High energy coastal sedimentary deposits; an evaluation of depositional processes in Southwest England

Sediments left by coastal flooding have been observed worldwide and have been variously ascribed to the action of storm surges and tsunami waves. To date, no study has attempted to unequivocally establish on stratigraphical, sedimentological, and palaeoecological grounds the means by which these two different processes might be distinguished in coastal sedimentary sequences. This paper examines the evidence for historical storm surges and tsunamis and shows that both high magnitude events have been documented over the past 250 years in southwest England. Sand layers of varying thickness are present within Holocene lagoonal and peat sequences of several shallow lakes of the Scilly Isles. Detailed analysis of Big Pool, St Agnes, indicates that the basal peats date from around 1000 BP. Within the basal peats are numerous thin sand layers. Above the basal peat is an extensive sand layer 15 to 40 cm thick. The base of this latter layer probably dates from the early to mid 18th century. Particle size, grain surface morphology, diatom, and mineral magnetic studies are used to try and determine the most likely mode of deposition. The results of all analyses are inconclusive, but the weight of evidence suggests that the increasing frequency of thin sand layers in the upper part of the basal peat may be related to the increasing frequency and intensity of Atlantic storms during the Little Ice Age superimposed upon a rising sea level. The thick sand layer may have been deposited by the tsunami wave generated by the Lisbon earthquake of November 1,1755. Due to the difficulties in distinguishing depositional processes in coastal environments known to have been affected by storm surges and tsunami waves, it is suggested that generally accepted sedimentological techniques are inadequate for discriminating depositional processes in coastal environments.     

滇西地区九种岩石的P波Q值

本文叙述了以波传播和谱振幅比法测得的滇西地区九种岩石在不同应力状态下(σ_1=σ_2=σ_3=σ)的衰减系数,P波速度及Q值。在σ≤90 MPa时,随压力增加,衰减系数减小,P波速度及Q值增大;在σ≥120 MPa以后,衰减系数、P波速度及Q值变化甚小。其中团山砂岩及砖窑村片麻岩Q值较低。     

Fault proximity surveys in coal using guided seismic waves—Numerical modelling

Guided seismic waves are generated at lateral discontinuities in coal seams as an integral part of the mechanics of scattering downgoing body waves from a surface energy source.Numerical and laboratory model studies undertaken for both thin and thick coal seam waveguides show that small faults generate recognizable diffraction patterns by convertingP andS waves into channel wave arrivals at in-seam geophones on a walkawayVSP profile.These results suggest that in-seam geophones can be used during subsequent surface reflection surveys so as to improve mine profitability by optimising longwall layout in faulted ground.     

The experimental study of parameters of rocks under ultrahigh-pressure state

The parameters of rocks in ultrahigh-presure state are very important to understanding the geodynamics and explosion engineering of rocks. In these experiments the plane wave dynamite lenses were used to drive the flying plates. The flying plate hits the target and produces the ultrahigh-pressure shock wave. The propagation velocity and the particle velocity of shock wave in rock samples were measured by probes, and the relation between density of rocks and pressure was obtained by Rankine-Hugoniot formulas. The maximum pressure in experiments was higher than 70 GPa. The ultrahigh-pressure state formulas obtained are: Limestoneρ=3.22+0.016p; Graniteρ=3.20+0.024p. In which the unit for ρ is g/cm³, the unit forp is GPa. The Chinese version of this paper appeared in the Chinese edition ofActa Seismologica Sinica,13, 96–103, 1991.     

Vertical S waves in soil stratum over halfspace

An analytical model of a soil layer resting on a halfspace with vertically propagating shear waves is studied under a harmonic excitation on the surface. The soil is treated as a Kelvin-Voigt continuum, using the one-dimensional wave equation. The material parameters can be strain dependent. A hyperbolic model proposed by Hardin & Drnevich is used for describing the nonlinear stress-strain relationship. Based on this, a solution to a problem where thirty thousand people were jumping to the rhythm of rock music on the field in a stadium is given. The soil stratum is a 5–60m thick uniform soft high-plastic clay. The particle velocity was estimated to range from 22·5 mm/s to 45 mm/s in the first overtone. These velocity values exceeded the critical value, of 20 mm/s, of the German building code DIN 4150 which may explain the occurence of damages.     

Analysis of Rayleigh waves in saturated porous elastic media by finite element method

A numerical procedure for the analysis of Rayleigh waves in saturated porous elastic media is proposed by use of the finite element method. The layer stiffness matrix, the layer mass matrix and the layer damping matrix in a layered system are presented for the discretized form of the solid-fluid equilibrium equation proposed by Biot. In order to consider the influence of the permeability coefficient on the behavior of Rayleigh waves, attention is focused on the following states: ‘drained’ state, ‘undrained’ state and the states between two extremes of ‘drained’ and ‘undrained’ states. It is found from computed results that the permeability coefficient exerts a significant effect on dispersion curves and displacement distributions of Rayleigh waves in saturated porous media.     

Effect of anelastic and scattering structures of the lithosphere on the shape of local earthquake coda

A simple model of single acoustic scattering is used to study the dependence of the shape of local earthquake coda on the anelastic and scattering structures of the lithosphere. The model is applied to the coda of earthquakes located near Stone Canyon, central California, and provides an explanation for the features observed in the data, which include an interesting temporal variation in the coda shape. A surficial layer with aQ of 50 and thickness of 10 or 25 km underlain by a zone with aQ of 1000 extending to the bottom of the lithosphere, together with a scattering scale length,a, that varies with depthz according to the relationa=0.3 exp[-(z/45)²] are found to constitute the simplest structure of the medium compatible with the coda data and with body and surface wave attenuation data. The profile of heterogeneity sizes implies that the scattering strength increases strongly with depth, a constraint required by the necessity to boost the energy of the later coda without forcing the intrinsicQ to be excessively high in the uppermost mantle. This constraint is viewed as an artifact of the single scattering model which overstimates the scattering coefficient due to the neglect of multiple scattering. The observed temporal variation of the signal is difficult to explain by a simple change of the intrinsicQ at some depth. Rather, it is suggested that the scattering properties at depth changed with time through a variation of the fractional rms velocity fluctuation on the order of one percent.     

Propagation and attenuation characteristics of the crustal phaseLg

TheLg wave consists of the superposition ofS waves supercritically reflected, and thus trapped, in the crust. This mode of propagation explains the strong amplitude of this phase and the large distance range in which it is observed. The numerical simulation leads to successful comparison between observed seismograms in stable continental areas and synthetics computed for simple standard crustal models. In regions with strong lateral variations, the influence of large-scale heterogeneities on theLg amplitude is not yet clearly established in terms of the geometrical characteristics of the crustal structure.The analysis of the decay of amplitude ofLg with epicentral distance allows the evaluation of the quality factor ofS waves in the crust. The results obtained show the same trends as codaQ: a clear correlation with the tectonic activity of the region considered, both for the value ofQ at 1 Hz and for its frequency dependence, suggesting that scattering plays a prominent part among the processes that cause the attenuation.The coda ofLg is made up of scatteredS waves. The study of the spatial attenuation of the coda indicated that a large part of the arrivals that compose the coda propagate asLg. The relative amplitude of the coda is larger at sites located on sediments because, in these conditions, a part ofLg energy can be converted locally into lower order surface modes.     

风浪作用下水生植物对水流结构的影响——以太湖中两种典型沉水植物为例

为研究风浪作用下水生植物对水流结构的影响，选取太湖中两种典型沉水植物（苦草与马来眼子菜）为研究对象，分别对苦草植物斑块与马来眼子菜植物斑块内外水体的瞬时流速进行野外现场测量，利用瞬时流速的能量谱分布将波浪流速与紊动流速分离，分别分析水生植物对时均流速、波浪流速以及紊动能的影响。风浪影响下，水体中存在流向与测量时近水面处盛行风向一致的水流；波浪流速以垂向流速为主，且波浪流速自水面向床底逐渐减小；紊动能在水面处达到最大值，并向床底方向逐渐减小。与无植被条件相比，苦草与马来眼子菜的存在减小了时均流速、波浪流速以及紊动能。两种植物形态上的差异，导致其对水流结构的影响不同：苦草叶片阻流面积在冠层中部达到最大，使得时均流速与波浪流速在苦草中部位置的减小程度最大；马来眼子菜叶片主要集中于冠层顶部（水面附近），其对时均流速以及波浪流速的减小作用在水面处达到最大。