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1.
Fourier spectrum amplitudes of horizontal and vertical earthquake accelerations recorded at the foundation levels of 57 buildings in the Los Angeles metropolitan area have been used to study the dependence of spectral amplitudes on the building foundation sizes. Comparison of these amplitudes with those predicted by empirical models for scaling ‘free field’ Fourier amplitude spectra does not indicate any significant dependence of the spectral amplitudes on the size of the foundation. Third degree polynomials have been employed to smooth the spectra of the accelerations recorded inside the buildings and their coefficients have been examined as functions of the foundation plan dimensions. These results also indicate no significant dependence of the spectral amplitudes on the foundation dimensions. A qualitative analysis of the spectral amplitudes for possible effects caused by the phenomena associated with soil-structure interaction indicates that the Fourier spectra of the recorded accelerations may experience some amplification as the relative ‘density’ of the foundation-structure system increases.  相似文献   

2.
The spatial relationship between areas with severely damaged (red-tagged) buildings and areas with large strains in the soil (indicated by reported breaks in the water distribution system), observed during the 1994 Northridge earthquake, is analysed. It is shown that these areas can be separated almost everywhere. Minimal overlapping is observed only in the regions with very large amplitudes of shaking (peak ground velocity exceeding about 150 cm s−1). One explanation for this remarkable separation is that the buildings on ‘soft’ soils, which experienced nonlinear strain levels, were damaged to a lesser degree, possibly because the soil absorbed a significant portion of the incident seismic wave energy. As a result, the total number of severely damaged (red-tagged) buildings in San Fernando Valley, Los Angeles and Santa Monica may have been reduced by a factor of two or more. This interpretation is consistent with the recorded peak accelerations of strong motion in the same area. It is concluded that significant reduction in the potential damage to wood frame single family dwellings may be expected in areas where the soil experiences ‘large’ strains (beyond the linear range) during strong earthquake shaking, but not significant differential motions, settlement or lateral spreading, near the surface.  相似文献   

3.
The Ms 5.9 earthquake of 1999 produced valuable records in three underground structures, as follows: (a) in the just completed cut-and-cover station of Sepolia two accelerographs recorded the free-field and the station-base motion; (b) in the still under-construction tunnelled station of Monastiraki an accelerograph recorded the ground surface motion, and (c) in the nearby Kerameikos station, abandoned for non-technical reasons, the temporary prestressed-anchor piled (PAP) wall was still in place and produced a record of total seismic displacement at its top. Directly or indirectly utilising these records, the article outlines the results of numerical analyses aimed at ‘recovering’ the complete seismic response of the three underground structures. Particular emphasis is given to Sepolia station, where the developed accelerations (with PGA of about 0.17 g at the station base and 0.43 g at the station roof) are shown to have been almost exactly equal to the design accelerations according to the seismic code under the assumption that the station responds as an aboveground structure. The successful performance of the two temporary structures, in Monastiraki and Kerameikos (which had been designed against minimal acceleration levels but experienced ground-surface high-frequency accelerations of the order of 0.50 g) is explained through dynamic response analyses.  相似文献   

4.
The areas that experienced large strains and differential motions in the soil (indicated by breaks in the water and gas pipe distribution systems) and the areas with severely damaged buildings showed remarkable separation during the March 10, 1933, Long Beach, California earthquake. With analogous results for the 1994 Northridge, California earthquake [Soil Dynam. Earthquake Engng. 17 (1998) 41], the observations summarized in this paper show the fallacy of simplistic and popular interpretations, such as those that hold that in the near field the damage to buildings is caused by ‘soft’ or ‘bad ground’ conditions. In fact, significant reduction in the potential damage to buildings may be expected in the areas where the soil experiences ‘moderate to large’ strains.  相似文献   

5.
Time series measurements from an array of temperature miniloggers in a line at constant depth along the sloping boundary of a lake are used to describe the ‘internal surf zone” where internal waves interact with the sloping boundary. More small positive temperature time derivatives are recorded than negative, but there are more large negative values than positive, giving the overall distribution of temperature time derivatives a small negative skewness. This is consistent with the internal wave dynamics; fronts form during the up-slope phase of the motion, bringing cold water up the slope, and the return flow may become unstable, leading to small advecting billows and weak warm fronts. The data are analysed to detect ‘events’, periods in which the temperature derivatives exceed a set threshold. The speed and distance travelled by ‘events’ are described. The motion along the slope may be a consequence of (a) instabilities advected by the flow (b) internal waves propagating along-slope or (c) internal waves approaching the slope from oblique directions. The propagation of several of the observed ‘events’ can only be explained by (c), evidence that the internal surf zone has some, but possibly not all, the characteristics of the conventional ‘surface wave’ surf zone, with waves steepening as they approach the slope at oblique angles.  相似文献   

6.
Phase equilibrium experiments were performed on typical ‘oceanic’ and ‘cratonic’ peridotite compositions and a Ca, Al-rich orthopyroxene composition, to test the proposal that garnet lherzolites exsolved from high-temperature harzburgites, and to further our understanding of the origin of ancient cratonic lithospheres. ‘Oceanic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1450–1600°C, but at 5 GPa and temperatures less than 1450°C, crystallize clinopyroxene to become true lherzolites. ‘Cratonic’ peridotites crystallize a garnet harzburgite assemblage at pressures above 5 GPa in the temperature range 1300–1600°C. Garnet-free harzburgite crystallizes from both ‘cratonic’ and ‘oceanic’ peridotite at temperatures above 1450°C and pressures below 4.5–5 GPa. Phase relations for the high Ca, Al-rich orthopyroxene composition essentially mirror those for ‘oceanic’ peridotite.The complete solution of garnet and clinopyroxene into orthopyroxene observed in all three starting compositions at temperatures near or above the mantle solidus at pressures less than 6 GPa supports the hypothesis that garnet lherzolite could have exsolved from harzburgite. The inferred cooling path for the original high-temperature harzburgite protoliths of garnet lherzolites differs depending on bulk composition. The precursor harzburgite protoliths of garnet lherzolites and harzburgites with ‘cratonic’ bulk compositions apparently experienced simple isobaric cooling from formation temperatures near the peridotite solidus to those at which most of these peridotites were sampled in the mantle (< 1200°C). The cooling histories for harzburgite protoliths of sheared garnet lherzolites with ‘oceanic’ compositional affinity are speculated to have involved convective circulation of mantle material to depths deeper than those at which it was originally formed.Phase equilibria and compositional relationships for orthopyroxenes produced in phase equilibrium experiments on peridotite and komatiite are consistent with an origin for ‘cratonic’ peridotite as a residue of Archean komatiite extraction, which has since cooled and exsolved clinopyroxene and garnet to become the common low-temperature, coarse-grained peridotite thought to comprise the bulk of the mantle lithosphere beneath the Archean Kaapvaal craton.  相似文献   

7.
Normal faults within the Ptolemais coal field and large seismogenic faults in the northwestern Aegean remain fractal for displacement values larger than about 1m. The kinematic parameters on reverse drag profiles such as length of rollover, footwall uplift and wavelength of footwall uplift show that all three parameters have a power law relationship, expressed by a c exponent of about 1, with the maximum displacement which take place across the fault. Footwall uplift/hanging wall subsidence ratio is about 1/2.The displacement analysis help us to propose a growth model for larger seismogenic faults in the NW Aegean, as is the ‘Hepiros fault set’ and the ‘Aliakmon fault zone’. Faults within the ‘Aliakmon fault zone’ were independently developed, at the first stages of deformation, by tip line deformation and out-of plane bifurcation, whereas later, deformation continued by segment linkage. One of these faults the ‘Sarakina fault’ was reactivated during the 1995 earthquake to produce a 25 km long surface rupture. A long term slip rate of about 0.3 mm a−1 has been estimated by taking into consideration that over the past 6 Ma a maximum displacement of 1700 m across this fault has taken place.  相似文献   

8.
Earthquake hazard in Marmara Region, Turkey   总被引：2，自引：0，他引：2  
Earthquake hazard in the Marmara Region, Turkey has been investigated using time-independent probabilistic (simple Poissonian) and time-dependent probabilistic (renewal) models. The study culminated in hazard maps of the Marmara Region depicting peak ground acceleration (PGA) and spectral accelerations (SA)'s at 0.2 and 1 s periods corresponding to 10 and 2% probabilities of exceedance in 50 yrs. The historical seismicity, the tectonic models and the known slip rates along the faults constitute the main data used in the assignment. Based on recent findings it has been possible to provide a fault segmentation model for the Marmara Sea. For the main Marmara Fault this model essentially identifies fault segments for different structural, tectonic and geometrical features and historical earthquake occurrences. The damage distribution and pattern of the historical earthquakes have been carefully correlated with this fault segmentation model. The inter-event time period between characteristic earthquakes in these segments is consistently estimated by dividing the seismic slip estimated from the earthquake catalog by the GPS-derived slip rate of 22±3 mm/yr. The remaining segments in the eastern and southern Marmara region are also identified using recent geological, geophysical studies and historical earthquakes. The model assumes that seismic energy along the segments is released by characteristic earthquakes. For the probabilistic studies characteristic earthquake based recurrence relationships are used. Assuming normal distribution of inter-arrival times of characteristic earthquakes, the ‘mean recurrence time’, ‘covariance’ and the ‘time since last earthquake’ are developed for each segment. For the renewal model, the conditional probability for each fault segment is calculated from the mean recurrence interval of the characteristic earthquake, the elapsed time since the last major earthquake and the exposure period. The probabilities are conditional since they change as a function of the time elapsed since the last earthquake. For the background earthquake activity, a spatially smoothed seismicity is determined for each cell of a grid composed of cells of size 0.005°×0.005°. The ground motions are determined for soft rock (NEHRP B/C boundary) conditions. Western US-based attenuation relationships are utilized, since they show a good correlation with the attenuation characteristics of ground motion in the Marmara region. The possibility, that an event ruptures several fault segments (i.e. cascading), is also taken into account and investigated by two possible models of cascading. Differences between Poissonian and renewal models, and also the effect of cascading have been discussed with the help of PGA ratio maps.  相似文献   

9.
In this paper we present an approach for 3-D travel-time tomography, which works well in reconstructing high contrast velocity anomalies in both location and strength. It uses a revised ‘irregular’ approach to the shortest-path method as the ray tracer and a damped minimum norm, and constrained least-squares CG approach as the inversion solver. In ray tracing, the advantages of the revised ‘irregular’ over the ‘regular’ approach are that the secondary nodes introduced on the cell surfaces significantly improve accuracy of computed travel times, without dramatically increasing the total number of cells and nodes; the tri-linear velocity function defined across the cell guarantees accurate ray tracing in a high velocity contrast medium; and the capacity to calculate a relatively large 3-D model, due to the fast run speed (at least one order of magnitude over the ‘regular’ approach) and less number of total nodes. The introduction of ‘soft’ and ‘hard’ bounds into the inversion process changes the conditioning and makes the solution meaningful in a physical sense. Thus the artifacts caused by noise and high velocity contrasts are substantially suppressed and the image quality is considerably improved, making the solution realistic with noisy or inconsistent travel-time data. Several numerical tests indicate that we can obtain good quality images even for high velocity contrast anomalies (say more than 20%) in the target region. This means the inversion algorithm is an efficient and effective procedure. Meanwhile, the inversion procedure is not very sensitive to the quality of the travel-time data, which is promising for practical usage.  相似文献   

10.
Analysis of the refracted arrivals on a seismic reflection profile recorded along the wall of a tunnel at an iron mine near Thabazimbi, South Africa, shows variations in P-wave velocity in dolomite away from the de-stressed zone that vary between 4.4 and 7.2 km/s, though values greater than 5.8 km/s predominate along most of the profile. The seismic velocities at the tunnel wall, however, vary between 4.2 and 5.2 km/s. Time–depth terms are in the range from 0.1 to 0.9 ms, and yield thicknesses of the zone disturbed by the tunnel excavations of between 2 and 9 m. The very low seismic velocities away from the tunnel wall in two regions are associated with alcoves or ‘cubbies’ involving offsets in the wall of up to 10 m. The large variations in seismic velocity resolved over distances less than 15 m with signals of wavelength around 6–9 m are attributed to variations in the sizes and concentrations of fracture systems and cracks, and in the degree of groundwater saturation of the fracture systems. The results suggest that seismic velocity variations from reflection surveys may also assist modelling studies of the stress regime in deep mines, particularly if both P and S wave velocity variations can be determined. The seismic velocity variations inferred also show that application of refraction static corrections in the processing of ‘in-mine’ seismic reflection profiles is as important as in surface surveys, because of the higher frequencies of the seismic energy recorded in the deep mine environment.  相似文献   

11.
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.  相似文献   

12.
Fuzzy neural network models for liquefaction prediction   总被引：1，自引：0，他引：1  
Integrated fuzzy neural network models are developed for the assessment of liquefaction potential of a site. The models are trained with large databases of liquefaction case histories. A two-stage training algorithm is used to develop a fuzzy neural network model. In the preliminary training stage, the training case histories are used to determine initial network parameters. In the final training stage, the training case histories are processed one by one to develop membership functions for the network parameters. During the testing phase, input variables are described in linguistic terms such as ‘high’ and ‘low’. The prediction is made in terms of a liquefaction index representing the degree of liquefaction described in fuzzy terms such as ‘highly likely’, ‘likely’, or ‘unlikely’. The results from the model are compared with actual field observations and misclassified cases are identified. The models are found to have good predictive ability and are expected to be very useful for a preliminary evaluation of liquefaction potential of a site for which the input parameters are not well defined.  相似文献   

13.
Several new empirical equations of the frequency dependent duration of strong earthquake ground motion are presented. The duration is considered as being composed of two parts: (1) the duration of stong motion as it is observed at recording stations located on basement rocks, and (2) the prolongation of this duration for stations located on sediments. The first part, called the ‘basic duration’, is modelled in terms of the Modified Mercalli intensity and (in some cases) the hypocentral distance. The depth of the sediments under the station, the distance from the station to the rocks surrounding it, and the angular measure of the size of those rocks (as seen from the station) are chosen as the parameters for modelling the prolongation of the duration. The new empirical equations are compared (a) with each other, (b) with our previous models which used similar ‘prolongation’ terms, but the ‘basic duration’ was expressed in terms of the magnitude of the earthquake and the source-to-station distance, and (c) with models with ‘intensity-type’ ‘basic duration’, but with a simplified ‘prolongation’ term (the geological conditions at the stations are modeled by lumping all the sites into three groups: basement rock, sediments and intermediate geology). This collection of models is found to have good internal consistency. The choice of the proper model depends on the availability of the earthquake and site parameters. The residuals of the empirical regression equations are found to have similar distribution functions for all the models. An explicit functional form for such distributions is proposed, and the frequency dependent coefficients are found for all the models of duration. This allows one to predict (for each set of earthquake and site parameters) the probability of exceedance of any given level of duration of strong ground motion at a given frequency.  相似文献   

14.
Blackfly communities from five German stream types out of two ecoregions (small and mid-sized siliceous gravel-bed mountain streams of the Central mountains, ecoregion 9, and organic type brooks, small and mid-sized sand-bottomed Central lowland streams of ecoregion 14) are compared. Ecoregional, as well as stream type-specific biocoenotic differences are discussed. The presence of Prosimulium sp. was restricted to mountain streams, whereas Simulium lineatum seemed to prefer mid-sized sand-bottomed Central lowland streams, and S. vernum showed a clear preference for Central lowland streams. An Index of Morphological Stress (IMS), derived from 22 geo-hydromorphological parameters recorded during sampling in the field, is presented. According to the IMS values, sites are divided into morphologically ‘unstressed’ sites (high or good morphological quality) and ‘stressed’ sites (moderate, poor or bad morphological quality), and biocoenotic differences of the two categories are discussed. Two stream types and the entire data set showed significantly higher numbers of taxa at ‘unstressed’ sites. Linear Multiple Regression (LMR) was used to identify geo-hydromorphological parameters that significantly explain the variance of the three most constant taxa, Prosimulium sp., P. hirtipes and Simulium sp. in the LMR model.  相似文献   

15.
A numerical model of deep, uniform, oscillatory, rough-turbulent boundary-layer flow is described. The model is based upon the governing horizontal momentum equation and a closure scheme involving the turbulent-energy equation and various turbulence-scaling laws. Finite difference solutions of these equations are obtained for a range of values of the ‘relative roughness’ (A0/ks), whereA0 is the excursion amplitude of the water particles in the free-stream flow andks is the ‘equivalent bed roughness’. Typical vertical profiles of horizontal velocity, turbulence energy and eddy viscosity, and time-series of the bed shear stress are presented. The model results are then used to determine the wave drag coefficient, boundary-layer thickness and phase lead of the bed shear stress over the free-stream velocity, each as a function ofA0/ks. These results are shown to be in generally good agreement with previous experimental and theoretical results. Finally, the model is used to test for the existence of a universal velocity distribution for uniform oscillatory (sinusoidal) rough-turbulent flow. The ‘law of the wall’ and the ‘defect law’ proposed by Jonsson (1980, Ocean Engineering, 7, 109–152) are well supported by the model, and the existence of a logarithmic ‘velocity overlap layer’ in which both of these laws are valid is demonstrated forAo/ks30.  相似文献   

16.
This paper compares the distribution of damage from the San Fernando, 1971, and Northridge, 1994, earthquakes. Both events had similar size, occurred on blind thrust faults beneath the densely populated San Fernando Valley of the Los Angeles metropolitan area, and hence offer a rare opportunity to compare the effects of the two earthquakes. In a previous study of the distribution of red-tagged (‘unsafe’) buildings and of breaks in the water distribution system caused by the Northridge earthquake, the authors discovered that buildings were damaged less where the soil response was not linear (as indicated by the breaks in the water pipes), except in localized areas of very severe shaking (peak ground velocity exceeding 150 cm/s). The study in this paper shows that the same applies to the damage caused by the San Fernando earthquake, and that the areas with severely damaged buildings (so called ‘gray zones’) for both earthquakes overlapped. This reoccurrence of damage within the same area is interpreted to result from some specific properties of local soil and geology. These properties are not fully understood at present, but should be explored to provide a basis for a new tool for forecasting microzonation maps, and reducing future seismic hazard.  相似文献   

17.
We describe a magma mingling episode from Ruapehu volcano between two andesite magmas, one very much minor in volume relative to the other. The event acted to trigger eruption of the andesitic Pourahu pyroclastic flow which is preserved in a thick sequence of tephras and laharic deposits in the southeastern ring plain of the volcano. The predominant andesite is pale brown coloured and porphyritic containing phenocrysts of plagioclase-clinopyroxene-orthopyroxene-Fe-Ti oxides. Rare clasts of a darker andesite are different texturally, less vesicular, and contain distinctive microphenocrysts of plagioclase and quench olivine. Equally rare clasts, of streaky pumice consisting of interbanded ‘dark’ and ‘light’ andesite attest to mingling between these two andesite components.Chemical analyses of discrete clasts demonstrate that the Pourahu pyroclastic flow andesites span much of the compositional spectrum of Ruapehu andesites. This observation demonstrates heterogeneity in the products of a relatively small eruption. The darker clast analyses and those from associated distal fall deposits lie within the fields defined by the dominant light coloured clasts. Phenocryst and microphenocryst geothermometry suggest slightly higher temperatures in the dark component. However, glasses from groundmass and phenocryst inclusions in the same specimen may differ considerably, leading us to conclude that many phenocrysts are in fact xenocrystic and were incorporated in the melts as they migrated towards the surface.We prefer a model in which a small volume of hot andesite magma injects a vent-feeding magma chamber, triggering vesiculation and eruption. We infer that the process of magma withdrawal extended downward into the magma body causing the dark component to intermingle with the lighter (dominant) component, ‘sucking’ more dark magma into the chamber. Our observations are entirely consistent with the existence of a plexus of small, possibly interlinked magma chambers beneath Ruapehu.  相似文献   

18.
Structures in bedrock and sediments suggest that the Fennoscandian Shield was subjected to a higher seismicity at the end of the last glaciation than it is today. We suggest that the annual varves of the glacial clays document this seismicity. In the Stockholm area, three paleoseismic events have so far been recorded in the varved clay chronology. Liquefaction structures have also been documented. The occurrence of heaps of ‘sharp-edged blocks’ (large caves in crystalline bedrock) might be another sign of this seismicity.  相似文献   

19.
The paper will describe a number of new characteristic phenomena of sand under cyclic loading, which have been discovered by triaxial cell testing, at the Soil Mechanics Laboratory at Aalborg University using test specimens with equal height and diameter. These phenomena have been compared with results from tests executed on specimens with double height. It will be explained why the new phenomena ‘Stabilization’ and ‘Instant Stabilization’ are seldom observed when the test is executed on specimens with double height. The conclusion recommends that the study of pore pressure variation due to cyclic loading should only be performed with smooth pressure heads and specimens with equal height and diameter because the phenomena leading to stress hardening would otherwise not be observed.  相似文献   

20.
Estimating severity of liquefaction-induced damage near foundation   总被引：1，自引：2，他引：1  
An empirical procedure for estimating the severity of liquefaction-induced ground damage at or near foundations of existing buildings is established. The procedure is based on an examination of 30 case histories from recent earthquakes. The data for these case histories consist of observations of the damage that resulted from liquefaction, and the subsurface soil conditions as revealed by cone penetration tests. These field observations are used to classify these cases into one of three damaging effect categories, ‘no damage’, ‘minor to moderate damage’, and ‘major damage’. The potential for liquefaction-induced ground failure at each site is calculated and expressed as the probability of ground failure. The relationship between the probability of ground failure and the damage class is established, which allows for the evaluation of the severity of liquefaction-induced ground damage at or near foundations. The procedure presented herein represents a significant attempt to address the issue of liquefaction effect. Caution must be exercised, however, when using the proposed model and procedure for estimating liquefaction damage severity, because they are developed based on limited number of case histories.  相似文献   

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