velocity range for the Abrigo pyroclastic flows is 1.3 to 87 m s⁻¹.